3 Quality of Service (QoS)
We said many times that VoIP applications require a real-time data streaming cause we
expect an interactive data voice exchange. Unfortunately, TCP/IP cannot guarantee this kind of
purpose, it just make a "best effort" to do it. So we need to introduce tricks and policies that
could manage the packet flow in EVERY router we cross. So here are:
• TOS field in IP protocol [3] to describe type of service: high values indicate low
urgency while more and more low values bring us more and more real-time urgency
• Queuing packets methods:
o FIFO (First in First Out), the more stupid method that allows passing
packets in arrive order.
o WFQ (Weighted Fair Queuing), consisting in a fair passing of packets (for
example, FTP cannot consume all available bandwidth), depending on kind
of data flow, typically one packet for UDP and one for TCP in a fair fashion.
o CQ (Custom Queuing), users can decide priority.
o PQ (Priority Queuing), there is a number (typically 4) of queues with a
priority level each one: first, packets in the first queue are sent, then (when
first queue is empty) starts sending from the second one and so on.
o CB-WFQ (Class Based Weighted Fair Queuing), like WFQ but, in addition,
we have classes concept (up to 64) and the bandwidth value associated for
each one.
• Shaping capability, that allows to limit the source to a fixed bandwidth in:
o download
o upload
• Congestion Avoidance, like RED (Random Early Detection).
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