Even if you follow networking technology only casually, you probably haven't been able to avoid seeing something about Integrated Services Digital Network (ISDN). According to ISDN fans, this technology is the best wide-area connection choice available. It gives you crystal-clear, high-speed digital connections, and lets you drive data and phone connections off one RJ-45 or RJ-11 home or office phone jack.
Heck, ISDN might even walk your dog or wash your car if you install it just right on your Windows NT system. But make no mistake about it: Installing ISDN is no walk in the park. To be successful, you must identify an appropriate application for ISDN technology, identify and purchase the appropriate ISDN interface for your application, navigate your local phone company's order entry system, and then configure the ISDN interface to work with your local phone company's switching equipment. No, ISDN is not plug-and-surf technology like dial-up modems; getting ISDN to work takes forethought and work. With this warning in mind, you can explore the choices you'll meet on the road to ISDN connectivity.
A No-Nonsense View
What is ISDN? It's a network of high-speed digital connections. (See "ISDN and Windows NT," Windows NT Magazine, January 1996.) You establish an end-to-end connection through the network by dialing the phone number for the end point you want to reach. In function, ISDN is similar to the analog phone system. As with an analog phone line, to use ISDN to dial into a computer, you enter the phone number into the software that drives the connection. As with an analog phone line, your phone company bills you for connect costs, including long distance fees, if applicable. Unlike an analog phone line, an ISDN connection does not use a modem. Instead, a special ISDN interface lets you establish the connection.
The most important requirement is that to do anything, you need an ISDN connection on both ends. For example, for ISDN Internet access, you need to find an Internet Service Provider (ISP) that supports ISDN. To launch a network of remote connections with ISDN, you must have ISDN at the central site and at all the remote sites. To implement ISDN between business locations, you have to deploy ISDN at both sites.
The requirement of end-to-end ISDN connectivity seems obvious, but if you get involved with a large ISDN project, you will quickly discover that ISDN availability on a national (let alone international) scale is a problem that continues to plague its success. Each Regional Bell Operating Company (RBOC) provides ISDN, and each RBOC can devise any rate structure and deployment schedule it wants. Getting an ISDN line into Ventura, California, is no problem, but you probably won't be so successful in Lansing, Michigan. Fortunately, this situation is rapidly improving as more and more RBOCs see the wisdom in offering reasonably-priced ISDN service to anyone who wants it.
ISDN is packaged two different ways. You can buy a Primary Rate Interface (PRI) composed of 23 digital channels (called B channels). Each channel can carry 64Kbits per second (Kbps) of data. Alternatively, you can purchase a Basic Rate Interface (BRI), which comes with two B channels capable of data rates of 64Kbps per channel. Both the PRI and BRI include an extra channel (called the D channel) for call setup and signaling. From a consumer perspective, you can forget about the D channel and focus on the B channels, which you will use to service your voice and data needs.
A PRI circuit is rarely necessary, unless you are an ISP who needs to accommodate numerous ISDN connections from your customers. Another exception is a large corporation that is building large remote access facilities for a central site. One BRI circuit can service most applications. Some situations will require multiple BRI circuits. A good reason to stick with BRI instead of PRI is that PRI circuits have much higher tariffs than BRI circuits.
With a BRI circuit, you get two digital lines and two corresponding phone numbers. What can you do with those two lines? The answer depends on the ISDN interface you choose.
Hello, Terminal Adapter
Two devices sit between your computer (or phone) and the ISDN jack the phone company installs in the location of your choice. In ISDN lingo, these devices are a Network Terminator 1 (NT1) and a Terminal Adapter (TA). Most ISDN interface manufacturers combine the NT1 and TA functions in one device. You end up with a single unit that plugs into the ISDN jack and handles both functions.
The ISDN interface--which most people just call a Terminal Adapter--is the key to which capabilities and features your ISDN connection provides. Your choice of ISDN interface directly affects which ISDN options you need to specify when you order an ISDN circuit, so your quest for ISDN connectivity needs to start with the ISDN interface.
You evaluate an ISDN interface from two perspectives. First, you ask how it interfaces with your computer, and second, what feature set it contains.
ISDN interfaces come packaged three ways. First, you can purchase an ISDN interface as a PC adapter that slips into a bus slot on your Intel-compatible system. Second, you can buy an external ISDN interface that connects to your computer via a serial port. Third, you can get a LAN-attached ISDN interface that can service all the systems on your network. Each approach has pros and cons.
A PC adapter ISDN interface is an excellent solution if you have one Intel-based computer. Because an adapter-based ISDN interface functions as an integrated component, you avoid all the logistical and technical problems associated with a serial-based interface. The WinISDN specifications define the interaction between the adapter and Windows NT (or any other Windows operating system). In the NT environment, this definition means you can simply click on ISDN when you set up your Remote Access Service (RAS) interface, as screen 1 shows. The disadvantages of a PC adapter are that it is useful for only one computer, you can't easily move it from one system to another, it requires an available slot in the system, and it requires an Intel-compatible system.
The second solution, using a serial-attached ISDN interface, is the easiest to install and configure. In fact, hooking up a serial-attached ISDN interface is just like attaching a modem. You cable the interface to your serial port and configure it like a modem. Under Windows NT, you set up this interface under RAS by clicking the Modem button and configuring this interface as a high-speed modem, as shown in screen 2. The ISDN interface will then respond to AT-type modem commands.
