Fast Ethernet network technology. Ethernet technology
INTUIT Internet Courses Exam Questions: 267. LAN Basics
In which topologies is the CSMA / CD control method applied?
In which network using the CSMA / CD access method, all other things being equal, will there be fewer collisions?
When are modulation techniques listed correctly in order of increasing immunity to interference?
When are the listed technologies correctly ranked in order of increasing maximum achievable transfer rate?
What is the main difference between FDDI management and Token-Ring management?
What is the main advantage of the FDDI network over other standard networks?
What is the difference between a Class I hub and a Class II hub?
What is the main disadvantage of ring topology?
What is the main distinguishing feature of a local network?
What is the main purpose of a local area network?
What is the main advantage of using a dedicated server on the network?
What is the main advantage of UTP twisted pair cable?
What is the main advantage of Arcnet over Ethernet?
What is the main disadvantage of the Manchester code?
What is the main disadvantage of the marker control method?
What is the main disadvantage of fiber optic cable?
What is fundamental difference deterministic access methods versus random ones?
What are the main advantages of certified structured cabling systems (SCS) in comparison with cable systems created "in-house"?
To what frequency are the performance characteristics of cable lines supporting Class D applications determined according to SCS standards?
To what level of the OSI model does the formation of network packets of a specified type belong?
What type of UTP cable provides maximum signal attenuation at a given frequency?
How model 2 accounts for latency network adapters and hubs?
How does Model 2 account for cable attenuation?
How does Fast Ethernet account for Packet Gap Gap (IPG) reduction?
How will the maximum possible data transfer rate change in discrete channel with an increase in the bitness of data by 4 times?
How does the delay of the next packet transmission after collision change in the CSMA / CD access method?
How to ground coaxial cable?
How to correctly arrange the active network equipment for the indicated types according to the level of price increase local area networks?
How are twisted pair functions distributed in the 100BASE-T4 segment?
What is the maximum network length that can be implemented on 10BASE2 segments without repeaters?
What is the maximum rated speed provided on an ADSL line?
Which international organization is the developer of the SCS standard?
What error is not logged and corrected by repeater hubs?
Which network provides Ethernet compatibility at the packet format level?
What is the faster transfer rate?
What function is not performed by the network adapter?
What quantities need to be calculated using Model 2 Ethernet Topology Assessment?
Which of the active network devices prevail quantitatively in the composition of the enterprise network?
Which of the following measures do not belong to the complex of measures to protect information?
Which of the listed technologies are fundamentally asymmetric (the speed of information transfer from user to provider and back is different)?
Which of the modern technologies listed below are used to transmit information over analog telephone lines?
What modulation techniques are used in high speed modems?
What control methods guarantee the size of the access time?
What mistakes in the organization of the cable system primarily affect the speed of information transfer?
What subsystems, in accordance with the standards, does the SCS include?
What connectors are used to connect cables on a 10BASE-T network?
Which Fast Ethernet segments use the same encoding system?
What network devices do not perform any information processing?
What are the standards for using different modems for the user and the provider?
What devices do not allow all packets to pass through?
What are the primary factors limiting the transmission rate of wireless (radio) links?
What characteristics of cables are most important for protecting the information transmitted through it from the influence of external electromagnetic radiation and reducing the radiation of the cable itself?
What kind digital elements includes cyclic encoder and decoder?
What is the main disadvantage of LANs?
What is the main disadvantage of the 10BASE-T segment?
What is the main disadvantage of asymmetric encryption methods?
What is the size of the MAC address of subscribers in Ethernet networks?
What is the key length in the standard GOST 28147-89 encryption method?
What is the length of an Ethernet / Fast Ethernet packet without a preamble?
What is the length of the SPARK signal used in the CSMA / CD access method to increase the likelihood of collision detection?
What should be the value of the matching impedance in relation to the characteristic impedance of the cable?
What is the maximum allowable Ethernet packet gap reduction?
What is the main purpose of configuring network OS settings?
What is the typical characteristic impedance for a coaxial cable?
What is the main advantage of centralized management methods?
What is the main advantage Wi-Fi networks over Ethernet / Fast Ethernet?
What is the minimum distance allowed between computers on a 10BASE2 segment?
What is the purpose of a hub in a 100VG-AnyLAN network?
What is the main advantage of WLAN?
What is the main advantage of Token-Ring over Ethernet / Fast Ethernet?
What are the possible exchange modes in a 10Gigabit Ethernet network?
What are the main benefits of Fast Ethernet?
What are the main benefits of a bus topology?
What are the features of a peer-to-peer network?
Which of the following definitions of network utilization is correct?
What is the most difficult device to use to troubleshoot a live network?
What is the maximum number of segments that a path between subscribers in an Ethernet network can contain according to the model 1 rules?
What is the maximum relative number of errors in received data allowed by the ITU-T standard?
Which network device parses the contents of the packet data field?
What network device is not capable of supporting communication between segments at different rates?
Which network device provides the lowest packet relay latency?
Which of the "classical" encryption methods generally leads to a change in the composition of the alphabet in an encrypted message?
Which type of analog modulation is most susceptible to interference and noise?
Which of the following codes is not self-synchronizing?
Which computer interface is the most suitable for Fast Ethernet?
What code is used on the 100BASE-T4 segment?
Which code requires the minimum bandwidth for a given baud rate?
Which code is self-synchronizing?
What access method is used on WLANs?
What method cannot be used to overcome the limitations on the size of the Ethernet network (conflict zone)?
What is the main disadvantage of the FDDI network when compared to other standard networks?
What parameter of the network adapter does not affect the integrated speed of information exchange over the network?
What is the least important parameter of the network server?
Which protocol does not guarantee packet delivery?
Which Ethernet / Fast Ethernet segment provides the longest distance between the computers on the network?
What is the IEEE specification for Ethernet LAN?
What is the standard for thick coaxial Ethernet?
Which standard segment provides the maximum length on an electrical cable?
What type of segment is not recognized by the Auto-Negotiation mechanism?
What type of transmission medium does not require galvanic isolation?
What type of transmission medium provides the maximum noise immunity and secrecy of information transmission?
What type of transmission medium provides the maximum data transfer rate?
What type of telephone line is preferred for LAN to WAN communications?
Which factor has the least impact on network performance?
What information does the control field contain in an Ethernet / Fast Ethernet packet?
What is the function of the encoder in the modem?
What is the function of a Token-Ring hub?
What is the function of the equalizer in the modem?
What function is the Active Token-Ring Monitor not performing?
When should you use a crossover cable on a 10BASE-T network?
Who determines the physical address (MAC address) of Ethernet subscribers?
Can rate in characters per second (cps) be derived from rate in bits / s divided by 8?
At what level of the OSI model is the correctness of the packet transmission checked?
At what level of the OSI model does the switch operate?
At what level of the OSI model do routers operate?
Is backing up files related to one of the information protection methods?
Why "classic" encryption methods (substitution, permutation, and gamming) do not provide complete cryptographic protection information?
What devices can be used to remove any restrictions on the size of the network?
Which network system protocols exactly correspond to the layers of the OSI model?
What kind of connectors are not used on the 10BASE-FL segment?
How many hubs can there be in a Fast Ethernet network (conflict zone) according to Model 1 rules?
What layers of the OSI model does the network adapter driver do?
What can a dedicated server achieve on the network?
What determines the choice of local network topology in the first place?
What is the difference between Arcnet exchange control and Token-Ring exchange control?
What is the difference between a 100BASE-TX segment and a 10BASE-T segment other than the transmission speed?
What is the practical limit for the maximum transmission rate of a conventional analog telephone line?
What is the maximum allowable collision window in Ethernet / Fast Ethernet networks?
What is not the responsibility of a network system administrator?
What is not a good thing about a coaxial cable?
What does the Auto-Negotiation mechanism provide?
What do Ethernet and Gigabit Ethernet have in common?
What do the layers of the OSI model define?
What does the statistical data compression operation automatically perform on a dial-up connection implies?
What does the datagram method suggest?
What happens on an Ethernet / Fast Ethernet network if the number of data bytes transferred is too small?
What is considered a disadvantage of NetWare network operating systems?
What is (or who is) a network system administrator?
What is a voice - modem?
What is a protocol analyzer?
What is a network adapter driver?
What is packet encapsulation?
What is a computer network client?
What is a method of managing the exchange of the network?
What is Minimum Code Distance?
What is the network number included in an IP address?
What is the longest path in an Ethernet / Fast Ethernet network?
What is a computer network server?
What is a passive star topology?
What is the disadvantage of server-based networking?
What is the advantage of 100BASE-FX over 100BASE-TX?
What is the advantage of the 10BASE2 segment?
What is the advantage of Token-Ring over Ethernet and Arcnet?
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Easy to install.
Well known and most widely used networking technology.
Low cost of network cards.
The possibility of implementation using various types of cables and cabling schemes.
Disadvantages of Ethernet
A decrease in the real data transfer rate in a heavily loaded network, up to its complete stop, due to conflicts in the data transmission medium.
Difficulties in troubleshooting: if the cable breaks, the entire LAN segment fails, and it is quite difficult to localize a faulty node or section of the network.
Brief characteristics of Fast Ethernet.
Fast Ethernet (Fast Ethernet) is a high-speed technology proposed by 3Com for the implementation of an Ethernet network with a data transfer rate of 100 Mbit / s, retaining to the maximum extent the features of 10 Mbit Ethernet (Ethernet-10) and implemented in the form of the 802.3u standard (more precisely, an addition to the standard 802.3 as chapters 21 to 30). The access method is the same as in Ethernet-10 - CSMA / CD of the MAC level, which allows you to use the old software and management tools for Ethernet networks.
