Multiaccess Networks

Within a single premises (or campus), it is usual for computers to be interconnected using a Local Area Network, or LAN, instead of using point-to-point links[1].
Characteristics include: [1] But see later this lecture.

Ethernet/802.3

Ethernet (also known as IEEE 802.3 and ISO 88023) is the dominant LAN technology at present. Ethernet defines a Medium Access Control (MAC) technology for operation over various types of cabling in a bus architecture. At La Trobe, Bendigo, we predominantly use so-called thin wire[2] Ethernet cabling. A typical installation looks like:

[2] Also called "cheapernet" or "10Base2"

CSMA/CD Medium Access Control

Defines how computers using Ethernet can share a common medium: Carrier Sense, Multiple Access, with Collision Detection.
Multiple Access
all computers have equal access: there are no masters or slaves, etc. If the channel is clear, a computer may begin to transmit immediately.
Carrier Sense
if the channel is busy, continue to listen, and attempt to transmit as soon as it becomes available.
Collision Detection
if a collision is detected (ie, another computer started to transmit at the same time), immediately cease transmission. Wait a random period of time, then start all over.

Ethernet/802.3 Repeaters

A group of computers connected to an Ethernet is called a segment, thus:

A thin wire segment has a maximum length of 185 metres. This can be extended using a repeater[3], which behaves somewhat like an amplifier. The resulting larger LAN acts like a single large segment, and is referred to as a "collision domain".

[3] a maximum of 4 repeaters is allowed between any two stations.

Twisted Pair (10BaseT) Ethernet

 For various reasons, this has become the preferred Ethernet technology. In this system, stations are "star-wired" to a central hub, using a 4-wire variation on standard telephone cabling:

The hub acts as a repeater, so whilst this appears to be a group of point-to-point links, all stations still see each other's transmissions, just the same as the bus topology.

Ethernet/802.3 Frames

Ethernet transmits data in frames, thus:

Preamble
7 bytes of 0101010101... This is used to synchronise the receiver.
Start Of Frame
1 byte, thus: 01010111
Source and Destination Address
each 6 bytes (48 bits!), and are uniquely assigned by IEEE. This is called a station's MAC-level address. All stations on a segment examine the destination address of all frames to see if it was addressed to them.
Type field
indicates which higher-level protocol created this frame. In 802.3 this field gives the length (in bytes) of the data field.
Data field
between 46 and 1500 bytes of data. NB: minimum frame size is thus 64 bytes.

Ethernet/802.3 Bridges

A bridge copies frames from Ethernet LAN to another, thus:

Bridges make decisions about which frames to copy, based on observations of source addresses in received frames. This makes them extremely useful for traffic management in LANs. A remote bridge can connect geographically separated LANs, using point-to-point telecom links:

New Technologies

The following are some technologies which seem to be evolving as successors to 10 Mbps Ethernet: Note: we have not covered token ring LANs in this lecture. See the assignment topics if interested.
This lecture is also available in PostScript format. The tutorial for this lecture is Tutorial #11.
Phil Scott