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Lecture #12
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 (coax) 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. In other words, it's
still a "shared medium" network.
Ethernet/802.3 Frames
Data on an Ethernet is transmitted in frames. This
term is used to describe, in general, any "link level" (ie,
within-network) PDU which encapsulates higher-layer data for local
delivery over a LAN. The Ethernet frame format is:
- Preamble
- 7 bytes of
0101010101...
This is used to
synchronise the receiver's clock circuitry.
- Start Of Frame
- 1 byte, thus:
01010111
-- notice the two "1" bits
at the end.
- CRC
- A 4-byte checksum which allows the receiver to
be absolutely certain that the frame does not contain any
physical errors. If it does, then the frame is simply discarded
at the receiver.
Ethernet/802.3 Frames, Continued
- 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. If the destination address matches its own address, it
accpets the frame for processing.
- Mac addresses are in two parts: 3 bytes of manufacturer ID
and 3 bytes of serial number.
- Type field
- indicates which higher-level protocol created this frame, eg
0x0800
for IP. In 802.3[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.
[3]
802.3 is rarely used to carry IP datagrams,. It has historically been
used by various proprietory protocol architectures such as DECnet,
Novell (early versions) and Appletalk. An 802.3 frame always
encapsulates an 802.2 LLC frame, see our companion subject Data Communications for more
information.
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 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.
NB: 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 diminishing.
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 fiendishly 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 probably become the dominant LAN technology.
- Note: we have not covered token
ring LANs in this lecture. See the assignment topics
if interested.
The tutorial for this lecture is
Tutorial #12.
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Copyright © 2001 by
Philip Scott,
La Trobe University.