Trunking And Grade of Service in Cellular System Design

Presented By:

Debabrata SinghAsst.Prof. ,Dept Of CSIT,ITER,SOA University

CELLURAL SYSTEM DESIGN FUNDAMENTALS

1. Frequency reuse2. Channel assignment strategies3. Handoff strategies4. Interference and system capacity5. Trunking And Grade of Service6. Improving coverage and capacity in cellular system

i. cell splittingii. Sectoringiii. Repeaters for range extensioniv. A microcell zone concept

Trunking

Cellular radio systems rely on � trunking to accommodate a large number of users in a limited radio spectrum.

It allows a large number of users to share the relatively small �number of channels in a cell by providing access to each user, on demand, from a pool of available channels.

In a trunked radio system, each user is allocated a channel on a �per call basis, and upon termination of the call, the previously occupied channel is immediately returned to the pool of available channels.

Telephone company uses trunking to determine the number of �telephone circuit that need to be allocated for office building with hundreds of telephones.

There are two major classes of trunked radio systems:

• Lost Call Cleared(LCC)

• Lost Call Delayed (LCD)

Lost Call Cleared(LCC)1. Queuing is not provided for call requests.2. When a user requests service, the user is given immediate

access to a channel if one is available. 3. If no new channels are available, the call is blocked without access to the system. 4. The user is free to try the call again later. Calls are assumed to arrive with a Poisson distribution.5. The time between successive call is exponentially distributed.6. Assumed that there are infinite number of users.7. Assume that all blocked calls are instantly returned to an infinite user pool, and may be retried at any time in the future.

Lost Call Delayed system:

1. Queues are used to hold call requests that are initially blocked.

2.When a channel is not immediately available, the call request may be delayed until a channel becomes available.3. The Erlang C formula gives the likelihood that all channels

are in use. It is also necessary to know the probability of how long the cal must be delayed before a channel is available.

4.The GOS is measured by the probability that calls will have delayed greater than t seconds.5.Assume that all calls in the queue are eventually serviced.

GOS(grade of service)

• It is a measure of the ability of a user to access a trunked system during the busiest hour• It’s the wireless designer’ s job to estimate the maximum required capacity and to allocate the proper number of channels in order to meet the GOS.

For a delayed call system, the Grade of Service is measured using three separate terms:•The mean delay   – Describes the average time a user spends waiting for a connection if their call is delayed.•The mean delay   – Describes the average time a user spends waiting for a connection whether or not their call is delayed.•The probability that a user may be delayed longer than time t  while waiting for a connection. Time t is chosen by the telecommunications Service provider so that they can measure whether their services conform to a set Grade of Service.

GOS CONTINUE…

A = Expected traffic intensity in Erlangs N = Number of circuits in group

•Grade of service is the probability of a call in a circuit group being blocked or delayed for more than a specified interval.•This is always with reference to the busy hour when the traffic intensity is the greatest.• Grade of service may be viewed independently from the perspective of incoming versus outgoing calls, and is not necessarily equal in each direction or between different source-destination pairs.

Example :1If total 33Mhz of band width is allocated to a particular FDD cellular telephony system ,which uses Two 25KHz simplex channel to provide full duplex voice and control channels, Compute the number of channels available per cell if a system uses 4 cell reuse, 7 cell reuse, 12 cell reuse , if 1MHz of allocated spectrum is dedicated to control channel.Ans:Bandwidth =33MHzChannel bandwidth =25KHz × 2 =50KHz for duplex channelTotal available channels =33000/50 =660 channels•For N=4 Total no. of channels available per cell =660/4 =165 channels.(5 control channel and 160 voice channel)•For N=7 Total no. of channels available per cell =660/7 =95 channels.(3 control channel and 92 voice channel)•For N=12 Total no. of channels available per cell =660/12 =55 channels.(2control channel and 53 voice channel)

Channel available per cell

Traffic intensity

It measure the channel time utilization, which is the average channel occupancy measured in Erlangs.

Types of trunked systems

There are two which are commonly used.

The first type offers � no queuing for call requests. (lost call cleared)

The second kind is one in which � a queue Is provided to hold calls which are blocked. (lost call delayed)

The first type offers no queuing for call requests.That is, for every user who requests service, it isassumed there is no setup time and the user is givenimmediate access to a channel if one is available.

If no channels are available, � the requesting user is blocked without access and is free to try again later.

This type of trunking is called � blocked calls clearedand assumes that calls arrive as determined bya Poisson distribution.

The Erlang B formula: determines the probability that a call is blocked and is a measure of the GOS for a trunked system which provides no queuing for blocked calls.

C- is the number of trunked channels offered by a trunked radio systemA -is the total offered traffic.

Erlang B Trunking GOS

Erlang B

� The way in which channels are grouped can substantially alter the number of users handled by a trunked system. For example, 10 trunked channels at a GOS of 0.01 can support 4.46 Erlangs of traffic. It can be offered a particular GOS with a particular configuration of fixed channels.

Clearly, 10 channels trunked together support 60% more traffic �at a specific GOS than do two 5 channel trunks! It should be clearthat the allocation of channels in a trunked radio system has a major impact on overall system capacity.

CONCLUSION

Thanking You