All the differences between Fast Ethernet and Ethernet-10 are focused on the physical layer. 3 types of cable systems are used:
multimode FOC (2 fibers are used);
Network structure- a hierarchical tree structure based on hubs (like 10Base-T and 10Base-F), since no coaxial cable is used.
Net diameter Fast Ethernet has been reduced to 200 meters, which is explained by a 10-fold reduction in the transmission time of a minimum frame length due to a 10-fold increase in transmission speed compared to Ethernet-10. Nevertheless, it is possible to build large networks based on Fast technology Ethernet, thanks to the widespread adoption of low-cost, high-speed technologies, and the explosive development of switch-based LANs. When using switches, the Fast Ethernet protocol can operate in full-duplex mode, in which there are no restrictions on the total length of the network, and only restrictions on the length of the physical segments connecting neighboring devices (adapter - switch or switch - switch) remain.
The IEEE 802.3u standard defines 3 specifications for the physical Fast level Ethernet incompatible with each other:
100Base-TX - data transmission over two unshielded pairs of category 5 (2 pairs of UTP category 5 or STP Type 1);
100Base-T4- data transmission over four unshielded pairs of categories 3, 4, 5 (4 pairs of UTP categories 3, 4 or 5);
100Base-FX- data transmission over two fibers of a multimode FOC.
What is the transmission time of the minimum (maximum) frame length (including the preamble) in bit intervals for a 10Mbps Ethernet network?
? 84 / 1538
What is PDV (PVV)?
PDV - the time it takes for the collision signal to propagate from the farthest node in the network - the time of the double turnover (Path Delay Value)
PVV - reduction of interframe interval (Path Variability Value)
What is the PDV Limit (PVV)?
PDV - no more than 575 bit intervals
PVV- when passing a sequence of frames through all repeaters, there should be no more than 49 bit intervals
How many bit slots is there a sufficient safety margin for PDV? 4
When is it necessary to calculate the maximum number of repeaters and the maximum network length? Why can't we just apply the “5-4-3” or “4-hubs” rules?
When different types of transmission media
List the basic conditions for the correct operation of an Ethernet network consisting of segments of different physical nature.
number of stations no more than 1024
the length of all branches is not more than the standard
PDV not more than 575
PVV- when passing a sequence of frames through all repeaters, there should be no more than 49 bit intervals
What is the segment base when calculating PDV?
Repeater delays
Where in the worst case does the collision take place: in the right, left, or intermediate segment?
In the right - the host
When do you need to calculate PDV twice? Why?
If there are different segment lengths at the far ends of the network, because they have different base latency values.
Brief description of the Token Ring LAN.
Token Ring (token ring) - a network technology in which stations can transmit data only when they own a token that continuously circulates around the ring.
The maximum number of stations in one ring is 256.
The maximum distance between stations depends on the type of transmission medium (communication line) and is:
Up to 8 rings (MSAU) can be bridged.
The maximum network length depends on the configuration.
Purpose of Token Ring network technology.
The Token Ring network was proposed by IBM in 1985 (the first option appeared in 1980). The purpose of Token Ring was to network all types of computers manufactured by the company (from PCs to mainframes).
What is the international standard for Token Ring networking?
Token Ring is currently an international IEEE 802.5 standard.
What bandwidth is provided on a Token Ring LAN?
There are two variants of this technology, providing data transfer rates of 4 and 16 Mbps, respectively.
What is the MSAU Multiple Access Device?
The MSAU hub is a self-contained unit with 8 connectors for connecting computers using adapter cables and two outer connectors for connecting to other hubs using trunk cables.
Several MSAUs can be constructively combined into a group (cluster / cluster), within which subscribers are connected in a ring, which allows increasing the number of subscribers connected to one center.
Each adapter connects to the MSAU using two bi-directional links.
Draw the structure and operation of a Token Ring LAN based on one (several) MSAUs.
One - see above
Several - (continued) ... The same two multidirectional communication lines included in the trunk cable can connect the MSAU in a ring (Figure 3.3), in contrast to the unidirectional trunk cable, as shown in Figure 3.2.
Each LAN node receives a frame from a neighboring node, restores signal levels to nominal, and transmits the frame to the next node.
The transmitted frame can contain data or be a marker, which is a special service 3-byte frame. The node that owns the token has the right to transfer data.
When the PC needs to transmit a frame, its adapter waits for the token to arrive, and then converts it into a frame containing data generated according to the protocol of the corresponding layer and transmits it to the network. The packet is transmitted over the network from adapter to adapter until it reaches the destination, which sets certain bits in it to confirm that the frame was received by the destination, and relays it further to the network. The packet continues to travel through the network until it returns to the sending node, where the correct transmission is verified. If the frame was transmitted to the destination without errors, the node passes the token to the next node. Thus, frame collisions are not possible on a token passing LAN.
What is the difference between the physical and logical topology of a Token Ring LAN?
The physical token ring topology can be implemented in two ways:
1) "star" (Fig. 3.1);
The logical topology in all modes is a "ring". The packet is passed from node to node along the ring until it returns to the node where it was originated.
Draw possible options for the structure of a Token Ring LAN.
1) "star" (Fig. 3.1);
2) "expanded ring" (Fig. 3.2).
Brief description of the functional organization of the Token Ring LAN. See # 93
The concept and functions of an active monitor in a Token Ring LAN.
When initializing a Token Ring LAN, one of the workstations is assigned as active monitor , which is assigned additional control functions in the ring:
temporary control in the logical ring in order to identify situations associated with the loss of a marker;
formation of a new marker after detecting the loss of a marker;
the formation of diagnostic personnel under certain circumstances.
When an active monitor fails, a new active monitor is assigned from many other PCs.
What mode (method) of token transfer is used on a 16 Mbps Token Ring LAN?
To increase network performance, Token Ring with a speed of 16 Mbps uses the so-called early token transfer mode (Early Token Release - ETR), in which the PC transmits the token to the next PC immediately after transmitting its frame. In this case, the next RS has the opportunity to transmit its frames without waiting for the completion of the transmission of the original RS.
List the frame types used on a Token Ring LAN.
marker; data frame; completion sequence.
Draw and explain the format of the token (data frame, termination sequence) of the Token Ring LAN.
Marker format
KO - final limiter - [J | K | 1 | J | K | 1 | PC | OO]
Data frame format
SPK - start sequence of the frame
BUT - initial delimiter - [J | K | 0 | J | K | 0 | 0 | 0]
UD - access control - [P | P | P | T | M | R | R | R]
UK - personnel management
AN - destination address
AI - source address
Data - data field
KS - checksum
PKK - sign of the end of the frame
KO - final limiter
SC - frame status
Completion sequence format
The structure of the Access Control field in a Token Ring LAN frame.
UD- access control(Access Control) - has the following structure: [ P | P | P | T | M | R | R | R ] where PPP is the priority bits;
the network adapter has the ability to assign priorities to the marker and data frames by writing in the priority bits field of the priority level in the form of numbers from 0 to 7 (7 is the highest priority); The RS has the right to send a message only if its own priority is not lower than the priority of the token that it received; T- marker bit: 0 for marker and 1 for data frame; M- monitor bit: 1 if the frame was transmitted by the active monitor and 0 otherwise; when the active monitor receives a frame with a monitor bit equal to 1, it means that the message or marker bypassed the LAN without finding the addressee; RRR- Reservation bits are used in conjunction with priority bits; The PC can reserve further use of the network by placing its priority value in the reservation bits, if its priority is higher than the current value of the reservation field;
after that, when the transmitting node, having received the returned data frame, generates a new token, it sets its priority equal to the value of the reservation field of the previously received frame; thus, the token will be passed to the node that has set the highest priority in the reservation field;
The assignment of the priority bits (marker bit, monitor bit, reservation bits) of the Access Control field in the Token Ring LAN token. See above
What is the difference between MAC frames and LLC frames?
Of the Criminal Code- frame control(Frame Control - FC) defines the frame type (MAC or LLC) and MAC control code; a single byte field contains two areas:
, where FF- frame format (type): 00 - for a MAC-type frame; 01 - for LLC level frame; (values 10 and 11 are reserved); 00 - unused reserve digits; CCCC- MAC-frame code MAC (physical control field), defining to what type (defined by the IEEE 802.5 standard) MAC layer control frames it belongs to;
Which field of the data frame indicates the MAC (LLC) type? In the UK field (see above)
The length of the data field in Token Ring LAN frames.
There is no special limitation on the length of the data field, although in practice it arises due to limitations on the permissible time for a network to be occupied by a separate workstation and is 4096 bytes and can reach 18 Kbytes for a network with a transmission rate of 16 Mbit / s.
What additional information and why does the Token Ring LAN frame end delimiter contain?
KO is an end limiter containing, in addition to a unique sequence of electrical impulses, two more areas 1 bit long each:
tween bit (Intermediate Frame), which takes values:
1 if the frame is part of a multi-burst transmission,
0 if the frame is the last or the only one;
error detected bit (Error-detected), which is set to 0 at the moment of creating a frame in the source and can be changed to 1 in case of an error detected while passing through the network nodes; thereafter, the frame is retransmitted without error control in subsequent nodes until it reaches the source node, which in this case will try to transmit the frame again;
How does Token Ring function when the error detected bit in the frame trailing separator is set to 1?
thereafter, the frame is retransmitted without error control in subsequent nodes until it reaches the source node, which in this case will try to transmit the frame again;
The structure of the packet status field of the Token Ring LAN data frame.
SC- (condition) frame status(Frame Status - FS) is a one-byte field containing 4 reserved bits (R) and two internal fields:
bit (indicator) address recognition (A);
bit (indicator) copy packet (C): [ ACRRACRR]
Since the checksum does not cover the SP field, each one-bit field in the byte is duplicated to ensure the reliability of the data.
