26 April, 1997

Installing ISDN

Normal residential telephone service provides four wires (two pairs). A phone line uses one pair, so this typical connection allows a home to have two traditional analog lines, one on each pair of wires. Since a pair of wires can be converted into two ISDN phone lines, one could in theory install four phone lines in a home. However, the phone company will probably resist converting more than one pair of wires to ISDN. An ordinary analog telephone gets power from the phone line. It can continue to work when electricity has been cut off in a house. To provide the greatest protection for fire, break in, and other emergency calls, the phone company would probably prefer to keep one analog line at each location. Adding an ISDN connection to the second pair of wires provides the opportunity of three "phone lines," one analog and two digital. Other countries may provide entirely digital service, but they will use more expensive equipment with a battery backup for emergencies.

During installation, the phone company will connect its end of the wire pair to digital equipment at the central office. Your end of the connection will be a plain phone jack that delivers what is known as the U-interface. The signaling conventions on the U-interface are designed to transfer data over a fairly long distance (up to 18000 feet), at a fairly high rate, on one pair of wires, in both directions at once, with rather crummy environmental considerations (it runs outside on telephone polls).

For ISDN to work, the telephone company needs to do two things right. They have to connect the wires at your house to digital termination equipment at the central office that generates the clock signal. Then they have to program the central office switch with the SPIDs that represent telephone number and type of service (data, voice, or both).

Until recently, ISDN installation was an ordeal involving trial and error by a sequence of repairmen. Of course, progress depends on where you are, but most phone companies have been learning. In the last installation in New Haven, the installer was no expert ("They trained me on this three months ago and since then I did one other ISDN install"). There were mistakes in the paperwork (the installer came a day early, and the switch was not programmed with the SPIDs). The whole process took two hours. Once the two 14-digit SPID values were entered into the Windows configuration utility for the Motorola Bitsurfer, the computer was online and even the attached phones worked correctly.

Lots of mistakes were made, but the problems were solved routinely. Success did not depend on being lucky or getting the right people. ISDN may now be ready to move into the mass market.


An ordinary phone line can be connected to several jacks and several phone devices. An ISDN line must run to one jack and connect to a device that terminates the loop to the central office with an NT1 function. In Europe the phone company supplies a separate NT1 device that they own and maintain. In the US, the phone company only provides the wire and you have to supply your own NT1. The most cost-effective solution is to buy a single device that combines the NT1, a data interface, and even an interface to ordinary telephones.

ISDN Modems

The simplest solution is to replace an existing modem with an external ISDN device. It connects to the Personal Computer through the same EIA RS-232 interface (the COM port). It responds to an extension of the "AT" command set and generates the same set of responses as a modem. It uses the same control signals like DTR, RTS, CTS to manage state and pace the data.

In common use, these devices are called "ISDN modems." Since a modem converts data to sound, and ISDN uses a digital signal, they are not really modems. However, it makes no sense to argue fine points, and "If it walks like a duck, quacks like a duck, Ö, then itís a duck."

The author's current personal favorite in this category is the Motorola Bitsurfer Pro. It is reliable and easy to setup. Always check the Motorola Web site for updated information and the latest version of microcode (the flash memory can be upgraded with a supplied utility). The Motorola unit is a good choice for casual home use because it provides two ordinary phone jacks that allow regular telephones (or fax machines, or conventional modems) to use the ISDN phone lines. If you have kids, you may need the lines. Since the unit is separately powered, the phone lines are available even when the computer is turned off.

USR also makes an ISDN modem, but its phone jack won't ring the telephone in response to an incoming call. Hayes is now producing a device that has not been reviewed or tested.

The 3Com Impact has the most interesting special feature. Its "TollMizer" transmits data over the ISDN protocol normally used for digitized voice instead of data. This is attractive in locations where the Phone Company charges a per-minute surcharge for ISDN data calls, but bills voice calls at a flat rate.

The strength and weakness of an external ISDN modem is the COM port. Since all systems support ordinary modems, they implicitly support an external ISDN device as simply a faster modem. However, the COM port was not designed to run at high speeds. The maximum baud rate is 115200 bits per second, and asynchronous framing uses 10 bits per byte of data. This means that the maximum data transfer is 11.5K bytes per second instead of the 16K that the ISDN technology would permit. An ordinary 16550 UART only provides a 16 character receive buffer, and at high speed the CPU of the Personal Computer must respond quickly enough to empty the buffer before it is overrun with more data. One can buy a special high-speed COM port card, but that adds to the cost.

