Specifications

Administrator’s Guide for SIP-T2xP IP Phones
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 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 supported 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:
 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 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
For VoIP transmissions to be 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.
SIP QoS