t the traffic restrictions. Default PHB should result in best-effort service. Some PHB types are: ELL: emulated leased line EVLL: emulated virtual leased line QAMS: quantitative assured media playback service EF: expedited forwarding EF is a defined PHB, like assured forwarding. EF PHB offers low loss, low delay and low latency. EF could be used for VoIP, possibly also for IP-VPN, for example based on business aspects such as SAP traffic. Otherwise assured forwarding (AF) could be used for IP-VPN and other traffic types with a need for certain priority. AF PHB offers a kind of PHB internal priorities, expressed as AF 1, AF 2, etc. From the point of view of a customer AF offers the possibility to tell what traffic could be dropped first, if the network is congested. Diffserv does not give any guarantees. It is like travelling by train without knowing when you will arrive. The standard is a compromise between reservation (you get a seat, but this could also be cancelled) and prioritization (you travel faster than packets with a lower priority). Diffserv is considered a scalable technology that can offer differentiated services for various needs in large networks. It is therefore a viable alternative in the packet backbone. Diffserv does not use any signalling. Diffserv is an IETF standard. It only addresses layer 3 and makes no assumptions regarding the underlying transport, as does MPLS. Network Design One or more Diffserv domains can form a Diffserv region with some type of common traffic-handling rules. Between domains, service level agreements (SLAs) and traffic con- dition agreements (TCAs) are used. Each corporate or ISP network is a DiffServ domain. Meter Network Packets Classifier Marker Shaper (DS domain) Figure 12.18 DiffServ traffic conditioner block (TCB) Packet Backbone For true QoS, the entire IP path that a packet travels must be DiffServ enabled. An example of service policy might be that EF gets 10 %, Gold 40 %, Silver 30 %, Bronze 10 %, and best-effort traffic (default class/PHB) the remaining 10 