Switching to VoIP

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Chapter 13: Network Infrastructure for VoIP

system monitoring companies with their customers. Dry lines can be used only to

link sites that are served by the same CO. This means the sites must be within a 3- or

4-mile radius in urban areas. In rural areas, dry lines may not be practical because of

distance-imposed attenuation problems.

Not all telephone companies offer dry lines because, if you use the

right hardware, dry lines can compete directly with their other, more

expensive services such as POTS and DSL. Other telephone compa-

nies may have a policy of not selling dry lines to customers who plan

to use them for voice applications. Check with your telephone com-

pany to be sure.

Private Digital Trunks

T1s and ISDN BRI connections are used to connect PBXs that have the appropriate

digital interfaces, though BRI’s practicality in this role has evaporated. T1s are also

used to connect groups of TDM phones to the PBX by way of a device called a chan-

nel bank. This permits 24 TDM phones to be used with a single T1 port on the PBX.

In order to connect two PBXs by T1, a DSU/CSU (data and channel service unit)

device is required at both ends of the T1. The DSU/CSU’s role is to provide low-level

signaling and diagnostic feedback for the T1 data link. DSU/CSUs are used in both

legacy