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Chapter 5Upstream channel capacity and characterisation prequire more time to be captured and buffered. Hence, a trade-off exists between packetoverhead and local latency for audio packet transfer.The optimal point for the number of audio frames per packet is between seven and eightfor G.723.1 codecs. According to [28], these values should be used by terminals thattarget packet efficiency as well as low latency for audio packets. However, sometimes,the audio local latency is reduced at the expense of packet overhead, in order to achievelatencies acceptable to users. In fact, a few applications are willing to sacrifice someprotocol overhead for achieving better audio latency. Some H.323 terminals use a valueof 3 or 4 for the number of frames per audio packet in order to reduce this latency [28].Therefore, in order to be consistent with these figures for H.323 terminals, 4 audioframes will be used per packet. Then, the following protocol overheads are added inorder to yield a complete VoIP packet.Each audio packet has a Real Time Protocol (RTP) header of 12 bytes that carriessequence numbers, a synchronisation source identifier and time-stamps. A UserDatagram Protocol (UDP) header of 8 bytes is needed to carry a UDP with an unreliabletransmission service. In addition a 20-byte IP header is needed to transfer routinginformation. A 3-byte Logical Link Control (LLC) and a 5-byte SubNetworkAttachment Point (SNAP) headers are used to carry a PDU with a connectionlessservice and to identify the type of the bridged media, respectively. Finally, an 18-byteMAC header is used to transmit the PDU to its final destination over the shared media.Thus, by adding the complete headers one obtains an improved VoIP stream of 9.7 kbpsas indicated in Table 5.2. Hence, in our performance analysis, VoIP streams of 9.7 kbpswill be considered.Table 5.2- VoIP encapsulation with and without header suppression.Frame/Header 9.7 kbps Streams 12.4 kbps StreamsVoice frame 80 bytes 20 bytesRTP headerUDP header6 bytes12 bytesIP header 8 bytes 20 bytesLLC headerSNAP header3 bytes5 bytesEthernet MAC header18 bytes5-5Total Size 146 bytes (4 ATM cells) 40 bytes (1 ATM cells)