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Qos on the voice

We have satellite link of: 256kbps/128kbps with satellite modem connect to Cisco 871.

One of the LAN outputs of the Cisco is connected to VOIP gateway with 4 FXS phones, and the other Cisco LAN ports connect to PC's for internet.

We need to configure the QOS so that the voice will have priority on the data always.

We need that the customer will have the possibility to use the 4 lines simultaneously (we are working with SIP and codec G729 and every call is taking 30kbps).

If someone is downloading a file and suddenly there are 4 calls, we need to have a priority to the calls, and the data will use the rest.

Our current QOS configuration is: Priority- 20% for SIP and 55% for RTP/RTCP - the total maximum is 75%.

1. Is this the best QOS that we can configure? Or we need less for SIP?

2. Can we not get more that 75% priority on the QOS? As we understand it is not enough for our BW.

3. It is better to use the QOS on the BW in this case?

4. In this configuration, if there is one phone call -it takes 55% from the BW?

Thanks for helping!!


Re: Qos on the voice

1. Take a look at the QoS SRND below. Specifically, see the WAN Aggregator QoS Design -> WAN Edge Classification and Provisioning Models section. A standard model to use is the 3 Class WAN Edge Model. This model allocates 33% for media traffic, 5% for signalling, and 62% for best effort data. SIP should fall into the signalling traffic category.

2. You can use the "max-reserved-bandwidth" command to allocate more than 75% of the bandwidth of a given interface.

3. You definitely want to use QoS in this case.

4. No, 55% is the maximum bandwidth that will be reserved for media traffic with your configuration. A single g.729 call uses 8k plus overhead.

Hope this helps. If so, please rate the post.



Re: Qos on the voice

You either need fewer calls, more bandwidth, or RTP header compression.

For acceptable performance you need to leave some overhead for router control traffic. This is especially true on a low speed 128kb link where exceeding the 75% max would not be a good idea. If you have 4 calls taking 30kb each, there is no "rest" left for data on the uplink side. That may be a problem for data users.

Is the modem interface serial or ethernet? If it is serial running HDLC or PPP, and the other end is another router you should be able to use RTP header compression which would get your per call bandwidth down to 13 kb, which would give you some breathing room. Not sure if compression is supported on a 871, but it might be worth upgrading if need be.

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