Trunking and Grade of service
Cellular radio systems rely on trunking to accommodate a large number of users in a limited radio spectrum. The concept of trunking 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.
- In a trunked mobile radio sy 4tem, when a particular user requests service and all of the radio channels are already in use, the user is blocked, or denied access to the system.
- To design trunks radio systems that can handle a specific capacity at a specific "grade of service”
- The grade of service (GOS) is a measure of the ability of a user to access a trunked system during the busiest hour
- The busy hour is based upon customer demand at the busiest hour during a week, month, or year. The busy hours for cellular radio systems typically occur during rush hours, between 4 p.m. and 6 p.m. on a Thursday or Friday evening.
- Set-up Time:The time required to allocate a trunked radio channel to a requesting user.
- Blocked Call: The Call which cannot be completed at time of request, due to congestion is known as Blocked calls. It also referred to as a lost call.
- Holding Time:It is defined as average duration of a typical call. It is denoted by H (in seconds).
- Traffic Intensity: Measure of channel time utilization, which is the average channel occupancy measured in Erlangs. This is a dimensionless quantity and may be used to measure the time utilization of single or multiple channels. Denoted by A.
- Load:Traffic intensity across the entire trunked radio system, measured in Erlangs.
- Grade of Service (GOS):A measure of congestion which is specified as the probability of a call being blocked (for Erlangs B), or the probability of a call being delayed beyond a certain amount of time (for Erlangs C).
- Request Rate: The average number of call requests per unit time. Denoted by lambda per second
There are two types of trunked systems which are commonly used.
- The first typeoffers no queuing for call requests.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 cleared
- The second typeof trunked system is one in which 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. This type of trunking is called Blocked Calls Delayed
- The average delay D for all calls in a queued system is given by
D = Pr[delay>0]H/C-A
Where the average delay for those calls which are queued is given by H/(C—A).