Wireless Communication

Lecture 4 – Trunking and  Grade of Service

 

Text Sections 3.5.2, 3.6,7,8

 

 

Trunking:

Sharing a pool of lines or channels between a larger number of users.  Statistically, they are not all likely to be on the phone at the same time.  But as the # of lines is reduced, an individual user may sometimes be unable to access a free line.

 

Queue:  May be used to hold users waiting for a line and increase the quality of service for the user.  The user just waits for (hopefully a short period of time) for a dial tone.

 

Table 3.3:  Quality of Service or Grade of Service (QOS, GOS)

            Erlang = total use of one channel (1 call per hour that lasts an hour)

            GOS = typical likelihood that a call will be blocked       (Erlang B)

                        Or that a user will have to wait beyond a certain time (Erlang C)

            Set-up time = length of time to allocate a trunked radio channel

            Blocked call = call that cannot be completed due to lack of channels = lost call

            Holding time = H  =average duration of call (in seconds)

            Traffic Intensity = A = average channel usage (in Erlangs, dimensionless)

            Request Rate = l = average call requests per unit time (calls requests/second)

            Number of users = U

            Number of available channels = C

 

Traffic intensity of a single user

            Au = l H

Total traffic intensity of the system

            A = UAu

Traffic intensity per channel (if evenly distributed throughout the channels)

            AC = A / C = U Au / C

This is the traffic that is requested by the system, but may not be carried by the system if the capacity is not large enough.

            Maximum capacity = C (Erlangs)

 

Example:

            AMPS  Allows GOS = 2% (which means that 2 out of 100 calls will be blocked during busiest hour, 4-8 pm on Thursday and Friday nights)

           

Types of Trunked Systems

            Blocked Calls Cleared (no queue) (Erlang B)

            Blocked Calls Delayed        (Erlang C)

Assume:

1)      Calls arrive according to a Poisson distribution (in time)

2)      There are an infinite # of users

3)      All users, including those that are blocked, may request  a channel at any time.

4)      Probability of a user occupying a channel is exponentially distributed … longer calls happen exponentially less often (a poor assumption at times when people are using the internet).

5)      There are a finite # of channels C

 

Blocked Calls Cleared = Erlang B (calls that are blocked are dropped, no queue)

 

Probability of a blocked call

TABLE 3.4 (be sure you can interpret this table)

            Trunking efficiency:

            How many users a configuration of channels can support.

            10 channels with GOS = 0.01 can support 4.46 users

whereas 2 groups of 5 channels (also a total of 10 channels) can support 2x1.36 = 2.72 users

 

Blocked Calls Delayed = Erlang C (blocked calls are held in a queue and delayed)

 

Assume also that blocked users don’t hang up.

 

Equations 3.13-3.19

Probability of a blocked call (delay > 0)

 

 

This is greater than the delay for an Erlang B system, because users are waiting in line for the next available line.

 

Probability a user will wait more than t seconds

 

Average delay for all calls in a queued system

 

FIGURES 3.6 and 3.7

 

Work Examples 3.4-3.7

 

Methods of Improving Capacity

 

Cell Splitting (Figure 3.8,9)

Sectoring (Figure 3.10)

Microcell Zone (Figure 3.12)

 

Work examples 3.8-9