Lecture 12: Multiaccess Networks
Network Technology
Revision:
- The Internet is composed of many interconnected networks
and/or subnets.
- Networks are interconnected by routers.
Most networks (sometimes called LANs) have the
following characteristics:
- Usually employ some form of shared medium --
the networked systems all "see" one another's transmissions.
- Data rates are high,, 10Mbps or greater, commonly 100Mbps.
- Privately owned; no involvement of public telcommunications
providers, usually limited geographical area.
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.
Until recently, the predominant form of Ethernet used so-called
thin wire coaxial[1]
cabling. A typical installation looked like:
[1] Also called "cheapernet" or "10Base2"
CSMA/CD Medium Access Control (MAC)
This 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 shared 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 a thin-wire Ethernet cable is called
a segment, thus:
A thin wire (coaxial cable) segment has a maximum length of 185 metres.
This can be extended using a repeater[2], which behaves somewhat like an
amplifier. The resulting larger LAN acts like a single large segment,
and is referred to as a "collision domain".
[2] a maximum of 4 repeaters is allowed
between any two stations.
Twisted Pair (10BaseT) Ethernet
For various reasons, this is now 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 superficially looks like 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
Data on an Ethernet is transmitted in frames:
- 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 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, eg
0x0800
for IP. 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. The data field usually
contains an IP datagram.
Ethernet/802.3 Switches
An Ethernet switch has a similar function to a hub
(see earlier) -- switches are sometimes called "switching hubs".
The difference is that a switch examines the destination address of
every frame it receives, and transfers it directly to the appropriate
port, without other ports being aware of the communication. Many such
transfers can occur simultaneously, which has the effect of increasing
overall "system" throughput.
NB: A switch builds a table mapping source addresses to ports which
it then uses to make switching decisions. It's obvious that switches
are significantly more complex than simple hubs, and this is reflected
in their price.
Newer Technologies
- 100 Mbps Ethernet (100baseT)
- so-called "Fast Ethernet".
- Modern switches have at least one 100BaseT port, and
100BaseT hubs are also invariably switches.
- Modern "Ethernet cards" are usually auto-detecting "10/100Mbps".
- "Full-duplex" systems can simultaneously send and receive
at 100Mbps.
- Fibre Distributed Data Interface
- FDDI operates
at 100 Mbps. It has been the Big New Thing for a decade, but has
never been widely adopted due to its complexity and high cost.
- Asynchronous Transfer Mode (ATM)
- This is a
system which allows integrated voice/video/data networks, currently
at bit rates between 25Mbps and 625Mbps, with the most common version
running at 155Mbps. Complex and expensive, but becoming very
popular.
- Gigabit Ethernet
- A variation which is
compatible with 10 and 100 Mbps Ethernet, but runs at 1000Mbps.
Still very expensive, but will become important.
- 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.
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Copyright © 2000 by
Philip Scott,
La Trobe University.