Quality of Service (QoS) in Computer Network

QoS is an overall performance measure of the computer network.

Important flow characteristics of the QoS are given below:

1. Reliability
If a packet gets lost or acknowledgement is not received (at sender), the re-transmission of data will be needed. This decreases the reliability.
The importance of the reliability can differ according to the application.
For example:
E- mail and file transfer need to have a reliable transmission as compared to that of an audio conferencing.

2. Delay
Delay of a message from source to destination is a very important characteristic. However, delay can be tolerated differently by the different applications.
For example:
The time delay cannot be tolerated in audio conferencing (needs a minimum time delay), while the time delay in the e-mail or file transfer has less importance.

3. Jitter
The jitter is the variation in the packet delay.
If the difference between delays is large, then it is called as high jitter. On the contrary, if the difference between delays is small, it is known as low jitter.
Case1: If 3 packets are sent at times 0, 1, 2 and received at 10, 11, 12. Here, the delay is same for all packets and it is acceptable for the telephonic conversation.
Case2: If 3 packets 0, 1, 2 are sent and received at 31, 34, 39, so the delay is different for all packets. In this case, the time delay is not acceptable for the telephonic conversation.

4. Bandwidth
Different applications need the different bandwidth.
For example:
Video conferencing needs more bandwidth in comparison to that of sending an e-mail.

Integrated Services and Differentiated Service

These two models are designed to provide Quality of Service (QoS) in the network.

1. Integrated Services( IntServ)

Integrated service is flow-based QoS model and designed for IP.
In integrated services, user needs to create a flow in the network, from source to destination and needs to inform all routers (every router in the system implements IntServ) of the resource requirement.

Following are the steps to understand how integrated services works.

I) Resource Reservation Protocol (RSVP)
An IP is connectionless, datagram, packet-switching protocol. To implement a flow-based model, a signaling protocol is used to run over IP, which provides the signaling mechanism to make reservation (every applications need assurance to make reservation), this protocol is called as RSVP.  

ii) Flow Specification
While making reservation, resource needs to define the flow specification. The flow specification has two parts:
a) Resource specification
It defines the resources that the flow needs to reserve. For example: Buffer, bandwidth, etc.
b) Traffic specification
It defines the traffic categorization of the flow.

iii) Admit or deny
After receiving  the flow specification from an application, the router decides to admit or deny the service and the decision can be taken based on the previous commitments of the router and current availability of the resource.

Classification of services

The two classes of services to define Integrated Services are:

a) Guaranteed Service Class
This service guarantees that the packets arrive within a specific delivery time and not discarded, if the traffic flow maintains the traffic specification boundary.
This type of service is designed for real time traffic, which needs a guaranty of minimum end to end delay.
For example: Audio conferencing.

b) Controlled Load Service Class
This type of service is designed for the applications, which can accept some delays, but are sensitive to overload network and to the possibility to lose packets.
For example: E-mail or file transfer.

Problems with Integrated Services.

The two problems with the Integrated services are:

i) Scalability
In Integrated Services, it is necessary for each router to keep information of each flow. But, this is not always possible due to growing network.

ii) Service- Type Limitation
The integrated services model provides only two types of services, guaranteed and control-load.

2. Differentiated Services (DS or Diffserv):

  • DS is a computer networking model, which is designed to achieve the scalability by managing the network traffic.
  • DS is a class based QoS model specially designed for IP.
  • DS was designed by IETF (Internet Engineering Task Force) to handle the problems of Integrated Services.
The solutions to handle the problems of Integrated Services are explained below:

1. Scalability
The main processing unit can be moved from central place to the edge of the network to achieve the scalability. The router does not need to store the information about the flows and the applications (or the hosts) define the type of services they want every time while sending the packets.

2. Service Type Limitation
The routers, route the packets on the basis of class of services define in the packet and not by the flow. This method is applied by defining the classes based on the requirement of the applications.