Information Technology Reference
In-Depth Information
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.
PSTN
Chicago
PSTN
PSTN
Private Tie Lines
Atlanta
San Diego
Houston
PSTN
Figure 14-1
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. All of this bandwidth is allocated for
voice calls and cannot be used for data transport.
PSTN switches are not private. They can scale up to tens of thousands of phones and use
open standards because they have to communicate with other switches, PBXs, fax ma-
chines, and home telephones. PSTN switches normally are located at the CO of the local
or interexchange carrier.
Local Loop and Trunks
Depending on the dialed digits, a call routes through the local loop, one or more trunks,
and the destination local loop to reach the destination phone. The local loop is the pair of
wires that runs from the CO to the home or business office.
the trunk is connecting. The term
tie line
is frequently used instead of
