VOIP: Interoperability Made Easy

If you work with voice over IP (VoIP) networks, ensuring compatibility
between equipment is a constant challenge. Even basic call-connections can be
challenging because of the variety of standards-based signaling protocols–H.323,
session initiation protocol (SIP), media gateway control protocol (MGCP), H.248,
and so on–and the varying vendor implementations. With supplementary services,
interoperability is even more of an issue. It is to address these issues that
the industry has evolved the H.450 standard.

The H.450 is a collection of standards that define the various supplementary
services that can be available in an IP-telephony environment. The ITU currently
defines 12 recommendations (H.450.1—H.450.12) for supporting various
supplementary services in an H.323 network. This article focuses on H.450.2
(call transfer) and H.450.3 (call diversion).

With H.450.2 call transfer, three parties exist: user A, the party initiating
the transfer; user B, the party being transferred; and user C, the party
receiving the transferred call. User A has an established call with user B and
initiates a transfer to user C. This transfer initiation is either consultative,
with user A privately conversing with user C before completing the transfer, or
blind, where user A transfers user B without conversing with user C.

In H.450.3 call diversion or forwarding also, three parties exist: A, the
calling endpoint; B, the diverting endpoint; and C, the diverted-to endpoint.
Endpoint A calls endpoint B; endpoint B does not answer, and the call is sent to
endpoint C, as a voice-mail system or a cell phone number. H.450.3 passes on the
reason for the call diversion at endpoint B, which can be ring-no-answer, busy,
or unavailable. This allows endpoint C to behave differently depending on the
reason, such as playing different prompts for busy versus ring-no-answer.

In both cases, the intermediate party is removed from the call, freeing
resources and bandwidth for other calls. But these recommendations typically,
require that all three parties support H.450. While many VoIP devices support
H.323 signaling, only a subset offers H.450 support, leading to interoperability
problems when trying to implement supplementary services.

Until recently, support for H.450.2 and H.450.3 on voice gateways from key
vendors was available only through a tool command language (TCL) application and
was limited to receiving requests only. The only exception was Cisco CallManager
Express (formerly IOS Telephony Service). Beginning with version 2.1, Cisco
CallManager Express also initiates call transfers and forwards using the H.450
recommendations.

To configure an IOS voice gateway for H.450 support, the H.450 TCL
application and supporting files were loaded onto the gateway and applied to all
dial-peers that would be involved in the call transfers or forwards. If other
TCL-enabled services, such as prepaid calling card support, were required on the
same dial-peer, a custom TCL application was required to combine the
functionalities, because dial-peers support only one application. Potentially
complicated and time-consuming, this led to longer deployment times for such
services.

Now, with Cisco IOS Software Release 12.3(4)T, the default session
application in IOS includes H.450.2 and H.450.3 support. The same limitations on
initiating H.450 requests still apply, but the configuration to accept H.450
requests is eliminated. While Release 12.3(4)T eases implementation of
H.450-based supplementary services, interoperability issues with devices that do
not support H.450 still exist. The most common example is Cisco CallManager,
which uses an H.323 mechanism, empty capability set (ECS), to initiate call
transfers and forwards.

In Release 12.3(4)T, Cisco CallManager Express provides for a VoIP-VoIP
hairpin for call transfers or forwards to non-H.450 devices using H.450 TCL
version 2.0.0.8 or later. Cisco CallManager Express first attempts the H.450
transfer or forward, and if that fails, falls back to hairpinning the call.

This procedure provides a good balance between end-user experience (call
transfer or forward is completed), network resources (only non-H.450 calls are
hairpinned), and service provider configuration tasks (no special dial-plan
provisioning).

What’s Next?

Cisco IOS Software Release 12.3(6)T integrates this hairpin function into
the default session application. H.450.12 capabilities exchange allows Cisco
CallManager Express to auto-detect the ability to support H.450 services per
call and attempt a hairpin transfer only if H.450 is unavailable. Finally, Cisco
IOS Software Release 12.3(6)T allows Cisco 2600, 3600, or 3700 series routers to
act as H.450 proxies for non-H.450 devices such as Cisco CallManager, the Cisco
BTS 10200 Softswitch, and the Cisco PGW 2200 Softswitch. This capability
eliminates the double-bandwidth limitation of these non-H.450 devices.

All in all, the H.450 standard is expected to address the interoperability
issue for supplementary PBX solutions. With this protocol, we believe that some
of the finer glitches in adopting IP-telephony into enterprise communication
networks may finally be addressed.

B Ashok VP, Cisco Systems (India and
Saarc)