In choosing a gateway, attention should be given to ensure it supports three
important features:
CallManager Redundancy
DTMF Relay
Supplemental Services
Whether the installation is a large enterprise deployment with multiple
CallManager clusters or a simple two-node cluster, it’s important CallManager
Redundancy is supported by the gateway of choice. CallManager Redundancy is
required since an AVVID network needs to have the same high level of availability
as the traditional PBX.You do not want your access to a PBX or PSTN
compromised by a single point of failure. DTMF uses two frequencies, a high and
low tone to distinguish numbers on a telephone keypad.This signaling is usually
carried over a 64 Kbps voice circuit, and accomplished with little problem, but
with lower-bit CODEC the signal can be lost or unrecognizable.The gateways
provide out-of-band support for passing DTMF signals across the VoIP network
via the gateway protocols, as illustrated in Table 3.4.The AVVID gateway needs to
provide support for other user telephony services such as Call Hold, Call
Handling, and N-way Conference.These are normal traditional voice services,
which should be considered basic requirements for an AVVID network.