Specifications

Network Configuration Concepts
209
Network Priority Mechanisms
There are two areas where priority mechanisms operate in the network to ensure that voice
traffic maintains high priority:
• Layer 2 in the LAN through use of IEEE 802.1p/Q
• Layer 3 in the WAN through use of DiffServ/TOS/Precedence
The following figure highlights an Ethernet packet format, and the location of the Layer 2 Priority
and Layer 3 Priority fields. This view is of a tagged frame, since it included IEEE 802.1p/Q
information. The values in Figure 30 are based on a voice call that uses a G.729a CODEC and
20 ms Frame Rate.
Figure 30: Ethernet Packet Format
LAN layer 2 priority
The priority mechanism used relies on that described in IEEE 802.1p. This is a subsection of
IEEE 802.1Q also known as VLAN tagging.
CAUTION: If a PC is introduced into the same subnet as the IP phones,
whether it is behind a phone or even connected to a Layer 2 device within the
subnet, the Quality of Service cannot be guaranteed without the use of VLAN
and careful network engineering. VLAN should be used when phones and PC
co-exist on the same network infrastructure. TOS or DiffServ should also be
used on WAN connections where data and voice share a common connection.