PBX and PSTN Switches

Traditional switches and PBXs route voice using TDM technology and use 64-kbps circuits. The CCDA must understand some of the differences between these devices. The PBX, as its name states, is used in a private network and uses proprietary protocols. The PBX is located in the enterprise's data center. Each PBX may scale up to 1000 phones. Companies deploy PBX networks to obtain enterprise features and to prevent PSTN long-distance charges.

PBXs are customer-owned voice switches. Enterprise companies install and configure their own PBXs to provide telephony service, four-digit dialing, remote-office extensions, voice mail, and private-line routing within other features. Organizations can reduce toll charges by using private tie-lines between their switches. Calls that are placed between offices through the private voice network are called on-net. If a user needs to place a call outside the private network, the call is routed to the local PSTN. If the call is forwarded to the PSTN, it is called off-net.

Figure 15-1 shows a PBX network for an enterprise. Callers use the PBX network when they place calls from San Diego to Chicago, Atlanta, or Houston. The enterprise reduces toll charges by using its private voice network. A separate private network is in place for data traffic. If a user places a call from San Diego to Los Angeles, it is routed to the PSTN from the San Diego PBX. Then, toll charges are incurred for the call.

Figure 15-1 PBX Network

Figure 15-1 PBX Network

Another issue in the design is the limitation on the number of calls per private line. If the private lines are T1s, they are each limited to carrying 24 concurrent calls at a time. This is because each call takes 64 kbps of bandwidth with the g.711 codec, and 24 calls times 64 kbps/call equals 1.536 Mbps, the bandwidth of a T1.

PSTN switches are not private. They scale up to 100,000 phones and use open standards because they have to communicate with other switches, PBXs, fax machines, and home telephones. PSTN switches normally are located at the CO of the local or interexchange carrier.

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