issues. Shared media, for example, can cause problems if collisions occur (because Network delay 50 msec Depends on network at least one user may need to be delayed). Prioritization load. of traffic on the LAN allows QoS requirements to be Jitter buffer 50 msec signaled to LAN switches and routers. The IEEE ratified a standard for QoS prioritization in IEEE802 LANs in Total 125 msec 1999. The IEEE802.1p specification defines three bits within the IEEE802.1Q field in the MAC header (which is a part of OSI Layer 2). The IEEE802.1 field was Packet loss requirements are tighter for short, contin- initially designed to support VLAN interoperability but uous packet loss than for random packet loss over has been extended to support traffic priorities. This in- time. Losing ten contiguous packets is worse than formation can be used as a "signal" to any device that can losing ten packets evenly spaced over an hour decode the bits. Figure 3.2-10 illustrates the approach. timespan. Three bits allows up to eight settings that can be used Packet loss may be more noticeable for larger voice for classes of traffic and priorities. Typically, an NIC card payloads than for smaller ones because more voice in a LAN-attached system sets the bits according to its is lost in a larger payload. needs, and this information can be used by Layer 2 switches to direct the forwarding processes. If multiple Available bandwidth distinct LANs are interconnected via routers (i.e., Layer 3 switches), then the Layer 2 bits must be used to drive IP network transmission adds considerable overhead to Layer 3 QoS mechanisms. Although the IEEE802.1p/Q any given data stream. Each protocol layer adds header mechanism cannot operate on an end-to-end basis in an and/or trailer information to be transmitted. Depending internetwork, it does provide a relatively simple method on their design, IP networks may show hard bandwidth of defining and signaling an end system's requirements limits or soft limits in which delays and/or losses increase within a LAN enviro