Problems

• Excessive delay due to slow link and MTU-sized (large) packets

• Jitter (variable delay) due to variable link utilization

When considering delay between two hops in a network, queuing delay in a router must be considered because it may be comparable to—or even exceed—the serialization and propagation delay on a link. In an empty network, an interactive or voice session experiences low or no queuing delay, because the session does not compete with other applications on an interface output queue. Also, the small delay does not vary enough to produce considerable jitter on the receiving side.

In a congested network, interactive data and voice applications compete in the router queue with other applications. Queuing mechanisms may prioritize voice traffic in the software queue, but the hardware queue (Tx ring) always uses a FIFO scheduling mechanism. Therefore, after packets of different applications leave the software queue, the packets will mix with other packets in the hardware queue (TxQ), even if their software queue processing was expedited. Therefore, a voice packet may be immediately sent to the hardware TxQ where two large FTP packets may still be waiting for transmission. The voice packet must wait until the FTP packets are transmitted, thus producing an 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.

Serialization Delays

56 kbps 64 kbps 128 kbps 256 kbps 512 kbps 768 kbps 1536 kbps

56 kbps 64 kbps 128 kbps 256 kbps 512 kbps 768 kbps 1536 kbps

©2006 Cisco Systems, Inc. All rights reserved.

Serialization delay is the fixed delay that is required to clock a voice or data packet onto the network interface. Serialization delay is directly related to the link speed and the size of the packet.

The figure shows the serialization delay as a function of the link speed and packet size.

For example, the serialization delay for a 1500-byte packet over a 56-kbps link will be 214 ms, while the serialization delay is only 7.5 ms over a 1.536-Mbps link for the same 1500-byte packet.

Link Fragmentation and Interleaving

This topic describes LFI operation and how LFI reduces the delay and jitter of VoIP packets.

Link Fragmentation and Interleaving

Voice

DATA

1 Voice

DATA

DATA

DATA

Voice

DATA

1 Voice

fragment size

• LFI reduces the delay and jitter of small packets (for example, VoIP).

fragment size

• LFI reduces the delay and jitter of small packets (for example, VoIP).

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 TxQ, which uses FIFO when a VoIP packet is sent to the front of the software queue. The serialization of the large packet can cause the VoIP packet to wait for a long time before it can be transmitted out.

For example, the serialization delay of a 1500-byte packet over a 56-kbps link will be 214 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 TxQ on a 56-kbps link can cause the end-to-end delay of the voice packet to be over the budget of 150 ms.

The solution to this problem 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 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 milliseconds. Depending on the LFI mechanisms being configured, the fragment size is either configured in bytes or in milliseconds.

Was this article helpful?

0 0

Post a comment