/mcpQuality of Service for Voice over IP Differentiated Services for VoIP 19 QoSVoIP.mif in Frame Relay and CLP-bit marking in ATM. These mechanisms allow the Layer 2 network to make intelligent drop decisions for nonconforming traffic during periods of congestion. DS allows similar operation over an IP network. The sixth bit must be set to 0 to indicate to the network devices that the classes have been set according to the DS standard. The DS architecture defines a set of traffic conditioners that are used to limit traffic into a DS region and place it into appropriate DS classes. Meters, markers, shapers, and droppers are all traffic conditioners. Meters basically are policers, and class-based policing (which you configure using the police policy-map configuration command under a class in Modular QoS CLI) is a DS-compliant implementation of a meter. You can use class-based marking to set the DSCP and class-based shaping as the shaper. Weighted Random Early Detect (WRED) is a dropper mechanism that is supported, but you should not invoke WRED on the VoIP class. Per hop behavior (PHB) describes what a DS class should experience in terms of loss, delay, and jitter. A PHB determines how bandwidth is allocated, how traffic is restricted, and how packets are dropped during congestion. Three PHBs are defined in DS based on the forwarding behavior required: • Best-effort class—Class selector bits set to 000 • Assured Forwarding PHB—Class selector bits set to 001, 010, 011, or 100 • Expedited Forwarding PHB—Class selector bits set to 101 The Assured Forwarding (AF) standard specifies four guaranteed bandwidth classes and describes the treatment each should receive. It also specifies drop preference levels, resulting in a total of 12 possible AF classes, as shown in Table 5. Table 5 Possible Assured Forwarding Classes You would most likely use Assured Forwarding classes for data traffic that does not require priority treatment and is largely TCP-based. Expedited Forwarding more closely matches VoIP QoS requirements. Implementing DS for VoIP: Expedited Forwarding PHB (RFC 2598) Expedited Forwarding (EF) is intended for delay-sensitive applications that require guaranteed bandwidth. An EF marking guarantees priority service by reserving a certain minimum amount of bandwidth that can be used for high priority traffic. In EF, the egress rate (or configured priority bandwidth) must be greater than or equal to the sum of the ingress rates, so that there is no congestion for packets marked EF. You implement EF behavior by using the strict priority queue in low latency queueing (LLQ). Constant bandwidth is guaranteed for traffic belonging to the EF class, but at the same time if there is congestion, nonconforming packets exceeding the specified priority rate are dropped to assure that packets in other queues belonging to different classes are not starved of bandwidth. The recommended DSCP value for EF is 101110 (46). The first three bits of this EF value correspond to IP Drop Preference Levels Class AF1 Class AF2 Class AF3 Class AF4 Low Drop Precedence 001010 010010 011010 100010 Medium Drop Precedence 001100 010100 011100 100100 High Drop Precedence 001110 010110 011110 100110