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Designing Voice-over-Data Networks 599
In Europe and in most other parts of the world, the standard for digital transmission is E1. E1
uses 32 time slots for a total speed of 2.048 Mbps. The channel maintains synchronization and
passes control information; the 16th channel passes signaling information. Both CAS and CCS
use time slot 16. The difference between the two is their use of messages to pass signaling
information. In CAS, the signaling is conveyed in channel 16, and the signaling associated with
each channel is maintained via a fixed relationship. CCS eliminates the requirement for the
signaling to maintain a fixed relationship with the voice channel. In CCS, signaling is passed in
messages between processors that control the terminating switches.
NOTE
For CAS or CCS, Cisco can translate the voice signals and carry them over the data network.
Signaling is a three-step process:
Step 1
The line is seized.
Step 2
A path is established across the network.
Step 3
A remote peer acknowledges the call.
For a telephone call to be completed, signaling must occur. The instant the receiver is lifted
from the cradle, an off-hook signal is sent to the PBX. In an exchange that is defined as Station
Loop Signaling, the PBX responds with a dial tone and receives digits from the user phone. As
soon as the PBX receives the digits, decisions occur at the PBX. What is the call's destination?
Is the call local to the PBX? If not, how can the call best be routed? Should the call be placed
to the telephone company central office (CO) or on an internal network to another PBX via
a tie-line?
In the first scenario, the PBX signals to seize a trunk to the CO. Depending on the services, the
signaling might be analog or digital. If the facilities are analog, the PBX might use E&M
signaling. If the call is established, the same signaling then occurs at the remote end of the
network. The CO seizes a line to the PBX and forwards the digits. The PBX selects the
appropriate station and signals an alert to the station. The PBX makes the call-routing decision
based on its best-match voice routing table. Enterprise PBXs communicate with each other
through industry-standard protocols or proprietary protocols. These specialized protocols let
PBXs offer enhanced services between sites.
NOTE
Cisco does not support proprietary protocols. When considering integration with the PBX,
verify the standard protocol features that Cisco supports.
87200333.book Page 599 Wednesday, August 22, 2001 1:41 PM