• Networking
  • CCTV
  • Telecoms



First defined in 1988, the ISDN standard (which stands for Integrated Services Digital Network) was established to carry voice, video, data and other network services on the traditional telephone company network. Most modern digital PBXs (private branch exchanges — i.e., business phone systems) connect to the phone company using ISDN Primary Rate Interface (PRI) circuits — T1 trunks that carry 24 channels (23 bearer channels and one signaling channel, for a total of 1.544 megabits per second (1544 kbps) in North America).

whyupgrade02Then along came SIP trunking to challenge the status quo. Whereas ISDN was designed to enable voice networks to carry data, SIP was designed to enable data networks to carry voice — including advanced communications features ISDN never dreamed of.

Which trunking standard is best to connect your business telephone system to the outside world? Let’s take a look at how they compare on essential attributes:

Features. ISDN supports dozens of baseline features for business telephony, such as call forwarding, call waiting, caller ID, hunt groups, audio-conferencing and more. SIP supports a full set of ISDN-like features and more. SIP redefines the possibilities with new IP services such as IP contact centers, call follow-me, unified communications, and the ability to publish local numbers in different markets while having the calls handled at a central location.

But they key benefit of SIP trunking is that your existing broadband access connection — the same one you use for data and Internet access — can now connect voice calls to and from the public telephone network. There’s no need for separate Internet/data and voice trunks, so you save money while streamlining your communications infrastructure.

Business-grade voice quality. ISDN achieves voice quality by one-to-one provisioning — one voice call per 64-kbps channel (23 direct inward dial numbers per ISDN PRI trunk) — to support two-way conversations. With SIP, voice quality is achieved by prioritizing voice over data, since data (emails, text messages) can tolerate a few milliseconds of delay while voice needs immediacy. Most business-class routers have the bandwidth management capabilities to ensure that voice traffic gets the priority treatment it requires.

Simplicity. With ISDN, you have to buy, deploy, operate, manage, troubleshoot and upgrade separate access connections for voice and data. With SIP trunking, you can converge voice onto your broadband access connection and eliminate the traditional ISDN PRI connections to the phone company.

Bandwidth efficiency. SIP beats ISDN hands-down here. ISDN PRI lines are configured into 23 bearer channels, and each channel is configured either for voice or data. You can’t switch them up. You can’t use idle voice channels to handle sudden peak demands in data, or vice versa.

With SIP trunks, channels are virtual — set up and taken down as needed — so bandwidth can be much more efficiently used. If there are lots of concurrent calls, SIP sets up more channels for voice. When there are few people talking on the phone, that bandwidth is available for data and Internet access. You get more value out of the same broadband access connection.

Scalability. ISDN trunks are divided into 23 channels, and you can’t buy just a few of those channels. If you only need 15 voice channels, you still have to pay for 23. If you need more than 23 channels, you’d have to buy two trunks (46 channels).

Since SIP trunks are virtual, your broadband access connection can have as few or as many channels as you need, up to total available bandwidth. If you only need 15 voice channels, just buy 15. If you need more than 23, no problem. And if you suddenly find you need more channels than you thought, a feature known as call bursting enables the connection to handle those calls on virtual channels beyond the ones you’ve paid for.

Easy set-up. If you need to connect a Toshiba IPedge system to the public voice network, it is much easier to do it with native SIP trunking than to program a gateway to translate between PRI/analog and SIP. When the technician selects “Toshiba SIP Trunk” in the management system, the correct settings and parameters are automatically entered. There are no interface cards or gateways to configure.

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