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Which VoIP Standard to Use?
[December 23, 2005]

Which VoIP Standard to Use?

By Eric Dean, TMCnet "Inside VoIP Technology" Columnist

Implementing a standards-based VoIP network presents an initial challenge of first selecting the appropriate standard.  Over the past decade, different industry standards bodies have developed various VoIP standards each for specific telephony environments.  Enterprise IP telephony, carrier long distance, call center, residential class 5 switching, and computer telephony systems all require different features, integration capabilities, scalability, and expertise.  Carefully considering each standard’s strengths and weaknesses from both an academic and industry best-practice perspective will prevent future re-designs or accelerated equipment depreciation.   

One would think that using standards-based VoIP equipment equates to interoperability.  Since the century old PSTN heavily predates VoIP, some VoIP standards have evolved merely for PSTN integration while others extend beyond.  For instance, in the 1990s, H.323’s crafters originally intended for LAN-based telephony with off-net access to the PSTN.  Today, major VoIP companies interconnect VoIP-to-VoIP calls via the PSTN because VoIP peering has yet to establish a scaleable interoperable standard of comparable quality.  Meanwhile, the latest presence directories, like Skype and Yahoo, rely upon the Internet for free on-net member communications by incorporating SIP into their messaging clients using alias addressing rather than E.164 phone numbers.
Not every VoIP environment must incorporate all of each standard’s features.  As a result, some of the guiding factors for selecting the best suited VoIP standard include:
·        What scale environment is the VoIP network? 
o Is this a small business or a large-scale enterprise?
o Is this a central headquarters or distributed architecture?
o Will you rely upon IP with PSTN failover?
·        Are you integrating with outsourced/third party services providers? 
o Are you considering an outsourced hosted PBX service?
o Are you considering local and long distance VoIP services?
o Are you considering a managed, per-seat phone service?
·        Do you require integration with a legacy PBX or phone system? 
o Is the legacy PBX VoIP capable?  If so, what standard?
o Is the voicemail system VoIP capable?  If so, what standard?
·        Do you require support for Computer Telephony?
o Will users register phone extension remotely (home/hotel)?
o Are there required advanced CTI features (calendering/find-me/follow-me)?
o Do you require integrated messaging?
o Do you want to integrate your enterprise directory for alias presence dialing?
The first question identifies the general architecture of the VoIP network.  For larger-scale environments, a VoIP network must accommodate both a flexible and extensible design for future growth.  SIP works well in a distributed environment while MGCP favors a centralized design.  Smaller businesses that are primarily limited to a single office often implement low-cost, proprietary, key-like solutions but may consider per-seat outsourced phone services.
The second question evaluates whether third party IP service come into play.  Managed service providers like Broadsoft offer carrier and enterprise PBX outsourcing services for companies willing to take that leap forward.  Most all PBX MSPs use SIP for end-to-end VoIP services. With regards to wholesale VoIP local and long-distance services, H.323 used to be a preferred method, but carriers today are embracing SIP due to its simplicity and robustness.  The trend for per-seat phone management leans towards SIP’s distributed ability (and sometimes Cisco’s proprietary protocol Skinny) although MGCP still remains within some residential VoIP carrier environments.  Cisco’s Skinny provides a PSTN failover feature whereby phones re-register to a router-gateway for limited PSTN connectivity in the event of an IP network failure.
Integration with a legacy PBX mainly impacts vendor specific features rather than protocol oriented issues.  Obviously, if a PBX or voicemail system only supports a specific VoIP protocol, that may affect the architectural design unless an analog POTS interface is used instead.  Also of note, different vendors license their VoIP PBX interfaces at surprisingly high costs such that an analog interface may be preferred.
Computer Telephony Integration may not appear to be a critical design choice, but in 1995, neither was email.  No doubt, the gap between computer and telephone will close over the next few years.  Microsoft favors SIP as the successor to H.323, which NetMeeting once used, while Linux already supports extensive SIP applications such as SIP Express Router and Asterisk.  As more computer-integrated communications proliferate, companies will either replace their non-interoperable desk phones or migrate towards computer-based softphones. 
Other considerations include video-support which both H.323 and SIP support, yet with H.323 on a downward trend, VTC manufacturers like Polycom, Tandberg and Cisco are incorporating SIP based video.  Not surprisingly, SIP emerges as a ubiquitous answer addressing all concerns but some environments may accommodate other protocols as well.  Of course, a standard is only as good as the vendor implementation; however, SIP provides an excellent foundation for vendors to build atop.  Regardless, VoIP has amassed such momentum that the standard and the vendors are fusing together computers and telephony into the next generation communication services.
Eric Dean is a Principal Consultant with The Heiden Group with over a dozen years designing, building, and operating carrier-class networks and enterprise architectures.  For more information please visit

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