Yealink Skype for Business HD IP Phones Administrator Guide246Quality of ServiceQuality of Service (QoS) is the ability to provide different priorities for different packets in thenetwork, allowing the transport of traffic with special requirements. QoS guarantees areimportant for applications that require fixed bit rate and are delay sensitive when the networkcapacity is insufficient. There are four major QoS factors to be considered when configuring amodern 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 IP networks. It provides no guarantees fordata delivering, which means delay, jitter, packet loss and bandwidth allocation areunpredictable. Differentiated Services (DiffServ or DS) is the most widely used QoS model. Itprovides a simple and scalable mechanism for classifying and managing network traffic andproviding QoS on modern IP networks. Differentiated Services Code Point (DSCP) is used todefine DiffServ classes and stored in the first six bits of the ToS (Type of Service) field. Eachrouter on the network can provide QoS simply based on the DiffServ class. The DSCP valueranges from 0 to 63 with each DSCP specifying a particular per-hop behavior (PHB) applicable toa packet. A PHB refers to the packet scheduling, queuing, policing, or shaping behavior of anode on any given packet.Four standard PHBs available to construct a DiffServ-enabled network and achieve QoS: Class Selector PHB -- backwards compatible with IP precedence. Class Selector codepoints are of the form “xxx000”. The first three bits are the IP precedence bits. These classselector PHBs retain almost the same forwarding behavior as nodes that implement IPprecedence-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 (Bandwidth Allocations) canbe given different forwarding assurances. Default PHB -- specifies that a packet marked with a DSCP value of “000000” gets thetraditional 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 interferencefrom other lower priority traffic. VoIP can guarantee high-quality QoS only if the voice and theSIP packets are given priority over other kinds of network traffic. Skype for Business phonessupport the DiffServ model of QoS.