ISDN Internal Cards

An internal ISDN card plugs into the I/O bus of the computer. The computer powers it. It doesn't require a case or the EIA interface. It also lacks any of the nice blinking lights that are so helpful diagnosing problems.

The main advantage is price. Some cards are available for less than $150. Because they use the I/O bus and not a COM port, they can transfer data at the full 16K bytes per second possible data speed. However, the ISDN line is now connected to a single machine, and when the computer is turned off the line is unusable.

There is a standard interface between the operating system or application and an ISDN card. However, each adapter presents a different hardware interface and may require a specific device driver. Before buying a card, make sure there is software for your operating system.

ISDN Routers and Bridges

The best, but most expensive solution, is an external device that connects to an Ethernet instead of a COM port. These devices act as small routers to connect a home or office LAN to a central corporate or campus network.

The PC requires an Ethernet adapter card. If the external device is shared between several computers, you will also need a "hub" to connect all the devices and form a LAN. All modern operating systems support LAN hardware and provide network software.

A LAN is fast, so most native LAN protocols are "chatty". They frequently exchange housekeeping messages. A router filters out these messages and only sends real data over the ISDN line. Routers must understand specific protocols, such as TCP/IP used for the Internet and IPX used for Novell. A bridge is a less sophisticated device that usually forwards all the messages and uses more of the ISDN link for useless traffic. There are some hybrid devices that filter out some of the unnecessary traffic and "spoof" some of the routine messages and responses that would routinely be exchanged between the remote and central site.

An external LAN-attached ISDN router provides a simple way for several computers to share a single ISDN line. However, these boxes cost $800 and up. They also require specific router support at the central site, which may be expensive if you use an ISP. The same sort of thing can be done with an ISDN adapter card or modem, provided that the computer to which it is attached is running routing software.

Software Routers and Gateways

Normally, an ISDN adapter card or "modem" provides remote access to the Internet for one computer. A serious IDSN candidate probably has a fairly new machine and modern software. The most likely environment is the Dialup Network support that comes standard with Windows 95. After that, other candidates include Windows NT, OS/2, and Linux.

An external ISDN modem can be switched among several computers. You can simply move the cable, or purchase a separate ABC switch for $30-$40. Only one computer can use the line at a time.

Several computers in a home or small office may already be connected with Ethernet adapters and a small hub. This would allow them to share files and printers. Once this is done, the ISDN line can also be effectively "shared" with the right software.

WinGate 2.0 from Qbik provides a firewall/proxy/gateway solution. You install the WinGate server on a Windows 95 or Windows NT computer that owns the ISDN connection. WinGate appears to the other machines on the network as a Web Server, Mail Server, News Server, Name Server, Socks Server, FTP proxy, Real Audio Proxy, and VDOLive server. WinGate will forward any of these protocols through the ISDN line to the real Internet. However, if you need to use some other protocol from a client that doesn't support Socks, then WinGate cannot forward the traffic. WinGate is free if you only access the Internet from one additional machine at a time. Beyond that, you need to purchase a license for a specific number of machines.

The central site sees a single phone line and traffic from a single machine (the one running WinGate). This will work with any Internet support and requires no special hardware or configuration.

WinGate can be configured to automatically dial the phone line when it gets a request, and to hang up the phone after an idle period. Since it typically takes less than 10 seconds to complete an ISDN call, this saves a lot of money in connect time and does not add much delay in normal use.

Any Unix system, including Linux, can be configured as a router. A Windows NT 4.0 Server can act as a router with the addition of "Steelhead" (available as of 4/97 as a Beta from www.microsoft.com). Steelhead is the most powerful alternative for ordinary Windows users, but the incremental cost of Windows NT Server over Windows NT Workstation will make it unattractive for home users.

Central Site: ISDN in Bulk

An ISDN link at home sounds nice, but it will not be useful unless there is a reasonable way to provide ISDN service from the central campus or corporate headquarters. Several vendors have cards that plug into existing communications routers and support eight standard Basic Rate Interface (BRI) ISDN connections, each with 2 B-channels. This provides a good solution for a limited amount of ISDN service. As the number of lines grows, however, racking up a large number of external NT1 boxes is not attractive.

An alternative is defined in the ISDN Primary Rate Interface (PRI). A Primary interface uses the same CSU/DSU equipment that currently delivers what is commonly called "T1" phone service. It supplies 23 B-channels.

The author does not personally feel any pressing need to solve this problem right now. The phone company in Connecticut has decided not to publish a PRI rate, so only BRI is currently a standard service.

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Copyright 1996 PCLT -- The Storm Before the COM -- H. Gilbert