GE MDS ORBIT ECR, Техническое руководство

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290 MDS Orbit MCR/ECR Technical Manual MDS 05-6632A01, Rev. F
Quality of Service (QoS) 3.8.17
Understanding
Quality of service (QoS) allows the MCR radio to classify network traffic giving preference to different
types of traffic before it is transmitted out of the MCR. Each interface has a packet queue that will hold
packets that are going to be transmitted out of the interface while the interface is busy. If the interface is
saturated with traffic, the interface will report busy and the packet queue will hold the packet. Normally
the packet the queue sends the packets to the interface in the order it receives them, but QoS allows a
different behavior. Depending on the policy applied to the interface, the packets that are in the queues
backlog can be sent to the interface in a different order that it was received based on fairness or the
priority of the packet.
There are currently three types of QoS policies that can be applied to an interface; prioritization, shaping
and fairness.
Prioritization implements a strict priority scheduler that requires classifiers be set up to give traffic
different priorities. The prioritization policy will always send highest priority traffic first. Excessive high
priority traffic can prevent any lower priority traffic from being sent.
The fairness policy attempts to split up the traffic into different groups based on the packets IP addresses
and IP protocols. It services these groups in a round robin fashion to ensure one traffic flow does not
prevent others from using the link. The fairness policy determines traffic flows on its own and does not
need the user to set up classifiers for it.
Traffic shaping is used to set minimal and maximal rates for a class of traffic. For example, with business
critical traffic like SCADA, traffic shaping can be setup to guarantee that this class of traffic will always
have at least 100Kbyte/s of an 800Kbyte/s link, regardless of the amount of other traffic. The remaing
unclassified traffic can use the entire 800Kbyte/s link as long as there is no SCADA traffic, but as soon as
SCADA traffic resumes, it will be given at least 100Kbyte/s allocation of the bandwidth. Additionally, a
maximal rate could be applied to a class of traffic to prevent that class from consuming too much of a
link. For example, a video stream could be limited to using 400Kbyte/s of an 800Kbyte/s link to prevent it
from interfering with any other traffic or to prevent it from saturating a radio interface.
A special QoS policy type, called modify , provides the ability to modify fields in an IP packet before they
egress an interface. This policy does not need to be applied to an interface for it to be used. Creating the
policy with at least one classifier enables the policy and any traffic matching the classifiers will be
modified accordingly. The modifications that can be performed are setting the ToS or DSCP value in IP
packets.
The modify policy is particularly useful when using GRE tunnels over a cellular interface and the cellular
interface is capable of prioritizing packets based on ToS/DSCP values. In this scenario, the GRE tunnel
inherits the ToS/DSCP value of the tunneled traffic. When the proper classifiers are created, designated
traffic flows inside the tunnel will have priority when egressing the cellular interface.
Any QoS policy can be applied to the MCR's non-virtual interfaces (i.e. Wi-Fi, NX915, LNxxx, etc.) or a
class in a class-full policy.
The classifiers mark the packets as they travel through the system. This mark is used when the packet
gets to the queue, to put it in its proper class. Packets can be classified based on the following parameters: