b signalling: 10%* Step 1. Calculate the voice and circuit switched data channels/BTS. The first assumption is that due to user behaviour the use of the voice service and circuit switched data services are independent consumption decision events and as such are statistically handled separately. The second assumption is that even if in 3G no circuits are assigned for services, describing user connections as circuits and using Erlang B as the connectivity quality measure for them on average is close enough. Voice traffic/BTS = 12 mErl/subscriber X 1000 subscribers/BTS: 12 Erl/BTS Based on Erlang B, with 2 % blocking, the total number of channels = 19 Soft handover = 40' % Hence, the total number of voice service user connections/legs over lub = 19 1.4 = 26.6 = Circuit switched data traffic/BTS = 10 mErl/subscriber X 1000 subscribers/BTS = 10 Erl/BTS Based on Erlang B, with % blocking, the total number of channels required = 17 Soft handover = 40% Hence, the total number of circuit switched data service user connections/legs over 1.4 = 23.8 = ~24 * The Iub signalling value is dependent upon the topology, system parameter values and vendor. Interface Planning Step 2. Special considerations for voice service. Voice service user data rate in the active mode in the U interface = 12.2 kbps Voice service user data rate including ATM overheads in the active mode = 18.9 kbps and in the silent mode = 4.5 kbps Using voice activity factor 4= 50 %, Hence, the average voice service user data rate = 18.9 X 50 % + 4.5 X 50 % = 11.7 kbps Step 3. Calculate the total circuit switched user data load in the Iub interface. As the user data rate for the circuit switched data service is 64 kbps, total circuit switched data in kbps/BTS = 64 x 24 = 1536 kbps/BTS As the average voice service user data rate is 11.7 kbps, the total voice service user data rate in kbps/BTS = 27 X 11.7 = 315.9 kbps/BTS Adding ATM overheads (23%) on circuit switched data services = 1536 X 1.23 = 1889.2 kbps/BTS Iub signalling for this example was assumed to be 10