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ONT – QoS basics

Quality of Service. Three words that describe (almost) a full exam.
When normal person hear these words they think about the supermarket cashier or the drugstore pharmacy. Some people might think of their last call to their utility company, where the automatic message announced that “this call is monitored for quality of service purposes”.

If you read this blog regularly (and you’re not my wifi) you are not a normal person. You are twisted, think only about networks and you understand that there is no such thing as quality of service, it is called QoS 🙂

What is it good for? There are few problems that QoS try to attend:

  • Lack of bandwidth – QoS cannot help when there is no bandwidth left
  • Packet loss – Voice packet loss affect the quality of call. While data transfer (Internet or FTP) is hardly affected and the user will not notice small hiccups, voice users will notice immediately.
  • Delay – Same issue as in packet loss. A regular data usage will not be notice small delays while voice\video are heavily affected.
  • Jitter = Delay Variation – Variable form of delay. A difference from when a packet is sent to the next packet => overall delay

QoS Tools:

  • Classification – Identify and group different traffic types. Not critical apps and Important apps. Matching the different types of applications.
    MATCHing is done using ACL – this is very processor intensive.
  • Marking – Taging the packet so it can be quickly recognized elsewhere on the network. Marking put a tag in the header so other routers can process it faster (and save the local processing resources)
  • FIFO = First In First Out – Whoever came first will be forwarded, when the buffer is full it will drop the rest of the traffic without looking at the data.
  • Random Early Detection = RED – When the buffer is close to full the router can start freeing space and drop packets out of the queue.
  • Weighted Random Early Detection = WRED – Cisco proprietary, allow the router to aim at the traffic it drops. This is RED with some brain.

The following will be covered in future posts so I’ll mention them but will not detail:

  • Policing -> drop or mark packets when the limit is reached
  • Shaping -> queue packets when the limit is reached, not dropping it
  • Queuing -> method to priorities packets
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