ISDN to the Rescue

Suffering the trials and tribulations of ISDN installation

Even if you follow networking technology only casually, you probably haven'tbeen able to avoid seeing something about Integrated Services Digital Network(ISDN). According to ISDN fans, this technology is the best wide-area connectionchoice 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 oroffice phone jack.

Heck, ISDN might even walk your dog or wash your car if you install it justright on your Windows NT system. But make no mistake about it: Installing ISDNis no walk in the park. To be successful, you must identify an appropriateapplication for ISDN technology, identify and purchase the appropriate ISDNinterface for your application, navigate your local phone company's order entrysystem, and then configure the ISDN interface to work with your local phonecompany's switching equipment. No, ISDN is not plug-and-surf technology likedial-up modems; getting ISDN to work takes forethought and work. With thiswarning in mind, you can explore the choices you'll meet on the road to ISDNconnectivity.

A No-Nonsense View
What is ISDN? It's a network of high-speed digital connections. (See "ISDNand Windows NT," Windows NT Magazine, January 1996.) Youestablish an end-to-end connection through the network by dialing the phonenumber for the end point you want to reach. In function, ISDN is similar to theanalog phone system. As with an analog phone line, to use ISDN to dial into acomputer, you enter the phone number into the software that drives theconnection. As with an analog phone line, your phone company bills you forconnect costs, including long distance fees, if applicable. Unlike an analogphone line, an ISDN connection does not use a modem. Instead, a special ISDNinterface lets you establish the connection.

The most important requirement is that to do anything, you need an ISDNconnection on both ends. For example, for ISDN Internet access, you need to findan Internet Service Provider (ISP) that supports ISDN. To launch a network ofremote connections with ISDN, you must have ISDN at the central site and at allthe remote sites. To implement ISDN between business locations, you have todeploy ISDN at both sites.

The requirement of end-to-end ISDN connectivity seems obvious, but if youget involved with a large ISDN project, you will quickly discover that ISDNavailability on a national (let alone international) scale is a problem thatcontinues to plague its success. Each Regional Bell Operating Company (RBOC)provides ISDN, and each RBOC can devise any rate structure and deploymentschedule it wants. Getting an ISDN line into Ventura, California, is no problem,but you probably won't be so successful in Lansing, Michigan. Fortunately, thissituation is rapidly improving as more and more RBOCs see the wisdom in offeringreasonably-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 cancarry 64Kbits per second (Kbps) of data. Alternatively, you can purchase a BasicRate Interface (BRI), which comes with two B channels capable of data rates of64Kbps per channel. Both the PRI and BRI include an extra channel (called the Dchannel) for call setup and signaling. From a consumer perspective, you canforget about the D channel and focus on the B channels, which you will use toservice your voice and data needs.

A PRI circuit is rarely necessary, unless you are an ISP who needs toaccommodate numerous ISDN connections from your customers. Another exception isa large corporation that is building large remote access facilities for acentral site. One BRI circuit can service most applications. Some situationswill require multiple BRI circuits. A good reason to stick with BRI instead ofPRI is that PRI circuits have much higher tariffs than BRI circuits.

With a BRI circuit, you get two digital lines and two corresponding phonenumbers. What can you do with those two lines? The answer depends on the ISDNinterface you choose.

Hello, Terminal Adapter
Two devices sit between your computer (or phone) and the ISDN jack the phonecompany installs in the location of your choice. In ISDN lingo, these devicesare a Network Terminator 1 (NT1) and a Terminal Adapter (TA). Most ISDNinterface manufacturers combine the NT1 and TA functions in one device. You endup 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 thekey to which capabilities and features your ISDN connection provides. Yourchoice of ISDN interface directly affects which ISDN options you need to specifywhen you order an ISDN circuit, so your quest for ISDN connectivity needs tostart with the ISDN interface.

You evaluate an ISDN interface from two perspectives. First, you ask how itinterfaces with your computer, and second, what feature set it contains.

ISDN-to-Computer Interface
ISDN interfaces come packaged three ways. First, you can purchase an ISDNinterface as a PC adapter that slips into a bus slot on your Intel-compatiblesystem. Second, you can buy an external ISDN interface that connects to yourcomputer via a serial port. Third, you can get a LAN-attached ISDN interfacethat can service all the systems on your network. Each approach has pros andcons.

A PC adapter ISDN interface is an excellent solution if you have oneIntel-based computer. Because an adapter-based ISDN interface functions as anintegrated component, you avoid all the logistical and technical problemsassociated with a serial-based interface. The WinISDN specifications define theinteraction between the adapter and Windows NT (or any other Windows operatingsystem). In the NT environment, this definition means you can simply click onISDN when you set up your Remote Access Service (RAS) interface, as screen 1shows. The disadvantages of a PC adapter are that it is useful for only onecomputer, you can't easily move it from one system to another, it requires anavailable slot in the system, and it requires an Intel-compatible system.

