Quality Of Service (Qos) - Matrix Sparsh VP110 User Manual

Quality of Service (QoS)

Quality of Service (QoS) is the ability to provide different priorities for different packets in the network, allowing the
transport of traffic with special requirements. QoS guarantees are important for applications that require fixed bit
rate and are delay sensitive when the network capacity is insufficient. There are four major QoS factors to be
considered when configuring a modern QoS implementation: bandwidth, delay, jitter and loss.
QoS provides better network service through the following features:
• Supporting dedicated bandwidth
• Improving loss characteristics
• Avoiding and managing network congestion
• Shaping network traffic
• Setting traffic priorities across the network
The Best-Effort service is the default QoS model in IP networks. It provides no guarantees for data delivering,
which means delay, jitter, packet loss and bandwidth allocation are unpredictable. Differentiated Services (DiffServ
or DS) is the most widely used QoS model. It provides a simple and scalable mechanism for classifying and
managing network traffic and providing QoS on modern IP networks. Differentiated Services Code Point (DSCP) is
used to define DiffServ classes and stored in the first six bits of the ToS (Type of Service) field. Each router on the
network can provide QoS simply based on the DiffServ class. The DSCP value ranges from 0 to 63 with each
DSCP specifying a particular per-hop behavior (PHB) applicable to a packet. A PHB refers to the packet
scheduling, queuing, policing, or shaping behavior of a node on any given packet.
Four standard PHBs available to construct a DiffServ-enabled network and achieve QoS:
• ClassSelector PHB -- backwards compatible with IP precedence. Class Selector code points are of the
form "xxx000". The first three bits are the IP precedence bits. These class selector PHBs retain almost the
same forwarding behavior as nodes that implement IP precedence-based classification and forwarding.
• Expedited Forwarding PHB -- the key ingredient in DiffServ model for providing a low-loss, low-latency,
low-jitter and assured bandwidth service.
• Assured Forwarding PHB -- defines a method by which BAs (Bandwidth Allocations) can be given
different forwarding assurances.
• Default PHB -- specifies that a packet marked with a DSCP value of "000000" gets the traditional best
effort service from a DS-compliant node.
VoIP is extremely bandwidth- and delay-sensitive. QoS is a major issue in VoIP implementations, regarding how to
guarantee that packet traffic not be delayed or dropped due to interference from other lower priority traffic. VoIP can
guarantee high-quality QoS only if the voice and the SIP packets are given priority over other kinds of network
traffic. IP phones support the DiffServ model of QoS.
Voice QoS
In order to make VoIP transmissions intelligible to receivers, voice packets should not be dropped, excessively
delayed, or made to suffer varying delay. DiffServ model can guarantee high-quality voice transmission when the
voice packets are configured to a higher DSCP value.
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