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Lecture 4th

Lecture 4th. Hnadoff and Trunking. Handoff. When a mobile moves to a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to a new BS

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Lecture 4th

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  1. Lecture 4th Hnadoff and Trunking

  2. Handoff • When a mobile moves to a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to a new BS • Handoff operation not only involves identifying a new BS but also requires that the voice and control signals be allocated to channels associated with the new BS • Many handoff strategies prioritize handoff requests over call initiation requests when allocating unused channels in a cell site. • Handoffs must be imperceptible to users. Thus system designers must specify an optimum signal level at which to initiate a handoff

  3. Basics of Handoff • Initiation: either mobile or network identifies need for handoff and begins the process • Resource reservation: required resources necessary to support handoff are allocated • Execution: actual handoff takes place and mobile uses new network resources • Completion: unneeded resources are freed

  4. Handoff • Once a particular signal level is specified as the minimum usable signal for acceptable voice quality at the BS receiver a slightly stronger signal level is used as a threshold at which a handoff is made • This margin given by delta= P r handoff –P r minimum usable, • cannot be too large or too small—has to be chosen carefully • If delta=too large there may be insufficient signal strength to continue the call. • If delta=too small: : unnecessary handoffs which burden MSC may occur

  5. Improper Handoff Situation

  6. Improper Handoff Situation • Calls are dropped when: • there is excessive delay by MSC in assigning a handoff • when the threshold (delta) is set too small for handoff time in the system • Excessive delays can occur: • During high traffic conditions due to computational loading at MSC • When there are no channels available on any of the nearby BSs (MSC has to wait until channel in a nearby cell becomes free)

  7. Proper Handoff Situation

  8. In deciding when to handoff, it is important to ensure that: • Drop in measured signal level is not due to momentary fading • The mobile is actually moving away from the serving BS • Decision about handoff may be based on mobile speed • Vehicular speeds require fast handoff so we cannot spend much time deciding whether or not to perform handoff • Pedestrian speeds can tolerate more waiting time to determine whether handoff is really necessary • Dwell time: time a call is maintained within a cell • Function of propagation, interference, distance to BS, etc. • Varies greatly depending on speed of mobile

  9. Trunking and Grade of Service Cellular radio systems rely on trunking to accommodate a large number of users in a limited radio spectrum. Trunking system A mechanism to allow many users to share fewer number of channels. Not every user calls at the same time Penalty: Blocking Effect. If traffic is too heavy, call is blocked!! Small blocking probability is desired. There is a trade-off between the number of available circuits and blocking probability.

  10. Trunking Trunking exploits the statistical behavior of users so that a fixed number of channels may accommodate a large random user community There is a trade-off between the number of available telephone circuits and the likelihood of a particular user finding that no circuits are available during the peak calling times As the number of phone lines decreases, it becomes more likely that all circuits will be busy for a particular user In a trunked mobile radio system, when a particular user requests service and all of the radio channels are already in use, the users is blocked or denied access to the system. In some systems a queue may be used to hold requesting users until a channel becomes available

  11. Erlang The fundamentals of truncking theory were developed by Erlang, a Danish mathematician who, in the late 19th century embarked on the study of how a large population could be accommodated by a limited number of servers. Erlang: a “dimensionless unit,” The basic unit of telecom traffic intensity carried a channel that is completely occupied (one call-hour/hour or one call-min/min) Since a single circuit used continuously carries 60 minutes of calling in one hour, one Erlang is usually defined as 60 minutes of traffic A radio channel that is occupied for 30 minutes during an hour carries 0.5 Erlangs of traffic

  12. Grade of Service GOS is a benchmark used to define the desired performance of a particular trunked system Grade of Service (GOS): probability that a call is blocked (or delayed). The probability that all servers will be busy when a call attempt is made. For example, on a trunk group: GOS of 2% means that there is a 2% probability of getting a busy signal (being “blocked”) when you have a given amount of traffic and a given number of trunks.

  13. Grade of Service It is a wireless designer’s job to estimate the maximum required capacity and to allocate the proper number of channels in order to meet GOS AMPS is designed for GoS of 2% blocking. This means that channel allocations for cell sites are designed so that a maximum of 2 out of 100 calls will be blocked due to channel occupancy during the busiest hour

  14. Common Terms of Trunking Theory Set-up Time:The time required to allocate a trunked radio channel to a requesting user Blocked Call:Call which cannot be completed at time of request, due to congestion. (lost call) Holding Time:Average duration of a typical call. Denoted by H Traffic Intensity:Measure of channel occupancy measured in Erlangs. Denoted by A Load Request Rate

  15. The traffic intensity offered by each user is equal to the call request rate multiplied by the holding time. Each user generates a traffic intensity of Au Erlangs given by H is the average duration of a call λ =avg No. of call requests per unit time for each user

  16. For a system containing “U” users and an unspecified No. of channels, Total offered traffic Intensity=A=UAu In a “C” channel trunked system, if the traffic is equally distributed among the channels, traffic intensity per channel, Ac = UAu / C

  17. Important! Offered traffic is not necessarily the traffic which is carried by the trunked system It is only that which is offered to the trunk system When the offered traffic exceeds the maximum capacity of the system, the carried traffic becomes limited due to the limited availability of the No. of channels

  18. Block Calls cleared/Erlang B System In Erlang B System, a call request is simply denied if all channels in the pool are in use Assumptions Any user can require a channel any time Probability of user occupying a channel is exponential Finite trunk channels available No queuing, for call requests if no channels are available, requesting user is blocked it is assumed that there is no setup time and user is given immediate access to a channel if one is available

  19. Probability of Blocking The GOS measure for Block Calls Cleared System is the probability that a user’s call request is blocked The Erlang B formula determines the blocking probability, p, given a certain total offered traffic intensity, A, and a certain number of channels C in the pool A is the total offered traffic Since some calls are blocked, A is not the traffic carried by the system

  20. Erlang-B Formula

  21. Erlang B Chart

  22. Blocked Calls delayed/Erlang C A queue is provided to hold calls which are blocked If a channel is not available immediately, the call request may be delayed until a channel becomes available Its measure of GOS is defined as the probability that a call is blocked after waiting a specific length of time in the queue To find the GOS, it is first necessary to find the likelihood that a call is initially denied access to the system This likelihood of a call not having immediate access to a channel is determined by Erlang C formula

  23. Erlang-C Formula

  24. Erlang C Chart

  25. Example

  26. Example

  27. Example

  28. Example

  29. Example

  30. Conclusion Trunking efficiency is a measure of No of users which can be offered a particular GOS with a particular configuration of fixed channels Way in which channels are grouped substantially alters the number of users handled by a trunked system 10 trunked channels at a GOS of 0.01 can support 4.46 Erlangs of traffic But two groups of 5 trunked channels can support 2 × 1.36 Erlangs, or 2.72 Erlangs of traffic 10 channels trunked together support 60% more traffic at a specific GOS than do two five channel trunks! So allocation of channels in a trunked radio system has a major impact on overall system capacity Do look at remaining examples!

  31. Assignment Pr 3.4, 3.5, 3.10, 3.11, 3.13, 3.15

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