Installation guide
TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
15
3.1.2 IEEE 802.1q Priority Field
The structure of the VLAN Tag defined in 802.1Q is illustrated in fi
gure 1.
Figure 2.
87 6543 21 87 654 321
2
1
Prirority Mark
VLAN Identifier (VID)
= 1 Bit
Octets
002
Figure 1. Structure of a VLAN Tag.
Note: The use of the 802.1Q VLAN tag does not require an implementation 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.
3.1.3 DiffServ, DSCP Value
The structure of the use of the ToS Field for both the
DSCP (new standard) value and IP
Precedence (ol
d standard) is illustrated in figure 2.
Figure 3.
87 6543 21
IP Precedence
ToS
003
DSCP
Unused
Figure 2. Diffserv Redefinition of ToS Field.
Note: Which version of the standard used depends on the software implementa
tion of the
switch port. An older device receiving a DSCP field set using the 6 bit code ma
y interpret this
as a 3-bit code and drop the last 3 bits, thus efficiently changing the value when the packet
is forwarded.
3.2 End-to-End QoS
To achieve QoS for a phone call, it is important that QoS is enabled or managed all the way
between 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.
3.2.1 Uplink, VoWiFi Handset to AP
The prioritization in the uplink (from
VoWiFi Handset to AP) is handled
by the VoWiFi
Handset. An internal classification 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 VoWiFi
Handset marks the DSCP field with the appropriate standard value for real-time data.
3.2.2 Downlink to Wired Network
The AP will preserve the 802.1D user priority by copying the value into the 802.1p
priority
tag. The IP DSCP value will be unaffected by th
e transition to the wired network.