The transmitting node sets bits to 0 A and WITH.
The receiving node after receiving the frame sets the bit A in 1.
If, after copying the frame to the buffer of the receiving node, no frame errors are detected, then the bit WITH also set to 1.
Thus, a sign of a successful frame transmission is the return of the frame to the source with bits: A= 1 and WITH=1.
A = 0 means that the destination station is no longer in the network or the PC is out of order (turned off).
A = 1 and C = 0 means that an error has occurred on the path of the frame from the source to the destination (this will also set the error detection bit in the trailing separator to 1).
A = 1, C = 1 and the error detection bit = 1 means that an error occurred on the return path of the frame from the destination to the source, after the frame was successfully received by the destination node.
What does the value of the "address recognition bit" ("packet copying bit to buffer"), equal to 1 (0), indicate?- See above
Is the maximum number of stations in one Token Ring LAN equal to ...?-256
What is the maximum distance between stations on a Token Ring LAN?
The maximum distance between stations depends on the type of transmission medium
(communication lines) and is:
100 meters - for twisted pair (UTP category 4);
150 meters - for twisted pair (IBM type 1);
3000 meters - for fiber optic multimode cable.
Token Ring pros and cons.
Token Ring Advantages:
no conflicts in the data transmission medium;
guaranteed access time for all network users;
Token Ring network functions well even under heavy loads, up to 100% load, in contrast to Ethernet, in which access time increases significantly even at 30% load or more; this is extremely important for real-time networks;
the larger allowable size of the transmitted data in one frame (up to 18 Kbytes), in comparison with Ethernet, provides a more efficient network operation when transferring large amounts of data;
the real data transfer rate in the Token Ring network may turn out to be higher than in the ordinary Ethernet (the real speed depends on the characteristics of the hardware of the adapters used and on the speed of the network computers).
Disadvantages of Token Ring:
the higher cost of the Token Ring network compared to Ethernet, because:
more expensive adapters due to the more complex Token Ring protocol;
additional costs for the purchase of MSAU concentrators;
the smaller size of the Token Ring network compared to Ethernet;
the need to control the integrity of the marker.
In which LANs are there no conflicts in the data transmission medium (guaranteed access time for all network users)?
On a LAN with token access
Brief description of LAN FDDI.
The maximum number of stations in a ring is 500.
The maximum length of the network is 100 km.
Transmission medium - fiber-optic cable (twisted pair can be used).
The maximum distance between stations depends on the type of transmission medium and is:
2 km - for fiber-optic multimode cable.
50 (40?) Km - for single-mode fiber optic cable;
100 m - for twisted pair (UTP category 5);
100 m - for twisted pair (IBM type 1).
The access method is marker.
The data transfer rate is 100 Mbps (200 Mbps for full duplex transmission).
The limit on the total length of the network is due to the time limit full passage signal on the ring to ensure the maximum allowable access time. The maximum distance between subscribers is determined by the attenuation of the signals in the cable.
What does the abbreviation FDDI stand for?
FDDI (Fiber Distributed Data Interface) is one of the first high-speed LAN technologies.
Purpose of FDDI network technology.
The FDDI standard is focused on high data transfer rates - 100 Mbit / s. This standard was conceived to be as close as possible to the IEEE 802.5 Token Ring standard. Slight differences from this standard are determined by the need to provide higher data transfer rates over long distances.
FDDI technology provides for the use of optical fiber as a transmission medium, which provides:
high reliability;
flexibility of reconfiguration;
high data transfer rate - 100 Mbit / s;
long distances between stations (for multimode fiber - 2 km; for single-mode when using laser diodes - up to 40 km; maximum length of the entire network - 200 km).
What bandwidth is provided on the FDDI LAN?
TechnologyEthernet
Ethernet is the most widely used standard for local area networks today.
Ethernet is a networking standard based on the experimental Ethernet Network that Xerox developed and implemented in 1975.
In 1980, DEC, Intel, and Xerox jointly developed and published the Ethernet version II standard for a coaxial cable network, which was the latest version of the proprietary Ethernet standard. Therefore, the proprietary version of the Ethernet standard is called the Ethernet DIX standard, or Ethernet II, on the basis of which the IEEE 802.3 standard was developed.
On the basis of the Ethernet standard, additional standards were adopted: in 1995 Fast Ethernet (an addition to IEEE 802.3), in 1998 Gigabit Ethernet (IEEE 802.3z section of the main document), which are in many ways not independent standards.
For transmission binary information over the cable for all variants of the physical layer of Ethernet technology, providing a throughput of 10 Mbit / s, the Manchester code is used (Fig. 3.9).
The Manchester code uses the potential drop, that is, the pulse front, to encode ones and zeros. In Manchester encoding, each bar is divided into two parts. Information is encoded by potential drops that occur in the middle of each clock cycle. One is encoded by the slope from low to high signal level (the leading edge of the pulse), and zero is coded by the falling edge (trailing edge).
Rice. 3.9. Differential Manchester Coding
The Ethernet standard (including Fast Ethernet and Gigabit Ethernet) uses the same media separation method - the CSMA / CD method.
Each PC operates on Ethernet according to the principle “Listen to the transmission channel before sending messages; listen when you send; stop working in case of interference and try again. "
This principle can be deciphered (explained) as follows:
1. No one is allowed to send messages while someone else is already doing it (listen before you send).
2. If two or more senders start sending messages at about the same moment, sooner or later their messages will "collide" with each other in the communication channel, which is called a collision.
Collisions are easy to recognize because they always generate a jamming signal that does not look like a valid message. Ethernet can recognize interference and forces the sender to pause transmission and wait a while before re-sending the message.
The reasons for the widespread use and popularity of Ethernet (advantages):
1. Cheapness.
2. Extensive experience of use.
3. Continuing innovations.
4. A wealth of equipment selection. Many manufacturers offer Ethernet-based networking equipment.
Disadvantages of Ethernet:
1. Possibility of message collisions (collisions, interference).
2. In the case of a large network load, the transmission time of messages is unpredictable.
TechnologyTokenRing
Token Ring networks, like Ethernet networks, are characterized by a shared data transmission medium, which consists of lengths of cable connecting all stations on the network in a ring. The ring is considered as a shared resource, and access to it requires not a random algorithm, as in Ethernet networks, but a deterministic one, based on the transfer of the right to use the ring to stations in a certain order. This right is conveyed using a special format frame called a token or token.
Token Ring technology was developed by IBM in 1984 and then submitted as a draft standard to the IEEE 802 committee, which adopted the 802.5 standard on its basis in 1985.
Each PC operates in Token Ring according to the principle “Wait for a token, if you need to send a message, attach it to a token when it passes by. If the marker passes, remove the message from it and send the marker further. "
Token Ring networks operate at two bit rates - 4 and 16 Mbps. Mixing stations operating at different speeds in one ring is not allowed.
Token Ring technology is more complex than Ethernet. It has the properties of fault tolerance. The Token Ring network defines network control procedures that use a ring-shaped feedback structure - a sent frame is always returned to the sending station.
Rice. 3.10. TOKEN RING technology principle
In some cases, detected errors in the network operation are eliminated automatically, for example, a lost token can be restored. In other cases, errors are only recorded, and their elimination is performed manually by the service personnel.
To monitor the network, one of the stations acts as a so-called active monitor. The active monitor is selected during ring initialization as the station with the maximum MAC address. If the active monitor fails, the ring initialization procedure is repeated and a new active monitor is selected. Token Ring can have up to 260 nodes.
A Token Ring hub can be active or passive. A passive hub simply interconnects the ports with interconnects so that the stations connected to those ports form a ring. The passive MSAU does not perform signal amplification or resynchronization.
An active hub performs signal regeneration functions and is therefore sometimes referred to as a repeater, as in the Ethernet standard.
In general, a Token Ring network has a combined star-ring configuration. End nodes are connected to MSAUs in a star topology, and the MSAUs themselves are combined through special Ring In (RI) and Ring Out (RO) ports to form a physical backbone ring.
All stations in the ring must operate at the same speed, either 4 Mbps or 16 Mbps. The cables connecting the station to the hub are called lobe cables, and the cables connecting the hubs are called trunk cables.
Token Ring technology allows the use of various types of cable to connect endpoints and hubs:
- STP Type 1 - shielded twisted pair(Shielded Twistedpair).
It is allowed to combine up to 260 stations into a ring with a branch cable length of up to 100 meters;
- UTP Type 3, UTP Type 6 - unshielded twisted pair (Unshielded Twistedpair). Maximum amount stations are reduced to 72 with the length of branch cables up to 45 meters;
- fiber optic cable.
The distance between passive MSAUs can be up to 100 m using STP Type 1 cable and 45 m using UTP Type 3 cable. Between active MSAUs, the maximum distance increases to 730 m or 365 m, respectively, depending on the type of cable.
The maximum ring length of Token Ring is 4000 m. The restrictions on the maximum ring length and the number of stations in a ring in Token Ring technology are not as stringent as in Ethernet technology. Here, these limitations are mainly related to the turnover time of the marker around the ring.
All of the timeout values on the network adapters of the Token Ring hosts are configurable, so you can build a Token Ring network with more stations and longer ring lengths.
Token Ring Technology Advantages:
· Guaranteed message delivery;
· High speed of data transfer (up to 160% Ethernet).
Disadvantages of Token Ring technology:
· Expensive devices for access to the environment are required;
· The technology is more difficult to implement;
· 2 cables are required (to improve reliability): one incoming, the other outgoing from the computer to the hub;
· High cost (160-200% of Ethernet).
TechnologyFDDI
Fiber Distributed Data Interface (FDDI) technology is the first local area network technology to use fiber as the transmission medium. The technology appeared in the mid-80s.
