QoS - Major Issues

QoS isn't just for fun, it aims to resolve the following key issues experienced in enterprise networks:

Available bandwidth: Obviously traffic flow is limited by the link with least bandwidth (bottleneck). Available bandwidth = total bandwidth/number of flows. Methods to fix lack of bandwidth include: upgrade links (costly), classify and mark traffic and use queuing techniques. Another solution is to use compression such as cRTP, this can be CPU intensive.

End to end delay: Four types of delay-

1. Processing delay -time taken for device to move packet from ingress to egress interface
2. Queuing delay - time spent in queue before serialisation
3. Serialisation delay - time taken to send all bits onto medium
4. Propagation delay - time taken to traverse medium

Variation of delay (jitter): Packets within a flow arrive out of order, and end to end delay may vary. Real-time applications like video and voice require packets to arrive in the order that they were sent. To resolve the problem a de-jitter buffer is used on the router, it (where delay is minimal) re-orders packets before passing them to the application.

Packet loss: Packet loss occurs due to router buffers filling up, and packets get dropped. Routers may drop some packets in order to make room for higher priority traffic. UDP traffic is connectionless, without the flow control of TCP, this means that packets have to be resent, which is costly for VoIP and video applications.