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Traffic engineering
Endpoint specifications
Normally, a customer who submits a Request for Proposal (RFP) specifies the number of each
type of station to place in each site, in each Communication Manager system in the network.
Certain customers might want to specify station placement more precisely. For example, a
customer might specify the exact population of circuit-switched stations on a Media Gateway.
The majority of customers know exactly how many of each station type are needed at each site,
based on the population of their anticipated end-users. However, the issue of trunk sizing is not
as straightforward. Trunk traffic is tightly coupled with station traffic because at least one party in
every Communication Manager trunk call is a Communication Manager station (except in
relatively rare cases in which a Communication Manager system is used to tandem calls
between non-Communication Manager endpoints). That being the case, station traffic
effectively induces trunk traffic. The given topology of trunk groups dictates which pairs of
Communication Manager systems are directly connected by trunks, and which Communication
Manager systems are directly connected to the PSTN (or other non-Communication Manager
systems). However, the size of each trunk group must be engineered with consideration of the
amount of traffic that each such trunk group is anticipated to carry. A traffic engineer should
either specify the number of trunks in each trunk group directly, or allow the configuration
algorithm to size the trunk groups to a specified Grade of Service (GOS). This GOS is usually
P01, which is 1% blocking. In some cases, customers might choose to over-engineer or
under-engineer certain trunk groups based on nontraffic considerations, such as reliability, cost,
security, and so on.
Endpoint traffic usage
Traffic usage is typically expressed in Erlangs, which represent the average number of busy
servers in a given server group. For example, if a group of stations carries 100 Erlangs of call
usage, that means the average number of those stations that are busy at any given time is 100.
The usage of a single station, when expressed in Erlangs, represents the fraction of time that
the station is in use. So, a station that carries 0.1 Erlang of usage is in use 10% of the time.
The most common way to specify total station usage is to multiply the usage per station by the
total number of stations. A traffic engineer can either explicitly specify the per-station usage for
each group of stations, or allow the configuration algorithm to specify per-station usages
automatically, using default values. Common defaults for station traffic usage in general
business scenarios are:
Light traffic—0.056 Erlangs per station (stations average 5.6% usage)
Moderate traffic—0.11 Erlangs per station (stations average 11% usage)
Heavy traffic—0.17 Erlangs per station (stations average 17% usage)
144 Avaya Application Solutions IP Telephony Deployment Guide
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