Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 182)

Chapter 8 Quality of Service

WAN QoS Model

8-8

Cisco IP Telephony Network Design Guide

78-11103-03

schemes, such as G.729, can squeeze a 64-kbps call down to an 8-kbps payload.

Cisco gateways and IP phones support a range of codecs that can enhance

efficiency on these low-speed links.

The link efficiency can be further increased by using compressed RTP (cRTP),

which compresses a 40-byte IP + UDP + RTP header to approximately two to four

bytes. In addition, voice activity detection (VAD) takes advantage of the fact that,

in most conversations, only a single party is talking at a time. VAD recovers this

empty time and allows data to use the bandwidth.

Note cRTP is currently supported only for leased lines and Frame Relay

media. Cisco IOS Release 12.1(2)T, which greatly enhances

performance, is the recommended system software for cRTP.

For low-speed links (less than 768 kbps), it is necessary to use techniques that

provide link fragmentation and interleaving (LFI). This places bounds on jitter by

preventing voice traffic from being delayed behind large data frames. The three

techniques that exist for this purpose are Multilink PPP (MLP) for point-to-point

serial links, FRF.12 for Frame Relay, and MLP over ATM for ATM connections

(available in Cisco IOS Release 12.1(5)T). Figure 8-4 depicts the general

operation of LFI.

Figure 8-4 Link Fragmentation and Interleaving (LFI) Operation

Voice Data

Before

After

Data VoiceData Data

214-ms serialization delay

for 1500-byte frame at 56 kbps

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 182