在帧中继上配置VoIP and VoFR与配置普通数据整形注意点!

Network Diagram

This diagram illustrates the network topology for the sample scenarios used in this document:

Sample Scenario: Frame Relay Traffic Shaping for Data Only

Assume this scenario: A 128Kbps frame relay circuit with a CIR PVC of 64Kbps. The user wants to burst to port speed (128Kbps) and throttle down to CIR rate (64 kbps) if BECNs are received to avoid data loss.

FRTS for Data PVCs

This is a typical FRTS configuration for data PVCs:

Relevant FRTS Commands

·         frame-relay traffic-shaping—This command enables FRTS for the interface. Every DLCI under this interface is traffic shaped with either user-defined or default traffic shaping parameters. User-defined parameters can be specified in two ways:

o        U
sing the command class class_name under the frame-relay interface-dlci configuration or

o        Using the command frame-relay class under the serial interface.

In the example above, class my_net is used under the DLCI configuration.

·         class class_name —Use this command to configure FRTS parameters for a specific DLCI. In the above example, the class is defined as "my_net". The class parameters are configured under the command map-class frame-relay class_name .

·         map-class frame-relay class_name —Use this command to configure the FRTS parameters for a specified class. There can be multiple class-maps in a configuration. Each DLCI can have a separate class or DLCIs can share a single map class.

·         frame-relay adaptive-shaping becn —This command configures the router to respond to frame relay frames that have the BECN bit set. When a frame is received on that PVC with the BECN bit set, then the router throttles traffic down on that PVC to the MINCIR value. The CIR is usually set to the port speed or a value higher than the true CIR of the PVC. The MINCIR value is then set to the true CIR of the PVC.

·         frame-relay cir bps —Use this command to specify the incoming or outgoing committed information rate (CIR) for a Frame Relay virtual circuit.

·         frame-relay bc bits —Use this command to specify the incoming or outgoing committed burst size (Bc) for a Frame Relay virtual circuit.

·         frame-relay be bits —Use this command to specify the incoming or outgoing excess burst size (Be) for a Frame Relay virtual circuit.

·         frame-relay mincir bps —Use this command to specify the minimum acceptable incoming or outgoing committed information rate (CIR) for a Frame Relay virtual circuit. This is the rate at which traffic will be throttled down to when using adaptive shaping.

Frame Relay Traffic Shaping For Voice

When configuring FRTS for voice, data performance may suffer at the expense of good voice quality. Here are some guidelines to enhance voice quality when configuring FRTS for voice:

·         Do not exceed the CIR of the PVC
对于语音，最好不要超额使用实际线路的CIR，避免使用BECN等流量整形方法。

Most users have difficulty following this recommendation because the result is the router will no longer be able to burst to port speed. Because voice quality cannot tolerate much delay, any queueing of voice packets within the Frame Relay cloud must be minimized. When CIR is exceeded (PVC CIR, not the router configured CIR), depending on the provider and how congested the rest of the Frame Relay network is, packets may begin queue in the Frame Relay network. By the time the Frame Relay switch queues have backed up enough to trigger BECNs, the voice quality is already diminished. Because customers have many different Frame Relay providers and differing amounts of congestion across their sites, it is difficult to forecast what configuration works. Maintaining values at (or below) CIR on the PVCs that transport voice has proven to work consistently.

Some providers sell a Frame Relay service of 0 CIR. Obviously, not exceeding CIR in this case would prevent any voice from being sent across the frame link. A service of 0 CIR may be used for voice but there needs to be a Service Level Agreement (SLA) with the provider to guarantee minimal delay and jitter for a certain bandwidth across the 0 CIR PVC.

·         Do not use frame relay adaptive shaping

If the configured CIR within the frame relay map class is the same as the true CIR of the PVC, there is no need to throttle down traffic due to BECNs. If CIR is not exceeded, BECNs are not generated.

·         Make Bc small so that Tc (shaping interval) is small (Tc = Bc/CIR)
设置BC足够小，这样可以使得TC间隔变小。

The minimum Tc value is 10 ms, which is ideal for voice. With a small Tc value, there is no risk of large packets using all the shaping credits. Large Tc values can lead to large gaps between packets sent because the traffic shaper waits an entire Tc period to build up additional credits to send the next frame. Making Bc = 1000 bits is usually a low enough value to force the router to use the minimum Tc of 10ms. This setting should not affect data throughput.

·         Set Be = zero

To ensure the CIR value is not exceeded, Be is set to zero so there is no excess burst within the first shaping interval.

Note:  A good solution employed by some customers is to use separate PVCs for data and voice. This solution enables the customer to transmit up to port speeds in the data only PVC while maintaining a load at or below CIR on the voice PVC. Some frame providers may not find the solution appropriate depending on the frame switch and its queuing structure. If possible, have the Frame Relay provider prioritize the voice PVC over the data one so that there is not any queuing delay because of the data packets.

Sample Scenario: Frame Relay Traffic Shaping for Voice

Assume the following scenario: A 128Kbps frame relay circuit with a CIR PVC of 64Kpbs. The Frame Relay PVC is used to transport voice and data traffic.

Traffic Shaping Configuration for Voice over IP (VoIP) over Frame Relay

This is a typical configuration for traffic shaping for Voice over IP (VoIP) over Frame Relay:

Traffic Shaping Configuration for Voice over Frame Relay (VoFR)

This is a typical configuration for traffic shaping for VoFR:

Relevant FRTS Commands

Relevant FRTS commands (not discussed in the Frame Relay Traffic Shaping For Data section) are explained in this section.

·         vofr cisco—(Applicable only for VoFR) This command enables VoFR for the PVC.

·         frame-relay voice bandwidth bps —Applicable only for VoFR) Use this command to specify how much bandwidth is reserved for voice traffic on a specific data link connection identifier (DLCI). This command gives voice traffic a bandwidth ceiling.

·         frame-relay fragment bytes —Use this command to enable fragmentation of Frame Relay frames for a Frame Relay map class. For more information refer to: Frame Relay Fragmentation for Voice. Be aware that every PVC that shares an interface with a voice PVC will need fragmentation depending on the lowest link speed between the two routers, even if the PVC is data only. Since the voice PVC may share the same physical interface as other PVCs, large datagrams going out on these other PVCs may cause delay for voice packets trying to go out the same physical interface on a voice PVC.

·         no frame-relay adaptive-shaping—This command disables adaptive shaping.

·         frame-relay cir 64000 —Use this command to force the router to transmit at the same rate of the PVC CIR (In the above example, 64kbps even though the port speed is 128Kbps).

·         frame-relay bc 1000—Use this command to configure the router to use a small Tc or shaping interval.

·         frame-relay be 0—Since the PVC CIR is not exceeded, be is set to 0 so that there is no excess burst in the first shaping interval