Figure 6-1 Layer 2 Diagram (802.1Q) - NEC Sl2100 Networking Manual

ISSUE 1.0
• Change Layer 2 Protocols:
Ethernet is most commonly used for IP packets. Unfortunately, Ethernet has a fairly large overhead
of 34 bytes. So every IP voice packet going over Ethernet has a 34-byte Ethernet header attached
to it. As the number of packets add up, this header data can become significant. Frame Relay has a
7-byte header and Point-to-Point Protocol (PPP) has a 6-byte header. With this decrease in header
length at layer 2, some significant savings in bandwidth use can be achieved.
The down side to this is that most networks may not have these services available, where Ethernet
is very widely used. This is usually outside the control of the installer and therefore NEC strongly
advises users to do more research on other layer 2 protocols before trying to implement them in
their voice network.
• Implement Quality of Service (QOS):
Now, assume a derivative of the above example. The user needs only one voice line over their
64Kbps connection. They are using G.729 with a 30ms fill time. This requires 34Kbps of their
available bandwidth. Also assume that this line is used at certain times of the day for data
connectivity. This data connectivity is very light, only 20Kbps or so during most of the day, but does
spike to 50Kbps during certain points of the day. This data is not time sensitive like the voice data,
so if necessary it could be forced to wait.
Therefore, the user can implement a Quality of Service mechanism on the IP network. At its most
basic form, this denotes certain IP packets as being more important than others. So they would tell
this 64Kbps line that IP packets with voice deserve a higher priority than those without voice. This
allows the network devices to give priority to the other data, so the quality of the call is not
compromised.
2.3 Types of Classifications for Traffic for QoS
Classification uses information from a packet (or frame) to define the type of data and therefore how
the data should be handled for QoS on the network. Using packet classification, you can partition
network traffic into multiple priority levels or Types of Service (ToS). NEC SL2100 supports methods of
marking a packet with its classification information in the Layer 2 or 3 headers.
• VLAN (802.1Q):
Virtual LANs work at Layer 2 of the OSI model and can be equated to a broadcast domain. More
specifically, VLANs can be seen as a group of end stations, perhaps on multiple physical LAN
segments that are not constrained by their physical location and therefore, communicate as if they
were on a common LAN. Packets can be marked as important by using layer 2 classes of service
(CoS) settings in the User Priority bits of the 802.1Pq header. Refer to Program
Setup on page 5-23 for information for VLAN configuration.
PREAM
Networking Manual
Layer 2
802.1Q/p
SFD DA SA
Type
PRI CFI
Three bits used for CoS
(user priority)

Figure 6-1 Layer 2 Diagram (802.1Q)

TAG
PT
4 Bytes
VLAN ID
SL2100
84-09 : VLAN
Data FCS
6-3