"2B or Not 2B", The ISDN Primer

ntegrated Services Digital Network (or ISDN) provides local access, cost efficient, high-speed DIGITAL communications – up to 128 Kbps (BRI), perfect for your telecommuting and Internet access requirements.

Why ISDN?

ISDN offers a highly cost-effective means of achieving bandwidth! 128 Kbps of bandwidth to address a multitude of applications: small office/home office (SOHO) applications, telecommuting, consolidated communications (telephone, modem, and facsimile), Internet access, large file transfer (image, video and sound files), and the list goes on.

Why not ISDN?

Well, ISDN has a few peculiarities that cause a few raised eyebrows ...

The Telcos have “sporadic” ISDN expertise. While it has grown better (and more consistent throughout the years), there still are some challenges in the correct and timely implementation of the service.
ISDN is reliable, but it is a switched service! And, as a switched service, can be susceptible to Telco switch ‘blips’ and resets. While ISDN is an excellent remote site access technology, it is not sufficiently robust to consistently (and competently) handle host backhaul traffic. Stick with leased lines, intelligent services (X.25 and Frame Relay) for your backhaul implementations. ISDN is deployed on a switch-by-switch basis, meaning that even though a city might be classified as an ISDN-present city, ISDN may NOT be available at your site’s location (through your local telco central office – CO).
ISDN is a local service, and is subject to long distance charging (for calls to another site beyond the local calling area). If your ISDN calls are limited to a local calling pattern, where no long distance charging is incurred, ISDN is a highly cost effective means of communications. If your ISDN calls can incur long distance charging, you should investigate alternate high speed communications means (and costs) versus the costs of long distance.

How does ISDN meet our “need for speed”?

ISDN comes in two flavors; BRI (Basic Rate Interface) and PRI (Primary Rate Interface). BRI has two - 64 Kbps B Channels (the “B” is for Bearer) and one - 16 Kbps D Channel (the “D” is for Data).

Figure 3. Primary Rate Interface

You get to decide on what your “B” channels transport; data, voice, video, whatever! Your “D” channel provides the “access path” for out-of-band signaling, and can provide an additional access medium for your X.25 data.

PRI has twenty-four - 64 Kbps B Channels (the ‘B’ is for Bearer) and one - 64 Kbps D Channel (the “D” is for Data).

What is needed for ISDN connectivity?

Well, loads more acronyms for one! As depicted in Figure 4 and Figure 5, components are classified as ISDN - ready (one assumes that non - ISDN - ready devices are classified as ISDN - free).

Figure 4. TE1 & TE2 Figure 5. TE1 & TE2

ISDN - ready (TE1) components require an NT1 termination only; ISDN - free (TE2) components require both a Terminal Adapter (TA) and an NT1 termination.

What are ISDN “references”?

ISDN references are “hard and fast” demarcation points, and identify where the ISDN components should reside and what their functionality should be.

Is there anything else I should know?

Yes. There are two more ISDN acronyms that you should know: SPID (Service Provider Identifier), and DN (Directory Number). These are defined in Table 3, below...

So, how do I get ISDN?

Well, coming from a programming background – a flowchart for the selection of high-speed services and, eventually, ISDN is most appropriate.

Any last words on ISDN?

“The world” has changed since ISDN’s introduction, and acceptance as the preferred Internet access medium. Its competition includes cable, ADSL, LMCS/LMDS (Local Multipoint Communications/Distribution Systems), improved modem technologies, radio-based systems and so on. All of which are available (or becoming available shortly) that meet or exceed ISDN’s bandwidth capabilities. This high-speed local access market finds many competitors: the local Telco, cable company, wireless provider, and the list grows as the CRTC rules on local access relax. T < G