Lecture 13: Multiaccess Networks
- The Internet is composed of many interconnected networks and/or subnets.
- Networks and subnets are interconnected by routers.
In today's lecture we investigate the technology of networks -- that is, the
technology used to physically connect a group of computers. 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. This is much less
true than it once was, but the core technology is designed around the
assumption of a shared medium.
- Data rates are high, usually at least 10Mbps (ie, 10 million bits per
second -- data is always transmitted "serially", or one bit at a time),
commonly 100Mbps, occasionally 1Gbps.
- Privately owned; no involvement of public telcommunications providers,
usually limited geographical area such as campus, business premises, etc.
Ethernet/802.3Ethernet (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.
the predominant form of Ethernet used so-called thin wire
cabling. A typical installation might have looked like:
Whilst this is no longer
the most common hardware used for Ethernet/802.3, it exemplifies the
shared medium idea that it is based on. Note the use of
"T-connectors" on each computer, and the "Terminators" at each end of the
 OK it's not so recent.
Basically this cabling technology was no longer being installed by about the
 Historical note: this was also
called "cheapernet" or "10Base2". Exercise: cheaper than what?
CSMA/CD Medium Access Control (MAC)This is an algorithm (implemented in
the network-card hardware) which defines how computers using Ethernet/802.3 can
efficiently share a common medium: Carrier Sense, Multiple Access, with
- Multiple Access
- All computers have equal access: there are no masters or slaves, etc. If
the shared channel is clear (ie, not in use), a computer may begin to transmit
- 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, so the signal is garbled), immediately cease
transmission. Wait a random period of time, then start all over.
Ethernet/802.3 RepeatersA group of computers connected to a thin-wire
(coax) Ethernet cable was called a segment, thus:
A thin wire (coaxial
cable) segment had a maximum length of 185 metres, plus a few
other rules as to numbers of machines, and the distance between them. The
185m segment length can be extended using a repeater,
which behaves somewhat like a bi-directional amplifier. The resulting extended
LAN acts like a single larger segment, and is referred to as a
"collision domain", because every computer in each of the
component segments still "sees" everyone else's tranmissions.
 a maximum of 4 repeaters was
permitted between any two stations on the network.
Twisted Pair (10BaseT) EthernetFor various reasons,
this is now the preferred Ethernet technology. In this system, stations are
"star-wired" to a central hub, using a high-performance 4-pair
twisted pair cable, thus:
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
coax-cable bus topology. In other words, it's still a "shared medium" network,
and the attached computers still occupy a single collision domain.
The cable is usually "Category-5" (always abbreviated to "Cat5") unshielded
twisted pair (UTP) or better, although the older, lower performance Cat-3 cable
is also sometimes encountered. In practice, there are various hardware
technologies involved in a typical installation: in-wall fixed wiring, patch
cables, patch panels, etc. There are also strong limits on "cable-run" lengths.
 We will discuss these reasons in
Ethernet/802.3 FramesData on an Ethernet is transmitted in
- 7 bytes of
0101010101... This is used to synchronise the
- Start Of Frame
- 1 byte, thus:
- Source and Destination Address
- each 6 bytes (48 bits!), and are uniquely assigned by
IEEE. This is called a station's MAC address (or 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, eg
0x0800 for IP. In 802.3 (rarely used to carry IP packets) 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 SwitchesAn 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 MAC-level destination address of
every frame it receives, and transfers it directly to the appropriate port,
without other ports being involved in, or aware of, the communication. Many such
transfers can occur simultaneously, which has the effect of increasing overall
"system" throughput. Typical switches can also do full-duplex
transfers, simultaneously transmitting and receiving frames.
- A switch builds a table mapping source addresses to ports which it
subsequently uses to make switching decisions. It's obvious that switches are
significantly more complex than simple hubs, and this is reflected in their
price. However, as with all technologies, the difference is rapidly
Philsophical note: it's interesting that switches
essentially remove the possibility of collisions. We should therefore imagine
that network hardware is no longer executing the CSMA/CD protocol, because a
host can send a frame at any time. This is ony partly true -- collisions can, in
fact, still occur, and the important point is that an Ethernet switch can still
interoperate with older, non-switching hardware. The retention
of the same frame format is an important aspect of this
Address Resolution Protocol (ARP)Recall that IP datagrams are forwarded
over networks from host to router, router to router and router to host, and that
every host and router has (at least one) unique IP address. From today's
lecture, we also see that they also have a separate, unique MAC
address -- used to address frames in the network.
The Address Resolution Protocol provides a mapping between
IP addresses and MAC addresses. For example, in the case of IP local
delivery (see earlier)
the router or host knows the IP address of the destination machine, and also
knows (from examination of the network/subnet address) that it is connected to
the same network. In order to deliver the datagram, it:
- First broadcasts an ARP-request. In
effect, the broadcast asks (eg) "Which one of you guys is
- The host whose IP address is
with its MAC address.
- The datagram is then encapsulated into a frame with the correct
destination MAC address and placed "on the wire". The destination system notes
its own MAC address as the frame's destination and picks up the frame,
delivering the datagram to the IP software.
Systems which use IP keep an ARP cache of recent IP-to-MAC
mappings to avoid the need for repeated ARP-requests. This "ARP Table" can
usually be examined by the system manager. ARP is considered to be the "last
hop" routing protocol for IP packets.
- 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" and will
run at 100Mbps if possible.
- "Full-duplex" systems can simultaneously send and receive at 100Mbps.
- Requires Cat5 cable as a minimum, and is commonly used over dual fibres
instead of UTP, giving a distance advantage.
- Fibre Distributed Data Interface
- FDDI operates at 100 Mbps. It has been the Big New Thing for more than a
decade, but has never been widely adopted due to its complexity and fiendishly
high cost. Dead technology now.
- 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
for "campus-wide" networks -- eg, La Trobe's microwave network is actually an
- Gigabit Ethernet
- A variation which is compatible with 10 and 100 Mbps Ethernet, but runs at
1000Mbps. Still very expensive (original version only ran
over fibre, for example) but will probably become the dominant LAN technology
- Note: we have not covered token ring LANs
in this lecture. Maybe see the assignment topics if interested.
Copyright © 2004 by Philip
Scott, La Trobe University.