ISDN: saved by the net

Published Saturday, 13 April 1996 2:14AM CST by in Technology

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The initial killer application for ISDN was videoconferencing. Before it happened, most of us woke up and realized that we sort of enjoyed the anonymity or at least insulation provided by voice communications. There was something comforting about knowing that you could answer the phone in various states of undress without the person on the other end of the connection knowing. Wags have been predicting the death of ISDN for years. They’re wrong. ISDN was saved by the Net and holds great promise for connecting geographically dispersed individuals and organizations.

At the end of the day, ISDN is neat. It offers what all of us want: fast, cheap, and ubiquitous connectivity. Problem is, there are all sorts of genuflections and contortions you have to go through to get ISDN up and running. But once it’s up and properly configured, it’s rock-solid and you’ll think it was worth all the hair pulling and teeth gnashing.

Some estimates are that about 1 million ISDN lines are currently being used in the United States, an increase of more than 300 percent from 1995.

Standard ISDN—called Basic Rate Interface (BRI) ISDN—is delivered as two 64 Kbps “bearer,” or B, channels and a single 16 Kbps “data,” or D, channel. Because it’s the phone company, it has to be confusing: the B channels are used to carry information; the D channel is used for signalling, call setup, and X.25 data.

Institutional ISDN—called Primary Rate Interface (PRI) ISDN—is delivered as twenty-three 64 Kbps B channels and a single 64 Kbps D channel.

Standard ISDN provides speeds up to 128 Kbps (up to 512 Kbps with compression). This is somewhat confusing because of the acronym alphabet soup used to describe how standard ISDN’s two channels are combined for maximum throughput. Bandwidth on Demand (BOND) refers to the ability to aggregate multiple B channels for added bandwidth when necessary. Multilink PPP (MPPP) refers to the ability to aggregate multiple B channels with the point-to-point (PPP) protocol for added bandwidth. Think of ISDN as a sort of Lego set for telecommunications. Virtual circuits from 128 Kbps all the way up to 1.544 Mbps can be constructed using no more than two pairs of ordinary telephone wires—the equivalent of two standard (analog) telephone lines.

ISDN is also usable for regular telephone, fax, and modem communications and can replace the functionality of two regular phone lines. (Regular phone lines are referred to as POTS—Plain Old Telephone Service—by telecom geeks.) Using standard ISDN, you can connect to the Net at 64 Kbps and simultaneously have a voice conversation on the other channel. When you’re done talking, your ISDN connection can be configured to automatically upshift to 128 Kbps throughput when needed.

ISDN does not require new telephone lines—only new equipment at both the telephone company and your end of the connection. The biggest barrier to ISDN availability is that you must be located within 18,000 feet from your telephone company’s central office in your neighborhood. This 18,000 foot limit can be remedied through the use of repeater equipment, but you’ll pay higher rates. The second biggest barrier to ISDN availability is the lack of the necessary equipment at your telephone company’s central office in your neighborhood. As of early 1996, about 80% of the U.S. population enjoys ISDN availability.

Ordering and Installation

Common sense would indicate that to get ISDN you need only call your local telephone company, place the order, and connect your equipment. Unfortunately it’s not that simple, although it is getting much easier than it was even a year ago.

Here’s a list of links to telephone companies that provide ISDN service in the United States:

Once you’ve determined that you live close enough to the telephone company’s central office in your neighborhood and that the telephone company has the necessary equipment, you’re still not home free. Ordering ISDN service is the most difficult part of the process because of a multitude of ISDN configuration options. Each B channel can be configured to handle voice, data, or both voice and data.

A circuit-switched data (CSD) option supports data transmission speeds of up to 64 Kbps for each B channel. “Circuit switched” refers to a call setup that establishes a fixed path through the telephone system for the duration of the call and is necessary for establishing a connection to the Net.

To support voice communications (including traditional fax and modem communications), at least one of the B channels must be configured for circuit-switched voice (CSV).

To support both voice and data, a B channel must be configured for circuit-switched voice/circuit-switched data (CSV/CSD).

If you want to use more than one device at a time, you have to specify a multipoint configuration option. If you’ll only be connecting to a single host, you’ll need only a point-to-point configuration.

If you plan to aggregate your B channels—technically called multichannel bonding—your line will have to be configured to support what’s called “multiuse bearer service.” In English, this ensures that all of your calls follow identical routes to the same destination (each B channel initiates its own call as needed). If your telephone company doesn’t support this, you’ll have a hard time getting all the bandwidth you’re paying for—especially for long distance calls.

So, here’s what you want in order to enjoy all of ISDN’s flexibility:

  • Both B channels configured for circuit-switched voice/circuit-switched data
  • Multipoint configuration
  • Multiuse bearer service

The good news is that almost all modern ISDN equipment comes with an ISDN ordering code (IOC). Give this code to your telephone company, and installation is—in theory—almost painless. In reality this is seldom the case.

During the installation process, your telephone company will assign a Service Profile Identifier (SPID) number for each B channel or each device connected to your ISDN line. SPIDs let the switch know which ISDN services a device can access. Be sure you get the SPIDs correct—misconfigured SPIDs are the single most common reason for connection failures. Some telephone companies let you order more than one SPID for each channel for an additional fee. This may be worth your while if you want to use some of the more advanced calling features like caller id, conferencing, forwarding, rejection, etc. Use two SPIDs to identify each of your data channels and a third SPID to identify one of channels for advanced calling features.

Your telephone company will also provide you with information about the type of central office switch being used for your line. A “switch” is the electronic equipment that routes telephone calls. AT&T’s 5ESS (Electronic Switching System) uses either Custom or National ISDN 1 (NI-1) software. Northern Telecom’s DMS-100 uses only National ISDN 1 (NI-1) software. Most recent national ISDN designations in the United States are vendor-independent and referred to simply as NI-1 or NI-2.