FDDI technology relies heavily on Token Ring technology, supporting the token passing access method.
The FDDI network is built on the basis of two fiber-optic rings, which form the main and backup data transmission paths between the network nodes. Having two rings is the primary way to improve resiliency in an FDDI network, and nodes that want to take advantage of this increased reliability potential must be connected to both rings.
In normal network operation, data passes through all nodes and all cable sections of only the Primary ring, this mode is called Thru mode - "through", or "transit". Secondary ring is not used in this mode.
In the event of some type of failure, where part of the primary ring cannot transmit data (for example, a cable break or node failure), the primary ring is combined with the secondary ring, again forming a single ring. This mode of operation of the network is called Wrap, that is, "folding" or "folding" the rings. The folding operation is performed by means of hubs and / or FDDI network adapters.
Rice. 3.11. IVS with two cyclic rings in emergency mode
To simplify this procedure, data on the primary ring is always transmitted in one direction (in the diagrams this direction is shown counterclockwise), and along the secondary - in the opposite direction (shown clockwise). Therefore, when a common ring of two rings is formed, the transmitters of the stations still remain connected to the receivers of neighboring stations, which makes it possible to correctly transmit and receive information by neighboring stations.
The FDDI network can fully restore its operability in the event of single failures of its elements. With multiple failures, the network splits into several unconnected networks.
Rings in FDDI networks are considered as a common shared data transmission medium, therefore a special access method is defined for it. This method is very close to the Token Ring access method and is also called the token ring method.
The differences in the access method are that the retention time of the token in the FDDI network is not constant. This time depends on the loading of the ring - with a small load it increases, and with large overloads it can decrease to zero. These changes in the access method apply only to asynchronous traffic, which is not critical to small frame transmission delays. For synchronous traffic, the token retention time is still a fixed value.
FDDI technology currently supports cable types:
- fiber optic cable;
- unshielded twisted pair of category 5. The last standard appeared later than optical and is called TP-PMD (Physical Media Dependent).
Fiber optic technology provides the necessary means for transmitting data from one station to another via optical fiber and determines:
Use of 62.5 / 125 µm multimode fiber optic cable as the main physical medium;
Requirements for the power of optical signals and the maximum attenuation between network nodes. For standard multimode cable, these requirements lead to a maximum distance between nodes of 2 km, and for single mode cable, the distance increases to 10–40 km, depending on the quality of the cable;
Requirements for optical bypass switches and optical transceivers;
Parameters of optical connectors MIC (Media Interface Connector), their marking;
Use for transmitting light with a wavelength of 1.3 nm;
The maximum total length of the FDDI ring is 100 kilometers, and the maximum number of double-connected stations in the ring is 500.
FDDI technology was developed for use in critical network sections - on backbone connections between large networks for example building networks, and for connecting high-performance servers to a network. Therefore, the main requirements for the developers were ( dignity):
- ensuring high speed of data transfer,
- fault tolerance at the protocol level;
- long distances between network nodes and a large number of connected stations.
All these goals have been achieved. As a result, the FDDI technology turned out to be of high quality, but very expensive ( flaw). Even the appearance of a cheaper twisted pair option did not significantly reduce the cost of connecting one node to the FDDI network. Therefore, practice has shown that the main area of application of FDDI technology has become the backbone of networks consisting of several buildings, as well as a network of the scale of a large city, that is, of the MAN class.
TechnologyFastEthernet
The need for high-speed yet inexpensive technology to connect powerful workstations to a network of powerful workstations led in the early 90s to the creation of an initiative group that began to look for a new Ethernet, the same simple and effective technology, but operating at a speed of 100 Mbps. ...
The specialists split into two camps, which eventually led to the emergence of two standards, adopted in the fall of 1995: the 802.3 committee approved the Fast Ethernet standard, which almost completely repeats the 10 Mbps Ethernet technology.
Fast Ethernet technology has kept the CSMA / CD access method intact, keeping the same algorithm and the same time parameters in bit intervals (the bit interval itself has decreased 10 times). All the differences between Fast Ethernet and Ethernet are manifested at the physical level.
The Fast Ethernet standard defines three physical layer specifications:
- 100Base-TX for 2 pairs of UTP category 5 or 2 pairs of STP Type 1 (coding method 4V / 5V);
- l00Base-FX for multimode fiber optic cable with two optical fibers (coding method 4V / 5V);
- 100Base-T4, operating on 4 pairs of UTP category 3, but using only three pairs simultaneously for transmission, and the rest - for collision detection (8B / 6T coding method).
The l00Base-TX / FX standards can operate in full duplex mode.
The maximum diameter of a Fast Ethernet network is approximately 200 m, and the exact value depends on the specification of the physical medium. In the Fast Ethernet collision domain, no more than one class I repeater is allowed (allowing to translate 4B / 5B codes into 8B / 6T codes and vice versa) and no more than two class II repeaters (not allowing codes translation).
Fast Ethernet technology when working on twisted pair allows two ports to choose the most effective regime operation - speed 10 Mbit / s or 100 Mbit / s, as well as half-duplex or full-duplex mode.
Gigabit Ethernet technology
Gigabit Ethernet technology adds a new 1000 Mbps step in the speed hierarchy of the Ethernet family. This stage makes it possible to effectively build large local networks, in which powerful servers and backbones of the lower network levels operate at a speed of 100 Mbit / s, and the Gigabit Ethernet backbone unites them, providing a sufficiently large margin bandwidth.
The developers of Gigabit Ethernet technology have retained a great deal of continuity with Ethernet and Fast Ethernet technologies. Gigabit Ethernet uses the same frame formats as previous Ethernet versions, operates in full and half duplex modes, supporting the same CSMA / CD access method on shared media with minimal changes.
To ensure an acceptable maximum network diameter of 200 m in half-duplex mode, the technology developers decided to increase the minimum frame size by 8 times (from 64 to 512 bytes). It is also allowed to transmit several frames in a row, without freeing up the medium, on an interval of 8096 bytes, then the frames do not have to be padded to 512 bytes. The rest of the access method and maximum frame size parameters remained unchanged.
In the summer of 1998, the 802.3z standard was adopted, which defines the use of three types of cable as the physical medium:
- multimode fiber optic (distance up to 500 m),
- single-mode fiber optic (distance up to 5000 m),
- double coaxial (twinax), through which data is transmitted simultaneously over two shielded copper conductors at a distance of up to 25 m.
To develop a variant of Gigabit Ethernet on UTP category 5, a special group 802.3ab was created, which has already developed a draft standard for working on 4 pairs of UTP category 5. Adoption of this standard is expected in the near future.
Ethernet, consisting of segments of various types, many questions arise, primarily related to the maximum allowable size (diameter) of the network and the maximum possible number of different elements. The network will be operational only if propagation delay the signal in it will not exceed the limit value. It is determined by the chosen exchange control method CSMA / CD based collision detection and resolution.First of all, it should be noted that to obtain complex Ethernet configurations from individual segments, intermediate devices of two main types are used:
- Repeater hubs (hubs) are a set of repeaters and do not logically separate the segments connected to them;
- Switches transfer information between segments, but do not transfer conflicts from segment to segment.
When using more complex switches, conflicts in individual segments are resolved on the spot, in the segments themselves, but do not propagate through the network, as in the case of using simpler repeater hubs. This is of fundamental importance for choosing an Ethernet network topology, since the CSMA / CD access method used in it assumes the presence of conflicts and their resolution, and the total length of the network is precisely determined by the size of the conflict zone, the collision domain. Thus, the use of a repeater concentrator does not divide the conflict zone, while each switching hub divides the conflict zone into parts. In the case of using a switch, it is necessary to evaluate the operability for each network segment separately, and when using repeater hubs - for the network as a whole.
In practice, repeater hubs are used much more often, since they are both simpler and cheaper. Therefore, in the future, we will focus on them.
There are two basic models used when choosing and evaluating an Ethernet configuration.
Model 1 rules
The first model formulates a set of rules that must be followed by the network designer when connecting individual computers and segments:
- A repeater or hub connected to a segment reduces by one the maximum number of subscribers connected to the segment.
- A complete path between any two subscribers should include no more than five segments, four hubs (repeaters) and two transceivers (MAUs).
- If the path between the subscribers consists of five segments and four concentrators (repeaters), then the number of segments to which the subscribers are connected should not exceed three, and the remaining segments should simply connect the concentrators (repeaters) together. This is the already mentioned "5-4-3 rule".
- If the path between subscribers consists of four segments and three concentrators (repeaters), then the following conditions must be met:
- the maximum length of a 10BASE-FL segment fiber-optic cable connecting hubs (repeaters) should not exceed 1000 meters;
- the maximum length of a 10BASE-FL segment fiber-optic cable connecting hubs (repeaters) with computers should not exceed 400 meters;
- computers can connect to all segments.
If you follow these rules, you can be sure that the network will be operational. No additional calculations in in this case not required. Compliance with these rules is believed to guarantee acceptable network latency.
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Introduction
During the time that has passed since the appearance of the first local area networks, several hundred of the most diverse network technologies have been developed, but few have become noticeable. This is primarily due to the high level of standardization of networking principles and their support by well-known companies. Nevertheless, standard networks do not always have record-breaking characteristics and provide the most optimal exchange modes. But the large volumes of production of their equipment and, consequently, its low cost give them huge advantages. It is also important that manufacturers software tools also primarily target the most common networks. Therefore, the user who chooses standard networks has a full guarantee of the compatibility of hardware and software.
The purpose of this course work is to consider the existing technologies of local area networks, their characteristics and advantages or disadvantages over each other.
I chose the topic of LAN technologies, due to the fact that, in my opinion, this topic is especially relevant now, when mobility, speed and convenience are appreciated all over the world, with the least waste of time as possible.
