Qos - Quality Of Service - Siemens HiPath 5000 Service Manual

ip_.fm
IP Fundamentals
IP Network Requirements
13.2.4

QoS - Quality of Service

Introduction
Data transmission in IP networks can encounter:
Time problems due to large e-mail attachments and web surfing
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Network bottlenecks as a result of multiple requests for the same web page
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Reduced voice quality due to:
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– Delay (decline in voice quality due to delay)
– Variable delays (decline in voice quality due to jitter)
– Packet loss (decline in voice quality due to packet loss)
Quality of Service includes various methods which guarantee certain transmission properties
in packet-orientated networks (IP).
It is thus important, for example, to ensure a minimum bandwidth for Voice over IP for the entire
duration of the transfer operation. When several equal-access applications are operating via IP,
the available transmission link bandwidth (for example, an ISDN B channel, 64K bps) is shared
between these applications. Consequently, packet loss may occur during a voice connection,
causing poor voice quality.
HiPath 3000/5000 QoS
In HiPath 3000/5000 networks, different types of traffic are transported via a single IP infra-
structure. It is the aim of Quality of Service to fulfill the data traffic and voice traffic requirements.
To prevent voice traffic from being suppressed by data traffic, the former must be given high
priority. In addition, it must be specified that high-priority traffic is transmitted before traffic with
lower priority. This is specified on layers 2 and 3:
The three bits in the 802.1p field, which is part of the 802.1Q tag, are used on layer 2.
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On layer 3, the six bits in the DiffServ Code Point (DSCP) field are used in the IP headers'
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Type of Service (ToS) byte.
It is also essential that sufficient bandwidth is available within the QoS class in use (Codepoint/
IEEE802.1p User Priority). The following table lists the QoS values used for DiffServ and the
User Priority IEEE802.1p field.
13-4
HiPath 3000 V6.0, HiPath 5000 V6.0, Provisional Service Manual
P31003-H3560-S403-54-7620, 09/05