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Yealink CP860 Руководство администратора онлайн [275/417] 166926

Yealink CP860 Руководство администратора онлайн [275/417] 166926

Configuring Advanced Features

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be considered when 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 network

The Best-Effort service is the default QoS model in the 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:

 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, regarding how to guarantee that packet traffic not to 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.

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