/mcpQuality of Service for Voice over IP Fragmentation and Interleaving 11 QoSVoIP.mif before being sent. If we assume that a VoIP packet will need to wait behind one data packet, and that the data packet can be, at most, equal in size to the MTU (1500 bytes for serial and 4470 bytes for high-speed serial interfaces), we can calculate the wait time based on link speed. For example, for a link speed of 64 kbps and MTU size of 1500 bytes, we have: Serialization delay = (1500 bytes * 8 bits/byte) / (64,000 bits/sec) = 187.5 ms Therefore, a VoIP packet may need to wait up to 187.5 ms before it can be sent if it gets delayed behind a single 1500-byte packet on a 64-kbps link. VoIP packets usually are sent every 20 ms. With an end-to-end delay budget of 150 ms and strict jitter requirements, a gap of more than 180 ms is unacceptable. You need a mechanism that ensures that the size of one transmission unit is less than 10 ms. Any packets that have more than 10-ms serialization delay need to be fragmented into 10-ms chunks. A 10-ms chunk or fragment is the number of bytes that can be sent over the link in 10 ms. You can calculate the size by using the link speed: Fragmentation size = (0.01 seconds * 64,000 bps) / (8 bits/byte) = 80 bytes It takes 10 ms to send an 80-byte packet or fragment over a 64-kbps link. On low speed links where a 10-ms-sized packet is smaller than the MTU, fragmentation is required. Simple fragmentation is insufficient, though, because if the VoIP packet must wait behind all the fragments of a single large data packet, the VoIP packet still will be delayed beyond the end-to-end delay limit. The VoIP packet must be interleaved or inserted in between the data packet fragments. Figure 2 illustrates fragmentation and interleaving. Figure 2 VoIP Packet Fragmentation and Interleaving Serialization Delay = --------------------------------- Frame Size Link Speed 214-ms Serialization Delay for 1500-Byte Frame at 56 kbps Before After using LFI Tools 1500-Byte Data Frame Data Data Voice Data 60-Byte Voice 60060