MNS-BB
Software User Guide
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11.0
QoS (Quality of Service)
11.1 Overview
Quality of Service (QoS)
refers to the capability of a network to provide better service to selected
network traffic over various technologies, including Frame Relay, Asynchronous Transfer Mode
(ATM), Ethernet and 802.1 networks, SONET, and IP-routed networks that may use any or all of
these underlying technologies. The primary goal of QoS is to provide priority including dedicated
bandwidth.
11.2 QoS
Concepts
Fundamentally, QoS enables you to provide better service to certain flows. This is accomplished by
either raising the priority of a flow or limiting the priority of another flow. When using congestion-
management tools, you try to raise the priority of a flow by queuing and servicing queues in different
ways. The queue management tool used for congestion avoidance raises priority by dropping lower-
priority flows before higher-priority flows. Policing and shaping provide priority to a flow by limiting
the throughput of other flows.
The LE2425A and LEV2525A switches support QoS as specified in the IEEE 802.1p and 802.1Q
standards. QoS can be 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.
QoS was designed to address this problem. The 802.1p standard outlines eight levels of priority, 0 to
7, with 0 the lowest priority and 7 the highest. The LE2425A and LEV2525A switches have two
priority queues, 1 (low) and 0 (high). When a tagged packet enters a switch port, the switch responds
by placing the packet into one of the two queues
11.3
IP Precedence: Differentiated QoS
IP precedence utilizes the 3 precedence bits in the IPv4 header's Type of Service (ToS) field to
specify class of service for each packet. You can partition traffic in up to eight classes of service
using IP precedence. The queuing technologies throughout the network can then use this signal to
provide the appropriate expedited handling.
This Diagram Shows the IP Precedence ToS Field in an IP Packet Header
The 3 most significant bits (correlating to binary settings 32, 64, and 128) of the Type of Service
(ToS) field in the IP header constitute the bits used for IP precedence. These bits are used to provide a
priority from 0 to 7 for the IP packet.
Because only 3 bits of the ToS byte are used for IP precedence, you need to differentiate these bits
from the rest of the ToS byte.
11.4 DiffServ
QoS (quality of service) refers to the level of preferential treatment a packet receives when it is being
sent through a network. QoS allows time sensitive packets, such as voice and video, to be given
priority over time insensitive packets, such as data. Differentiated services (DiffServ or DS) is a set of
technologies defined by the IETF (Internet Engineering Task Force) to provide quality of service for
traffic on IP networks.
DiffServ is designed for use at the edge of the enterprise where corporate traffic enters the service
provider environment. DiffServ is a layer-3 protocol and requires no specific layer-2 capability,
