Back in Chapter 6 I discussed the difference between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). You should recall that DTE equipment is usually the source or destination of a network communication session, such as a router, computer, or terminal. In order for DTE equipment to establish communication over a service provider’s data communications network, DCE equipment is required. Common examples of DCE equipment include:
Modems. A modem provides the ability to connect a DTE device to a service provider’s analog communications facilities. It does this by modulating the digital signal output by a DTE device into the analog signals used on the local loop portion of the Public Switched Telephone Network (PSTN). At the receiving end, a modem demodulates the analog signal back to its digital form. While modems can be used to form a WAN connection between locations, they are typically relegated to “backup duty” because of their relatively slow (56K or less) connection speeds.
Terminal Adapters. A terminal adapter is used in ISDN communications to connect a DTE device to a service provider’s ISDN network. Terminal adapters are similar in function to a modem, in that they are used to dial into a network. However, they do not convert digital signals to analog or vice versa, since an ISDN networks are completely digital.
CSU/DSUs. Channel Service Unit / Data Service Units are the modem-like devices that act as an intermediary between DTE equipment (such as a router) and the service provider’s digital circuit. The CSU/DSU handles the sending and receiving of data over the service provider’s circuit, as well as clocking functions.