_________   SWAT MAGAZINE ISSUE SEVENTEEN: May 1999   __________
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 /	           		 Wan                        	    \
/                         By -= Slasher =-                           \
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Hi! 
	Well this is my first attempt at an article for the group and i
hope you all enjoy it.

	WAN stands for wide area network, it is a network of computers 
which could span over several continents. A good example of this is 
the Internet.  Communication is achieved by local phone lines, 
satellite and microwave technology.  The use of WAN's has increased 
emensely in the last couple of years, this is due to the technology is 
now cheap and more efficient examples of this are digital phone line 
(ISDN- Intergrated Services Digital Network) instead of analogue lines.
The cost of connection has been reduced by the introducion of "free 
servers" and emerging technology like cable modems(ADSL connection) 
which offer perminent and fast connection for a set price. Also there 
are now efficient compression techinque to aid in sending and recieving
of data. Compression makes it more efficient as there is less data to 
be sent.

	Modems are the interface between a computer and the telephone
system. Modem means "MOdulator DEModulator".  At the moment the 
fastest analogue modems run at speed to 56K, but ISDN lines allow 
speeds of 128k and ADSL allow speed faster than that.  ADSL connection 
doesn't require a modem because there is no conversion form analogue to
digital like on a normal phone line. An ADSL connection use a network
card instead.

	 A large network eg. in a university would use a multiplexor 
this hardware which combines more than one input into one data signal 
for transmission. This makes more efficient use of the modem or 
connection, and also saves having individual channels.  At the reciving
end the sign will be spilt up in to the separate data by another 
multipexor or de-multiplexor. A communications processor is sometime 
used to handle the data, this reduces stress on the main processor.

	There are two types of multiplexing time division and frequency 
division.  In time division each terminal is alotted an inteval of 
time to transmit data even if there is nothing to transmit. Statistical 
multiplexing is the same as time division but if a terminal has no 
information to send then it is given no time.  Frequency division uses 
several carrier wave to carry the one lot of data. This means that the 
de-multplexor can distinguish between the sets of data form each 
terminal.

	When data is tranfered it could be subject to interference from
electrical goods and other signals.  This interference must be picked 
up and dealt with. The method use to check this is parity and 
checksums. These are extra binary numbers added to the code being 
transmitted. They can be odd or even. eg. You are using odd parity and 
you have the binary code "0110110" the number of ones is even so the 
extra digit added is one. you have 0110100 so the extra digit is zero. 
The problem comes when there are two errors. eg. 0101001 is 
transmitted. The extra digit is zero, the code received is 01110110 
the last digit is zero and i use odd parity and there are five one and 
so it is odd but it is wrong because the are two errors.
This is where check sums come in, 
eg.
you have a batch of data

0111001  1
1010100  0
1000110  0
0011010  0

1001110  0 This is the check sum. it check the code vertically and so 
if there is an error in transmittion then the data can be sent again.