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MULTILINK ML1200 MANAGED FIELD SWITCH – INSTRUCTION MANUAL 14–1
Multilink ML1200
Managed Field Switch
Chapter 14: Quality of Service
Digital Energy
Multilin
Qual ity of Ser vice
14.1 QoS Overview
14.1.1 Description
Quality of Service (QoS) refers to the capability of a network to provide different priorities to
different types of traffic. Not all traffic in the network has the same priority. Being able to
differentiate different types of traffic and allowing this traffic to accelerate through the
network improves the overall performance of the network and provides the necessary
quality of service demanded by different users and devices. The primary goal of QoS is to
provide priority including dedicated bandwidth.
14.1.2 QoS Concepts
The MultiLink ML1200 Managed Field Switch supports QoS as specified in the IEEE 802.1p
and IEEE 802.1q standards. QoS is important in network environments where there are
time-critical applications, such as voice transmission or video conferencing, which can be
adversely effected by packet transfer delays or other latency in a network.
Most switches today implement buffers to queue incoming packets as well as outgoing
packets. In a queue mechanism, normally the packet which comes in first leaves first (FIFO)
and all the packets are serviced accordingly. Imagine, if each packet had a priority
assigned to it. If a packet with a higher priority than other packets were to arrive in a
queue, the packet would be given a precedence and moved to the head of the queue and
would go out as soon as possible. The packet is thus preempted from the queue and this
method is called preemptive queuing.
Preemptive queuing makes sense if there are several levels of priorities, normally more
than two. If there are too many levels, then the system has to spend a lot of time managing
the preemptive nature of queuing. IEEE 802.1p defines and uses eight levels of priorities.
The eight levels of priority are enumerated 0 to 7, with 0 the lowest priority and 7 the
highest.