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Quality of Service guidelines
For these reasons, shortening the MTU is only recommended as a last resort. The techniques
described later in this section are more efficient, and should be used before changing the values
of the MTU. When changing the MTU, size it such that the serialization delay is less than or
equal to 10 ms. Thus, for a 384-kbps circuit, the MTU should be sized as follows: 384000 bps
*0.01 second (10 ms)/8 bits/byte = 480 bytes. As the circuit size diminishes, however, care
should be taken to never reduce the MTU below 200 bytes. Below that size, telephony signaling
and bearer (voice) packets can also be fragmented, which reduces the link efficiency and
degrades voice performance.
LFI
Link Fragmentation and Interleaving (LFI) is an enhancement to Multilink PPP (MLP) that
fragments packets at the Layer 2 (PPP) level. Fragmenting at the IP layer, as with MTU
reduction, forces the addition of a new 20-byte IP header and an 8-byte PPP header. However,
fragmenting at the data link (PPP) layer only forces generation of an 8-byte PPP header, which
greatly increases the efficiency of the link.
Avaya recommends use of LFI functionality instead of MTU manipulation when transmitting IP
Telephony packets over PPP links. As with MTU, Avaya recommends sizing packets so that the
serialization delay is approximately 10 ms or less.
FRF.12
FRF.12 is a Frame Relay standard for fragmentation. It works for Frame Relay in the same way
that LFI works for PPP, with similar increases in efficiency over MTU manipulation. When
implementing a Frame Relay network, Avaya recommends using FRF.12 for fragmentation, and
sizing the fragments so the serialization delay is no more than 10 ms.
RTP
RTP header compression is a mechanism that reduces the protocol overhead that is associated
with IP Telephony audio packets. It is a function of the network, and not a function of the IP
Telephony application. Along with the benefits of using RTP header compression, there are also
cautions.
This section discusses the following topics:
Application perspective
Network perspective
The test
Configuration
324 Avaya Application Solutions IP Telephony Deployment Guide
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