ity is also utilised for buffering. Usually 10-15 % of the overheads are utilised for each of the two features. Iu Interface The I interface is of two types: one for CS and one for PS. The Iu-cs interface carries the circuit switch traffic (e.g. voice and real-time data) and connects the RNC and the MGW. The Iu-ps interface carries the packet switch traffic (e.g. non-real-time data) and connects the RNC with the SGSN. The blocking probability is quite low (e.g. ~0.1 The overheads are quite similar to those of the Iub interface. However, as the calls are terminated at the RNC, there are no overheads related to soft handovers. Also, the signalling is on of the lower side of the assumption mentioned above, ~1%-2 However, the Iu-ps Interface Planning I u-ps Figure 3.59 Interfaces in the UMTS transport network interface overheads are quite different. The data are routed to the SGSN through the tunnelling protocol (GTP). Tunnelling of the user data requires the GTP protocol plus UDP and IP (encapsulated) running on AAL5 ATM transmission media. Protocols GPT and UDP contain headers that are 12 bytes and 8 bytes long respectively. The IP layer also adds a header of variable lengths with the minimum being 20 bytes long. Over the ATM adaptation layer 5, IP uses LLC encapsulation, leading to overheads of around 8 bytes per datagram. Then there are trailer bytes, which are added to the end of the packet data unit. Adding all these bytes together leads to substantial overheads on the Iu-ps interface. Signalling on the Iu-ps interface is similar to that of the I-cs interface, i.e. ~ 1-2%. Iur Interface The Iur interface connects two RNCs. The Iur is not a physical interface. It is a logical interface that connects two RNCs via an MGW. For capacity calculation purposes, it is assumed that the Iur carries traffic that is a small percentage of the total I traffic. Example: Interface Planning As mentioned before, access interface planning is the most important aspect of transmission network planning. Consider the following inputs 