unacceptable delay in the voice path. Because links are used variably, the delay varies with time and may produce unacceptable jitter in jitter-sensitive applications such as voice. In the example shown in Figure , the serialization delay of a 1500-byte packet over a 512-kbps link will be 24.3 ms. For VoIP traffic, the maximum recommended one-way, end-to-end delay is 150 ms. Therefore, having a 1500-byte packet ahead of a VoIP packet in the hardware queue on a 512-kbps link can cause the end-to-end delay of the voice packet to be over the budget of 24.3 ms. Note
It is highly recommended that you run the same version of code on both sides of the WAN link to ensure compatibility and correct compression results.
Content 4.8 Understanding WAN Link Efficiency Mechanisms 4.8.6 Link Fragmentation and Interleaving The use of a hybrid queuing method such as low latency queuing (LLQ) can provide low latency and low jitter for VoIP packets while servicing other data packets in a fair manner. But, even if VoIP packets are always sent to the front of the software queue, there is still the issue of serialization delay. A large packet may be on its way out of the hardware queue, which uses FIFO. When a VoIP packet is sent to the front of the software queue, the serialization of the large packet in the hardware transmit queue can cause the VoIP packet to wait for a long time before it can be transmitted out. The solution is to fragment the large packets so that they never cause a VoIP packet to wait for more than a predefined amount of time. The VoIP packets must also be allowed to transmit in between the fragments of the larger packets (interleaving), or there will be no point in doing the fragmenting. When you are configuring the proper fragment size to use on a link, a typical goal is to have a maximum serialization delay of around 10 to 15 ms. Depending on the LFI mechanisms being configured, the fragment size is either configured in bytes or in milliseconds, as shown in Figure .
Content 4.8 Understanding WAN Link Efficiency Mechanisms 4.8.7 Applying Link Efficiency Mechanisms Use the following guidelines for applying link efficiency mechanisms: Figure summarizes these guidelines. Example
Figure shows an example of a network using LFI. Header compression and LFI are typically configured at the WAN edge for WAN links below T1 or E1 speeds to optimize the use of the WAN link and to prevent long serialization delay. Layer 2 payload compression is less commonly deployed on WAN links, especially without the use of hardware-assisted payload compression. In this case, use TCP and RTP compression, as well as LFI mechanisms, because this network carries converged network traffic.