ISDN Hardware

What hardware do I need for ISDN?

An ISDN line requires a Digital Service Unit (DSU) and a Terminal Adaptor (TA).

What is a DSU?

A DSU terminates the ISDN line at the customer site.

What is a TA?

A TA interfaces the ISDN line with your communications equipment, typically performing digital-to-analog and analog-to-digital conversions for analog telephone, fax, or modem equipment, and digital data rate adaption for computers.

Do I need a DSU and TA?

Yes, you need both units. Many TAs have a DSU built-in.

What is the advantage of a TA with a built-in DSU?

A TA/DSU combination means one less box, providing a neater installation.

Do separate TA and DSU have any advantages?

With a TA/DSU combination, if you upgrade your TA you'll have to replace the DSU as well. DSU design is unlikely to change, but TA design probably will. However, some TA/DSU units feature flash ROM memories for future upgrades.

Some TA/DSU units do not have an ISDN output jack (S/T) for connection to other ISDN equipment. If a single TA fulfills your ISDN requirements, this won't be a problem. If you intend to use a plug-in ISDN card in your computer or use an ISDN telephone or G4 fax machine, make sure the TA/DSU has an S/T output.

How many analog devices can I connect to a TA?

That depends on the TA. Some have just one analog port, some two, and some three. A TA with three analog ports is more flexible.

What if I need more than three analog ports?

Several TAs can be connected to a single ISDN line.

What about TAs that plug into computers?

TAs fall into two main groups: external and internal, or as I call them plug-in ISDN cards.

Which is better, an external TA or an ISDN card that plugs into my computer?

An external TA/DSU with two or three analog ports is more flexible, but a plug-in ISDN card provides better performance for computer applications (i.e., Internet access, file transfers).

Most plug-in cards don't have analog ports, so you can't connect your existing telephone equipment. Those that do usually only have one analog port. Furthermore, if you connect analog equipment to a plug-in card, you must leave your computer switched on all the time because the computer supplies power to the card. Turn off your computer, and your telephone and fax stop working. Plug-in cards don't usually feature a built-in DSU. So you'd still need one of those.

If you intend to use an ISDN line with just your computer, perhaps you plan to install a new line, an external DSU and plug-in card may be sufficient. If you have only one line, however, it makes sense to convert it to ISDN and use it for telephone, fax, and computer applications, as shown below.

External TA

If you still want the added performance of a plug-in card for Internet access and file transfers, you can connect one to the S/T jack on your external TA/DSU, as shown below.

Plug-in card

If I upgrade to ISDN telephone and G4 fax, do I still need a TA?

No. These devices have the equivalent of a TA built-in, and can be connected directly to a DSU or ISDN bus.

Can I connect several analog devices to the same analog port on my TA, like I do with my analog telephone line?

I don't think so. Each analog device must be connected to its own analog port on the TA, because ports have to be configured to work with specific devices (e.g., telephone or fax).

Can I configure the TA myself?

Yes. Most TAs come with configuration software, and some can be configured using a push-button telephone. Note that TAs sold in Japan come with Japanese user manuals, and the configuration software may need a Japanese operating system.

Do all TAs work with Macs and PCs?

Some do, some don't. The OKI PC Link series, for example, is available in Macintosh and Windows versions. The only difference being the supplied software and connecting cables. The NEC Aterm IT55DSU, however, comes with software and connecting cables for both Macintosh and Windows computers.

Do TAs sold in Japan come with English software and manuals?

Not usually. Manuals tend to be Japanese only, and you'll need a Japanese-capable computer to use the supplied setup utility, otherwise, menus and dialog boxes appear as mojibake. (Note: some TAs can be setup using a regular telephone.) Drivers and script files should work okay with the English versions of Windows and Mac OS. (From first-hand experience, I can confirm that the driver supplied with the Aterm IT55DSU works okay with the English version of Windows 95, and that the script file supplied with the Aterm IT50 works okay with the English Mac OS.

How many ISDN devices can be connected to a single ISDN line?

Up to eight ISDN devices can be connected to a single ISDN line using a bus wiring system. A TA counts as one device, so with one TA you can connect up to seven more devices.

How many devices can be used at the same time?

Two. This can be any combination of telephone, fax, and computer. Even with a TA that has three analog ports you can still use just two devices simultaneously.

If I use two devices simultaneously, do I have to pay more?

Yes. You pay the relevant charges for each.

Which external TAs are popular?

Impress reports that NEC holds the biggest share of the TA market with 38.1%, followed by NTT with 35.4%. Sales of TAs reached 1.1 million in 1997 and is expected to reach 1.37 million in 1998.

Since new TAs are being introduced quicker than I can update this page, I recommend that you pick up the latest catalogs and choose a TA to suit your individual requirements. Things to consider include, the number of analog ports and Internet access speed. If you intend to connect an analog telephone, fax, and fax/modem, you'll need a TA with three analog ports. (If you subscribe to Sky PerfecTV, you'll need a analog port to connect your digital tuner.) If your ISP doesn't have plans to offer 128 kbps Internet access, you may not need 128 kbps and BOD (Bandwidth On Demand).

As an example, here's the feature list for the NEC Aterm IT55DSU, the TA I've been using happily since April 1997:

NEC's latest Aterm TAs feature the following:

Macintosh Aterm Note: To use the Aterm Raku-Raku configuration utility on Macintosh computers running the JLK (Japanese Language Kit), you must first register it with the Japanese Language Register. Otherwise, menus and dialog boxes appear as mojibake. Thanks to Julian for pointing this out.

