Wired LAN and Backbone Requirements
End-to-End Quality of Service
NOTICE:
The use of the 802.1Q VLAN tag does not require an implementa-
tion of a full-blown VLAN system since by default all devices belong to the same
VLAN and thus can communicate with each other. This VLAN is often called the
native VLAN, and often has a VLAN ID of 0.
DiffServ, DSCP Value
The structure of the use of the Type of Service (ToS) Field for both the DSCP (new standard) val-
ue and IP Precedence (old standard) is illustrated in the
Figure 2: Diffserv Redefinition of ToS
on page 11.
8
7
6
5
4
3
2
1
IP Precedence
ToS
DSCP
Unused
Figure 2: Diffserv Redefinition of ToS Field
NOTICE:
The version of the standard used depends on the software implemen-
tation of the switch port. An older device receiving a DSCP field set using the 6-
bit code may interpret this as a 3-bit code and drop the last 3 bits, thus efficiently
changing the value when the packet is forwarded.
End-to-End Quality of Service
To achieve QoS for a phone call, it is important that QoS is enabled or managed all the way be-
tween the two endpoints. By following a speech packet as it travels along the path between the
endpoints, it is possible to identify all network segments and transitions where QoS needs to be
managed.
End-to-End QoS for voice traffic guarantees that high load on a part of the network does not
cause delays for the voice packets which would cause short disruptions/delays in the voice call.
Other types of data traffic can handle the delays in a graceful way and while the traffic will go
slower, it would not significantly affect the user experience. Hence, for other data traffic than
voice, assuming default QoS classification and delivery in the network as “best effort”, there is no
requirement of End-to-End QoS.
Uplink : Handset to Access Point
The prioritization in the uplink (from handset to AP) is handled by the handset. An internal clas-
sification is done at the low-level MAC software and ensures that voice packets are transmitted
prior to any other data. All voice packets are marked both with an 802.1D user priority (Layer 2)
as well as IP DSCP (Layer 3). By default, the handset marks the DSCP field with the appropriate
standard value for real-time data.
Downlink to Wired Network
The AP preserves the 802.1D user priority by copying the value into the 802.1p priority tag. The
IP DSCP value is unaffected by the transition to the wired network.
A31003-M2000-P103-01-76A9, 29/04/2020
System Planning, Planning Guide
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