GSM traffic example:
a. An area is served by a GSM transceiver. A GSM transceiver uses 2X200 KHz BW to provide 8 TDM communication channels. There are 30 users during non- business hours (i.e. after 6:00 pm) in the area. The average call arrival rate l 1 and average holding time 1/m per user are 1 per 2 hours and 15 minutes, respectively. What is the GOS?
b. User profile changes during business hours. Number of users increase to 40 and l 1 and 1/m becomes 3 per hour and 6 minutes respectively. How many transceivers are needed to keep GOS below 1%? Below 2%?
c. Since there is no point in keeping an installed transceiver idle, the traffic carried during off-hours must be improved. What are the guide figures to determine the off-hour call tariffs to improve the revenues? Please discuss this problem.
Soln:
a. l 1/m = 0.125 Erlang; Au= 0.11111 Erlang; A = 3.333 Erlangs;
From the table
GOS =0.005 at A = 3.033 for U=30 and C=8
and
GOS =0.01 at A = 3.444 for U=30 and C=8
Hence GOS is better than1%. Interpolating between two GOS values, we can find GOS as 0.0086.
b. l 1/m = 0.3 Erlang; Au= 0.2308 Erlang; A = 9.231 Erlangs;
From the table
GOS =0.01 at A = 9.825 for U=40 and C=16
Two transceivers are sufficient for GOS=1%.
c. If for example 2% GOS is an acceptable service, two transceivers can serve an offered traffic of 11.122 Erlangs for U=30. The off-hour users must be encouraged to increase their offered traffic intensity from 3.333 Erlangs to 11.122 Erlangs.
Although additional data on price sensitivity is necessary, a guideline can be drawn form above figures:
If a new tariff which is 1/3 (» 3.333/11.122) of the existing one increases the offered traffic to 11.122 Erlang, no extra revenues. Furthermore if the traffic intensity remains below this level, there will be loss.
A new tariff, which is 50% of the previous one will produce extra revenue for the traffic offered above 6.666 Erlang, etc.