Models

Model	Applications
Erlang B	Designing public phone systems where: blockage is very low and hence retries are uncommon (and can be neglected), or blockage may be high, but blocked calls overflow to some other facility (modeled separately) Grade of Service, non-USA Telcos Grade of service in commercial (low blockage) networks
Poisson	Grade of Service, USA Telcos Grade of service in commercial networks PBX trunk sizing in large offices
Erlang C	Calculating staffing levels (i.e.. required agents) for customer call centers Auto-Attendants, Auto Call Distributors (ACDs) Latency in data transmission circuits Bank Teller, Supermarket, etc. lines
Engset	Designing private phone systems (PBXs) where: blockage is very low and hence retries are uncommon (and can be neglected), or blockage may be high, but blocked calls overflow to some other facility (e.g. public phone system)
Binomial	Designing private phone systems (PBXs) where: Office size (sources) is small Where there is no overflow Networked servers (e.g. Fax Servers, Modem Pools,…) Help Desk phone traffic.
Delay	Networked servers (e.g. Fax Servers, Modem Pools,…) Repair turnaround.
Call Center	This is built on top of the Erlang C model. It is specifically designed for modeling telephone call centers, and allows you to calculate: Required numbers of agents to 'answer xx% of calls within yy seconds' Required number of incoming phone lines (trunks) Average delay, agents busy, etc.