The second solution, using a serial-attached ISDN interface, is the easiestto install and configure. In fact, hooking up a serial-attached ISDN interfaceis just like attaching a modem. You cable the interface to your serial port andconfigure it like a modem. Under Windows NT, you set up this interface under RASby clicking the Modem button and configuring this interface as a high-speedmodem, as shown in screen 2. The ISDN interface will then respond to AT-typemodem commands.

The advantages of a serial-attached ISDN interface are that it attaches toall Windows NT platforms (Intel, PowerPC, MIPS, and Alpha), and you can easilymove it from one system to another (or share it between systems on anon-concurrent basis). The disadvantage to this approach is that manyIntel-based serial interfaces can't drive the interface at full speed--128Kbpswhen both channels are in use for data.

The third option is a LAN-attached ISDN interface. This option is the mostflexible because it gives you a single point of attachment that all thecomputers in your network can use. Think of a LAN-attached ISDN interface as aspecialized router. For example, if you set up a LAN-attached ISDN interface forInternet access, all the computers on your network can access the Internetthrough that single device. Best of all, this approach has no specialconfiguration requirements for the individual systems on the network--they seeone unified network. However, the one significant disadvantage to this approachis price. LAN-attached ISDN interfaces are significantly more expensive than aPC adapter or serial-attached interfaces.

The networking industry has reached no agreement that one of these threeapproaches is the best. You must choose an interface based on your type ofsystem, your need to share ISDN connections on a concurrent or non-concurrentbasis, and of course, your budget.

Beyond Packaging
Beyond choosing an ISDN interface package, you also have to consideroptional feature sets. The three most important ones are telephony support andbonding and multilink Point-to-Point Protocol (PPP). In terms of telephony, mostISDN interfaces include at least one phone jack for a standard telephone (PlainOld Telephone Service, or POTS, in ISDN lingo). That jack is associated with a Bchannel--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 ordinaryanalog phone line. Similarly, you can use that same line for dial-out voice andfax connections.

Some ISDN interfaces have additional telephony support. For example, theyprovide two phone jacks so you can use both B channels for phone service. Andone emerging application for ISDN interfaces is to include full PBXcapabilities. The ISDN interface provides local services, such as extensiondialing, 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 youcombine both B channels and achieve a transmission rate of 128Kbps. As great asthis capability sounds, few applications (chiefly, video transmission and bulktransfer) can benefit from the higher data rate. Also, a side effect of bondingis that it ties up both B channels. An alternative is multilink PPP, whichsplits a transmission over both B channels. You get the same effective rate of128Kbps, but multilink PPP can drop to a single channel if a phone call comes inon one B channel. Multilink PPP is for use with TCP/IP, whereas bonding is ageneral 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 iscertainly 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'sISDN ordering center and find out what kind of switching equipment your phonecenter 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 5ESSswitch. The ISDN services you may run into include National ISDN-1 service, AT&Tmultipoint service, or AT&T point-to-point service.

This information is important because you need to look it up in thedocumentation your ISDN interface vendor provides, so you can determine whichoptions 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 servicethrough an AT&T 5ESS switch, one popular ISDN interface requires thefollowing options:

Term Type set to ACircuit Switched Voice (CSV) set to 1CSV Additional Call Offering (ACO) set to UnrestrictedCSV limit set to 2CSV Notification Busy (NB) limit set to 1Circuit Switched Data (CSD) set to 1CSD ACO is UnrestrictedCSD limit set to 2CSD NB set to 1Electronic Key Telephone System (EKTS) set to NoACO set to Yes

As you can imagine, there is no way you can come up with the right set ofoptions without help from the ISDN interface vendor or some experienced ISDNprofessional. And these options are one reason why you need to pick your ISDNinterface before you order your circuit.

Once you correctly order your ISDN circuit and the telephone companyinstalls it, you have one final step to take: You must configure your ISDNadapter 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 vendorsalso let you use an attached telephone for configuration. This is another areawhere, unfortunately, no rules or standards apply.

Any Way to Run a Network?
If you compare the ISDN procurement and installation process with the onefor ordering an additional analog phone line for a modem, you can see that ISDNhas a long way to go before it is accessible by non-technical business or homeusers. In fact, as one ISDN equipment manufacturer pointed out, finding someoneto 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 CSVsettings on your circuit are correct.

After the installation and configuration process, you'll find that ISDNconnectivity is superior to modem-based connections. The integration into theWindows NT environment is certainly smooth--NT's RAS supports bothserial-attached and PC adapter ISDN interfaces; no special support is necessaryfor LAN-attached ISDN interfaces because they appear as network routers. Allthings considered, ISDN is great solution for a variety of connectivityproblems--if you can get it installed where you need it.