Difference between QoS and CoS

QoS vs CoS

In computer networks, there are several ways to improve the quality of data transmission. The obvious way is to expand bandwidth and improve speed. But is there any way to improve this by keeping the existing hardware in packet-switched networks? This concept came as, classifying the data frames in terms of "data type", prioritizing them and transferring them over the network according to their priority levels. This allows data with higher priority levels to take precedence over low priority data. Data frames with higher priority levels will be more and more likely to use a transmission medium, which means greater bandwidth. This will lead to effective use of bandwidth. The class of service and the quality of service play a major role in the "classification" and "prioritization" of the data frames in order to meet the above requirements.

CoS (class of service)

The class of service (CoS) is a technique for grouping similar data types and assigning labels with "priority levels" to each group. The IEEE 802.1p standard of the IEEE 802.1 class (network and network management) provides layer 2 switches to perform classification and prioritization in data frames. It works in the Media Access Control (MAC) layer of the OSI model. The IEEE 802.1p frame header includes a 3-bit field to define eight priority levels.

PCP Network priority Acronym Traffic characteristics
1 0 (lowest) BK Context
0 1 BE Best effort
2 2 EE Excellent effort
3 3 California Critical applications
4 4 VI Video, <100 ms latency
5 5 VO Voice, <10 ms latency
6 6 IC Internetwork control
7 7 (highest) North Carolina Network control


According to this, 7th The priority level (highest) is assigned to the network control frames and to the last levels (0). th and 1 st ) are allocated to long distance traffic and best effort.

QoS (quality of service)

QoS is a mechanism for manipulating network traffic based on frame priority levels. Priority levels are defined by CoS and QoS uses these values ​​to manage traffic in the communication path according to the organization's strategy. In this way, existing network resources can be used efficiently to optimize data transmission. Several network characteristics are associated with quality of service. These are bandwidth (data transfer rate), latency (maximum delay for data transfer between source and destination), jitter (variation in latency) and reliability (percentage of packets rejected by a router) ..

There are several techniques for defining quality of service, such as Int-Serv (Integrated Services), Diff-Serv (Differential Services) and MPLS (Multiprotocol Label Switching). In the integrated service model, the resource reservation protocol (RSVP) is used to request and reserve resources in the network that can be used for hierarchical data. In the differential service model, Diff-Serv marks packets with different codes depending on the type of service. Routing devices use these marks to queue data frames according to their priorities. MPLS is a widely used protocol; The main objectives are to ensure bandwidth management and quality of service for IP protocols and others.

What is the difference between CoS and QoS ?


  • The CoS defines the priority levels and the QoS manipulates the traffic according to these defined priority levels.
  • CoS does not guarantee fixed bandwidth or delivery time, but QoS guarantees fixed bandwidth for critical applications.
  • CoS works later in layer 2 in OSI, while quality of service is implemented in layer 3.
  • Network administrators can efficiently configure the quality of service on the network according to the requirements of the organization. However, the changes made to CoS offer no more benefits than QoS.
  • CoS techniques are simpler and can evolve easily as the network grows. Compared to CoS, the quality of service becomes more and more complex as the network and the demand for hierarchical data increases.

Post a Comment

0 Comments