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Traffic Engineering 627

Legacy System Busiest Hour

If you have some traffic information, but it is not broken down according to the busiest hour,
you can extrapolate this information from your weekly or monthly traffic bill. The following
example shows how to calculate the number of Erlangs (E):

Average hourly traffic on the monthly bill = 1700 hours = 1700 E
Average number of business days in a month = 22
Average hourly traffic on an average day = 1700 / 22 = 77.27 E
Average daily busy-hour traffic (17% of the total) = 77.27

× 17% = 13.14 E

Using the Erlang B table, looking up 13.14 E with a GoS of 0.01, you calculate that you will
require 28 or 29 trunks (depending on whether you round up or down from 13.14).

Choosing the Proper Combination of Trunks

Finding a combination of trunks that is right for your organization is more of an economic
decision than a technical decision.

Cost per minute is the most commonly used measurement for determining whether to add
trunks. Ensure that all cost components are considered, such as accounting for additional
transmission, equipment, administration, and maintenance costs.

Consider the following two rules when optimizing the network for cost:

Use average-usage figures instead of the busy hour, which would overstate the number of
call minutes.

Use the least-costly circuit until the incremental cost becomes more expensive than the
next-best route.

Knowing how much traffic you have to deal with at peak loads (in Erlangs) and how the traffic
flows will determine how many and what type of trunks are required to support your
organization's calls. If the calling pattern suggests only local calls, you might require direct
connection to the central office (CO). Extensive long-distance dialing might require a dedicated
T1 connection to an Interexchange Carrier (IEC or IXC) for long-distance services. An inbound
call center that generates revenue or provides customer service and support might also require
a dedicated T1 connection. Or, several small companies sharing a leased facility might require
several small groups of COs.

Converting PSTN Traffic to IP Traffic

The last calculation you need to make is to equate Erlangs of carried traffic to packets per
second (pps). (If your system includes Asynchronous Transfer Mode [ATM] links, this
calculation is made in cells per second [cps] instead of packets per second.) The following
example illustrates one way to do this:

1 Erlang = 1.44 million packets (20-byte packets) or 400 pps

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