with a constant bit rate of 64 kbps and one sample is sent every 20 ms, the overhead consists of Nb_UP/RTP/UDP/IP, which totals 204 bytes every 20 ms, or 80 kbps (without the link level overhead; see Figure 4.15). Protocol Stack for the User Plane The Nb-UP Protocol is specified by 3GPP in specification 29.415. It is used to convey data between MGWs over the Nb interface. The Nb-UP Protocol can be either in the support mode or transparent mode. The transparent mode may be used when the bearer does not need any services of the Nb-UP Protocol except data transfer. Therefore Nb-UP frames are not generated in the transparent mode. The support mode is intended for use when the bearer requires other services of the Nb-UP Protocol besides data transfer. So far, the only support mode defined is the support mode for predefined SDU size. For example, an AMR codec might use this support mode because it requires some procedure control function, e.g. initialisation and rate change. The Nb-UP framing is identical to the Iu-UP framing defined in 3GPP standards TS 25.415. In the support mode, the Nb-UP overhead is 3 or 4 bytes, depending on whether the CRC is used or not. In the transparent mode, the Nb-UP header is absent, and thus the overhead is 0 bytes. IP Backbone RTP The Real-Time Transport Protocol (RTP) is a simple protocol used to encapsulate voice and compressed video codec information for end-to-end delivery. The purpose of the RTP is to offer a remedy for the delays and packet losses inherent in IP based networks. The RTP uses the UDP for transport. The services provided by the RTP, and its companion protocol, the Real-Time Control Protocol (RTCP), include: Payload type identification Sequence numbering Time stamping Delivery monitoring Core Network Planning and Optimisation Table 4.9 Data bit rates in the IP backbone G.711 (64 kbps) AMR (12.2 kbps) AMR VAD Nb-UP (3-4 bytes) RTP (12 bytes) UDP (8 bytes) IPv4 (20) IPv6 (40) IPv4 (20) IPv6 (40) IPv4 (20) IPv6 (40) Bit rate [kbps] Ethernet (42 bytes) Bit rate [kbps] 104