Yealink SIP-T4X Administration Manual

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Contents

Yealink SIP-T4X Administration Manual Manual pdf 191 page image

Administrator’s Guide for SIP-T4X IP Phones176Quality of Service (QoS) is the ability to provide different priorities to different packets inthe network, allowing the transport of traffic with special requirements. QoS guaranteesare important for applications that require fixed bit rate and are delay sensitive whenthe network capacity is insufficient. There are four major QoS factors to be consideredwhen 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 networkThe Best-Effort service is the default QoS model in the IP networks. It provides noguarantees for data delivering, which means delay, jitter, packet loss and bandwidthallocation are unpredictable. Differentiated Services (DiffServ or DS) is the most widelyused QoS model. It provides a simple and scalable mechanism for classifying andmanaging network traffic and providing QoS on modern IP networks. DifferentiatedServices Code Point (DSCP) is used to define DiffServ classes and stored in the first sixbits of the ToS (Type of Service) field. Each router on the network can provide QoSsimply based on the DiffServ class. The DSCP value ranges from 0 to 63 with each DSCPspecifying a particular per-hop behavior (PHB) applicable to a packet. A PHB refers tothe packet scheduling, queuing, policing, or shaping behavior of a node on any givenpacket.There are four standard PHBs available to construct a DiffServ-enabled network andachieve QoS: Class Selector PHB – backwards compatible with IP precedence. Class Selectorcode 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 nodesthat implement IP precedence-based classification and forwarding. Expedited Forwarding PHB – the key ingredient in DiffServ model for providing alow-loss, low-latency, low-jitter and assured bandwidth service. Assured Forwarding PHB – defines a method by which BAs can be given differentforwarding assurances. Default PHB – specifies that a packet marked with a DSCP value of “000000” getsthe traditional best effort service from a DS-compliant node.VoIP is extremely bandwidth- and delay-sensitive. QoS is a major issue in VoIPimplementations, regarding how to guarantee that packet traffic not to be delayed ordropped due to interference from other lower priority traffic. VoIP can guarantee