be a very useful feature to ease the process of connecting the call. Another bene- fit of DID trunks is that they make it seem like a private line goes directly to the user, but DID can support perhaps 100 different numbers with a group of only 25 to 35 trunks (traffic engineering is used to determine the proper number of trunks). DOD trunks-Direct outward dialing (DOD) trunks are used specifically for outgoing calls. DOD trunks are used when you dial an access code such as the number 9 or the number 8 to get an outside-line dial tone before you can dial the actual number that you want to reach. To service the key telephone systems, business lines connect the network ter- mination at the user end to the local exchange. Users who want to use the local exchange as if it were their PBX rent centrex trunks on a monthly basis. Large com- panies often access the network via leased lines, which can be a very expensive solution, and home users access the network via residential subscriber lines. Access lines can be either analog facilities or digital carrier services. Analog transmission is often called plain old telephone service (POTS). Three main types of digital services are offered over twisted-pair cable: Digital services involving T-1 access (at 1.5Mbps), E-1 access (at 2.048Mbps), and J-1 access (at 1.544Mbps) Narrowband ISDN (N-ISDN) services, including Basic Rate Interface (BRI) for residences and small businesses and Primary Rate Interface (PRI) for larger businesses xDSL services, including high-speed lines that enable the all-important applications of Internet access and multimedia exploration Chapter 2, "Traditional Transmission Media," describes these types of access in more detail. The PSTN Infrastructure Transport Services Transport services are the network switching, transmission, and related services that support information transfer between the originating and terminating access facilities. The underlying facilities include local exchanges and tandem switches, toll and transit switches, international 