Compressed voice or video voice or video ex. IP packets AAL 1 AAL 2 AAL 3/4 AAL 5 Physical Figure 7.15 Adaptation protocols in ATM Understanding Changing Telecommunications - Building a Successful Telecom Business header. The MAC header (the lower half of Layer 2 in the ISO OSI Model) is the only part of a packet that hubs or switches investigate in their scope of work. 802. 1p provides prioritization of packets traversing a subnet by the setting of a three- bit value in the MAC header. Thus, when the local segment becomes congested and the hub/switch workload results in the delay (dropping) of packets, those packets with flags that correspond to higher priorities will receive preferential treatment, and will be serviced before packets with lower priorities. 7.5.3 QOS IN IP NETWORKS In IP networks the routing is normally based on hop count and delay. This often leads to an uneven traffic load in the network. Some links become heavily congested while others remain almost idle. Resource allocation should therefore be complemented by traffic engineering and performance optimization. The initiatives on QoS in IP networks are conceptually similar to those in ATM networks and consist of: Admission policy (possible shaping). Assignation of specific paths for synchronous (real-time) traffic preventing the compe- tition from long IP data packets. Prioritization (certain packets can be dropped in case of congestion). All QoS methods have the same goal which is to open a differentiated lane for real- time communications out of the sometimes congested heavy traffic highway assigned to best-effort traffic. The main enablers are RSVP through resource reservation, MPLS through label switch- ing and differentiated services with a defined per hop behaviour in each router based on the traffic identification. See Figure 7.16. RSVP is normally used for reserving bandwidth in the access. RSVP offers guaranteed quality. It is heavy work to maintain RSVP in large backbones. At the edge nodes, when entering the Internet from the access 