Siemens optiPoint 420 economy, Administrator'S Manual

Page: 208 / 247

Alphabetical Reference
208
Quality of Service (QoS)
The QoS technology based on layer 2 and the two QoS technologies Diff-
serv and TOS/IP Precedence based on layer 3 are allowing the VoIP appli-
cation to request and receive predictable service levels in terms of:
• data through put capacity (bandwidth)
• latency variations (jitter)
• and delay.
Please note that all these technologies are just marking packets which al-
low the network to classify and prioritize the packets accordingly. This
means that the network decides which QoS configuration (marking) will be
used and should be set in the endpoints. The default values are well known
recommendations.
Layer 2 / 802.1p
QoS on layer 2 is using 3 Bits in the 802.1q/p 4-Byte VLAN Tag which has
to be added in the Ethernet header..
The CoS (class of service) value can be set from 0 to 7. The value 7 is de-
scribing the highest priority and is reserved for network management. Val-
ue 5 is used for voice (RTP-streams) by default. Value 3 is used for signaling
by default.
Layer 3 / Diffserv
Diffserv is classifying traffic flows, like voice (RTP-streams) or signaling
messages into 2 per-Hop-Behavior groups:
1. Expedited Forwarded (EF referred to RFC 2598)
Expedited forwarded is used for voice (RTP-streams) by default.
• High priority traffic to be handeled at the arrival rate
• DSCP value: 1 0 1 1 1 0 (DSCP = Diffserv Codepoint)
• Effectively creates a special low-latency path in the network
2. Assured Forwarding (AF referred to RFC 2597)
Assured forwarding is used for signaling messages by default (AF31) and
less stringent than EF in a multiple dropping system.
The AF values are containing two digits X and Y (AFXY), where X is describ-
ing the priority class and Y the drop level.
Four classes X are reserved for AFXY: AF1Y (High Priority), AF2Y, AF3Y and
AF4Y (Low Priority).
Three Bits Used for CoS
(User Priority)
PREAM. SFD DA SA
TAG
4 Bytes
PT DATA FCS