Windows 95 Aterm Note: Although the Aterm Raku-Raku Utility and documentation are in Japanese, the supplied drivers can be installed successfully on the English version of Windows 95. (I can confirm this for the drivers supplied with the IT55DSU.)

What is E-mail waiting?

One of the benefits of a dedicated, 24-hour connection to the Internet is that you receive your e-mail in real time. NEC offers subscribers to its Biglobe Internet service the next best thing with e-mail waiting notification. When e-mail arrives in the mailbox of a Biglobe subscriber, a notification message is sent via the ISDN D channel to the user's Aterm TA free of charge and a message-waiting indicator lights up. The user then connects to Biglobe as normal and downloads the waiting e-mail. As far as I know, this feature is not supported by other ISPs.

What about ISDN routers?

See Routers.

Is the cost of ISDN equipment likely to fall?

It already has. In 1990, an NTT TA cost ¥150,000. In 1997, an NTT TA with more features sells for just ¥50,000. In April 1995, NTT made the specifications of their DSU available to other manufacturers in a bid to encourage them to enter the market. At the same time NTT reduced the cost of their own I-DSU64 from ¥75,000 to ¥23,900.

Can ISDN equipment made for use in the United States be used in Japan?

Although ISDN is an international standard proposed by the CCITT, telephone companies can decide which features to implement, creating equipment variations between countries. NTT's INS-Net uses the same mu-law encoding as the United States for voice encoding, while other countries use A-law encoding. But that's a similarity, not an indication of compatibility. Then there's the question of approval. My OKI DSU, the device that connects to NTT's U-Interface, is marked with the numbers "U96-0013-0" and "N96-0009-0", which are JATE approvals. In Japan, the JATE (Japan Approval Institute for Telecommunications Equipment) approves telecommunications equipment, such as telephones, fax machines, and modems, for use with Japan's public telephone network. JATE-approved equipment usually bears one of the approval marks shown below, or an approval number.

JATE approvals

Most of the TAs sold in Japan are made by domestic manufacturers, such as NEC, NTT, and OKI. Personally, I wouldn't buy ISDN equipment from the United States, or any where else, unless it was approved by the JATE. A few years ago I bought a Supra 28.8 kbps fax/modem from the United States, but it was common knowledge at the time that modems from the United States worked with NTT's telephone network. ISDN, however, is very different, and when you apply for ISDN there are spaces on the application form for you to enter the DSU type and JATE approval numbers.

3Com (USR) and Zyxel are advertising NTT INS Net64 compatible ISDN TAs and routers on their Web sites. Does that mean I can buy in the United States?

No! These pages are somewhat misleading. What 3Com and Zyxel really mean is that they have models specifically for use in Japan (with JATE approval). Although the U.S. and Japanese versions may look the same, and probably are 99% identical, they're not, and it's that 1% that makes all the difference.

If you really want a 3Com or Zyxel TA, I'd suggest that you contact their offices in Japan for more information. (MX Engineering distribute Zyxel TAs and ISDN routers in Japan.) English manuals for U.S. 3Com and Zyxel models can be downloaded off the Web.

Motorola used to make a version of their BitSURF Pro ISDN TA for use in Japan, but have since exited the retail business altogether and discontinued it along with their entire range of modems. Last time I checked you could still download the English user manuals.

What about equipment location?

ISDN installation is quicker and easier if your NTT line jack and TA/DSU are sited reasonably close together. If they are, NTT's room wiring installation fee may be reduced from ¥3,800 to ¥1,200 (you can avoid this charge altogether if you do the wiring yourself). The computer connecting cable supplied with the OKI PC Link TA is one meter in length, and the owner's manual recommends a maximum cable length of three meters, as longer cables are likely to cause data errors. As the TA/DSU has to be connected to an AC outlet a location close to your computer with several AC outlets is probably convenient. Just like modems, some TAs have send (SD) and receive (RD) indicators for monitoring online activity, so you may want your TA in a place where it can be seen easily.

What happens if I plug an analog telephone into an ISDN jack?

It just won't work.

What happens if I plug an ISDN device into an analog telephone jack?

To start with it won't work, and when a call comes in your expensive ISDN device will receive 48 Volt pulses that it won't know what to do with. Well designed equipment should be able to handle this kind of abuse, but I don't recommend it.

Is ISDN equipment susceptible to lightning?

This may seem like an odd question, but having been without ISDN for an entire week due to lightning, I thought it worth mentioning, especially for those users who rely on their ISDN line and have no analog backup. To cut a long story short, lightning hit my ISDN line somewhere and blew the protection fuses in the DSU, leaving the line dead and the TA's ACT indicator flashing. Fortunately, I still had an analog line and was able to call NTT and NEC, and access the Internet via modem. Without that I'd have been stuck. One week later, after several telephone calls, an NEC service guy came to fix it, charging about ¥6,000 for 10 minutes work. He said that it wasn't the first time he'd carried out such a repair and went on to say that ISDN DSUs are more susceptible to lightning than good old analog equipment. Telephone line surge protectors can be licked up for a few thousand yen. A wise investment, methinks!

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Created: 16 May 1997. Modified: 10 October 1998.
Copyright (c) 1998 Paul Findon. All rights reserved.