Why H.323
Several advantages exist to using H.323 gateways as voice gateways:
■ Dial plans can be configured directly on the gateway: This makes it possible to
handle special calls, such as calls to directly connected analog devices locally on the
gateway without routing them to the Cisco Unified Communications Manager.
Another option is to route calls that are directed to other sites directly on the gateway
without sending them to the local Cisco Unified Communications Manager
cluster.
■ Translations can be defined per gateway: This makes it possible to meet regional
requirements such as calling party transformations or special number formats. It also
makes it possible to translate all incoming calls directly on the gateway to meet the
internally used number format and then process only calls with those internal numbers
on the Cisco Unified Communications Manager clusters within the network.
■ Call-routing configuration can be more specific than on Cisco Unified
Communications Manager: Cisco IOS gateways enable translating and matching to
called number and calling number, which can improve call routing. (Cisco Unified
Communications Manager matches only the called number.) This makes it possible
to route calls from unwanted people to a special destination, for example.
■ There is no need for extra Survivable Remote Site Telephony (SRST)-related callrouting
configuration: Because the call routing configuration is done directly on the
gateway, you do not need to configure the routing twice. This is because SRST is
using the same configuration parameters for call routing as the gateway does on the
H.323 gateway.
■ There is no dependency on the Cisco Unified Communications Manager version:
Because the configuration is performed on the gateway and the H.323 umbrella is a
peer-to-peer protocol, Cisco Unified Communications Manager does not need to
support a special Cisco IOS version, or vice versa.
■ More voice interface types are supported: Because Cisco Unified Communications
Manager does not need to control the interface cards within H.323 environments,
many more interface cards are supported when you use H.323 rather than MGCP or
any other protocol based on the client/server mode.
■ ISDN Nonfacility Associated Signaling (NFAS) is supported: The H.323 gateway
signaling protocol supports NFAS, which MGCP does not.
■ Fax support is improved: Fax support is better on H.323 gateways than on MGCP
gateways because H.323 supports T.37 and T.38, and an H.323 gateway can route a
fax direct-inward-dialing (DID) number directly to a Foreign Exchange Station (FXS)
port on the gateway.
■ Call preservation is enhanced: H.323 VoIP call preservation enhancements for WAN
link failures sustain connectivity for H.323 topologies where signaling is handled by
an entity that is different from the other endpoint, such as Cisco Unified
Communications Manager. Call preservation is useful when a gateway and the other
endpoint (typically a Cisco Unified IP Phone) are collocated at the same site, and the
call agent is remote and therefore more likely to experience connectivity failures.