Lesson 1: IEEE 802 Standards for Local Area Networks

There are three common, low-speed, versions in this set of standards for wired LANs:

The 802 standards describe the three lowest layers of the network architecture:

These standard LANs differ in their physical layer and their MAC sublayer but are compatible at their data link layer.

Medium Access (MAC) Sublayer

Remember that a broadcast channel (as opposed to a point-to-point channel) is one where many nodes share a single transmission medium. The Medium Access Control sublayer is located between the OSI model's physical layer and data link layer and controls each node's access to the medium to prevent collisions (where two or more nodes transmit simultaneously, interfering with each other). Each version of the 802 LAN standards utilizes a different technique to solve the access control problem.

The key issue in medium access control:

How do we determine who gets the channel when there is competition for it?

Medium access control is typically an issue for LANs, MANs, and satellite networks. These tend to use broadcast channels while most WANs use point-to-point channels (an exception being satellite networks).

IEEE 802 LAN specifications:

IEEE 802.3 Standard - CSMA/CD Bus

CSMA/CD - Carrier Sense, Multiple Access / Collision Detection.

This version uses a bus topology to connect multiple nodes to a single, shared transmission cable.

Xerox pioneered this technology, with a focus on office automation, and the resulting system was called Ethernet. It was soon backed by DEC and Intel and was later standardized by the IEEE.

802.3 LANs can carry data at up to 10 Mbps and can use one of several cable types:

The transmitted signal for 802.3 LANs uses a special format for sending bits, called Manchester encoding, that includes a synchronizing pulse between bits. This allows a long series of 0's or 1's to be sent without a loss of synchornization between the sending and receiving nodes.

CSMA/CD Bus Operation:

How do collisions occur?

It is possible for two different nodes to "hear" no traffic on the bus and both begin transmitting at the same time. In fact, since a LAN cable can be hundreds of feet long, one node could even start sending slightly before the other and still not be "heard" by the other node, because the signal hadn't traveled down the cable as far as the second node yet. The time interval between the start of a signal and its arrival elsewhere on the bus is called propagation delay. To reduce the chance of repeated collisions between two nodes after a collision has occured, they will each wait a different amount of time before trying again. Usually, one of them will start resending its frame before the other's waiting time has ended and the second one will not interfere. Designers also limit the length of a bus to reduce the possible delay from one end of the cable to the other.

Format for CSMA/CD Bus Frames

field
preamble
start of frame delimiter
destination address
source address
length of data
data
data padding
checksum bits
size (bytes)
7
1
6
6
2
0-1500
0-46
4

CSMA/CD fields and their uses:

IEEE 802.4 Standard - Token Bus

This version also uses a bus topology, but with a different way to control access to the transmission medium. General Motors pioneered this technology for factory automation and it was adopted as an IEEE standard, but it is not used much any more.

Token Bus LANs could transmit at 1, 5, or 10 Mbps speeds and were only approved to use coaxial cable to connect nodes. The physical layer specifications included several very complex ways of encoding data as electrical signals and makes it totally incompatible with 802.3 LANs.

Token Bus Operation

Token Bus Frame Format

field
preamble
start of frame delimiter
frame control
destination address
source address
data
checksum bits
end delimiter
size (bytes)
1
1
1
6
6
0-8174
4
1

Token Bus fields and their uses:

Token passing:

IEEE Standard 802.5 - Token Ring

IBM pioneered this ring-based technology with a focus on office automation.

Transmission speeds are typically 1, 4, or 16 Mbps with the fastest speed currently the most common.

Token Ring Operation

Token Ring Frame Format

field
start of frame delimiter
access control
frame control
destination address
source address
data
checksum bits
end delimiter
frame status
size (bytes)
1
1
1
6
6
no
limit
4
1
1

Token Ring fields and their uses:

Since no data is sent with a token, the token frame consists of only the first three fields of a normal frame (the start of frame, access control and frame control fields) with a special bit pattern in the frame control field to indicate that it is a token. Loss of the token and extra tokens are also handled automatically. The priority bits in the access control field are used to set some frames to a higher priority than others, supporting the delivery of real-time data.

A comparison of 802 LAN versions:

All three LANs use similar technology and provide similar performance.

CSMA/CD bus

Token bus and token ring.