Cisco Unified Contact Center Enterprise Solution Reference Network Design, Release 9.x
Sizing Contact Center Resources
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Sizing Contact Center Resources

Sizing Contact Center Resources

Central to designing a Cisco Unified Contact Center (or any contact center) is the proper sizing of its resources. This chapter discusses the tools and methodologies needed to determine the required number of contact center agents (based on customer requirements such as call volume and service level desired), the number of Unified IP IVR ports required for various call scenarios (such as call treatment, prompt and collect, queuing, and self-service applications), and the number of Voice Gateway ports required to carry the traffic volume coming from the PSTN or other TDM source such as PBXs and TDM IVRs.

The methodologies and tools presented in this chapter are based on traffic engineering principles using the Erlang-B and Erlang-C models applied to the various resources in a Unified CCE deployment. Examples are provided to illustrate how resources can be impacted under various call scenarios such as call treatment (prompt and collect) in the Unified IP IVR and agent wrap-up time. These tools and methodologies are intended as building blocks for sizing contact center resources and for any telephony applications in general.

Contact Center Basic Traffic Terminology

It is important to be familiar with, and to be consistent in the use of, common contact center terminology. Improper use of these terms in the tools used to size contact center resources can lead to inaccurate sizing results.

The terms listed in this section are the most common terms used in the industry for sizing contact center resources. There are also other resources available on the internet for defining contact center terms.

Busy Hour or Busy Interval

A busy interval can be one hour or less (such as 30 minutes or 15 minutes, if sizing is desired for such smaller intervals). The busy interval occurs when the most traffic is offered during this period of the day. The busy hour or interval varies over days, weeks, and months. There are weekly busy hours and seasonal busy hours. There is one busiest hour in the year. Common practice is to design for the average busy hour (the average of the 10 busiest hours in one year). This average is not always applied, however, when staffing is required to accommodate a marketing campaign or a seasonal busy hour such as an annual holiday peak. In a contact center, staffing for the maximum number of agents is determined using peak periods, but staffing requirements for the rest of the day are calculated separately for each period (usually every hour) for proper scheduling of agents to answer calls versus scheduling agents for offline activities such as training or coaching. For trunks or VRU ports, in most cases it is not practical to add or remove trunks or ports daily, so these resources are sized for the peak periods. In some retail environments, additional trunks can be added during the peak season and disconnected afterwards.

Busy Hour Call Attempts (BHCA)

The BHCA is the total number of calls during the peak traffic hour (or interval) that are attempted or received in the contact center. For the sake of simplicity, we assume that all calls offered to the Voice Gateway are received and serviced by the contact center resources (agents and Unified IP IVR ports). Calls normally originate from the PSTN, although calls to a contact center can also be generated internally, such as by a help-desk application.

Calls Per Second as reported by Call Router (CPS)

These are the number of call routing requests received by the Unified CCE Call Router per second. Every call will generate one call routing request in a simple call flow where the call comes in from an ingress gateway, receives some VRU treatment and is then sent to an Agent; however, there are conditions under which a single call will need more than one routing request to be made to the Unified CCE Call Router to finally get to the right agent.

An example of this is when the first agent who receives the call wants to transfer/conference to another agent by using a post route. This will generate an additional routing request resulting in the same call generating two routing requests to the Unified CCE Call Router. A routing request is made to the Unified CCE Call Router whenever a resource is required for a call/task. These requests also include multimedia requests for Email, Chat, Blended Collaboration, Callback and certain Outbound Calls. Call center administrators must take into account these additional call routing requests when they size their contact center.

The maximum supported call rate is the call rate reported by the Unified CCE Call Router and not the BHCA at the ingress gateway. These additional routing requests need to be factored into the calculation of BHCA at the ingress gateway. In general, the BHCA at the ingress gateway is lower than or equal to the corresponding CPS rate reported by the Unified CCE Call Router.

For example, consider the following situation. If the BHCA at the ingress gateway is 36,000, then the call rate at the ingress gateway is 10 CPS. If we assume that 10% of the calls are transferred through the Call Router, the CPS reported by Call Router is equal to 11 CPS. In this case, the Unified CCE platform needs a capacity of 11 CPS.


Servers are resources that handle traffic loads or calls. There are many types of servers in a contact center, such as PSTN trunks and gateway ports, agents, voicemail ports, and VRU ports.

Talk Time

Talk time is the amount of time an agent spends talking to a caller, including the time an agent places a caller on hold and the time spent during consultative conferences.

Wrap-Up Time (After-Call Work Time)

After the call is terminated (the caller finishes talking to an agent and hangs up), the wrap-up time is the time it takes an agent to wrap up the call by performing such tasks as updating a database, recording notes from the call, or any other activity performed until an agent becomes available to answer another call. The Unified CCE term for this concept is after-call work time.

Average Handle Time (AHT)

AHT is the mean (or average) call duration during a specified time period. It is a commonly used term that refers to the sum of several types of handling time, such as call treatment time, talk time, and queuing time. In its most common definition, AHT is the sum of agent talk time and agent wrap-up time.


Erlang is a measurement of traffic load during the busy hour. The Erlang is based on having 3600 seconds (60 minutes, or 1 hour) of calls on the same circuit, trunk, or port. (One circuit is busy for one hour regardless of the number of calls or how long the average call lasts.) If a contact center receives 30 calls in the busy hour and each call lasts for six minutes, this equates to 180 minutes of traffic in the busy hour, or 3 Erlangs (180 min/60 min). If the contact center receives 100 calls averaging 36 seconds each in the busy hour, then total traffic received is 3600 seconds, or 1 Erlang (3600 sec/3600 sec).

