Providing a good Quality of Service (QoS) for IP phones is important. Voice has a character of delay sensitive and some factors need to be considered for that which are:
1. Jitter, which is the interval of the receiving packages. For example the word hel-lo can be heard as h--ello if the jitter is bad.
2. Delay, which is the time for the data to travel from the moment one speaks and the other receives the voice signal. Large delays will provide uncomfortable conversations.
3. Packet loss, which is the amount of packets that can be dropped. Voice traffic is not packet sensitive, so it is allowed to have some packets losses but if the packets are dropped too often, the receiver is unable to identify what the speaker is saying.
The default QoS for all services is best-effort. In BE, all packets will be forwarded in the same order as they came in.
Another protocol for QoS is the Integrates Services Model (Intserv). IntServ uses Resource Reservation Protocol (RSVP), where RSVP creates and reserves a high priority path called as Guaranteed Rate Services (GRS) end-to-end before the voice traffic is being transmitted. This protocol can be disadvantageous because of the reserved bandwidth concept it uses. Therefore, if there are a lot voice traffic, the bandwidth will be reserved all for voice traffic.
To prevent unused reservations, another more advanced protocol is available which is Differentiated Services Model (DiffServ). Diffserv, does not reserve end to end bandwidth, but makes priorities based on per-hop decision. DiffServ has 2 tasks which are marking (tagging data with a value) and classification (classify the queuing and transmitting process of the data according to the marks).
To implement QoS on a switch's interface put the command:
(config-if)#mls qos
To trust incoming devices for its QoS values add the command:
(config-if)#mls qos trust
Trust CoS, DSCP and IP precedence means it will read those frames/packets and trust the QoS stated by them. However, trust device (for example trust device cisco-phone) means that the switch will only accept QoS values from a cisco phone. QoS values stated from device other than a cisco phone will be overwritten with the default configured value of QoS on the switch by the switch itself.
To see the interface's QoS settings put the command:
SWX#show mls qos interface
We can modify QoS values that comes from another device behind an IP Phone such as the PC. The command is:
(config-if)#switchport priority extend cos
(config-if)#switchport priority extend trust
-- 23 November 2010 --
1. Jitter, which is the interval of the receiving packages. For example the word hel-lo can be heard as h--ello if the jitter is bad.
2. Delay, which is the time for the data to travel from the moment one speaks and the other receives the voice signal. Large delays will provide uncomfortable conversations.
3. Packet loss, which is the amount of packets that can be dropped. Voice traffic is not packet sensitive, so it is allowed to have some packets losses but if the packets are dropped too often, the receiver is unable to identify what the speaker is saying.
The default QoS for all services is best-effort. In BE, all packets will be forwarded in the same order as they came in.
Another protocol for QoS is the Integrates Services Model (Intserv). IntServ uses Resource Reservation Protocol (RSVP), where RSVP creates and reserves a high priority path called as Guaranteed Rate Services (GRS) end-to-end before the voice traffic is being transmitted. This protocol can be disadvantageous because of the reserved bandwidth concept it uses. Therefore, if there are a lot voice traffic, the bandwidth will be reserved all for voice traffic.
To prevent unused reservations, another more advanced protocol is available which is Differentiated Services Model (DiffServ). Diffserv, does not reserve end to end bandwidth, but makes priorities based on per-hop decision. DiffServ has 2 tasks which are marking (tagging data with a value) and classification (classify the queuing and transmitting process of the data according to the marks).
To implement QoS on a switch's interface put the command:
(config-if)#mls qos
To trust incoming devices for its QoS values add the command:
(config-if)#mls qos trust
Trust CoS, DSCP and IP precedence means it will read those frames/packets and trust the QoS stated by them. However, trust device (for example trust device cisco-phone) means that the switch will only accept QoS values from a cisco phone. QoS values stated from device other than a cisco phone will be overwritten with the default configured value of QoS on the switch by the switch itself.
To see the interface's QoS settings put the command:
SWX#show mls qos interface
We can modify QoS values that comes from another device behind an IP Phone such as the PC. The command is:
(config-if)#switchport priority extend cos
(config-if)#switchport priority extend trust
-- 23 November 2010 --
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