Currently, the reduction in the number of types of networks used has become a trend. The fact is that increasing the transmission speed in local networks up to 100 and even up to 1000 Mbit / s requires the use of the most advanced technologies and expensive scientific research. Naturally, this can only be afforded by the largest firms that support their standard networks and their more advanced varieties. In addition, a large number of consumers have already installed some kind of networks and do not want to immediately and completely replace network equipment. In the near future, one should hardly expect that fundamentally new standards will be adopted.
The market offers standard local area networks of all possible topologies, so users have a choice. Standard networks provide a wide range of acceptable network sizes, number of subscribers and, just as important, equipment prices. But making a choice is still not easy. Indeed, unlike software, which is not difficult to replace, the equipment usually serves for many years, its replacement leads not only to significant costs, to the need to rewire the cables, but also to the revision of the computer system of the organization. As a result, hardware selection errors are usually much more expensive than software selection errors.
1. Ethernet and fast Ethernet networks
The most widespread among standard networks is the Ethernet network. It first appeared in 1972 (developed by the well-known company Xerox). The network turned out to be quite successful, and as a result, it was supported in 1980 by such major companies as DEC and Intel). Through their efforts in 1985, the Ethernet network became an international standard, it was adopted by the largest international standards organizations: the 802 IEEE committee (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association).
The standard was named IEEE 802.3 (in English it reads as "eight oh two dot three"). It defines bus type multiple access to a mono channel with collision detection and transmission control. Some other networks also met this standard, since the level of detail is low. As a result, networks of the IEEE 802.3 standard were often incompatible with each other in terms of both design and electrical characteristics. However, in recent times the IEEE 802.3 standard is considered to be the Ethernet standard.
Key features of the original IEEE 802.3 standard:
· Topology - bus;
· Transmission medium - coaxial cable;
· Transmission speed - 10 Mbit / s;
· Maximum network length - 5 km;
· The maximum number of subscribers - up to 1024;
· The length of the network segment - up to 500 m;
· The number of subscribers on one segment - up to 100;
· Access method - CSMA / CD;
· Transmission is narrowband, that is, without modulation (mono channel).
Strictly speaking, there are minor differences between the IEEE 802.3 and Ethernet standards, but they usually prefer not to be remembered.
Ethernet is now the most popular in the world (over 90% of the market), and it is expected to remain so in the coming years. This was largely due to the fact that from the very beginning the characteristics, parameters, protocols of the network were open, as a result of which a huge number of manufacturers around the world began to produce Ethernet equipment that is fully compatible with each other.
V classical network Ethernet used a 50-ohm coaxial cable of two types (thick and thin). However, in recent years (since the beginning of the 90s), the most widespread version of the Ethernet is using twisted pairs as a transmission medium. A standard has also been defined for the use of fiber optic cable in a network. Additions have been made to the original IEEE 802.3 standard to accommodate these changes. In 1995, an additional standard appeared for a faster version of Ethernet, operating at a speed of 100 Mbit / s (the so-called Fast Ethernet, IEEE 802.3u standard), using twisted pair or fiber-optic cable as the transmission medium. In 1997, a version with a speed of 1000 Mbit / s appeared (Gigabit Ethernet, IEEE 802.3z standard).
In addition to the standard bus topology, passive star and passive tree topologies are increasingly being used.
Classic Ethernet topology
The maximum cable length of the network as a whole (maximum signal path) can theoretically reach 6.5 kilometers, but practically does not exceed 3.5 kilometers.
Fast Ethernet does not have a physical bus topology, only a passive star or passive tree is used. In addition, Fast Ethernet has much more stringent requirements for the maximum network length. Indeed, if the transmission speed is increased by 10 times and the format of the packet is preserved, its minimum length becomes ten times shorter. Thus, the permissible value of the double signal transit time through the network is reduced by 10 times (5.12 μs versus 51.2 μs in Ethernet).
The standard Manchester code is used to transmit information on an Ethernet network.
Access to the Ethernet network is carried out using a random CSMA / CD method, which ensures the equality of subscribers. The network uses packets of variable length with structure.
For an Ethernet network operating at a speed of 10 Mbit / s, the standard defines four main types of network segments, focused on different media:
10BASE5 (thick coaxial cable);
10BASE2 (thin coaxial cable);
10BASE-T (twisted pair);
10BASE-FL (fiber optic cable).
The segment name includes three elements: the digit "10" means the transmission rate of 10 Mbit / s, the BASE word - transmission in the base frequency band (that is, without modulation of the high-frequency signal), and the last element - the permissible segment length: "5" - 500 meters, "2" - 200 meters (more precisely, 185 meters) or the type of communication line: "T" - twisted pair (from English "twisted-pair"), "F" - fiber optic cable (from English "fiber optic").
Likewise, for an Ethernet network operating at a speed of 100 Mbps (Fast Ethernet), the standard defines three types of segments, differing in the types of transmission media:
· 100BASE-T4 (twisted pair);
· 100BASE-TX (double twisted pair);
· 100BASE-FX (fiber optic cable).
Here, the number "100" means a transmission rate of 100 Mbit / s, the letter "T" is a twisted pair, the letter "F" is a fiber-optic cable. The types 100BASE-TX and 100BASE-FX are sometimes combined under the name 100BASE-X, and 100BASE-T4 and 100BASE-TX under the name 100BASE-T.
The evolution of Ethernet technology is moving away from the original standard. The use of new transmission media and switches can significantly increase the size of the network. Abandoning the Manchester code (on Fast Ethernet and Gigabit Ethernet) results in higher data rates and reduced cable requirements. Rejection of the CSMA / CD control method (with full-duplex exchange mode) makes it possible to dramatically increase the efficiency of work and remove restrictions on the length of the network. However, all of the newer types of networking are also referred to as Ethernet.
2. Token-Ring network
The Token-Ring (token ring) network was proposed by IBM in 1985 (the first option appeared in 1980). It was designed to network all types of computers made by IBM. The very fact that it is supported by IBM, the largest manufacturer of computer technology, suggests that she needs to be given special attention. But no less important is the fact that Token-Ring is currently the international standard IEEE 802.5 (although there are minor differences between Token-Ring and IEEE 802.5). This puts this network on the same level as Ethernet in status.
Developed by Token-Ring as a reliable alternative to Ethernet. Although Ethernet is now superseding all other networks, Token-Ring is not hopelessly obsolete. More than 10 million computers worldwide are connected by this network.
IBM has done everything to make its network as widespread as possible: detailed documentation has been released up to schematic diagrams adapters. As a result, many companies, for example, 3COM, Novell, Western Digital, Proteon and others, started to manufacture adapters. By the way, the NetBIOS concept was developed specifically for this network, as well as for another IBM PC Network. If in the previously created PC Network NetBIOS programs were stored in the built-in adapter permanent memory, then a NetBIOS emulation program was already used on the Token-Ring network. This made it possible to more flexibly respond to the peculiarities of the hardware and maintain compatibility with higher-level programs.
The Token-Ring network has a ring topology, although it looks more like a star in appearance. This is due to the fact that individual subscribers (computers) are not connected to the network directly, but through special hubs or multi-station access devices (MSAU or MAU - Multistation Access Unit). Physically, the network forms a star-ring topology. In reality, the subscribers are nevertheless united in a ring, that is, each of them transmits information to one neighboring subscriber, and receives information from another.
Star-ring token-ring network topology
At the same time, the hub (MAU) allows you to centralize the configuration task, disconnect faulty subscribers, monitor network operation, etc. It does not perform any information processing.
For each subscriber in the concentrator, special unit connection to the trunk (TCU - Trunk Coupling Unit), which provides automatic inclusion of the subscriber in the ring if it is connected to the hub and is working properly. If the subscriber disconnects from the hub or is faulty, the TCU automatically restores the integrity of the ring without participation this subscriber... The TCU is triggered by a DC signal (the so-called "phantom" current) that comes from a subscriber who wants to join the ring
The hub in the network may be the only one; in this case, only subscribers connected to it are closed in a ring. Outwardly, this topology looks like a star. If more than eight subscribers need to be connected to the network, then several hubs are connected by trunk cables and form a star-ring topology.
Ring topology is very sensitive to ring cable breaks. To increase the survivability of the network, Token-Ring provides a so-called ring folding mode, which allows you to bypass the break point.
In normal mode, the hubs are connected in a ring by two parallel cables, but information is transmitted only through one of them.
In the event of a single damage (breakage) of the cable, the network transmits through both cables, thereby bypassing the damaged section. At the same time, the order of bypassing the subscribers connected to the concentrators is preserved. True, the total length of the ring increases.
In the event of multiple cable faults, the network splits into several parts (segments) that are not connected to each other, but remain fully functional. The maximum part of the network remains connected, as before. Of course, this no longer rescues the network as a whole, but it allows, with the correct distribution of subscribers to concentrators, to preserve a significant part of the functions of the damaged network.
Several hubs can be structurally combined into a group, a cluster, within which subscribers are also connected in a ring. The use of clusters allows you to increase the number of subscribers connected to one center, for example, up to 16 (if the cluster includes two hubs).
At first, twisted pair, both unshielded (UTP) and shielded (STP), were used as a transmission medium in the IBM Token-Ring network, but then there were options for equipment for coaxial cable, as well as for fiber optic cable in the FDDI standard.
The main technical characteristics of the classic version of the Token-Ring network:
· The maximum number of concentrators such as IBM 8228 MAU - 12;
· The maximum number of subscribers in the network - 96;
· Maximum cable length between the subscriber and the concentrator - 45 meters;
· Maximum cable length between hubs - 45 meters;
· The maximum length of the cable connecting all the hubs is 120 meters;
· Data transfer rate - 4 Mbit / s and 16 Mbit / s.
