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 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. Calls generally 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 CCX 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 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 CCX Call Router to finally get to the right agent.

Servers

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, and voicemail 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 ends the call), 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.

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

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. 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, 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, 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 needed.

Queuing

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 CCX solution uses a 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 IVR ports to handle the different applications because each has a different average handle time and possibly a different call load.