IP-Based PBXs: Facts and Fictions (Part II)
This column is the conclusion to the Next Wave that
appeared in the November 1998 issue of INTERNET TELEPHONY. Read Part I here.
BY MIKE KATZ
Why stop and look at the standards for something as simple as setting up and tearing
down walls between IP PBXs, VoIP phones, and IP telephony gateways? Because without
standards and interoperability, each vendor's IP PBX is an island - incompatible with
other vendors' VoIP phones and IP telephony gateways. If an IP PBX stands any chance of
being successful in the market, it will be through its ability to create a homogenous
environment for telephony systems. Just as Internet telephony gateways enabled proprietary
PBXs to run voice calls on the corporate Intranet, IP PBXs have the potential to bring new
telephony applications to corporate desktops. Interoperable standards at both the vendor
and industry levels will allow the new facet of this relationship to emerge.
So, why do we care if these systems interoperate? Suppose you're the MIS manager of a
medium-sized corporation and want to try out IP telephony/IP PBXs on a mixture of sites
with both existing PBXs and older key systems. The key systems can be replaced with an IP
PBX of some type, but you will need compatible IP telephony gateways on the corporate side
of the network. You will probably purchase most of your equipment from one vendor, and in
order to avoid being captive to that single source, you will need a promise of
interoperability. But standards alone will not solve the interoperability puzzle, and in
fact they may make the performance of a product unacceptable, as illustrated in the H.323
IP PBX PROTOCOLS: THE WINNING COMBINATION
H.323 - The Standard
H.323 terminals are the client endpoints on a local-area network (LAN) that provide
real-time, two-way communications, while gateways are the means for an H.323 client
terminal to speak with a PSTN caller. The two are the major components of an IP PBX, and
how the International Telecommunication Union's (ITU) H.323 standard provides
interoperability between gateways has been a popular discussion topic. Details of the
H.323 standard are available at www.itu.org, or through the International Multimedia
Teleconferencing Consortium Web site at www.imtc.org/i/standard/itu/i_h323.htm. The
standard describes terminals, equipment, and services for multimedia communication over
LANs, which do not provide a guaranteed Quality of Service (QoS), since they can lose
transmitted data. Support for voice is mandatory under H.323, while data and video support
are optional. All terminals must be able to use a specified common mode of operation - to
understand how to initiate, receive, and render the audio, data, or video parts of a call.
How the application layer of one vendor's product will use H.323 to speak with another
vendor's implementation does not have to be defined under the standards.
The real issue here is how vendors use H.323 to make a call from their gateway to
another vendor's gateway or, in the case of an IP PBX, from the phone or terminal device
to the PSTN gateway. If that gateway's call control application is proprietary, then it's
unlikely that other vendors' products will interoperate. So, H.323 compliance does not
necessarily equal interoperability. The most well known H.323 client terminal software is
Microsoft's NetMeeting (www.microsoft.com/netmeeting). It has been used as the market's
benchmark measurement of the interoperability of a given vendor's H.323 Internet telephony
gateway. There have been many public H.323 interoperability events where NetMeeting was
tested with a given vendor's gateway, yet interworking with other vendors' products was
Another issue that will affect the acceptance of standards-based IP PBXs is call setup
time, a chief user interface characteristic referring to the period of time before the
user hears a ring after dialing. Using the existing H.323 protocol without any workarounds
requires up to 20 requests and replies to set up a call. On a LAN-based IP PBX this means
low overhead, however it may take too much time over a WAN link, and a caller would
probably hang up rather than wait for a ring.
SIP - Reducing Transactions
Another Internet Engineering Task Force (IETF) standard vying for market dominance is the
Session Initiation Protocol (SIP), taken from original work done by Henning Schulzrinne.
SIP reduces the number of protocol transactions required to set up an Internet telephony
session from 20 for H.323 to two for SIP. It provides the same call setup features as
H.323, and is also capable of being grafted on top of the key lower layers of H.323
including codecs and the real-time protocols. The number of high visibility players
considering or implementing SIP is growing, and includes Cisco, Lucent, and NetSpeak.
IPDC - A Broad New Alternative
A group of influential vendors working for Level 3 have submitted a new specification for
Internet Protocol Device Control (IPDC) to the IETF. The vendors include 3Com, Ascend,
Alcatel, Cisco, Ericsson, and Selsius, now a part of Cisco.
The IPDC protocol specifications cover four major component areas including:
- Signaling transport within an IP network.
- Device management - management of the IP telephony gateways.
- Media control - functionality, which allows for detection and generation of specific
- Connection control - within the media gateways.
IPDC does not supersede H.323 or SIP but instead works with them, covering the open
call control application area needed to enable interoperability between multiple vendors'
IP Telephony gateways and IP PBXs. Stay tuned, as many of the vendors will probably
introduce products that are IPDC-compliant.
INTEROPERABILITY: THE KEY TO THE FUTURE SET
Without open standards, our phone systems would not stand a chance of interworking with
the millions of other phones in the world. And the 2500 set, the analog phone of the
future, will depend on interoperability. Its design should be open, programmable, based on
LAN standards, and publicly available * to copy and mass market without an onerous patent
cost, otherwise we will have a Tower of Babel that offers no gain to anyone.
So, what will the communications appliance of the future be like? It's likely to be
intelligent, application-driven, and open to just about all of the competing protocols
discussed. Third-party developers of all sizes will be able to drive and control the
device. Perhaps our next generation 2500 set will be WinCE-based, Java-based, or a mixture
of the two, but with open APIs based on standards-driven appliances. This will stimulate
the start of a new market - the sale of applications for millions of end-user
communications appliances. I believe this market opportunity will happen like Internet
telephony did, practically overnight, and well within the next two years. The types of
applications might enable the future 2500 set with the same capabilities of a Palm PC and
make it your communications appliance as well as your PIM.
Proprietary closed phones and PBXs that do nothing more than the "status quo"
are a thing of the past. The arrival of the IP PBX and intelligent, open communications
appliances of the future will certainly see to that.
Mike Katz is vice president of marketing and business development at NetPhone, Inc. Headquartered in Marlborough,
Massachusetts, NetPhone is a provider of computer telephony solutions for small business
environments. For more information, contact the author at email@example.com.