Installation guide

TD 92408EN

9 December 2011 / Ver. G

System Planning

Ascom VoWiFi System

Note: The 802.1p priority tag is likely not preserved if VLANs are not configured throughout

the wired network. If the packets will travel across different subnets, the router

configuration needs to cope with preservation of the 802.1p priority tag.

Note: Any device

that assigns QoS information to a data frame must be connected to a port

in the LAN switch which is defined as a trunk port. A trunk port in a switch accepts a frame

as legal when it is extended with a VLAN tag.

Normally an access port in a sw

itch will not accept such a frame because the frame is not a

standard Ethernet frame.

Note: The priority tag can be changed by any interm

ediate device by an administrator

creating rules in the device.

3.2.3 Downlink, AP to VoWiFi Handset

As stated in the section about WMM, if QoS is configured

properly, vo

ice packets will gain

high priority and thereby minimize latency and packet inter-arrival jitter.

But how does an AP know which packets to prioritize? Two basic methods are defined:

• WMM default (Layer 2 to Layer 2 mapping).

The classification is done by translating the Layer 2 802.1p priority tag i

nto one of four

Access categories and vice versa. This requires that the 802.1p priority tag is preserved

in the wired network all the way to the APs Ethernet interface. In most cases, this

requires the use of VLAN. A VLAN header includes the 802.1p priority tag.

• IP DSCP mapping (Layer 3 to Layer 2 mapping).

All IP packets contain a field used for prioritization. This value

is called DSC

P -

Differentiated Services Code Point. In the AP, a rule can be created that map packets with

a specific DSCP value to the access category voice and thereby gain priority by using

WMM channel access.

If no classification is done, the downlink packets

(from th

e AP to the VoWiFi Handset) will

contend for transmission time on the same conditions as all other data traffic. The impact

will be bad speech at random occasions when other clients might create load on the system

by some heavy file transfer etc.