Lesson 22

Wide Area Networks

Every college in the HCT can swap e-mail with every other college. All of the individual LANs are connected. Together they form a WAN - a Wide Area Network.

This chapter looks in more detail at the options for connecting networks.

Analog Connections

Analog connections are ones originally intended for use with voice telephones. To use them to move computer data we must use a modem at each end. By computer standards the quality of these connections is poor and transmission speeds are low.

There are two kinds of analog lines that we may use.

1. PSTN - The Public Switched Telephone Network.

This is the dial-up telephone system that we are all familiar with. Its advantages are that it is cheap, and everyone has access to it. It disadvantage is that it has the lowest quality and speed of all connections.

2. Dedicated Lines

This is when we pay for a permanent connection between two sites. The telephone company do not run a new wire directly between our sites. Instead they set switches in their exchange so that our sites are always connected. (Nowadays, these switches are implemented in software.)

The advantage is that the quality of the connection will be better, so data transmission will be can be faster. This happens because the data passes through fewer switches, and those switches are of better quality.

Dedicated Digital Connections

Various dedicated digital connections can be leased. They are collectively known as DDS (Digital Data Service) lines. The faster they operate the more their rental costs.

To use such a system, the supplier will need to run a special high speed cable into your site.

The interface to a high speed digital link is called a CSU / DSU - a Channel Service Unit / Data Service Unit.

A T1 connection is a high speed, high quality connection. It runs at 1.544 Mbps. (This is far faster than any analog connection, but still much slower than an Ethernet which runs at 10 Mbps.)

Because T1 is so expensive to rent, some carriers let users share a T1 line.

T2, T3 and T4

Even faster lines are also available:

T2 - 6 Mbps

T3 - 45 Mbps

T4 - 275 Mbps

Packet Switching Networks

This refers to a network of high speed digital connections, maintained by companies such as CompuServe and MCI Mail, that connects many cities around the world. These service are also known as PDNs - Public Data Networks or VANs - Value Added Networks.

Data sent across such a network is broken into packets and the packets are sent one at a time across the network. Each packet might travel by a different route but they are all recombined when they reach their destination.

These networks have speeds typically in the range 56 Kbps to 1.544 Mbps - depending on cost. To use them you pay a monthly connection fee plus a charge dependent on the volume of data sent.

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