BHCA
Initialism of busy-hour call attempts.
BHCA: peak-hour traffic load that sizes your switching gear
BHCA stands for busy-hour call attempts. It measures the total number of call originations (not completions, and not counting transfers or retries as separate events) that occur during the single busiest hour of an exchange or switching system in a typical day. BHCA is the traffic engineering metric that determines how much switching capacity, trunking, and signaling bandwidth a telephone network must provision.
A switching system rated for 10,000 BHCA must be able to process 10,000 new call attempts during its peak hour without congestion or call blocking. The rating applies to the exchange itself, its outbound trunks, its signaling links, and its subscriber line cards. The distinction between call attempts and completed calls matters: a user who dials, hears a busy tone, and tries again counts as two attempts.
Relationship to traffic intensity and dimensioning
BHCA differs from Erlang traffic, which measures occupancy (the cumulative duration of all calls in progress). A system might handle 100 Erlangs of traffic with a very different BHCA depending on average call duration. Shorter calls mean more attempts per hour; longer calls mean fewer. Engineers use BHCA to size the switching fabric, processors, and common channel signaling networks, while Erlang ratings govern trunk count and subscriber line capacity.
Historical switching systems like step-by-step and crossbar machines had firm BHCA limits imposed by mechanical switching speed and relay recovery time. Modern digital exchanges (using stored-program control) can scale BHCA by adding processor capacity and memory, but must still accommodate signaling link congestion if BHCA climbs beyond the network's design point. Mobile networks track BHCA separately by cell site and by aggregate network load.
BHCA is also used informally in VoIP provisioning and private branch exchange (PBX) sizing, though these systems may use seat-count or concurrent-call metrics alongside it. The term remains essential in telecom capacity planning and in regulatory documents where network operators must declare their exchange ratings.