The advantages of a serial-attached ISDN interface are that it attaches to all Windows NT platforms (Intel, PowerPC, MIPS, and Alpha), and you can easily move it from one system to another (or share it between systems on a non-concurrent basis). The disadvantage to this approach is that many Intel-based serial interfaces can't drive the interface at full speed--128Kbps when both channels are in use for data.
The third option is a LAN-attached ISDN interface. This option is the most flexible because it gives you a single point of attachment that all the computers in your network can use. Think of a LAN-attached ISDN interface as a specialized router. For example, if you set up a LAN-attached ISDN interface for Internet access, all the computers on your network can access the Internet through that single device. Best of all, this approach has no special configuration requirements for the individual systems on the network--they see one unified network. However, the one significant disadvantage to this approach is price. LAN-attached ISDN interfaces are significantly more expensive than a PC adapter or serial-attached interfaces.
The networking industry has reached no agreement that one of these three approaches is the best. You must choose an interface based on your type of system, your need to share ISDN connections on a concurrent or non-concurrent basis, and of course, your budget.
Beyond choosing an ISDN interface package, you also have to consider optional feature sets. The three most important ones are telephony support and bonding and multilink Point-to-Point Protocol (PPP). In terms of telephony, most ISDN interfaces include at least one phone jack for a standard telephone (Plain Old Telephone Service, or POTS, in ISDN lingo). That jack is associated with a B channel--if someone dials the number associated with that B channel, the phone (or fax) will ring, and you (or the fax) can answer as if it were an ordinary analog phone line. Similarly, you can use that same line for dial-out voice and fax connections.
Some ISDN interfaces have additional telephony support. For example, they provide two phone jacks so you can use both B channels for phone service. And one emerging application for ISDN interfaces is to include full PBX capabilities. The ISDN interface provides local services, such as extension dialing, conferencing, and other traditional PBX services, to a set of phones. Of course, these advanced features come with an advanced price tag as well.
Other features to look for are bonding and multilink PPP. Bonding lets you combine both B channels and achieve a transmission rate of 128Kbps. As great as this capability sounds, few applications (chiefly, video transmission and bulk transfer) can benefit from the higher data rate. Also, a side effect of bonding is that it ties up both B channels. An alternative is multilink PPP, which splits a transmission over both B channels. You get the same effective rate of 128Kbps, but multilink PPP can drop to a single channel if a phone call comes in on one B channel. Multilink PPP is for use with TCP/IP, whereas bonding is a general function that is not protocol specific. Both features are well defined, and both are often implemented in the same ISDN interface.
ISDN adapters can come with a plethora of other features. Exploring them is certainly worth your time to see how they can enhance your ISDN experience.
Ordering and Configuring
Once you select an ISDN interface, you call your local telephone company's ISDN ordering center and find out what kind of switching equipment your phone center uses and what kind of ISDN service is available through that equipment. Most centers use either the Northern Telecom DMS 100 or the AT&T 5ESS switch. The ISDN services you may run into include National ISDN-1 service, AT&T multipoint service, or AT&T point-to-point service.
This information is important because you need to look it up in the documentation your ISDN interface vendor provides, so you can determine which options to order on your ISDN circuit. These options are called "provisioning" information, and frankly, they are beyond the comprehension of mere mortals (non-telephone company employees). For example, for National ISDN-1 service through an AT&T 5ESS switch, one popular ISDN interface requires the following options:
Term Type set to A
Circuit Switched Voice (CSV) set to 1
CSV Additional Call Offering (ACO) set to Unrestricted
CSV limit set to 2
CSV Notification Busy (NB) limit set to 1
Circuit Switched Data (CSD) set to 1
CSD ACO is Unrestricted
CSD limit set to 2
CSD NB set to 1
Electronic Key Telephone System (EKTS) set to No
ACO set to Yes
As you can imagine, there is no way you can come up with the right set of options without help from the ISDN interface vendor or some experienced ISDN professional. And these options are one reason why you need to pick your ISDN interface before you order your circuit.
Once you correctly order your ISDN circuit and the telephone company installs it, you have one final step to take: You must configure your ISDN adapter so it knows what type of service and what type of switch it is using. Usually, you perform this configuration through a PC program, but some vendors also let you use an attached telephone for configuration. This is another area where, unfortunately, no rules or standards apply.
Any Way to Run a Network?
If you compare the ISDN procurement and installation process with the one for ordering an additional analog phone line for a modem, you can see that ISDN has a long way to go before it is accessible by non-technical business or home users. In fact, as one ISDN equipment manufacturer pointed out, finding someone to install a turnkey ISDN connection is definitely worth the time and money. That way, someone else can be responsible for making sure the CSD and CSV settings on your circuit are correct.
After the installation and configuration process, you'll find that ISDN connectivity is superior to modem-based connections. The integration into the Windows NT environment is certainly smooth--NT's RAS supports both serial-attached and PC adapter ISDN interfaces; no special support is necessary for LAN-attached ISDN interfaces because they appear as network routers. All things considered, ISDN is great solution for a variety of connectivity problems--if you can get it installed where you need it.