Cisco Systems 1760 11

Note When you enter the show running-config command, the format of the ip rtp header-compression command changes to

ip rtp header-compression iphc-format.

Configuring VoIP on a Frame Relay link involves certain special considerations to ensure acceptable voice quality. For Frame

Relay links with slow output rates (64 kbps or less) and with data and voice being transmitted over the same permanent virtual

circuit (PVC), you should configure the following parameters:

•Lower maximum transmission unit (MTU) size—Voice packets are generally small. If you decrease the MTU size to 300

bytes, large data packets can be broken up into smaller data packets that are more easily interleaved with voice packets.

The following example configures an MTU size of 300 bytes over serial interface 0/0:

Router# interface serial 0/0

Router(config-if)# mtu 300

Note Lowering the MTU size affects data throughput speed.

•Resource Reservation Protocol (RSVP)—Configure RSVP on subinterfaces (which correspond to PVCs) to reserve

bandwidth for voice channels. Refer to the “Configuring RSVP Support for Frame Relay” chapter in the

Cisco IOS Quality of Service Solutions Configuration Guide, Release 12.2.

The following example configures RSVP over serial subinterface 0/0.1:

Router(config-if)# interface serial 0/0.1 point-to-point

Router(config-if)# ip address 192.168.19.0 255.0.0.0

Router(config-if)# ip rsvp bandwidth 48 48

•RTP header compression—Configure RTP header compression on the subinterfaces to minimize the size of the voice packet.

See the “Real-Time Transport Protocol Header Compression” section on page 33.

The following command configures RTP header compression on the selected subinterface:

Router(config-if)# frame-relay ip rtp header-compression

•Traffic shaping—Use traffic shaping to control the outbound traffic rate; otherwise, voice packets can be discarded. In

Cisco IOS Release 12.0T, Frame Relay traffic shaping is not compatible with RSVP. Use generic traffic shaping instead, and

set the committed information rate (CIR) equal to the port speed. This prevents the router from exceeding the CIR rate and

discarding the frames.

The following command configures generic traffic shaping with a CIR of 32000 bps:

Router(config-if)# traffic-shape rate 32000

For further information and more detailed examples of Frame Relay configuration, refer to the Cisco 1751 Router Voice-over-IP

Configuration Guide.

11 Obtaining Documentation

Cisco documentation and additional literature are available on Cisco.com. Cisco also provides several ways to obtain technical

assistance and other technical resources. These sections explain how to obtain technical information from Cisco Systems.

You can access the most current Cisco documentation at this URL:

http://www.cisco.com/univercd/home/home.htm

You can access the Cisco website at this URL:

http://www.cisco.com