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Improve VoIP Quality with Link Fragmentation and Interleaving

There are a number of different mechanisms that can be used to increase the likelihood that high priority Voice over IP (VoIP) traffic is not delayed across a network path. One of these features is link fragmentation and interleaving (LFI); this feature is used on low speed links (< 768kbps) to help high priority traffic obtain link access when other parallel large packet traffic is also being sent. This article provides a brief description of these features and their configuration.

Link Fragmentation and Interleaving

When transmitting voice over a packet network, traffic must be transmitted across the network within a specific amount of time in order to ensure that the conversation is high quality. One of the things that can introduce large amounts of delay along this path is combining high priority VoIP traffic, low priority large datagram traffic and a slow speed link. When transmitting large datagram packet traffic along a slow speed link, the amount of time it takes to transmit the packet across the link can be quite long.

The problem that occurs is what happens to a high priority packet if it needs to be transmitted across the link at the same time the link is committed to transmitting this large packet. Without LFI, the high priority packet will wait behind this large packet until it is sent completely; this will typically introduce a high amount of delay making the packet unusable for an active VoIP conversation. The LFI feature can be used in these situations by implementing an interleaving and a fragmentation action that ensures the high priority traffic is transmitted with as little delay as possible.

The interleaving feature, when enabled, works by ensuring that high priority VoIP traffic is transmitted across a link within a configured “delay budget”. This function works by interrupting lower priority traffic to give the higher priority traffic access to the link. This action allows the higher priority traffic the ability to be transmitted without unneeded delay.

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The fragmentation feature works by splitting (“fragmenting”) large datagram traffic into smaller pieces that allows it to more be more easily transmitted along with higher priority traffic without causing unneeded delay for either type of traffic.

Link Fragmentation and Interleaving Configuration

The configuration of either one of these features is rather simple; this article takes a look at what is required to configure them when using a serial interface.

The LFI feature works with VoIP traffic and is configured with a few short commands. The LFI feature is part of Multilink Point-to-Point Protocol (MPPP) and requires PPP to be used on the serial link.

As this article is focused on the LFI feature specifically, coverage of the creation of a multilink interface and policy map will not be covered but information on this can be found at http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/mlppp_over_serial.pdf.

When a multilink interface is created, the fragmentation is enabled with a default “delay budget” of 30 milliseconds. With this configuration, any packet that requires longer than 30 milliseconds to be transmitted is automatically fragmented.

If this default “delay budget” must be altered, the following command syntax can be used:

router(config-if)#ppp multilink fragment delay milliseconds {microseconds}

The interleave feature is not enabled by default and can be enabled with the following command syntax:

router(config-if)#ppp multilink interleave

Summary

The LFI feature is essential to utilize when implementing VoIP traffic over lower speed interfaces. The configuration is minimal, but does require the use of MPPP; in situations when this is not true it should be considered as it has little downside from existing alternatives on serial links. Hopefully this article has been able to provide enough information to those users with networks with these conditions and at least made them aware that these features exist and can be easily implemented.

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