Difference Between QoS and CoS

In computer networks, there are several ways to improve the quality of data transmission. The obvious way is to expand the bandwidth and improve the speed. But is there any way to improve this by keeping existing hardware in packet switched networks? This concept came as, classifying data frames in terms of “type of data”, prioritize them, and transfer in the network according to their priority levels. This helps data with higher priority levels to have precedence over low priority data. Data frames with higher priority levels will have more and more chances to use transmission medium, which means higher bandwidth. This will leads to the effective usage of the bandwidth. CoS (Class of Service) and QoS (Quality of Service) plays major roles in “Classifying” and “Prioritizing” data frames to meet above requirements.

CoS (Class of Service)

Class of Service (CoS) is a technique to group similar type of data together, and assign labels with “priority levels” to each group. IEEE 802.1p standard of IEEE 802.1 (networking and Network Management) class provides layer 2 switches to perform classification and prioritization in data frames. This works in the MAC (Media Access Control) layer in OSI model. IEEE 802.1p frame header includes 3-bit field to define eight priority levels.

According to this, 7^th (highest) level of priority is assigned to Network Control frames, and last levels (0^th and 1^st) are assigned to Background and Best Effort traffic. 

QoS (Quality of Service)

QoS is a mechanism to manipulate network traffic according to the prioritization levels of frames. Priority levels are defined by CoS, and QoS use these values to handle traffic in the communication path according to organization’s policy. By this way, existing network resources can be used in an effective manner, to optimize data transmission. There are several network characteristics associated with QoS. They are Bandwidth (The rate of data transfer), Latency (Maximum data transfer delay between source and destination), Jitter (The variation in latency) and Reliability (The percentage of packets discarded by t a router).

There are several techniques to define QoS such as Int-Serv (Integrated Services), Diff-Serv (Differential Services) and MPLS (Multiprotocol Label Switching). In Integrated Service model, Resource Reservation Protocol (RSVP) is used to request and reserve resource in the network which can be used to for prioritized data. In Differential Services model, Diff-Serv marks packets with different codes according to the type of service. Routing devices use these marks to queue data frames according to their priorities. MPLS is widely used protocol; primary objectives are to provide bandwidth management and quality of service for IP and other protocols.