Cisco Cisco IP Contact Center Release 4.6.2 Design Guide

Service level (percentage of calls that are answered within x seconds)

Grade of service, or percent blockage, desired for PSTN trunks and Unified IP IVR ports

The remaining sections of this chapter help explain the differences between the Erlang-B and Erlang-C 
traffic models in simple terms, and they list which model to use for sizing the specific call center 
resource (agents, gateway ports, and Unified IP IVR ports). There are various web sites that provide call 
center sizing tools free of charge (some offer feature-rich versions for purchase), but they all use the two 
basic traffic models, Erlang-B and Erlang-C. Cisco does not endorse any particular vendor product; it is 
up to the customer to choose which tool suits their needs. The input required for any of the tools, and 
the methodology used, are the same regardless of the tool itself.

Cisco has chosen to develop its own telephony sizing tool, called Cisco Unified CCE Resource 
Calculator. The version discussed here is designed to size call center resources. Basic examples are 
included later in this chapter to show how to use the Cisco Unified CCE Resource Calculator. Additional 
examples are also included to show how to use the tool when some, but not all, of the input fields are 
known or available. 

Before discussing the Cisco Unified CCE Resource Calculator, the next two sections present a brief 
description of the generic Erlang models and the input/output of such tools (available on the internet) to 
help the reader who does not have access to the Cisco Unified CCE Resource Calculator or who chooses 
to use other non-Cisco Erlang tools.

The Erlang-C model is used to size agents in call centers that queue calls before presenting them to 
agents. This model assumes:

Call arrival is random.

If all agents are busy, new calls will be queued and not blocked.

The input parameters required for this model are:

The number of calls in the busy hour (BHCA) to be answered by agents

The average talk time and wrap-up time

The delay or service level desired, expressed as the percentage of calls answered within a specified 
number of seconds

The output of the Erlang-C model lists the number of agents required, the percentage of calls delayed or 
queued when no agents are available, and the average queue time for these calls.

The Erlang-B model is used to size PSTN trunks, gateway ports, or Unified IP IVR ports. It assumes the 
following:

Call arrival is random.

If all trunks/ports are occupied, new calls are lost or blocked (receive busy tone) and not queued.

The input and output for the Erlang B model consists of the following three factors. You need to know 
any two of these factors, and the model will calculate the third:

Busy Hour Traffic (BHT), or the number of hours of call traffic (in Erlangs) during the busiest hour 
of operation. BHT is the product of the number of calls in the busy hour (BHCA) and the average 
handle time (AHT).