Class of Service (CoS) is a policy to enable QoS on the network. It puts a 3 bit field in an layer-2 Ethernet frame to define the priority of the traffic. The highest priority is marked with 7 (111) and the lowest priority is marked with 0 (000). Dot1q and ISL can both provide CoS with some differences:
- ISL uses 4 bits User field, where the last 3 bits is used for CoS.
- Dot1q uses 3 bits User field used for CoS.
Type of Service (ToS) is an allocated field in the layer 3 IPv4 header to provide QoS. It consists of 8 bits with 3 precedence bits in the beginning, 3 request bits and 2 unused bits. The first 3 bits represents the priority of the packet. As in CoS, 7 means highest priority and 0 means lowest priority. 3 request bits consists of 3 different type of service which is request low delay (100), request high throughput (010), or request high reliability (001) which can also be memorized as DTR bits.
The modern version of ToS is used in DiffServ. The DS field in DiffServ uses 6 Differentiated Services Code Point (DSCP) bits and 2 Explicit Congestion Notification (ECN) bits. The 6 DSCP bits contain of Class Selector Value (3 bits) and Drop Precedence value (3 bits).
The Class Selector Value is divided into 7 classes which are:
Class 7 (111) - Network Control, used of traffic controlling such as STP and routing protocol. This is the highest priority class.
Class 6 (110) - Internetwork Control, used for same purposes in class 7 but for internetwork traffic
Class 5 (101) - Expendited Forwarding, used for voice traffic and other time critical data. This traffic has characteristics of low delay, low loss and low jitter and is given strict priority queuing above all other traffic classes.
Class 1-4 (001-100) - Assured Forwarding, used for applying QoS for certain traffic where 4 is higher priority than 3, which is higher than 2, and in return is higher than class 1.
Class 0 (000) - Best-effort forwarding, used as default for every traffic if not configured for QoS.
The Drop Precedence value is used for Assured Forwarding which defines the importance of the packet. There are only 3 values high is high (3), medium (2) and low (1). Therefore AF21 means traffic will be handled as class 2 with a low drop precedence.
RTP Header Compression can also improve VoIP quality. The compression can reduce an IP/UDP/RTP header from 40 bytes to only 2-4 bytes. The command is:
(config-if)#ip rtp header-compression
-- 25 November 2010 --
- ISL uses 4 bits User field, where the last 3 bits is used for CoS.
- Dot1q uses 3 bits User field used for CoS.
Type of Service (ToS) is an allocated field in the layer 3 IPv4 header to provide QoS. It consists of 8 bits with 3 precedence bits in the beginning, 3 request bits and 2 unused bits. The first 3 bits represents the priority of the packet. As in CoS, 7 means highest priority and 0 means lowest priority. 3 request bits consists of 3 different type of service which is request low delay (100), request high throughput (010), or request high reliability (001) which can also be memorized as DTR bits.
The modern version of ToS is used in DiffServ. The DS field in DiffServ uses 6 Differentiated Services Code Point (DSCP) bits and 2 Explicit Congestion Notification (ECN) bits. The 6 DSCP bits contain of Class Selector Value (3 bits) and Drop Precedence value (3 bits).
The Class Selector Value is divided into 7 classes which are:
Class 7 (111) - Network Control, used of traffic controlling such as STP and routing protocol. This is the highest priority class.
Class 6 (110) - Internetwork Control, used for same purposes in class 7 but for internetwork traffic
Class 5 (101) - Expendited Forwarding, used for voice traffic and other time critical data. This traffic has characteristics of low delay, low loss and low jitter and is given strict priority queuing above all other traffic classes.
Class 1-4 (001-100) - Assured Forwarding, used for applying QoS for certain traffic where 4 is higher priority than 3, which is higher than 2, and in return is higher than class 1.
Class 0 (000) - Best-effort forwarding, used as default for every traffic if not configured for QoS.
The Drop Precedence value is used for Assured Forwarding which defines the importance of the packet. There are only 3 values high is high (3), medium (2) and low (1). Therefore AF21 means traffic will be handled as class 2 with a low drop precedence.
RTP Header Compression can also improve VoIP quality. The compression can reduce an IP/UDP/RTP header from 40 bytes to only 2-4 bytes. The command is:
(config-if)#ip rtp header-compression
-- 25 November 2010 --
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