• Change Layer 2 Protocols:Ethernet is most commonly used for IP packets. Unfortunately, Ethernet has a fairly large overheadof 34 bytes. So every IP voice packet going over Ethernet has a 34-byte Ethernet header attachedto it. As the number of packets add up, this header data can become significant. Frame Relay has a7-byte header and Point-to-Point Protocol (PPP) has a 6-byte header. With this decrease in headerlength 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 Ethernetis very widely used. This is usually outside the control of the installer and therefore NEC stronglyadvises users to do more research on other layer 2 protocols before trying to implement them intheir voice network.• Implement Quality of Service (QOS):Now, assume a derivative of the above example. The user needs only one voice line over their64Kbps connection. They are using G.729 with a 30ms fill time. This requires 34Kbps of theiravailable bandwidth. Also assume that this line is used at certain times of the day for dataconnectivity. This data connectivity is very light, only 20Kbps or so during most of the day, but doesspike 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 mostbasic form, this denotes certain IP packets as being more important than others. So they would tellthis 64Kbps line that IP packets with voice deserve a higher priority than those without voice. Thisallows the network devices to give priority to the other data, so the quality of the call is notcompromised.2.3 Types of Classifications for Traffic for QoSClassification uses information from a packet (or frame) to define the type of data and therefore howthe data should be handled for QoS on the network. Using packet classification, you can partitionnetwork traffic into multiple priority levels or Types of Service (ToS). NEC SL2100 supports methods ofmarking 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. Morespecifically, VLANs can be seen as a group of end stations, perhaps on multiple physical LANsegments that are not constrained by their physical location and therefore, communicate as if theywere 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 84-09 : VLANSetup on page 5-23 for information for VLAN configuration.Figure 6-1 Layer 2 Diagram (802.1Q)ISSUE 1.0 SL2100Networking Manual 6-3