All specifications are based on the use of an unshielded twisted pair cable. If a different transmission medium is used, the characteristics of the network may differ. For example, when using shielded twisted pair (STP), the number of subscribers can be increased to 260 (instead of 96), the cable length - up to 100 meters (instead of 45), the number of hubs - up to 33, and the total length of the ring connecting the hubs - up to 200 meters ... Fiber optic cable allows to extend the cable length up to two kilometers.
The Token-Ring network in the classic version is inferior to the Ethernet network both in the allowable size and in the maximum number of subscribers. In terms of transmission speed, there are currently 100 Mbps (High Speed Token-Ring, HSTR) and 1000 Mbps (Gigabit Token-Ring) versions of Token-Ring. Companies supporting Token-Ring (including IBM, Olicom, Madge) do not intend to abandon their network, seeing it as a worthy competitor to Ethernet.
Compared to Ethernet hardware, Token-Ring hardware is noticeably more expensive, since it uses a more complex exchange control method, so the Token-Ring network is not so widespread.
However, unlike Ethernet, Token-Ring network maintains a high load level much better (more than 30-40%) and provides guaranteed access time. This is necessary, for example, in industrial networks, where a delay in reaction to an external event can lead to serious accidents.
The Token-Ring network uses the classic token access method, that is, a token constantly circulates around the ring, to which subscribers can attach their data packets. This implies such an important advantage of this network as the absence of conflicts, but there are also disadvantages, in particular, the need to control the integrity of the token and the dependence of the functioning of the network on each subscriber (in the event of a malfunction, the subscriber must be excluded from the ring).
Interestingly, the faster version of Token-Ring (16 Mbps and higher) uses the so-called Early Token Release (ETR) method. It avoids network overhead while the data packet is looped back to its sender.
3. Arcnet network
Arcnet (or ARCnet from Attached Resource Computer Net) is one of the oldest networks. It was developed by the Datapoint Corporation back in 1977. There are no international standards for this network, although it is she who is considered the ancestor of the token access method. Despite the lack of standards, the Arcnet network until recently (in 1980-1990) was popular, even seriously competing with Ethernet. A large number of companies (for example, Datapoint, Standard Microsystems, Xircom, etc.) have produced equipment for this type of network. But now the production of Arcnet equipment is practically discontinued.
Among the main advantages of the Arcnet network in comparison with Ethernet are the limited amount of access time, high reliability of communication, ease of diagnostics, as well as the relatively low cost of adapters. The most significant disadvantages of the network include low data transfer rate (2.5 Mbit / s), addressing system and packet format.
To transfer information in the Arcnet network, a rather rare code is used, in which a logical one corresponds to two pulses during a bit interval, and a logical zero corresponds to one pulse. Obviously this is self-timing code that requires even more cable bandwidth than even Manchester's.
As a transmission medium in the network, a coaxial cable with a characteristic impedance of 93 Ohm is used, for example, of the RG-62A / U brand. Twisted pair options (shielded and unshielded) are not widely used. Fiber optic options have been proposed, but they haven't saved Arcnet either.
As a topology, the Arcnet network uses the classic bus (Arcnet-BUS) as well as a passive star (Arcnet-STAR). Hubs are used in the star. It is possible to combine bus and star segments into a tree topology using hubs (as with Ethernet). The main limitation is that there should be no closed paths (loops) in the topology. Another limitation is that the number of daisy chained segments using hubs must not exceed three.
Hubs are of two types:
· Active concentrators (restore the shape of incoming signals and amplify them). The number of ports is from 4 to 64. Active hubs can be interconnected (cascaded).
· Passive concentrators (simply mix the incoming signals without amplification). The number of ports is 4. Passive hubs cannot be connected to each other. They can only link active hubs and / or network adapters.
Thus, the Arcnet network topology is as follows.
Arcnet network topology of bus type (B - adapters for working in the bus, S - adapters for working in a star)
The main technical characteristics of the Arcnet network are as follows.
· Transmission medium - coaxial cable, twisted pair.
· The maximum length of the network is 6 kilometers.
· The maximum cable length from the subscriber to the passive concentrator is 30 meters.
· The maximum cable length from the subscriber to the active concentrator is 600 meters.
· The maximum cable length between active and passive hubs is 30 meters.
· The maximum cable length between active hubs is 600 meters.
· The maximum number of subscribers in the network is 255.
· The maximum number of subscribers on the bus segment is 8.
· The minimum distance between subscribers in the bus is 1 meter.
· The maximum length of a bus segment is 300 meters.
· Data transfer rate - 2.5 Mbit / s.
Arcnet uses token access (pass-through), but is slightly different from Token-Ring. This method is closest to the one provided in the IEEE 802.4 standard. The sequence of actions of subscribers with this method:
1. The subscriber who wants to transmit is waiting for the arrival of the token.
2. Having received the token, he sends a request to transmit information to the receiving subscriber (asks if the receiver is ready to receive his packet).
3. The receiver, having received the request, sends a response (confirms its readiness).
4. Having received confirmation of readiness, the sender subscriber sends his packet.
5. On receiving the packet, the receiver sends an acknowledgment of the packet.
6. The transmitter, having received an acknowledgment of packet reception, ends its communication session. After that, the token is passed to the next subscriber in descending order of network addresses.
As with Token-Ring, conflicts are completely eliminated in Arcnet. Like any token network, Arcnet holds the load well and guarantees the amount of network access time (as opposed to Ethernet). The total round trip time of all subscribers by the marker is 840 ms. Accordingly, the same interval determines the upper limit of the network access time.
The Arcnet package size is 0.5 KB. In addition to the data field, it also includes 8-bit receiver and transmitter addresses and a 16-bit cyclic checksum (CRC). Such a small packet size turns out to be not very convenient with a high traffic intensity over the network.
Arcnet network adapters differ from other network adapters in that they need to set their own network address using switches or jumpers (there can be 255 of them, since the last, 256th address is used in the network for broadcasting mode). The control over the uniqueness of each network address is entirely the responsibility of the network users. Connecting new subscribers becomes quite difficult at the same time, since it is necessary to set the address that has not yet been used. The choice of the 8-bit address format limits the number of network subscribers to 255, which may not be enough for large companies.
As a result, all this led to the almost complete abandonment of the Arcnet network. There were 20 Mbit / s versions of the Arcnet network, but these were not widely adopted.
4. FDDI network
FDDI network (from English Fiber Distributed Data Interface, fiber optic distributed interface data) is one of latest developments local area network standards. The FDDI standard was proposed by the American National Standards Institute ANSI (ANSI X3T9.5 specification). Then the ISO 9314 standard was adopted, corresponding to the ANSI specifications. The level of network standardization is quite high.
Unlike other standard local area networks, the FDDI standard was initially focused on high transmission rates (100 Mbit / s) and on the use of the most promising fiber-optic cable. Therefore, in this case, the developers were not constrained by the framework of the old standards, focused on low speeds and electrical cable.
The choice of fiber as a transmission medium determined such advantages of the new network as high noise immunity, maximum secrecy of information transmission and excellent galvanic isolation subscribers. The high transmission speed, which is much easier to achieve in the case of fiber-optic cable, allows you to solve many problems that are not available in lower-speed networks, for example, the transmission of images in real time. In addition, fiber optic cable easily solves the problem of transmitting data over a distance of several kilometers without retransmission, which makes it possible to build large networks, covering even entire cities and having all the advantages of local networks (in particular, a low error rate). All this determined the popularity of the FDDI network, although it is not yet as widespread as Ethernet and Token-Ring.
The FDDI standard was based on the token access method provided by the international standard IEEE 802.5 (Token-Ring). Insignificant differences from this standard are determined by the need to provide a high speed of information transmission over long distances. FDDI network topology is a ring, the most suitable topology for fiber optic cable. The network uses two multidirectional fiber-optic cables, one of which is usually in reserve, but this solution also allows the use of full-duplex information transmission (simultaneously in two directions) with twice the effective speed of 200 Mbit / s (with each of the two channels operating at a speed 100 Mbps). A star-ring topology with hubs included in the ring (as in Token-Ring) is also used.
Basic technical characteristics of the FDDI network.
· The maximum number of network subscribers is 1000.
· The maximum length of the network ring is 20 kilometers.
· The maximum distance between network subscribers is 2 kilometers.
· Transmission medium - multimode fiber-optic cable (electrical twisted pair can be used).
· Access method - marker.
· Information transfer rate - 100 Mbit / s (200 Mbit / s for duplex transmission mode).
The FDDI standard has significant advantages over all previously discussed networks. For example, a Fast Ethernet network with the same bandwidth of 100 Mbps cannot match FDDI in terms of network size. In addition, the FDDI token access method provides, unlike CSMA / CD, guaranteed access time and the absence of conflicts at any load level.
The limitation on the total network length of 20 km is associated not with the attenuation of signals in the cable, but with the need to limit the time for the complete passage of the signal along the ring to ensure the maximum allowable access time. But the maximum distance between subscribers (2 km with a multimode cable) is determined precisely by the attenuation of signals in the cable (it should not exceed 11 dB). The possibility of using a single-mode cable is also provided, in which case the distance between subscribers can reach 45 kilometers, and the total length of the ring is 200 kilometers.
To achieve high network flexibility, the FDDI standard provides for the inclusion of two types of subscribers in the ring:
· Class A subscribers (stations) (subscribers of dual connection, DAS - Dual-Attachment Stations) are connected to both (internal and external) rings of the network. At the same time, the possibility of exchange at a speed of up to 200 Mbit / s or redundancy of the network cable is realized (if the main cable is damaged, the reserve cable is used). Equipment of this class is used in the most critical parts of the network from the point of view of performance.
