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ECE 5221 Personal Communication Systems

ECE 5221 Personal Communication Systems. Prepared by: Dr . Ivica Kostanic Lecture 19: Traffic planning (3). Spring 2011. Outline . Traffic planning for voice service Busy hour Grade of service M/M/C/C system and Erlang B formula Kendall’s notation .

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ECE 5221 Personal Communication Systems

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

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

  3. 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 in equilibrium the number of calls leaving and entering the cell are approximately the same the signaling overhead due to handoff is small and usually handled by separate set of channels call holding time for voice is assumed exponentially distributed - “handing in” calls can be interpreted as continuation of “handing out” calls Traffic planning for voice service

  4. Busy hour • Busy hour - uninterrupted period 60 min during which the traffic volume is highest • Used for traffic dimensioning • Can be • fixed • bouncing • 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

  5. QoS in lossy systems - Grade Of Service (GOS) Definition of 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

  6. Trunking model for lossy systems - Erlang B State diagram of M/M/C/C system • Erlang B assumptions • Call arrival process is Poisson • Service time is exponentially distributed • There are C identical servers (channels) • There is no queue • Common QoS parameter in M/M/C/C systems is GOS Probability of blocked request (i.e. blocked call) Erlang B formula Offered traffic

  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 Average birth rate Average death rate Offered traffic Grade of service

  8. Erlang B - performance curves • Erlang B formula can be given in a form of • family of curves • table (Appendix A) Erlang B family of curves

  9. Erlang B - Summary of performance parameters

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

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