Subjects -> Computer Networks -> Lectures -> 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.
La Trobe Uni Logo

[Previous Lecture] [Lecture Index] [Next Lecture]