Cisco Unified Contact Center Express Solution Reference Network Design Release 9.0(1)
Basics of Call Center Sizing
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Basics of Call Center Sizing

This chapter introduces the basic concepts involved in call center sizing and contains the following sections:

Principal Design Considerations for Call Center Sizing

The figure below illustrates the principal steps and design considerations for sizing a call center.
Figure 1. Unified CCX Design Process – Call Center Sizing

This figure is a general overview of the design considerations for call sizing. For a detailed description of the call center sizing design process, refer to the section on sizing call center resources in the Cisco Unified Contact Center Enterprise Solution Reference Network Design Guide, available online at the following URL:


There are similar basic call center sizing considerations and steps for Unified CCE, and they also can be used in sizing a smaller contact center for Unified CCX. This call-sizing approach will provide you with the minimum number of IVR ports to support the total BHCA.

In addition, you should include the following design considerations, specific to Unified CCX, in your call center sizing calculations:
  • At a minimum, plan on enough capacity to replace your existing system. The replacement system should perform at least as well as the one it is replacing.
  • After all of the Erlang (C and B) calculations are complete for the call center sizing, any changes in queue times or agents will affect the total number of trunks and IVR ports required for an Unified CCX solution.
  • As you increase the size of the agent pool, very small changes in the average queue time and percentage of queued calls will affect the required number of gateway trunks and IVR ports.
  • Even if you perform all of the calculations for a call center, there are still some variables that you cannot plan for but that will affect the ports needed on a Unified CCX system. For example, one or more agents could call in sick, and that would affect the port count and queue time for each call. Just two agents calling in sick could increase the port count by over 12%. This would affect the price of the system and, if not planned for, would affect the ability of the call center to meet caller requirements. Properly sizing call center resources is integral to designing an effective Unified CCX system.


Not all of the Unified CCX system limits are available at the same time.

If all of the call sizing information is available, the next step is to apply Unified CCX sizing limits to the call center requirements. For this step, use the Cisco Unified Communications Sizing Tool, available online at:


The Unified Communications downloadable sizing tools help you with the task of sizing Unified Communications deployments.

Preliminary Information Requirements

System designers are advised to create a sizing document to do the following:
  • Scope out the preliminary configuration information for the Unified CCX server.
  • Size the gateways for the system.
To determine the size of the call center, obtain answers to the following questions:
  • How many IVR ports do you need?
  • How many PSTN gateway trunk ports do you need?
  • How many agents will answer incoming calls?
To answer these questions properly, you will need the sizing metrics and information listed in the following table.
Table 1 Call Center Sizing Metrics



Average handle time (AHT)

Average duration (talk time) of a call plus after-call work time, which is the wrap-up time after the caller hangs up.

Average IVR port usage time

The total time for prompt playout and/or menu navigation (if any) in the Unified CCX script. This should not include the queue time the caller spends waiting in queue before an agent becomes available. Queue time is calculated using Erlang-C automatically as shown in Figure 1.

Service level goal for agents

Percentage of calls answered by agents within a specific number of seconds.

Busy Hour Call Attempts (BHCA)

Average number of calls received in a busy hour.

Grade of service (% blockage) for gateway ports to the PSTN

Percentage of calls that get a busy tone (no gateway trunks available) out of the total BHCA.

All of the metrics in this table are basic call-sizing metrics. After these information are obtained, calculate the number of gateway trunk ports, IVR ports, and agents using standard Erlang B and C calculators as shown in Principal Design Considerations for Call Center Sizing.

See Figure 1 for an overview of the IP call center sizing process.


If the system being designed is a replacement for an existing ACD or an expansion to an installed Unified CCX or Cisco Unified IP IVR system, you might be able to use the historical reporting information from the existing system to arrive at the above metrics.

In addition, call sizing design considerations may vary if the call center is more self-service oriented.


The figure below illustrates the common port types and how they map to Unified CCX.
Figure 2. Call Center Port Types

Call center sizing differentiates the port types as follows:
  • Gateway or PSTN trunk ports — handle calls originating from the PSTN. They are purchased separately from Unified CCX.
  • Queue ports — are IVR ports that queue calls (when no agents are available) prior to transferring the caller to an available agent. These ports are included at no additional cost with Unified CCX Standard or Enhanced, but they must be sized for proper capacity planning for the Unified CCX server.
  • IVR ports — are full-featured IVR ports available with the Cisco Unified IP IVR and Unified CCX Premium product.

If you want additional supporting features, such as automatic speech recognition (ASR), text-to-speech (TTS), email notification, web server or client functionality, and database operations, you simply need to purchase the Premium package. Additional seats may also be purchased for IVR port licenses if the number of port licenses that come with the seat licenses is not sufficient.

The Unified CCX architecture differs slightly from the example TDM call center configuration in that IVR ports and queue ports (and P&C ports as well) are combined into one logical CTI port as shown in the figure above.