· Class B subscribers (stations) (subscribers of a single connection, SAS - Single-Attachment Stations) are connected to only one (external) ring of the network. They are simpler and cheaper than class A adapters, but lack their capabilities. They can be connected to the network only through a hub or bypass switch, which turns them off in case of an emergency.
In addition to the actual subscribers (computers, terminals, etc.), the network uses Wiring Concentrators, the inclusion of which allows you to collect all connection points in one place in order to monitor the operation of the network, diagnose faults and simplify reconfiguration. When using different types of cables (for example, fiber-optic cable and twisted pair), the hub also performs the function of converting electrical signals into optical ones and vice versa. Hubs are also available in Dual-Attachment Concentrator (DAC) and Single-Attachment Concentrator (SAC).
Example for FDDI Network Configuration
The FDDI standard also provides for the ability to reconfigure the network in order to maintain its operability in the event of a cable break.
Unlike the access method offered by the IEEE 802.5 standard, FDDI uses what is known as multiple token passing. If, in the case of Token-Ring network, a new (free) token is transmitted by the subscriber only after returning his packet to him, then in FDDI a new token is transmitted by the subscriber immediately after the end of the packet transmission to him (similar to how it is done with the ETR method in the Token- Ring). The sequence of actions is as follows:
1. The subscriber wishing to transmit waits for a marker that follows each packet.
2. When the token arrives, the subscriber removes it from the network and transmits his packet. Thus, there can be several packets on the network at the same time, but only one token.
3. Immediately after transmitting his packet, the subscriber sends a new token.
4. The recipient subscriber to whom the packet is addressed, copies it from the network and, having made a note in the packet status field, sends it further along the ring.
5. Having received his packet back on the ring, the subscriber destroys it. In the packet status field, it has information about whether there were errors and whether the receiver received the packet.
The FDDI network does not use the priority and redundancy system as in Token-Ring. But an adaptive load scheduling mechanism is provided.
In conclusion, it should be noted that, despite the obvious advantages of FDDI, this network has not become widespread, which is mainly due to the high cost of its equipment (on the order of several hundred and even thousands of dollars). The main area of application of FDDI today is backbone networks that connect multiple networks. FDDI is also used to connect powerful workstations or servers that require high-speed communication. It is assumed that the Fast Ethernet network may overtake FDDI, but the advantages of fiber optic cable, token control method and record allowable network size currently put FDDI out of competition. And where hardware cost is critical, the twisted-pair version of FDDI (TPDDI) can be used in non-critical areas. In addition, the cost of FDDI equipment can greatly decrease with an increase in its production volume.
5. 100VG-AnyLAN network
100VG-AnyLAN is one of the latest high-speed local area networks that has recently entered the market. It was developed by Hewlett-Packard and IBM and complies with the international IEEE 802.12 standard, so the level of its standardization is quite high.
Its main advantages are the high exchange rate, the relatively low cost of the equipment (about twice the cost of the equipment of the most popular network Ethernet 10BASE-T), a centralized exchange control method without conflicts, and packet format compatibility with Ethernet and Token-Ring networks.
In the name of the 100VG-AnyLAN network, the number 100 corresponds to a speed of 100 Mbit / s, the letters VG denote a cheap unshielded twisted pair cable of category 3 (Voice Grade), and AnyLAN (any network) denotes that the network is compatible with the two most common networks.
The main technical characteristics of the 100VG-AnyLAN network:
· Transfer rate - 100 Mbps.
· Topology - a star with the possibility of extension (tree). The number of cascading levels of concentrators (hubs) is up to 5.
· Access method - centralized, conflict-free (Demand Priority - with a priority request).
· Transmission media - quad unshielded twisted pair (UTP category 3, 4, or 5 cables), double twisted pair (UTP category 5 cable), double shielded twisted pair (STP), and fiber optic cable. Nowadays, quad twisted pair is mainly widespread.
· The maximum cable length between the hub and the subscriber and between the hubs is 100 meters (for UTP category 3 cable), 200 meters (for UTP category 5 cable and shielded cable), 2 kilometers (for fiber optic cable). The maximum possible network size is 2 kilometers (determined by the allowable delays).
· Maximum number of subscribers - 1024, recommended - up to 250.
Thus, the parameters of the 100VG-AnyLAN network are quite close to those of the Fast Ethernet network. However, the main advantage of Fast Ethernet is full compatibility with the most common Ethernet network (in the case of 100VG-AnyLAN, this requires a bridge). At the same time, the centralized management of 100VG-AnyLAN, which eliminates conflicts and guarantees the maximum amount of access time (which is not provided in the Ethernet network), also cannot be discounted.
Network structure 100VG-AnyLAN
The 100VG-AnyLAN network consists of a central (main, root) concentrator of level 1, to which both individual subscribers and concentrators of level 2 can be connected, to which subscribers and concentrators of level 3, etc. are connected, etc. Moreover, the network can have no more than five such levels (in the original version there were no more than three). Maximum size the network can be 1000 meters for an unshielded twisted pair.
Thus, the 100VG-AnyLAN network is an affordable solution for increasing the transmission speed up to 100 Mbps. However, it does not have full compatibility with any of the standard networks, so its further fate is problematic. In addition, unlike the FDDI network, it does not have any record parameters. Most likely, 100VG-AnyLAN, despite the support of reputable companies and a high level of standardization, will remain just an example of interesting technical solutions.
If we talk about the most common 100Mbit Fast Ethernet network, then 100VG-AnyLAN provides twice the length of UTP Category 5 cable (up to 200 meters), as well as a conflict-free method of exchange control.
6. Superfast networks
The speed of Fast Ethernet and other networks operating at a speed of 100 Mbit / s currently satisfies the requirements of most tasks, but in some cases even it turns out to be insufficient. Especially in those situations when it is necessary to connect modern high-performance servers to the network or build networks with a large number of subscribers that require a high exchange rate. For example, network processing of 3D dynamic images is becoming more widely used. The speed of computers is constantly growing; they provide ever higher rates of exchange with external devices. As a result, the network may turn out to be the weakest point of the system, and its bandwidth will be the main limiting factor in increasing performance.
Efforts to achieve a transmission speed of 1 Gbps (1000 Mbps) have been going on quite intensively in recent years by several companies. However, most likely, the most promising network will be Gigabit Ethernet. This is primarily due to the fact that the transition to it will be the most painless, cheapest and psychologically acceptable. After all, the Ethernet network and its version of Fast Ethernet today are far ahead of all their competitors in terms of sales and prevalence in the world.
Gigabit Ethernet is a natural evolutionary evolution of the concept found in standard Ethernet. Of course, it inherits all the shortcomings of its direct predecessors, for example, non-guaranteed network access time. However, the huge bandwidth makes it difficult to load the network to the levels where this factor becomes decisive. On the other hand, maintaining continuity allows you to simply connect the Ethernet, Fast Ethernet and Gigabit Ethernet segments into a network, and, most importantly, move to new speeds gradually, introducing gigabit segments only in the most stressed network sections. (Moreover, not everywhere is such a high throughput really necessary.) If we talk about competing gigabit networks, then their use may require a complete replacement of network equipment, which will immediately lead to high costs.
The Gigabit Ethernet network retains the same CSMA / CD access method, which has proven itself in previous versions, using the same packet (frame) formats and the same frame sizes. No protocol conversion is required at the Ethernet and Fast Ethernet connections. The only thing that is needed is the coordination of exchange rates, so the main area of application for Gigabit Ethernet will be, first of all, the connection of Ethernet and Fast Ethernet hubs with each other.
With the advent of ultra-fast servers and the proliferation of the most advanced high-end personal computers, the benefits of Gigabit Ethernet are becoming increasingly evident. Thus, the 64-bit PCI system bus, already a de facto standard, fully achieves the data transfer rate required for such a network.
Work on the creation of a Gigabit Ethernet network has been going on since 1995. In 1998, a standard was adopted that was named IEEE 802.3z (1000BASE-SX, 1000BASE-LX and 1000BASE-CX). The development is carried out by a specially created alliance (Gigabit Ethernet Alliance), which, in particular, includes such a well-known network equipment company as 3Com. In 1999, the IEEE 802.3ab (1000BASE-T) standard was adopted.
The nomenclature of Gigabit Ethernet network segments currently includes the following types:
· 1000BASE-SX - a segment on a multimode fiber optic cable with a light signal wavelength of 850 nm (up to 500 meters). Laser transmitters are used.
· 1000BASE-LX - a segment on a multimode (up to 500 meters) and single-mode (up to 2000 meters) fiber-optic cable with a light signal wavelength of 1300 nm. Laser transmitters are used.
· 1000BASE-CX - shielded twisted pair segment (up to 25 meters long).
· 1000BASE-T (IEEE 802.3ab standard) - a segment on a quad unshielded twisted pair cable of category 5 (up to 100 meters long). It uses 5-level coding (PAM-5), and in full duplex mode, transmission is carried out on each pair in two directions.
Especially for the Gigabit Ethernet network, a method of encoding the transmitted information 8V / 10V is proposed, built on the same principle as the 4V / 5V code of the FDDI network (except for 1000BASE-T). Thus, the eight bits of information to be transmitted are mapped to 10 bits transmitted over the network. This code allows you to maintain self-synchronization, easily detect the carrier (fact of transmission), but does not require doubling the bandwidth as in the case of Manchester code.
To increase the 512-bit Ethernet interval corresponding to the minimum packet length (51.2 μs in Ethernet and 5.12 μs in Fast Ethernet), special methods have been developed. In particular, the minimum packet length has been increased to 512 bytes (4096 bits). Otherwise, the 0.512 µs time interval would unduly limit the maximum length of the Gigabit Ethernet network. All packets less than 512 bytes in length are expanded to 512 bytes. The extension field is inserted into the packet after the checksum field. This requires additional processing of packets, but the maximum allowable network size becomes 8 times larger than without such measures.
