Call blocking probability

Study how call blocking probability varies as the load on a GSM network is continuously increased

Network Setup#

Open NetSim and click on Experiments> Cellular Networks> Impact of load on call blocking probability in GSM then click on the tile in the middle panel to load the example as shown in below Figure 23‑1.

Graphical user interface, application Description automatically
generated

Figure 23‑1: List of scenarios for the example of Impact of load on call blocking probability in GSM

NetSim UI displays the configuration file corresponding to this experiment as shown below Figure 23‑2.

Figure 23‑2: Network set up for studying the 4-Mobile Station

Procedure#

The following set of procedures were done to generate this sample:

Step 1: A network scenario is designed in NetSim GUI comprising of 4 Mobile Stations, 1 MSC, and 1 Base Station in the "Cellular Networks" Network Library.

Step 2: Ensure all the Mobile Stations are placed within the range of Base Station.

Step 3: In the Interface GSM > Data Link Layer Properties of MSC 2, Uplink BW Min and Uplink BW Max are set to 890 MHz and 890.2 MHz respectively Figure 23‑3.

Graphical user interface, application Description automatically
generated

Figure 23‑3: Data Link Layer Properties

Step 4: Right click on the Application Flow App1 ERLANG CALL and select Properties or click on the Application icon present in the top ribbon/toolbar.

The applications are set as per the below Table 23‑1.

Application Properties Application 1 Application2
Application type Erlang_call Erlang_call
Source_Id 3 5
Destination_Id 4 6
Call
Duration_ Distribution Exponential Exponential
Duration(s) 60 60
Inter Arrival Time (sec) 10 10
IAT_ Distribution Exponential Exponential
Codec Custom Custom
Inter Arrival Time distribution Constant Constant
Packet Distribution Constant
Service Type CBR CBR
Packet Size 33 33
Inter Arrival Time (µs) 20000 20000

Table 23‑1: Detailed Application Properties

Step 5: Enable plots and run the Simulation for 100 Seconds.

The following changes in settings are done from the previous sample:

Step 1: In the next sample, increase the number of Mobile Stations by 2 and add one more application between them.

Step 2: Enable plots and run the Simulation for 100 Seconds.

The following changes in settings are done from the previous sample:

Step 1: Similarly, increase the number of Mobile Stations by 2 up to 20 and set properties for different Samples by adding an application every time and changing Source ID and Destination ID.

Step 2: Enable plots and run the Simulation for 100 Seconds.

Output#

To view the output, go to the Cellular Metrics. In MS metrics, take sum of call blocking probability (It is the as ratio of Total call blocked to Total call generated).

Comparison Charts: Graphical user interface, application Description automatically
generated

Figure 23‑4: Plot of Call Blocking Probability vs. Number of Mobile Stations

*** All the above plots highly depend upon the placement of Mobile station in the simulation environment. So, note that even if the placement is slightly different the same set of values will not be got but one would notice a similar trend.

Inference#

When the number of MS is increased from 4 to 20 the call blocking probability increases from 0 to 3.46. As we increase the number of mobile stations more calls are generated. This increases the traffic load on the system & more calls generated implies more channel requests arrive at the base station, but the number of channels is fixed. So, when the base station does not find any free channel the call is blocked. An additional observation is that the call blocking is zero until 8 MS. This is because the number of channels is sufficient to handle all call that 6 MS may generate. Only after this the base station does not find free channels and blocks calls.