Use the following formula to calculate the Erlang value:

Traffic in Erlangs = (Number of calls in the busy hour * AHT in sec) / 3600 sec

The term is named after the Danish telephone engineer A. K. Erlang, the originator of queuing theory used in traffic engineering.

Busy Hour Traffic (BHT) in Erlangs

BHT is the traffic load during the busy hour and is calculated as the product of the BHCA and the AHT normalized to one hour:

BHT = (BHCA * AHT seconds) / 3600, or
BHT = (BHCA * AHT minutes) / 60

For example, if the contact center receives 600 calls in the busy hour, averaging 2 minutes each, then the busy hour traffic load is (600 * 2/60) = 20 Erlangs.

BHT is typically used in Erlang-B models to calculate resources such as PSTN trunks or self-service VRU ports. Some calculators perform this calculation transparently using the BHCA and AHT for ease of use and convenience.

Grade of Service (Percent Blockage)

This measurement is the probability that a resource or server is busy during the busy hour. All resources might be occupied when a user places a call. In that case, the call is lost or blocked. This blockage typically applies to resources such as Voice Gateway ports, VRU ports, PBX lines, and trunks. In the case of a Voice Gateway, grade of service is the percentage of calls that are blocked or that receive busy tone (no trunks available) out of the total BHCA. For example, a grade of service of 0.01 means that 1% of calls in the busy hour is blocked. A 1% blockage is a typical value to use for PSTN trunks, but different applications might require different grades of service.

Blocked Calls

A blocked call is a call that is not serviced immediately. Callers are considered blocked if they are rerouted to another route or trunk group, if they are delayed and put in a queue, or if they hear a tone (such as a busy tone) or announcement. The nature of the blocked call determines the model used for sizing the particular resources.

Service Level

This term is a standard in the contact center industry, and it refers to the percentage of the offered call volume (received from the Voice Gateway and other sources) that are answered within x seconds, where x is a variable. A typical value for a sales contact center is 90% of all calls answered in less than 10 seconds (some calls are delayed in a queue). A support-oriented contact center might have a different service level goal, such as 80% of all calls answered within 30 seconds in the busy hour. Your contact center's service level goal determines the number of agents needed, the percentage of calls that are queued, the average time calls spend in queue, and the number of PSTN trunks and Unified IP IVR ports needed. For an additional definition of service level within Unified CCE products, see the Unified CCE glossary, which is available in the Configuration Manager’s online help.


When agents are busy with other callers or are unavailable (after call wrap-up mode), subsequent callers must be placed in a queue until an agent becomes available. The percentage of calls queued and the average time spent in the queue are determined by the service level desired and by agent staffing. Cisco's Unified CCE solution uses a Unified IP IVR to place callers in queue and play announcements. It can also be used to handle all calls initially (call treatment, prompt and collect such as DTMF input or account numbers or any other information gathering) and for self-service applications where the caller is serviced without needing to talk to an agent (such as obtaining a bank account balance, airline arrival/departure times, and so forth). Each of these scenarios requires a different number of Unified IP IVR ports to handle the different applications because each has a different average handle time and possibly a different call load. The number of trunks or gateway ports needed for each of these applications will also differ accordingly.

Contact Center Resources and the Call Timeline

The focus of this topic is on sizing the following main resources in a contact center:
  • Agents
  • Gateway ports (PSTN trunks)
  • Unified IP IVR ports
It is helpful first to understand the anatomy of an inbound contact center call as it relates to the various resources used and the holding time for each resource. The following figure shows the main resources and the occupancy (hold/handle time) for each of these resources.
Figure 1. Inbound Call Timeline

Ring delay time (network ring) must be included, if calls are not answered immediately. This delay can be a few seconds on average, and it must be added to the trunk average handle time.

Erlang Calculators as Design Tools

Many traffic models are available for sizing telephony systems and resources. Choosing the right model depends on three main factors:
  • Traffic source characteristics (finite or infinite)
  • How lost calls are handled (cleared, held, delayed)
  • Call arrival patterns (random, smooth, peaked)

For purposes of this document, there are mainly two traffic models that are commonly used in sizing contact center resources, Erlang-B and Erlang-C. There are many other resources on the internet that give detailed explanations of the various models (search using traffic engineering).

Erlang calculators are designed to help answer the following questions:
  • How many PSTN trunks do I need?
  • How many agents do I need?
  • How many VRU ports do I need?
Before you can answer these basic questions, you must have the following minimum set of information that is used as input to these calculators:
  • The busy hour call attempts (BHCA)
  • Average handle time (AHT) for each of the resources
  • 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 next two sections of this chapter present a brief description of the generic Erlang models in simple terms. Also described are the input/output of the Erlang models and which model to use for sizing the specific contact center resource (agents, gateway ports, and Unified IP IVR ports). There are various web sites that provide contact 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.


The Erlang-C model is used to size agents in contact centers that queue calls before presenting them to agents. This model assumes:
  • Call arrival is random.
  • If all agents are busy, new calls are 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).
  • Grade of Service, or the percentage of calls that are blocked because not enough ports are available.
  • Ports (lines), or the number of Unified IP IVR or gateway ports.