MOBILE COMMUNICATIONS (650539) Part 3

Transcription

1 Philadelphia University Faculty of Engineering Communication and Electronics Engineering MOBILE COMMUNICATIONS (650539) Part 3 Dr. Omar R Daoud ١

2 The accommodation of larger number of users in a limited radio spectrum denoted by trunking a large number of users share a pool of available channels (relatively small number of channels) Trade off between the number of available telephone circuits (in the telephony system) and the likelihood of a particular users finding that no circuits on the peak time Dr. Omar R Daoud ٢

3 What will happen to the user if all channels are in use?? (in some systems a queue may be used to solve the blocking or denying process) In the 19 th century, Erlang (Danish mathematician) study how a large number of population could be accommodated by a limited number of services (today its known by the measure of traffic intensity) Dr. Omar R Daoud ٣

4 The Grade of Service (GOS) is a measure of the ability of a user to access a trunked system during the busiest hour By specifying a desired likelihood of a user obtaining channel access given a specific number of channels available in the system (It s the benchmark used to design the performance of a particular trunked system) It s the designer s job to estimate the max required capacity to allocate the proper number of channels to meet the GOS Dr. Omar R Daoud ٤

5 GOS is typically given as the likelihood that a call is blocked or experiencing a delay greater than a certain queuing delay time A u is defined as a traffic intensity and is given by A u = λh where λ is the average number of call requests per unit time for each user, H is the average duration of a call Dr. Omar R Daoud ٥

6 The total offered traffic intensity for U users is A= A u U In a C channel trucked system, the traffic intensity per channel, A c, is given by A c = A/C The max possible carried traffic is the total number of channels, C, in Erlangs Dr. Omar R Daoud ٦

7 Some needed definitions - Set-up time: the time required to allocate a trunked radio channel to a requesting user - Holding time: call which cannot be completed at time of request, due to congestion (lost calls) - Traffic intensity: measure of channel time utilisation, which is the average channel occupancy measured in Erlang. This is a dimensionless quantity and may be used to measure the time utilisation of single or multiple channels, A - Load: traffic intensity across the entire trunked radio system, measured in Erlang - GOS: A measure of congestion which is specified as the probability of a call blocked or a call being delayed beyond a certain amount of time - Request Rate: the average number of call requests per unit time Dr. Omar R Daoud ٧

8 AMPS system is designed for a GOS of 2% blocking Tow types of trunked systems - offers no queuing for call requests (no set up time and given immediate access to a channel if one is available or blocked if no available ones ) * assume infinite number of users * memoryless arrivals of requests * exponentially distributed probability of a user occupying a channel (longer calls are less to happened) * a finite number of channels are available in the pool Dr. Omar R Daoud ٨

9 Erlang B formula determine the probability that a call is blocked and is a measure of the GoS for a trunked system (which provides no queuing for blocked call) Pr( Blocking) GoS where C is the number of trunked channels offered by a radio system A is the total offered traffic A C 1 k 0 C A / C! k / k! Dr. Omar R Daoud ٩

10 Erlang B chart Dr. Omar R Daoud ١٠

11 Tow types of trunked systems the second type is one in which a queue is provided to hold calls which are blocked (called by blocked calls delayed, Erlang C) Pr( delay 0) A C A C! 1 C A C C 1 k 0 k A k! Dr. Omar R Daoud ١١

12 Erlang C chart Dr. Omar R Daoud ١٢

13 Tow types of trunked systems The GoS of a trunked system where blocked calls are delayed is given by ( ( C A) t ) H Pr( delay t) Pr( delay 0) e The average delay D for all calls in a queued system is given by D = Pr(delay>0) [H/(C-A)] Dr. Omar R Daoud ١٣

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