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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 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 

techniques to minimize serialization delay. 

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.