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Understanding TCP/IP > 3. Physical Layer - Pg. 33

3 Physical Layer Protocols of the physical layer are for the vast majority of users They are completely hidden protocols that describe signals on the connectors (commonly referred to as plugs) on the back part of the computer, to which a cable connecting the computer with the network is attached. Users tend to shift the responsibility to technical staff, whom they consider people "who take care of wires, by measuring something with a voltmeter." The situation today is completely different. A technician more or less administers software that controls all the mysterious boxes in locked rooms. This idea does not actually refer to the physical layer only, but encompasses the link layer as well. The users usually get involved only in the IP protocol (or network protocols), since in this protocol, they either see or do not see servers or neighbors. In contrast, the physical and link layers only provide communication with some kind of a box halfway down to the server, the existence of which is usually not known to regular users. Generally, we distinguish between two types of network: a Local Area Network (LAN) and a Wide Area Network (WAN). Regarding the physical layer, for one group of protocols are LAN protocols, while another group are WAN. However, the currently popular ATM protocol eliminates the differences between LAN and WAN, and it not only uses new protocols, but is also able to use the current WAN lines, including their protocols (for example, T1 lines in America or E1 lines in Europe). In addition, the ATM emulates protocols for the LAN protocol as well. LAN The LAN is used by several stations to communicate mostly on a shared medium. Within one LAN, the same link protocol is used (for example, Ethernet). Today, however, the term LAN also covers the so-called extended LANs that are composed of individual LANs. The extended LANs are created by connecting individual LANs via switches. Switches often have interfaces for various types of link protocols and are able to convert frames of one link protocol into frames of a different link protocol. An individual LAN composed of just two items, with one of them being the switch, is increasingly common. As for the physical layer, we will be interested in just the individual LAN since the extended LANs are viewed only as a complex of several individual LANs. LANs commonly use broadcasts. Routers are used to connect a LAN to a WAN. A router is a box that transfers an IP datagram from one network interface to another one, while each interface may be a part of a different LAN or may be an interface for the WAN.