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| 8.8 |  | QoS using Low Speed Links |
| 8.8.4 |  | Link fragmentation and interleaving (LFI): Multilink PPP |

While 1500 bytes is a common size for data packets, a typical VoIP packet carrying G.729 voice frames can be around 66 bytes, 20 bytes voice payload, 6 bytes Layer 2 header, 20 bytes RTP and UDP header, and 20 bytes ip header.

Now, imagine a 56 Kbps leased line link where voice and data traffic coexist. If a voice packet is ready to be serialized just when a data packet starts being transmitted over the link, then there is a problem. The delay-sensitive voice packet will have to wait 214 msec before being transmitted.

Large data packets can adversely delay delivery of small voice packets, reducing speech quality. Fragmenting these large data packets into smaller ones and interleaving voice packets among the fragments reduces jitter and delay. The Cisco IOS Link Fragmentation and Interleaving (LFI) feature helps satisfy the real-time delivery requirements of VoIP.

As seen in Figure , the amount of serialization delay, the time it takes to place the bits onto an interface, introduced on low-speed WAN links can be significant. Particularly when considering that the target end-to-end one-way delay should not exceed 150 ms. For voice applications, recommended serialization delay, on a per hop basis, is 10 ms and should not exceed 20 ms.

> Note: In cases with a WAN link of more than 768 Kbps, the fragmentation feature is not needed. However, there is still a need for some type of QoS mechanism, such as LLQ or IP RTP Priority. 768 Kbps offers enough bandwidth to allow voice packets to enter and leave the queue without delay issues.