Finally, once the line has been installed and configured at the central office, the telephone company will send out a technician to test your line. Treat these folks nicely; they are well-informed about most things ISDN and usually aren’t shy about sharing their knowledge.

ISDN Equipment

You can’t just plug your existing telephones, modems, and fax machines into your new ISDN line. You’ll need some special equipment determined by what you want to do with your ISDN line.

You’ll need a network termination device (NT1) regardless of what you plan to do with your ISDN line. The NT1 enables the ISDN network to recognize your connection. The NT1 device itself can be a standalone box or it can be integrated into another ISDN device. Be aware that an NT1 without ports will allow an ISDN connection for only a single PC. You won’t be able to connect other computers, telephones, or other equipment. Fancier NT1s include connection ports for both ISDN and analog devices. The Alpha Telecom Super NT1 is popular, as is IBM’s 7845 ISDN Network Terminator Extended.

I use the IBM 7845. It only has a single POTS port, and it gets confused from time to time and needs to be reset, but overall it works pretty well. Word on the street is that Farallon is working to include POTS ports to its Netopia line of ISDN routers. When that happens, I’ll buy one if only because it will eliminate three existing boxes (a LocalTalk to Ethernet bridge, the NT1, and my current router).

In addition to an NT1, you’ll need a terminal adapter (TA) to connect your computer or local area network (LAN) to the Net. Terminal adapters come in two flavors: U-interface and S/T-interface. The two flavors are easily differentiated. Just remember that the U-interface TA includes the NT1 function built-in and the S/T-interface requires a separate NT1. A terminal adapter with an S/T-interface is generally more flexible, allowing you to select whatever NT1 best meets your needs.

Most terminal adapters support the Point-to-Point Protocol/Multilink Protocol (PPP/MP) supporting connections using both B channels as needed. Make sure your Internet service provider supports PPP/MP on its side of the connection, though, or your connection will be limited to 64 Kbps.

Similarly, compression only works if it’s supported on both sides of the connection. Two de-facto compression standards exist for ISDN: Stac and Lempel Ziv. Make sure your equipment supports the compression standard used by your Internet service provider. Theoretically, a standard ensures compatibility across equipment from multiple vendors. Unfortunately, the reality is that the ISDN standards include enough wiggle-room that cross-vendor compatibility is only now beginning to happen.

The kind of terminal adapter you buy depends on how many computers you want to connect to the Net. ISDN “modems”—modem is a misnomer for a digital device connecting to a digital service like ISDN, but it’s a marketing term that seems to be sticking—are easier to install and configure than an ISDN bridge or router, but they have two drawbacks. ISDN “modems” currently can’t use the entire 128 Kbps bandwidth (start and stop bits reduce the total available throughput to about 92 Kbps). The Motorola BitSurfer Pro, IBM’s WaveRunner, 3Com’s Impact, and the US Robotics I-Modem are all popular alternatives.

If you have more than one computer, chances are you already have a Local Area Network (LAN). In this case, an ISDN bridge or router is your best bet because they allow multiple computers to share a single connection. Incoming and outgoing traffic is automatically transmitted across the bridge or router to the Internet as necessary. All connections are established quickly and transparently—there’s no need to physically “dial” into your Internet service provider. ISDN bridges and routers are more expensive than ISDN “modems,” but you only need a single bridge or router for your entire LAN. Ascend Communications Inc. has the greatest ISDN router market share, and its Pipeline 50 is one of the best products available. Last year Cisco Systems acquired Combinet and repackaged the Combinet Everyware series as the Cisco 750 series. Farallon was late to the ISDN routing game, but its Netopia series is worth a close look. Mac users will find the Netopia’s ability to bridge LocalTalk and route AppleTalk especially attractive.

ADSL

Lots of observers say that ADSL—Asymmetric Digital Subscriber Line—will be the death of ISDN. Of course, these are mostly the same observers that insisted ISDN would be still-born.

GTE Telephone Operations is the first telephone company to begin to provide ADSL data communications technology to its customers. Originally developed to deliver video over standard copper telephone lines, ADSL will represent the opening for the telephone companies in the bandwidth battle against the cable television companies. ADSL offers transfer speeds of 1.5 - 4 million bits per second compared to the 10 million bits per second transfer speeds promised by the cable television companies.

US West and MCI have both announced field trials for ADSL and HDSL (High-data-rate Digital Subscriber Line). US West will begin testing ADSL and HDSL in the second half of 1996 in Boulder, Colo. MCI has already begun ADSL testing in Sioux City, Iowa. US West states that its target price for ADSL is less than US$50 per month when it becomes available in two or three years, less than half what it currently charges for ISDN.

Unfortunately, ADSL (and cable modems, for that matter) are at best a half solution, according to some observers. ADSL and cable modems reportedly provide lots of bandwidth for inbound traffic but modem speed or less for all outbound traffic. If all you want to do is graze the Net, ADSL—or any other asymmetric solution—may be appropriate for you. But what the Net offers that every other medium to date has neglected is true interactivity. And true interactivity demands as much bandwidth going out as coming in.

Other experts insist that ADSL and HDSL provide adequate outbound bandwidth, while acknowledging that inbound bandwidth is greater. ADSL, according to some experts will provide 1.544 Mbps inbound and 640 Kbps outbound; HDSL will provide 768 Kbps both inbound and outbound.

SDSL—Symmetric Digital Subscriber Line—is probably five or six years out but because it’s a symmetric solution it will provide high-speed bandwidth for both inbound and outbound traffic. SDSL, not ADSL, will probably replace ISDN.

In the meantime, Dan Kegel’s ISDN page continues to be the most content-rich source of information for ISDN users.

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