Because of the limited bandwidth available on the WAN, using a compression codec allows efficient use of resources without a significant decrease in voice quality. IP telephony implementations across a WAN must use the G.729 codec with 20 ms packets. This configuration uses 24 kbps (excluding Layer 2 overhead), 30% of the bandwidth of the G.711 uncompressed codec (80 kbps).

To conserve 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 a 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 several Avaya secure routers (1000–series, 2330, 3120, and 4134) and on the Extreme, Juniper, Cisco, and other vendor routers.