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622 Chapter 16: Voice Techniques
NOTE
Because DID trunks do not provide a dial tone, they cannot be used for outgoing calls from the
PBX to the CO.
Wide-area transmission service (WATS) trunks provide toll-free service (in the United States
with 800 numbers) for users who make frequent calls to the same geographic area. Outgoing
WATS (OUTWATS) trunks are used exclusively for outgoing toll-free calls from a PBX to a
defined area or organization, and incoming WATS (INWATS) trunks are used exclusively for
incoming toll-free calls from a defined area or organization to the PBX.
When measuring network traffic, it is important to consider all of the following types of traffic:
·
Automatic call distribution (ACD)--High-priority traffic that is typically sent over
dedicated trunks
·
Incoming (non-ACD) traffic--Traffic that originates in the PSTN and does not pass
through an ACD call center
·
Outgoing off-net (non-ACD) traffic--Traffic that originates in the network and
terminates in the PSTN
·
On-net incoming and outgoing traffic--Traffic that originates and terminates within the
same network
·
Voice mail--Traffic that consists of spoken messages that are stored on a voice-
messaging system
·
Data traffic--Traffic that originates from computer systems and is passed to other
computer systems
Traffic Measurement Units
In the United States, voice traffic is typically measured in one of two ways--Centum call
seconds or Erlang:
·
Centum call seconds (CCS)--A unit that expresses the amount of time that a particular
line is in use. The CCS measurement is made per 100 seconds of usage. For example, a
telephone with 9 CCS per hour means that it is off-hook (in use) 900 seconds per hour
(15 minutes).
·
Erlang--A unit that expresses one hour of traffic on a trunk or trunk group in one hour.
For example, five calls of 1-hour duration equal five Erlangs of traffic, whereas five 12-
minute calls equal 1 Erlang of traffic.
So, 1 Erlang equals 36 CCS per hour, which is 3600 seconds.
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