To size a given group of branch gateways, three pieces of information are needed:

• Total call usage (expressed in Erlangs) involving circuit-switched endpoints within the particular group of branch gateways. Designate this usage by u_total for the purposes of this discussion.

• Call usage (expressed in Erlangs) associated with calls between two circuit-switched endpoints within the particular group of branch gateways. Designate this usage by u_tdm for the purposes of this discussion.

• Maximum call usage (expressed in Erlangs) supported by a single branch gateway at the specified grade of service (GOS). Designate this usage by u_GOS for the purposes of this discussion. The value of u_GOS is determined by applying the Erlang B model to the number of time slot pairs available for bearer traffic and announcements and a specified GOS.

  • For a GOS of P001, use a value of u_GOS = 200.8 for G450 and a value of 193.3 for G430 Branch Gateway.

  • For a GOS of P000001 (essentially non-blocking), use a value of u_GOS = 175.1 for G450 and a value of 168.2 for G430 Branch Gateway.

  • The term u_GOS represents the maximum port usage (expressed in Erlangs) supported by a single branch gateway.

The number of port networks required for the given group of branch gateways is the smallest integer that is greater than or equal to the following formula: