Saturday, August 25, 2012

Basic Cisco QoS for IP Phone System to IP Phone VoIP System Voice Garbled


Sample Cisco QoS for IP Phone System to IP Phone System to Solve Distorted Voice Quality

In this situation, the remote office's router is a Cisco 1841. The VoIP (voice) over a  legacy T1 Point-to-Point was garbled. The conversations sounded like the callers were underwater. There was need to clear up the voice quality and the best method to do so was to use QoS.  The unclear voice occurred on nearly all phone conversations both internal and external. The phone sets at the remote office were digital but the phone system is IP (VoIP) based. The remote office phone sytsem and the the phone system at the main office communicated via IP.  The simple solution, using the existing hardware already installed,  for this problem was to implement QoS on the router. The goal was to prioritize the voice packets that were sent in and out of the router from one system to another. At the same time, the goal was to also keep the solution as simple as possible.

Cisco Remote Office Router - QoS class-map, policy-map, and access-list on router for Voice Quality

This simple basic QoS example firstly required a simple access list that contained both of the phone systems' ip addresses. After the simple access-list, a class-map and policy-map were also configured on the router. In the end, the the policy map is applied to an interface.
 First, in configuration mode of the router,  the access-list. is created.  The access list basically will contain a lists of our phone system ip addresses for the main office and the remote.

Replace the ip addresses shown below with the ip addresses of your phone system. Also, if the access-list number is already taken on your router for another access-list, use another number. This example uses 101 for an extended access list.

   access-list 101 permit ip any host
   access-list 101 permit ip any host

 Next, create a class-map on the branch router. The class-map is basically a map or list of the match criteria.If you haven't guessed it already, the match criteria will be the ip addresses of the phone systems. In the class-map, we add "match access-group 101. 101 is the access list that contains our phone systems' ip addresses.
    class-map match-any IPPhoneSys-to-IPPhoneSys
   match access-group 101

 After the access-list and class-map is configured, the class-map is applied to the policy map. The policy map can contain more than one class-map.  So the policy-map is like a list of class-maps. The policy-map is  applied to an interface. In this example it will be the serial interface corresponding to the T1 (point-to-point T1).: The bandwidth staement is key to what we are trying to accomplish. This is a simple example of QoS so I have just merely taken 20 percent of the bandwidth and pretty much assigned it to voice (VoIP)

    policy-map QoS_Policy_Priority
    class IPPhoneSys-to-IPPhoneSys
    bandwidth percent 20
    class class-default fair-queue

In this example it is a legacy point-to-point T1 that connects the two offices. The interface you use may differ. To apply the policy-map that contains the class-map with 20 percent bandwidth defined for voice change to the interface level commands by entering the interface on which you plan to apply the QoS policy map. In this example it is a serial interface for the t1 (S0/0/0)

    config# interface serial0/0/0

 Then apply the policy on the interface by running the command

    service-policy output QoS_Policy_Priority

 The above is a simple basic QoS policy using one of the many Cisco Modular QoS solutions. Once applied, it cleared the voice issues immediately. In the example above the bandwidth reserved for voice over ip (VoIP) was 20 percent. You can change that percentage to fit your environment.

Sunday, January 15, 2012


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