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Quality of service

The Quality of Service (QoS) feature allows the management of the level of service and preference given to the various types and sources of traffic going through the firewall so that the traffic that is important to the services and functions connecting through the firewall gets the treatment required to ensure the level of quality that is required.

QoS uses the following techniques:

Traffic policing	Packets are dropped that do not conform to bandwidth limitations
Traffic Shaping	Assigning minimum levels of bandwidth to be allocated to specific traffic flows to guarantee levels of servers or assigning maximum levels of bandwidth to be allocated to specific traffic flows so that they do not impede other flows of traffic.

This helps to ensure that the traffic may consume bandwidth at least at the guaranteed rate by assigning a greater priority queue if the guarantee is not being met. Traffic shaping also ensures that the traffic cannot consume bandwidth greater than the maximum at any given instant in time. Flows that are greater than the maximum rate are subject to traffic policing.

Queuing

Assigning differing levels priority to different traffic flows so that traffic flows that are adversely effected by latency are prevented from being effected by traffic flows that are not subject to the effects of latency. All traffic in a higher priority traffic queue must be completely transmitted before traffic in lower priority queues will be transmitted.

An example of where you would want to use something like this is if you had competing traffic flows of Voice over IP traffic and email traffic. The VoIP traffic is highly susceptible to latency issues.If you have a delay of a few seconds it is quickly noticeable when it is occurring. Email on the other hand can have a time delay of much longer and it is highly unlikely that it will be noticed at all.

By default, the priority given to any traffic is high, so if you want to give one type of traffic priority over all other traffic you will need to lower the priority of all of the other traffic.