PBX Call Processing Power: BHC Rating

PBX system call processing capability is rated with the BHC benchmark. BHC is simply defined as the maximum of number of calls processed in 1 hour by the PBX system. There are two types of BHC measurements, Busy Hour Call Attempts (BHCAs) and Busy Hour Call Completions (BHCCs). BHCAs indicate the total number of placed calls that can be processed by the PBX system. BHCA calls include successful and unsuccessful calls. Unsuccessful call attempts include the following call types: no answer, busy, misdial, and abandoned. BHCCs indicate the only total number of successful placed calls. Measured call attempts and completions include station-originated calls and incoming trunk calls.

The BHCA rating of a PBX system will always be greater than its BHCC rating because unsuccessful calls included in the BHCA measurement have a lesser call processing burden than successful calls. The larger the number of unsuccessful call attempts, the greater the BHCA rating. BHCCs are successful calls that require a switched call connection, which is continually supervised and monitored by the call processing system, and require a teardown process when the call is terminated. PBX manufacturers may provide data on either BHC measurement but do not describe the benchmark tests used to determine the rating.

A manufacturer can test its PBX system to determine its BHCA or BHCC rating by using the system test parameter of provisioning a dial tone within a target period. Station users expect to receive dial tone immediately upon picking up their handsets, but some manufacturers perform BHC rating tests with acceptable dial tone delays of 1.5 seconds. In addition to dial tone delay parameters, BHC ratings are heavily dependent on the following three parameters:

1. System design configuration

2. Call connection type

3. Feature/function activity

The system design configuration defines the number and type of station terminals used for call testing purposes, the types of trunk circuits used for incoming or outgoing calls, and adjunct equipment that may be used to answer or support calls, such as VMSs and recorded announcers. Terminal type and complexity may have a major effect on the call processing rating. For example, a digital multiline telephone model with a softkey display field and add-on module options typically will require more processing power to place and receive calls than a basic analog telephone with no display or options. There are different call processing load factors associated with two-way analog trunks as compared with digital trunk circuits used for ISDN PRI services. An intercom call between two analog telephones is more likely to use less processing resources than an incoming ISDN PRI trunk circuit call, with ANI received by a digital displayphone.

The type of call connections determining the BHC rating will also affect the results. For example, trunk calls are usually more processing intensive than intercom calls. Calls between stations connected to the same local TDM bus may require less processing than calls between stations connected to different local TDM buses that require a center stage switched connection. Availability and physical location of incoming registers, outgoing registers, and sender tone receivers also will affect the BHC rating.

The call processing burden imposed by some PBX features and functions may be the most significant factor affecting BHC ratings. ACD and networking features require significantly greater processing resources than simpler features such as hold or transfer. ACD operations typically will require call screening, routing, queuing, and treatment operations before a connection is completed to an agent position. Intensive implementation of complex ACD features can reduce BHC ratings by factors greater than 50 percent. Figure 1 shows feature/function load factor effects on BHC rating.

Figure 1: Busy hour call processing

Networking operations typically require routing table look-up and analysis before trunk circuits can be selected and calls are routed. If a manufacturer’s benchmark testing procedure does not include some type of feature/function activation and implementation during a reasonable percentage of placed calls, the resulting BHC rating will not mirror the reality of a customer’s actual system installation.

The published PBX system BHC ratings, BHCA or BHCC, based on testing procedures may not adequately reflect the call processing capability of a customer-installed system configuration. The actual rating of an installed PBX system likely will be less than the published number. Fortunately for customers today, even if the true installed system rating is half of the published BHC ratings, the call processing capacity will be far greater than required by the customer. For example, small PBX system models from the leading manufacturers typically have call processing ratings in the range of 10,000 to 50,000 BHCC. Assuming a system configuration of 100 stations with associated trunking, the call processing rating of the system will far exceed the realistic call handling requirements. It is highly unlikely that, during the BHC period, all 100 stations will place 100 or 500 calls per hour. It is more likely that the total number of BHCs for a typical PBX system with 100 stations will be a few hundred.

Many of today’s intermediate/large PBX systems have call processing ratings greater 100,000 BHCC, and some are greater than 500,000 BHCC. Only very large systems approaching maximum port capacity will come close to approaching the maximum call processing capacity of the system. If the PBX is used for ACD applications, customers may need to install a larger system model than necessary for its greater BHCC rating, but it is highly unlikely that call processing limits will be reached, except in extreme circumstances.