In addition, Gigabit Ethernet provides frame bursting. In this case, a subscriber who has received the right to transmit and has several packets for transmission can transmit not one, but several packets, sequentially, and addressed to different recipient subscribers. Additional transmitted packets can only be short, and the total length of all packets in a block must not exceed 8192 bytes. This solution allows you to reduce the number of network captures and reduce the number of collisions. When using block mode, only the first packet of the block is expanded to 512 bytes in order to check for collisions in the network. The rest of the packets up to 512 bytes may not expand.
Using Gigabit Ethernet to Connect Groups of Computers
Using Gigabit Ethernet to Connect High-Speed Servers
Gigabit Ethernet is primarily used in networks that connect computers in large enterprises that are located in several buildings. It allows, using appropriate switches that convert the transmission rates, to provide communication channels with high bandwidth between individual parts of a complex network or the communication line of switches with ultra-fast servers.
Probably, in some cases, Gigabit Ethernet will replace the FDDI fiber-optic network, which is now increasingly used to connect several local networks into a network, including Ethernet. True, FDDI can connect subscribers that are much farther from each other, but in terms of information transfer rate, Gigabit Ethernet significantly exceeds FDDI.
But even a Gigabit Ethernet network cannot solve some problems. A 10 Gigabit Ethernet version is already being proposed, called 10Gigabit Ethernet (IEEE 802.3ae standard, adopted in 2002). It is fundamentally different from previous versions. Only fiber optic cable is used as a transmission medium. The electric cable can sometimes be used only for communication over short distances (about 10 meters). The exchange mode is full duplex. The Ethernet packet format is the same. This is probably the only thing that remains of the original Ethernet standard (IEEE 802.3).
In conclusion, a few words about an alternative solution to the ultra-fast network. It is about a network with ATM technology (Asynchronous Transfer Mode). This technology is used in both local and global networks. The main idea is to transmit digital, voice and multimedia data over the same channels. Strictly speaking, there is no rigid standard for ATM equipment.
Initially, a transmission speed of 155 Mbit / s was chosen (for desktop systems - 25 Mbit / s), then - 662 Mbit / s, and now work is underway to increase the speed to 2488 Mbit / s. ATM competes successfully with Gigabit Ethernet in speed. By the way, ATM appeared earlier than Gigabit Ethernet. ATM technology assumes the use of fiber-optic cable and unshielded twisted pair as a medium of information transmission in a local network. The codes used are 4V / 5V and 8V / 10V.
The fundamental difference between ATM and other networks is the rejection of the usual packets with addressing, control and data fields. All transmitted information is packed in micropackages (cells) 53 bytes long. Each cell has a 5-byte header that allows the IEDs to sort the cells and ensure they are sent in the correct sequence. Each cell has 48 bytes of information. Their minimum size allows for hardware error correction and routing. It also ensures the uniformity of all information flows of the network and minimum time waiting for network access.
The header includes identifiers of the path, delivery channel, information type, a delivery priority indicator, and a header checksum to determine if there are transmission errors.
The main disadvantage of networks with ATM technology is that they are completely incompatible with any of the existing networks. A smooth transition to ATM is, in principle, impossible, all equipment must be changed at once, and its cost is still very high. True, work is underway to ensure compatibility, and the cost of equipment is also decreasing. Moreover, there are more and more tasks for transferring images over computer networks.
In the not too distant past, ATM technology was considered promising and versatile, capable of squeezing out the usual local networks. However, at the moment, due to the successful development of traditional local networks, the use of ATM is limited only to global and backbone networks.
7. Wireless networks
wireless ethernet high speed
Until recently, wireless communication in local networks was practically not used. However, since the late 90s of the 20th century, there has been a real boom in wireless local area networks (WLAN - Wireless LAN). This is primarily due to the success of technology and the convenience that wireless networks can provide. It is projected that the number of wireless users in 2005 will reach 44 million, and 80% of all mobile computers will have built-in access to such networks.
In 1997, the IEEE 802.11 standard for wireless networks was adopted. Now this standard is actively developing and already includes several sections, including three local area networks (802.11a, 802.11b and 802.11g). The standard contains the following specifications:
· 802.11 was the original WLAN standard. Supports data transfer rates from 1 to 2 Mbps.
· 802.11a is a high-speed WLAN standard for the 5 GHz frequency. Supports 54Mbps baud rate.
802.11b is a WLAN standard for the 2.4 GHz frequency. Supports 11Mbps baud rate.
· 802.11e - sets the quality requirements of the request required for all IEEE WLAN radio interfaces.
· 802.11f - describes the communication order between peer access points.
· 802.11g - Installs an additional modulation technique for the 2.4GHz frequency. Designed to provide data transfer rates up to 54 Mbps.
· V802.11h - describes 5 GHz spectrum management for use in Europe and Asia.
· 802.11i - Fixes existing security issues in the areas of authentication and encryption protocols.
The Wi-Fi Alliance is responsible for the development and support of the IEEE 802.11 standard. The term Wi-Fi (wireless fidelity) is used as a generic name for the 802.11a and 802.11b standards and all subsequent wireless local area networks (WLANs).
Wireless network equipment includes Access Points and wireless adapters for each subscriber.
Access points act as hubs that provide communication between subscribers and among themselves, as well as the function of bridges that communicate with the cable LAN and the Internet. Several nearby access points form a Wi-Fi access zone, within which all subscribers equipped with wireless adapters access the network. Such Hotspots are created in crowded places: airports, college campuses, libraries, shops, business centers, etc.
Each access point can serve several subscribers, but the more subscribers, the lower the effective transmission rate for each of them. The network access method is CSMA / CD. The network is built on a cellular basis. The network provides for a roaming mechanism, that is, it supports automatic connection to an access point and switching between access points when subscribers move, although the standard does not establish strict roaming rules.
Since the radio channel does not provide a high degree of protection against eavesdropping, a special built-in information protection mechanism is used in the Wi-Fi network. It includes authentication tools and procedures to prevent unauthorized network access and encryption to prevent eavesdropping.
The IEEE 802.11b standard was adopted in 1999 and, due to its focus on the mastered 2.4 GHz band, has won the greatest popularity among equipment manufacturers. It uses DSSS (Direct Sequence Spread Spectrum) as a basic radio technology, which is highly resistant to data distortion, interference, including deliberate interference, and detection. Since 802.11b equipment operating at a maximum speed of 11 Mbps has a shorter range than at lower speeds, the 802.11b standard provides for an automatic reduction in speed when signal quality deteriorates. The throughput (theoretical 11 Mbps, real - from 1 to 6 Mbps) meets the requirements of most applications. Distances are up to 300 meters, but usually up to 160 meters.
The IEEE 802.11a standard is designed to operate in the 5 GHz frequency range. Data transfer rates up to 54 Mbps, which is about five times faster than 802.11b networks. It is the most broadband of the 802.11 family of standards. Three mandatory speeds are defined - 6, 12 and 24 Mbps and five optional - 9, 18, 36, 48 and 54 Mbps. Orthogonal frequency division multiplexing (OFDM) is adopted as a signal modulation method. Its most significant difference from DSSS methods is that OFDM assumes the parallel transmission of the desired signal simultaneously over several frequencies in the range, while spread spectrum technologies transmit signals in series. The result is increased channel bandwidth and signal quality. The disadvantages of 802.11a include the high power consumption of radio transmitters for 5 GHz frequencies, as well as a shorter range (about 100 m). In addition, devices for 802.11a are more expensive, but over time, the price gap between 802.11b and 802.11a products will narrow.
The IEEE 802.11g standard is a new standard governing the construction of WLANs operating in the unlicensed 2.4 GHz frequency band. Orthogonal frequency division multiplexing (OFDM) technology maximum speed data transmission in wireless networks IEEE 802.11g is 54 Mbps. Equipment that supports the IEEE 802.11g standard, such as wireless access points, provides simultaneous network connectivity for IEEE 802.11g and IEEE 802.11b wireless devices. The 802.11g standard is an evolution of 802.11b and is backward compatible with 802.11b. In theory, 802.11g shares the virtues of its two predecessors. Among the advantages of 802.11g are low power consumption, long distances (up to 300 m) and high signal penetration.
IEEE 802.11d specification. sets universal requirements for physical layer(channelization procedures, pseudo-random frequency sequences, etc.). The 802.11d standard is still under development.
The IEEE 802.11e specification will enable the creation of multiservice wireless networks for corporations and individual consumers. While maintaining full compatibility with the current 802.11a and b standards, it will expand their functionality by servicing streaming multimedia data and guaranteed quality of service. So far, a preliminary version of the 802.11e specifications has been approved.
The IEEE 802.11f specification describes the Inter-Access Point Protocol (IAPP), which is necessary for building distributed wireless data transmission networks. Under construction.
The IEEE 802.11h specification provides the ability to complement existing algorithms for efficient frequency selection for office and outdoor wireless networks, as well as spectrum management, radiated power monitoring and reporting. Under construction.
Among the Wi-Fi equipment manufacturers are such well-known companies as Cisco Systems, Intel, Texas Instruments and Proxim.
Thus, wireless networks are very promising. Despite their disadvantages, the main of which is the insecurity of the transmission medium, they provide a simple connection of subscribers that does not require cables, mobility, flexibility and scalability of the network. In addition, importantly, users are not required to have knowledge of network technologies.
Conclusion
Following from the progress that network technologies have been able to achieve in recent years, it is not difficult to guess that in the near future the data transfer rate over the local network will at least double. The usual ten-megabit Ethernet, which has occupied a leading position for a long time, at least from Russia, is being actively replaced by more modern and significantly faster data transmission technologies.
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