load. The CPS-PDU payload may carry zero, one or more complete or partial CPS packets. The CPS packet may overlap one or two ATM cells, and the boundary can be anywhere, including the CPS packet header. The SSCS consists of three parts: SSSAR (service specific segmentation and reassembly), which supports payloads up to 65 536 bytes long; SSTED (service specific transmission error detection); SSADT (service specific assured data transfer). Only the SSSAR is used and implemented on the Iu-CS and Nb interfaces. The SSSAR does not add any overhead to the SSCS-SDU. Backbone Bandwidth Dimensioning The backbone bandwidth dimensioning depends on the type of codec used (G.711, AMR, etc.) as well as on the total number of voice calls. In the previous paragraphs it has been shown how to calculate the bit rate required for a voice call in the backbone depending on the different codecs, IP backbones and physical layers (e.g. Ethernet) used. Example 1. Given the traffic in Erlangs, calculate the total number of voice calls by applying the Erlang B formula and the required bandwidth for a given codec on the IP or ATM backbone: X = traffic (Erl) Y = blocking probability (%) Z = voice calls = number of traffic channels = Erlang B calculator (X, Y) D = data bit rate B = bandwidth (kbps) = ZD Core Network Planning and Optimisation Example 2. Calculate the required bandwidth for: Traffic = 6540 Erl Blocking probability = 0.1% IP backbone using the AMR 12.2 codec, without including the Ethernet header and IPv4 Solution: Voice calls = Erlang B calculator (6540,0.1%) Z = 6687 calls Bandwidth (kbps) = 6687 X 45.2 = 295.1684 Mbps Example 3. Calculate the required bandwidth for Traffic = 6540 Erl Blocking probability = 0.1% On an ATM backbone using the AMR 12.2 codec Solution: Voice calls = Erlang B calculator (6540,0.1%) Z = 6687 calls Bandwidth (kbps) = 6687 X 17.1 = 111.6677 Mbps ATM cells per second = 6687 X 45 = 300.915 kcells per second 4.4.3 Control Plane Planning The control plane planning process follows similar steps: Determine 