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Issue 6 January 2008 295
To conserve even more bandwidth, RTP header compression (cRTP) can be used on
point-to-point links. cRTP reduces the IP/UDP/RTP overhead from 40 bytes to 4 bytes. With
20-ms packets, this translates to a savings of 14.4 Kbps, making the total bandwidth required
for G.729 approximately 9.6 Kbps. The trade-off for cRTP is higher CPU utilization on the router.
The processing power of the router determines the amount of compressed RTP traffic that the
router can handle. Avaya testing indicates that a typical small branch-office router can handle
768 Kbps of compressed traffic. Larger routers can handle greater amounts. cRTP is available
on Avaya, Extreme, Juniper, and Cisco routers.
Serialization delay
Serialization delay refers to the delay that is associated with sending bits across a physical
medium. Serialization delay is important to IP Telephony because this delay can add significant
jitter to voice packets, and thus impair voice quality. See Layer 3 QoS
on page 317 for
techniques to minimize serialization delay.
Network design
Routing protocols and convergence
When designing a IP Telephony network across a WAN, some care should be taken when
selecting a routing protocol or a dial-backup solution. Different routing protocols have different
convergence times, which is the time that it takes to detect a failure and route around it. While a
network is in the process of converging, all voice traffic is lost.
The selection of a routing protocol depends on several factors:
● If a network has a single path to other networks, static routes are sufficient.
● If multiple paths exist, is convergence time an issue? If so, EIGRP and OSPF are
appropriate.
● Are open standards-based protocols required? If so, OSPF and RIP are appropriate, but
not EIGRP or IGRP, which are Cisco proprietary.
In general, Avaya recommends the use of OSPF when routing protocols are required. OSPF
allows for relatively fast convergence, and does not rely on proprietary protocols.
In many organizations, because of the expense of dedicated WAN circuits, dial-on-demand
circuits are provisioned as backup if the primary link fails. The two principal technologies are
ISDN (BRI) and analog modem. ISDN dial-up takes approximately 2 seconds to connect, and
offers 64 Kbps to 128 Kbps of bandwidth. Analog modems take 60 seconds to connect, and
offer up to 56 Kbps of bandwidth. If G.729 is used as the codec, either technology can support
IP Telephony traffic. If G.711 is used as the codec, only ISDN is appropriate. Also, because of
the difference in connect times, ISDN is the preferred dial-on-demand technology for
implementing IP Telephony.