Florida Institute of technologies ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 19: Traffic planning (3) Spring 2011

Florida Institute of technologies Page 2  Traffic planning for voice service  Busy hour  Grade of service  M/M/C/C system and Erlang B formula  Kendall’s notation Outline Important note: Slides present summary of the results. Detailed derivations are given in notes.

Florida Institute of technologies Page 3 Traffic planning for voice service  Circuit switched voice - dominant service in 1G and 2G cellular systems  Assumption - channel is held for the entire duration of the call - violated due to handoff  In practice the effect of handoff is usually neglected oin equilibrium the number of calls leaving and entering the cell are approximately the same othe signaling overhead due to handoff is small and usually handled by separate set of channels ocall holding time for voice is assumed exponentially distributed - “handing in” calls can be interpreted as continuation of “handing out” calls

Florida Institute of technologies Page 4 Busy hour  Busy hour - uninterrupted period 60 min during which the traffic volume is highest  Used for traffic dimensioning  Can be ofixed obouncing  In heterogeneous networks busy hours for different traffic types may not coincide  In cellular network, busy hour occurs at different time for different cells Typical daily traffic usage in a cellular system

Florida Institute of technologies Page 5 QoS in lossy systems - Grade Of Service (GOS)  Grade Of Service = probability of blocking in lossy system  Blocked traffic is considered lost  Usually expressed in percent  Reported as a part of switch statistics  Closely monitored  Typical GOS requirement for cellular systems is 2% or better Definition of GOS

Florida Institute of technologies Page 6 Trunking model for lossy systems - Erlang B  Erlang B assumptions oCall arrival process is Poisson oService time is exponentially distributed oThere are C identical servers (channels) oThere is no queue  Common QoS parameter in M/M/C/C systems is GOS State diagram of M/M/C/C system Probability of blocked request (i.e. blocked call) Erlang B formula Offered traffic

Florida Institute of technologies Page 7 Erlang-B example A cell site has 5 FDMA radios. The average call origination rate is 60 calls per hour. If the call holding times are distributed exponentially with an average of 90 sec, calculate the GOS Grade of service Average birth rate Average death rate Offered traffic

Florida Institute of technologies Page 8 Erlang B - performance curves  Erlang B formula can be given in a form of ofamily of curves otable (Appendix A) Erlang B family of curves

Florida Institute of technologies Page 9 Erlang B - Summary of performance parameters

Florida Institute of technologies Page 10 Erlang B - examples  Example 1 oDetermine the traffic capacity in erlangs for a 30-channel cell such that the GOS will not exceed a) 2% and b) 1% oSolution a) A = 21.9 E for N = 30 and GOS = 2% b) A = 20.3 E for N = 30 and GOS = 1%  Example 2 oDetermine the number of voice channels required to support 20 erlangs of traffic at a GOS of a) 2% and b) 1% oSolution a) N = 28 for A = 20.2 E and GOS = 2% b) N = 30 for A = 20.3 E and GOS = 1%