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Monday, September 23, 2013

What is meant by Quality of Service provided by network layer?

- The QoS or the quality of service is such a parameter that refers to a number of aspects of computer networks, telephony etc. 
- This parameter allows transportation of traffic as per some specific requirements. 
- Technology has advanced so much now computer networks can also be doubled up as the telephone networks for doing audio conversations. 
- The technology even supports the applications which have strict service demands. 
- The ITU defines the quality of service in telephony. 
It covers all the requirements concerning all the connection’s aspects such as the following:
Ø  Service response time
Ø  Loss
Ø  Signal – to – noise ratio
Ø  Cross – talk
Ø  Echo
Ø  Interrupts
Ø  Frequency response
Ø  Loudness levels etc.  

- The GoS (grade of service) requirement is one subset of the QoS and consists of those aspects of the connection that relate to its coverage as well as capacity. 
- For example, outage probability, maximum blocking probability and so on. 
- In the case of the packet switched telecommunication networks and computer networking, the resource reservation mechanisms come under the concept of traffic engineering. 
- QoS can be defined as the ability by virtue of which the different applications, data flows and users can be provided with different priorities. 
- It is important to have QoS guarantees if the capacity of the network is quite insufficient. 
- For example, voice over IP, IP-TV and so on. 
- All these services are sensitive to delays, have fixed bit rates and have limited capacities.
- The protocol or network supporting the QoS might agree up on some traffic contract with the network node’s reserve capacity and the software. 
- However, the quality of service is not supported by the best effort services. 
-Providing high quality communication over such networks provides a alternative to the QoS control mechanisms that are complex. 
- This happens when the capacity is over-provisioned so much that it becomes sufficient for the peak traffic load that has been expected. 
- Now since the network congestion problems have been eliminated, the QoS mechanisms are also not required. 
- It might be sometimes be taken as the level of the service’s quality i.e., the GoS. 
- For example, low bit error probability, low latency, and high bit rate and so on. 
- QoS can also be defined as a metric that reflects up on the experienced quality of the service.
- It is the cumulative effect that can be accepted. 
Certain types of the network traffic require a defined QoS such as the following:
Ø  Streaming media such as IPTV (internet protocol television), audio over Ethernet, audio over IP etc.
Ø  Voice over IP
Ø  Video conferencing
Ø  Telepresence
Ø  iSCSI, FCoE tec. Storage applications
Ø  safety critical applications
Ø  circuit emulation service
Ø  network operations support systems
Ø  industrial control systems
Ø  online games

- All the above mentioned services are examples of the inelastic services and a certain level of latency and bandwidth is required for them to operate properly. - On the other hand, the opposite kind of services such as the elastic services can work with any level of bandwidth and latency. 
- An example of these type of services is the bulk file transfer application based up on TCP.
- A number of factors affect the quality of service in the packet switched networks. 
- These factors can be broadly classified in to two categories namely technical and the human factors. 
The following factors are counted as the human factors:
Ø  reliability
Ø  scalability
Ø  effectiveness
Ø  maintainability
Ø  grade of service and so on.

- ATM (asynchronous transfer mode) or GSM like voice transmissions in the circuit switched networks have QoS in their core protocol. 

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