Cisco Systems VC-289 manual Quality of Service, Application-Specific Routing, VC-301

Configuring H.323 Gatekeepers and Proxies

H.323 Proxy Features

Table 25	Guidelines for Networks That Do Not Use NAT
For Networks Not Using NAT		Firewall with H.323. NAT	Firewall Without H.323 NAT
Firewall with Dynamic Access		Gatekeeper and proxy inside the	Gatekeeper and proxy inside the
Control		firewall	firewall
		Gatekeeper and proxy outside	Gatekeeper and proxy outside the
		the firewall	firewall
Firewall Without Dynamic		Gatekeeper and proxy inside the	Gatekeeper and proxy inside the
Access Control		firewall, with static access lists	firewall, with static access lists
		on the firewall	on the firewall

Quality of Service

Quality of service (QoS) enables complex networks to control and predictably service a variety of applications. QoS expedites the handling of mission-critical applications while sharing network resources with noncritical applications. QoS also ensures available bandwidth and minimum delays required by time-sensitive multimedia and voice applications. In addition, QoS gives network managers control over network applications, improves cost-efficiency of WAN connections, and enables advanced differentiated services. QoS technologies are elemental building blocks for other Cisco IOS-enabling services such as its H.323-compliant gatekeeper. Overall call quality can be improved dramatically in the multimedia network by using pairs of proxies between regions of the network where QoS can be requested.

When two H.323 terminals communicate directly, the resulting call quality can range from good (for high-bandwidth intranets) to poor (for most calls over the public network). As a result, deployment of H.323 is almost always predicated on the availability of some high-bandwidth, low-delay, low-packet-loss network that is separate from the public network or that runs overlaid with the network as a premium service and adequate QoS.

Adequate QoS usually requires terminals that are capable of signaling such premium services. There are two major ways to achieve such signaling:

•RSVP to reserve flows having adequate QoS based on the media codecs of H.323 traffic

•IP precedence bits to signal that the H.323 traffic is special and that it deserves higher priority Unfortunately, the vast majority of H.323 terminals cannot achieve signaling in either of these ways. The proxy can be configured to use any combination of RSVP and IP precedence bits.

The proxy is not capable of modifying the QoS between the terminal and itself. To achieve the best overall QoS, ensure that terminals are connected to the proxy using a network that intrinsically has good QoS. In other words, configure a path between a terminal and proxy that provides good bandwidth, delay, and packet-loss characteristics without the terminal needing to request special QoS. A high-bandwidth LAN works well for this.

Application-Specific Routing

To achieve adequate QoS, a separate network may be deployed that is partitioned away from the standard data network. The proxy can take advantage of such a partitioned network using a feature known as application-specific routing (ASR).

Cisco IOS Voice, Video, and Fax Configuration Guide

VC-301