GE T1000, Техническое руководство

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GE Reason Switches Chapter 4
– Functions
REASON SWITCHES-TM-EN-3 89
10 Quality of Service (QoS
Quality of service is a main topic that Ethernet technology does not allowed at its
conception. Ethernet frames were designed to be fair, that is, each frame should
have the same possibility to flow in the catenet. Besides, it was observed that it
should be useful to have some traffic prioritization, especially when sporadic peaks
at network traffic leads to everyone lose data. Older devices (that is, equipment
designed before IEEE 802.1p standard has been developed) used some specific tools
to ensure some traffic prioritization over Ethernet, such as shortening interframe gap
or inserting long preamble at the frame, thus inducing collision detection on other
devices.
These technologies tried to fill a gap at the necessity to do some prioritization at
determined traffic over other traffic. If the network is oversized of bandwidth, both on
average bandwidth needs or sporadic peaks, there is really no problem on treating
all traffic as the same. Besides, on sporadic peaks that the network cannot flow, this
could cause undesired behaviour, as traffic will be lost.
There are many ways to do traffic prioritization in different layering protocols, and
these philosophies are generally referred as Quality-of-Service (QoS). This chapter will
focus on the CoS (Class-of-Service) bits usage, over 802.1Q Ethernet frames, which is
one kind of QoS. As shown in the VLAN chapter, 802.1Q frames include a 3-bits field
for determination of the priority of that marked VLAN packet. Differentiated Services
Code Point (DSCP) over IP traffic is also supported by Reason Switches as explained in
this chapter.
10.1 Quality of Service Basics
As explained at the beginning of this chapter, Ethernet frames were not designed to
prioritize one kind of traffic over other one. Besides, as communication is increasing
in size and traffic, there are boundary situations sporadic traffic (or even the average
traffic) can overreach LAN switching capacity for a longer time than network
equipment’s buffering capacities. Thus, dat a is lost.
Consider the network capacity is oversized, as shown below, all incoming data is
processed and forwarded. The data packets can be understood as Ethernet frames
to be processed by the switch. Besides, this philosophy can be extrapolated to other
layering protocols.