14.2 BROADCAST NETWORKS 

Chapter 12 (section 12.3.2) introduced the concept of broadcast networks and the most common topologies. Remember that broadcast networks use a channel to which all the users are connected, so all the users receive any transmission made on the channel. The only wide area broadcast networks all use radio broadcast, either relatively local up to a few hundred kilometres or over much further distances using satellite transmission. Section 14.2.1 will examine this further. The most common examples of broadcast networks are local area networks. These may use twisted pair cables, coaxial cable or optical fibre cable as their transmission medium (see section 13.6). Comparison between twisted pair and coaxial cable is not helpful because there are many variants of each to meet the different requirements of bandwidth, loss, noise immunity, etc. In general, coaxial cable has higher noise immunity and bandwidth, but the cable is stiffer (which may or may not be helpful depending on whether it is being surface mounted or pushed through ducts). Both types can adequately serve most LAN environments, but coaxial cable has been dropping out of use. Optical fibre is particularly suited to environments which have high levels of electromagnetic radiation, or to meet demands for very high speeds of transmission. However, it is more difficult to tap into, which makes it more difficult and expensive for the installation of a LAN. 
LAN protocols have developed into the following layers: 
* physical layer identical to ISO layer 1
* medium access control (MAC) layer to manage communications over the link 
* logical link control (LLC) layer, which provides a form of multiplexing to handle multiple-source data (a number of users attached to one host). 
In addition, the LLC layer assembles the data into a frame complete with address and error checking bits and disassembles them on receipt. 
For a particular LLC protocol there may be several different MAC options provided, since this is the protocol layer in which the differences in topology are involved. 
The major standards activity for LAN networks has been developed by the US Institute of Electrical and Electronic Engineers (IEEE). Their work has been organised into a number of committees, of which some are as follows: 
* 802.2 Logical link control (LLC) 
* 802.3 CSMA/CD networks (Ethernet, etc.) 
* 802.5 Token ring networks
(Acknowledgment: S. Coope, J, Cowley & N. Willis. Computer Systems: Architecture, Networks and Communications. London: McGraw Hill. 2002)