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598 Chapter 16: Voice Techniques
network that is optimized for data, the result could be choppy and unintelligible voice traffic.
Unlike data traffic, which tends to be bursty, voice traffic tends to be constant. Although the
amount of bandwidth per call is not as large as with data traffic, the bandwidth needs to be
guaranteed and without delay.
In voice transmission, delay is intolerable. The actual words spoken carry only a part of the
meaning. When individuals carry a conversation, meaning is also conveyed in inflection,
intonation, and pace. A tiny pause has as much meaning as a verbalized part, and its timing must
be preserved. Voice networks must be designed to transport the voice conversation, reliably and
in synchronization with the originator's intent.
Regular phone service provides 99.999% reliability and availability. Failures within the
traditional phone network are extremely unusual. Cell phone users might accept and
acknowledge unintelligible voice traffic from time to time because of the convenience that a cell
phone offers. Computer users tolerate slow access to a Web site or a network's being down.
(Some look forward to it because it means extra time off!) However, regular phone users have
become accustomed to a 99.999% reliability and availability standard for voice quality, and
they will not (and should not) tolerate anything less.
Path Basics
Signaling, addressing, and routing are the major functions of a voice over data network. Each
is discussed in detail in the following sections.
Analog and Digital Signaling
The purpose of signaling in a voice network is to establish a connection. Mapping PBX
signaling to your data network requires an understanding of how the PBX handles PBX-to-PBX
signaling. The first PBXs used simple analog lines to achieve voice-band information. Analog
signaling has become outdated and has largely been replaced by less-expensive and more-
effective digital signaling.
Two methods can be employed for digital signaling:
·
Channel Associated Signaling (CAS), or Robbed-Bit Signaling
·
Common Channel Signaling (CCS)
In CAS, the signaling information is conveyed within the voice channel. CAS is sometimes
called robbed-bit because, in every sixth frame, a bit is stolen from the voice channel to signal
information. In North America, the standard for digital transmission is T1. T1 uses 24 time slots
for a total speed of 1.544 Mbps. In North American CCS, a signaling channel is designated on
the T1, and the signaling bits for all the other T1 channels are transmitted across the single
CCS channel.
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