ation. Links trans- local exchange companies) or their competitors fer data between one station and another in the network (competitive local exchange companies). They are and are responsible for access to the physical medium organized around a large number of feeders to a small (a wire or a radio link), packaging the packet in an number of telephone central offices, where traffic is appropriate format readable by the receiving station and aggregated. Most MANs deployed today are based on then reading received packets and producing the actual a hierarchical system of increasingly faster synchronous voltages or signals. (Note that an important function of data technologies. T-1 lines (in the United States) networks, correcting errors in messages, can go anywhere provide 1.5 megabits per second (Mbps) of data in the hierarchy and may be going on simultaneously over twisted pairs of copper wires and in sheer at some or all of the levels, leading on occasion to numbers still dominate over all other connections regrettable results.) to the MAN: there are thousands or tens of thousands Avery widely used example of this sort of hierarchical of these in a large central office. T-1 and T-3 (45 arrangement is the Open Systems Interconnect (OSI) Mbps) are further aggregated into faster connections, protocol stack. A simplified view of the OSI stack, usually using fiber optic transmission over synchronous arranged assuming a wireless LAN (WLAN) link to one optical network links: OC-3 at 155 Mbps, OC-12 at of the stations, is depicted in Figure 5.1-1. The ar- 622 Mbps, and SO on. Traditional MANs support both rangement is in general correspondence with the re- voice and data transfer. quirements noted above, though additional layers have Wide area networks (WANs): WANs connect cities been interposed to provide all the manifold functions and countries together. They are descended from needed in complex networks. the long-distance telephone services developed in The standards work of the Institute of Electrical and the mid-twen