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The telecommunications network

Whether you have a single standalone computer or a number of computers connected on a LAN, you can get even more from your technology by connecting to the internet. Depending on your setup, there are a number of different options available, each of which have pros and cons associated with them.

Broadly speaking, these options can be categorised as -
 

PSTN

ISDN

Leased line
 

In the coming years there will also be a technology known as ADSL, which is quite similar to today's leased line, but offering a number of advantages for internet access.

PSTN stands for public switched telephone network. This is the same network you use every day for making telephone calls. It's advantage is that it is readily available, reasonably priced and well understood by everyone. You can even use the existing telephone line in the school to access the internet, but of course this will prevent you from making or receiving phone calls while anyone is "surfing the net".

However, there are limitations and drawbacks with using the PSTN. The most fundamental one is that it wasn't originally designed to carry communications between computers. Ordinary telephones convert a persons speech into what is termed an analogue signal. As most science students know, sound is carried by variations in air pressure which are driven by the vibrations of, say, a persons larynx. Similarly on a telephone line the electrical varies in accordance with the same vibrations.

Computers however communicate using digital signals - basically streams of "1s" and "0s" which correspond to on/off in electrical terms. To convert from the digital format of a computer to the analogue format required on a normal telephone line requires a special piece of equipment called a modem.

The other main drawback of the PSTN is speed. Basically the PSTN has a kind of speed limit imposed on it by the technology used in telephone exchanges. In the US and North America this limit is 56kb/s, or in Europe it is 64kb/s. As most modems are manufactured to be sold all over the world, the maximum rate that can be supported is 56kb/s. This is adequate for a single computer, and might even support a small number of computers on a LAN, but it becomes extremely limiting if the number of computers in a school grows over a number of years.

The Telecom Eireann Information Age Schools initiative is providing PSTN as the default solution to schools around the country. It provides the quickest and most straightforward way of accessing the information age, and does so in a cost effective way that meets most schools initial requirements.

 


ISDN stands for integrated services digital network. If you have read the section on PSTN you might be beginning to feel it is not telling you the full truth. "Haven't we been hearing for years that Telecom Eireann has a digital network, and now we are being told that the network isn't suitable for computers?"

In fact the core of the Telecom Eireann network is effectively 100% digital, but a comparison can be made with the road network in this case - we have a national primary route network made up of N roads and motorways (which is like Telecom Eireann's digital core network) and we have a network of smaller roads to access them (which is like the Telecom Eireann access network - the telephone lines provided to every single customer). Digital telecommunications offer better quality, faster switching and advanced services, and customers receive most of the benefits of these even on the PSTN. However, to achieve the maximum benefit it is necessary to bring the digital capability right into the customers premises. This service is what is referred to as ISDN. ISDN uses an existing customer line to carry 144kb/s to the local exchange - this is broken up into 2 X 64kb/s data channels (which can carry voice or computer communications or both) and 1 X 16kb/s signalling channel (which controls the connection to the network). As such it is really two "telephone lines" to the telephone exchange.

The advantages of ISDN are increased quality and reliability, support of applications such as videoconference, and an opportunity to have a new "speed limit" set for you. The disadvantages include cost (ISDN is more expensive to install and rent, but the cost for a call is the same as PSTN), and complexity. The complexity becomes particularly acute if you have to interface a LAN with the ISDN line, as there re a number of possible solutions based on router or server technology.

 

 


Leased lines are a technology best suited to very high users of  the internet. They are basically direct, dedicated, connections into the internet, and are available at speeds of 64kb/s, and increments of 64kb/s up to 2Mb/s. However, the cost of this option is likely to be prohibitive for most schools. The best comparison to this is the concept of an "all you can eat for $25" type restaurant. This might be a worthwhile option if you are absolutely starving, but you will hardly go there if all you want is a cup of coffee! Similarly, there is a breakeven point where usage of the internet gets sufficiently high that the "all you can eat" price is attractive, but a school would probably need to be running up telephone bills of £5000?? per year before this would become an attractive option

 


ADSL stands for Asymmetric Digital Subscriber Line. This mouthful means a brand new technology that is currently in the R+D phase within Telecom Eireann. It can be seen as a combination of ISDN and leased line type technology, which is ideally suited for internet type applications. It will allow very high speed access to the internet and other services such as video on demand - and will also provide "always on" connection.

The technology works on the basis that the data rate from the user is low compared to that from the network (e.g. the amount of data sent to represent a mouse click or short text is very small, whereas the amount of data sent in the form of an internet page containing a lot of graphics is much larger). While the technology is still developing, it is already achieving "downstream" data rates of up to 8Mb/s. Telecom Eireann expect to commence roll out of services based on this technology around the middle of 1999.



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