Mitel MiVOICE BUSINESS, Engineering Manualline

Page: 227 / 381

Network Configuration Concepts
213
• 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.
IEEE 802.1p (Layer 2 priority) uses a field in the IEEE 802.1Q tag to provide eight levels of
priority. IEEE 802.1Q is the open VLAN standard that extends the Ethernet header by adding
an additional 4 bytes to tagged packets. Because the 802.1p priority is part of the VLAN header,
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.