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:
- Usually employ some form of shared medium: multiple
computers are connected to a single cable, using complex interface
technology.
- Data rates are high, typically 10Mbps or greater.
- Privately owned; no involvement of public telcommunications
providers.
- Interfaces often built-in on high-end workstations.
[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:
- 100 Mbps Ethernet (100baseT), and its competitor, 100base-VG
"AnyLAN"
- Switched 10 Mbps Ethernet
- Fibre Distributed Data Interface, FDDI, operating at 100 Mbps
- Distributed Queue, Dual Bus (DQDB) MAN-level networks
- Frame Relay and SMDS
- ATM and Broadband ISDN
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