Supplemental Guide – DOC. 8412A
UC-P100-S/UC-P110-S Phones for Skype for Business
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Shaping network traffic
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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:
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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.
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Expedited Forwarding PHB: the key ingredient in DiffServ model for providing a
low-loss, low-latency, low-jitter and assured bandwidth service.
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Assured Forwarding PHB: defines a method by which BAs (Bandwidth
Allocations) can be given different forwarding assurances.
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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. Skype for Business 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
guarantees high-quality voice transmission when the voice packets are configured to a
higher DSCP value.
SIP QoS
SIP protocol is used for creating, modifying and terminating two-party or multi-party
sessions. To ensure good voice quality, SIP packets emanated from phones should be
configured with a high transmission priority.
DSCPs for voice and SIP packets can be specified respectively.