These are the most common industry terms for sizing contact center resources.
- Busy Hour or Busy Interval
A busy interval is 1 hour or less. The busy interval is when the most traffic occurs during a period of the day. The busy hour or interval varies due to circumstances like weekends and seasonal effects. 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, you staff for the maximum number of agents based on peak periods. But, you calculate the requirements for the rest of the day separately for each period (usually every hour). This gives proper scheduling of agents to answer calls versus scheduling agents for offline activities such as training or coaching. For trunks or VRU ports, 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, extra 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 the contact center resources (agents and VRU ports) receive and service all calls offered to the Voice Gateway. 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)
This is the rate at which the Unified CCE Router receives call routing requests. Every call generates one call routing request in a simple call flow from ingress gateway to VRU treatment to routing to an agent. However, some calls need more than one routing request to be made to the Router to finally get to the right agent.
An example of this is when the first agent who receives the call wants to transfer or conference to another agent by using a post route. This generates an extra routing request resulting in the same call generating two routing requests to the Router. A routing request is made to the Router whenever a resource is required for a call or task. These requests also include multimedia requests for Email, Chat, Callback and certain Outbound Calls. Call center administrators must account for these additional call routing requests when they size their contact center.
The maximum supported call rate is the call rate reported by the Router and not the BHCA at the ingress gateway. Factor these additional routing requests 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 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 Router, the CPS reported by Router is equal to 11 CPS. In this case, your solution 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. Each type can require different resources.
- Talk Time
Talk time is the amount of time an agent spends talking to a caller. This includes any time an agent places a caller on hold and any time spent during consultative conferences.
- Wrap-Up Time (After-Call Work Time)
After the call terminates (the caller hangs up), and agent completes certain tasks to "wrap up" the call. The wrap-up time includes 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. Unified Contact Center Enterprise solutions sometimes call this period the after-call work time.
- Average Handle Time (AHT)
AHT is the mean (or average) call duration during a specified time period. It refers to the sum of several types of handling time, such as call treatment time for self-service calls or talk time for calls to agents. 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 of calls on the same circuit, trunk, or port. (One circuit is busy for 1 hour regardless of the number of calls or how long the average call lasts.) The formula to calculate the Erlang value is:
Traffic in Erlangs = (Number of calls in the busy hour * AHT in sec) / 3600 sec
If a contact center receives 30 calls of 6-minute length in the busy hour, this equates to 180 minutes of traffic in the busy hour, or 3 Erlangs. 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).
- 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 1 hour:
BHT = (BHCA * AHT seconds) / 3600
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.
- Grade of Service (Percent Blockage)
- This measurement is the probability that a resource or server is busy during the busy hour. 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. Caller are 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
The industry standard term for the percentage of the offered call volume (received from the Voice Gateway and other sources) that are answered within X seconds. 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 service level goal determines the necessary agents, the percentage of queued calls, the average time calls spend in queue, and the necessary PSTN trunks and VRU ports.
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. Your desired service level and agent staffing determines the percentage of calls queued and the average time spent in the queue. Contact center enterprise solutions use a VRU to place callers in queue and play announcements. The VRU initially handles all calls. It supplies call treatment and prompts for necessary information. The VRU handles self-service applications where the caller is serviced without needing to talk to an agent. Each of these scenarios requires a different number of VRU ports 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 differs accordingly.