Lesson 23

Advanced WAN Technologies

This chapter deals with technologies available for transmitting data at high speed across LANs and WANs.

FDDI - Fiber Distributed Data Interface

This is a specification for a high speed token ring network system.

FDDI uses a pair of fiber optic cables in a logical ring topology. The ring can have a total circumference of up to 100 kilometers. This means that FDDI is suitable for a campus network or a MAN - a Metropolitan Area Network - a network joining buildings within the same city. The rate of transmission is 100 Mbps - ten times faster than ordinary Ethernet.

FDDI is often used to interconnect mini computers and mainframes:

FDDI can also be used as a LAN in applications where high data transfer rates are needed. (But as FDDI is no faster than Fast Ethernet, but is more complicated, it may not be the best solution.)

Two rings are used for redundancy. If one fails, the other will keep the network working.

Due to the large size of this system more than one token can be on the ring at the same time.

X.25

This is a set of protocols for a packet switching WAN. Data is spilt into packets which are sent, by varying routes, from sender to receiver.

X.25 originally used ordinary telephone lines, so was slow and unreliable. The X.25 standard has been updated to give greater speed by using leased lines, but it is still being left behind by newer systems.

Frame Relay

This is a modern, high-speed packet switching system. It uses variable length packets, and because it is designed to use modern, highly reliable digital lines, it has less error-checking than X.25.

Frame Relay uses a PVC - a permanent virtual circuit. This means that when a transmission is initiated, the path from source to destination is fixed. This path is used for all of the packets of that transmission. This gives higher speed as no routing decisions need to be made. It is like having your own leased line. But you pay only for the data you transmit.

To use Frame Relay you will need a special high-speed connection between your office and the company providing the service. You will also need a special ‘Frame Relay Router’ which acts as the interface between the LAN and the WAN.

ATM - Asynchronous Transfer Mode

ATM is the most important of these new technologies. It is likely that ATM will be used increasingly in the future both for WANs and for high speed LANs.

ATM is a cell switching system. All transmissions are broken up into 53 byte ‘cells’. (A cell is like a packet but smaller. It has 48 data bytes + 5 control bytes.) Because the network knows that all cells are the same length it can work faster than systems where packets vary in length.

The ATM standard is defined by the CCITT - an international standards organization for electronic communications based in Switzerland. It can operate at various speeds. Most current systems operate at 155 Mbps. Others are available that work at 622 Mbps. In theory speeds up to 1.2 Gbps are possible.

ATM as a LAN

To use ATM in a LAN all networking hardware must be ATM compatible. This means throwing away the company’s existing network infrastructure and starting again with all new equipment.

(ATM can be made to work with traditional copper cable, this severely limits its capabilities. Fiber optic cables are recommended.)

New NICs, along with devices called ATM switches are needed. These do the same jobs as hubs and routers in a conventional network.

ATM in a WAN

Communication companies such as Etisalat in the UAE and AT&T in the USA are building a world wide ATM infrastructure. To use ATM in their WAN a company must buy service from one of these companies.

What Can We Use ATM For?

ATM can transfer any data that can be represented as binary digits. This includes: voice; data; fax; audio; photos and video.

Etisalat and ATM

Check out

Back to top