ontend networks are char- acterized by a large number of devices such as word processors, CRT terminals, printers and copiers. Backend networks serve computer-to-computer communication and computer-to-hard disks or high-speed device communication. Terminal Terminal Terminal Terminal Terminal Terminal Frontend networks Backend networks Laser speed printer speed Fig. 6-5. Frontend and backend networks. 86 TELECOMMUNICATIONS Backend networks typically have tens of nodes which need to com- municate at very high data rates in the 50 to 100 MBS range. Front- end networks can have hundreds of nodes and because of the human interface involved in all these, data rates of a magnitude lower can be adequate in many cases. Also backend networks have the nodes concentrated in shorter distances in the order of a kilometer, than frontend networks. Both bus and ring topologies are used in backend networks and rings can provide an upgrade to even higher speeds using fiber-optic cable as the medium. Frontend and backend networks also differ in terms of the imple- mentation cost. Frontend devices are usually low performance, low cost devices compared to backend devices which are quite expensive. Hence backend networks can incorporate more sophisticated fea- tures and hardware to obtain the performance they need. 6.2 LOCAL AREA NETWORKS The diversity of commercial local area networks (LANs) is one of the strongest considerations of the current marketplace. Each LAN product offers unique components, functions and applications. When a local area network is classified by topology, it is the access method of each product that sets it apart from its competitors. The access method is the set of procedures by which a user device is granted access to a data channel. General characteristics of an access method include distribution of the right to communicate on a fair and orderly basis. The right to access can be controlled by a central arbiter or shared by all stations in the network. Assignment of spe- cial priorities or different levels of