IEEE STANDARD
The standards are divided into parts.
The 802.1 standard gives an introduction to the set of standards & defines the interface primitives.The 802.2 standard describes the upper part of the data link layer.The parts 802.3 through 802.5 describes the 3 lan standards,the CSMA/CD,token bus & token ring standards respectively.Each standard covers the physical layer & MAC sublayer protocol.
IEEE Standard 802.3
This standard is for a 1-persistant CSMA/CD lan.E.g: when a station wants to transmit,it listen to the cable.if the cable is busy,the station waits until it goes idle;otherwise it transmits immediately. If two or more stations simultaneously begin transmitting on an idle cable,they will collide. All colliding stations then terminate their transmission,wait a random time,& repeat the whole process all over again.The real beginning of 802.3 was the ALOHA system constructed to allow radio communication.Later,carrier sensing was added,& Xerox PARC built a 2.94 Mbps CSMA/CD system to connect over 100 personal working stations on a 1-km cable. This system was called ETHERNET.THE Xerox ethernet was so successful that Xerox,DEC,& intel drew up a standard for a 10-Mbps ethernet.This standard formed the basis for 802.3.The published 802.3 standards differs from the ethernet specification in that it describes a whole family of 1-persistant CSMA/CD systems,running at speeds from 1 to 10 Mbps on various media.
802.3 Cabling
Four types of cabling are used.
10Base5 cabling,popularly called thick ethernet. It resembles a yellow garden hose,with markings every 2.5 metres to show where the taps go.Connections to it are generally made using vampire taps in which a pin is carefully forced halfway into the coaxial’s core.The notation 10Base5 means that it operates at 10 Mbps.
10Base2 or thin Ethernet, which, In contrast to the garden-hose-like thick Ethernet, bends easily.Connections to it are made using industry standards BNC connectors to form T junctions, rather than using vampire taps.Thin Ethernet is much cheaper & easier to install, but it can run for only 200 meters & can handle only 30 machines per cable segment.
Problems
Detecting cable breaks, bad taps, or loose connectors can be a major problem.For this reason, a pulse of known shape is injected into the cable. If the pulse hits an obstacle or the end of the cable, an echo will be generated & sent back. By carefully timing the intervel between sending the pulse & receiving the echo, it is possible to localize the origin of the echo. This technique is called time domain reflectometry.
The problem associated with finding cable breaks have driven systems toward a different kind of wiring pattern,in which all stations have a cable running to a central hub. Usually,these wires are telephony company twisted pairs,mostly used in buildings. This scheme is called 10Base-T.
This is expensive due to the cost of the connectors & terminators,but it has excellent noise immunity.
Each frame starts with a preamble of 7 bytes,each contatining the bit pattern of 10101010.
The frame contains two addresses,one for the destinatin & one for the source.The standard allows 2-bytes & 6-bytes addresses.The high order bit of the destination address is a 0 for ordinary addresses & 1 for group addresses.Group addresses allow multi stations to listen to a single address.When a frame is sent to a group address, all the stations in the group receive it. Sending to a group of stations is called multicast.The length field tells how many bytes are pesent in the datd field,from a min of 0 to max of 1500.When a transceiver detects a collision,it truncates the current frame,which means that stray bits & piece of frame appear on the cable all the time.To make it easier to distinguish valid frames from garbage,802.3 states that valid frames must be atleast 64 bytes long,from destination address to checksum.If the data portion of a frame is less than 46 bytes,the pad field is used to fill out the frame to the minimum size.
The frame contains two addresses,one for the destinatin & one for the source.The standard allows 2-bytes & 6-bytes addresses.The high order bit of the destination address is a 0 for ordinary addresses & 1 for group addresses.Group addresses allow multi stations to listen to a single address.When a frame is sent to a group address, all the stations in the group receive it. Sending to a group of stations is called multicast.The length field tells how many bytes are pesent in the datd field,from a min of 0 to max of 1500.When a transceiver detects a collision,it truncates the current frame,which means that stray bits & piece of frame appear on the cable all the time.To make it easier to distinguish valid frames from garbage,802.3 states that valid frames must be atleast 64 bytes long,from destination address to checksum.If the data portion of a frame is less than 46 bytes,the pad field is used to fill out the frame to the minimum size.
IEEE Standard 802.4: Token Bus
In this if there are n stations & it takes T sec to send a frame,no frame will ever have to wait more than nT sec to be sent. Every body liked the conceptual idea of a ring but did not like physical implementation because a break in the ring cable would bring the whole network down. Ring is a poor fit to the linear topology of most assembly lines. This standard,802.4 descirbes a LAN called a token bus. Physically,the token bus is a linear or tree shaped cable onto which the stations are attached. Logically,the stations are organized into a ring,with each station knowing the address of the stataion to its left & right. When the logical ring is initialized,the highest numbered station may send the first frame. After it is done,it passes permission to its immediately neighbor by sending neighbor a special control frame called token.
The token propagates around the logical ring,with only the token holder being permitted to transmit frames. Since only one station at a time holds the token, collisions do not occur. When a station passes the token,it sends a token frame specifically addressed to its logical neighbor in the ring. For the physical layer,the token bus uses the 75-ohms broadband coaxial cable used for cable television.
The 802.4 frame format
IEEE Standard 802.5: Token Ring
A ring consists of a collection of ring interfaces connected by point to point lines.Each bit arriving at an interface is copied into a 1-bit buffer & then copied out onto the ring again.While in the buffer,the bit can be inspected & possibly modified before being written out. This copying step introduces a 1-bit delay at each interface.
Comparison of 802.3 ‘Ethernet’ and 802.5 ‘Token Ring’
¡ Generally they have similar technology with similar performance.
¡ 802.3 Ethernet – Advantages
l Widely used at present. People are experienced in using this technology.
l Simple Protocol. New computers can be added with having to bring the network down.
l Almost zero delay at low load, there is no need to wait for a token, you can transmit when ready.
¡ 802.3 Ethernet – Disadvantages
l The electronics is more complicated for carrier sense and collision detection.
l Ethernet is non-deterministic system (possibility of repeated collisions). This means that Ethernet is not suitable for network applications that require guaranted delivery times.
l Poor performance at high loads as there can be lots of collisions reducing the number of messages that are successfully transmitted.
¡ 802.5 Token Ring – Advantages
l Token Ring uses point-to-point connections between ring interfaces so that the electronic hardware can be fully digital and simple. There is no need for collision detection.
l Can use any medium,twisted pair is cheap and easy to install but could equally use fiber optic if available.
l Throughput excellent at high loads since there is no possibility of collisions unlike 802.3.
¡ 802.5 Token Ring – Disadvantages
l Computers must wait for the token to arrive, therefore at load, a computer is delayed before sending.
l Each token ring has a monitor computer, to look after the ring (i.e. remove damaged frames, handle lost frames and lost tokens). This introduces a critical point of failure. If the monitor computer failed, the remaining computers would have to wait until it is replaced before being able to continue.
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