Thursday, 7 February 2019

Introduction Quality of Service

Quality of Service (QOS)


Quality of Service (QOS) help us to queue, limit, filter and make reservations for our network traffic. In QOS series you will find a variety of QoS topics.



lets see some converged issues.






  • Packet loss 
  • Delay
  • Jitter
1.Packet loss

Tail drop occur when the output queues is full. tail drops are common and happen when a link is congested. Now here is question comes what is the impact  of packet loss?. voice breaking up during phone calls, in Teleconferencing voice will not synchronized and picture become very jerky, file corrupted and so on.



2.Delay is the time to takes for a packet to get from the source to the destination, this is something called the one-way delay. The time it takes to get from a source to the destination and back to the source is called the round-trip delay

Here is types of delay:




  1. processing delay: the time it take for a router to take the packet from an input interface, examine it and put it into the output queue of the output interface.
  2. queuing delay: the time a packet resides in the output of a router.
  3. serializing delay the time it take place the "bits on the wire".
  4. propagation delay: the time it takes for the packet to cross the link from one end to the other

3.Jitter is the variation of one-way delay in a stream of packets. jitter is generally caused by congestion in the IP network. the congestion can occur either at the router interface or in a provider or carrier network in the circuit has no been provisioned correctly.


QOS Mechanisms 

  • Classification 
  • Marking 
  • Congestion management 
  • Congestion avoidance 
  • Policing 
  • Shaping 
Classification and Marking supported by a class-oriented QOS mechanism used to mark packets based on classification metering or both. 



Marking, coloring each packet as a member of a network class so that the packet's class can be quickly recognized throughout the rest of the network.

Congestion management is used to prioritize the transmission of packets, with a queuing mechanism on each interface.

Congestion avoidance is used to drop packets early to avoid congestion later in the network

Policing is used to enforce a rate limit by dropping  or marking down packets.

Shaping is used to enforce a rate limit by delaying packets using buffers.




PIM Sparse Mode

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