Yealink VC400 Administrator's Manual page 111

modern IP networks. Differentiated Services Code Point (DSCP) is used to define DiffServ classes
and is 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:
Class Selector 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,
with regard to guaranteeing how that packet traffic is not delayed or dropped due to
interference from other lower priority traffic. VoIP can guarantee high-quality QoS only if the
voice, video and data packets are given priority over other kinds of network traffic. Yealink video
conferencing systems support the DiffServ model of QoS. DSCPs for voice, video and data
packets that can be specified respectively.
Voice QoS
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.
Video QoS
To ensure acceptable visual quality for video, video packets emanated from the system should
be configured with a high transmission priority.
Configuring Network
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