Cisco H.323 VC-289 - Manual (Page 13)

Configuring H.323 Gatekeepers and Proxies
H.323 Proxy Features
VC-301
Cisco IOS Voice, Video, and Fax Configuration Guide
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).
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
Control Gatekeeper and proxy inside the
firewall
Gatekeeper and proxy outside
the firewall
Gatekeeper and proxy inside the
firewall
Gatekeeper and proxy outside the
firewall
Firewall Without Dynamic
Access Control Gatekeeper and proxy inside the
firewall, with static access lists
on the firewall Gatekeeper and proxy inside the
firewall, with static access lists
on the firewall