ISDN Connectivity

by Jeff Newman and David Willis

ISDN Equipment

Now that we've looked at some of the more popular applications of ISDN, we will look at what types of equipment best implements these applications, including NT1s, TAs, NICs, bridges and routers.

NT1s

ISDN abides by a set of standards that defines the layers of contact between the central office and the users equipment. Each point of contact has a specific function and communicates using a different layer of protocols according to the needs of the users' equipment. The different points are as follows: NT1, NT2, TE1 and TE2. About the only one that you need to worry about in our scenario is the NT1. It represents the actual termination of the ISDN circuit to the site and handles the communication to the telephone switch for the devi ces that it connects via TDM.

That is why a device accordingly called an Network Termination 1000 (NT1), must reside between the switch interface (called the "U" interface) and each ISDN-compatible equipment interface (called an "S" interface for equipment such as TAs and/or a "T" interface for other switching devices such as PBXes--often both are implemented into NT1 units, hence the S/T label of the interface). Other devices that do not abide by the ISDN standards, such as analog phones, can be accounted for by another interface ("R" interface) that is not usually implemented into simple NT1 units, but is commonly incorporated into SOHO-oriented ISDN devices or more complex NT1 devices.

NT1 units typically connect ISDN-compatible devices to the ISDN line. More complex NT1s connect non-compatible devices such as analog phones and fax machines as well as ISDN-compatible devices.

NT1 units can be purchased as external devices or can be purchased as built-in components to the ISDN equipment being connected. Every ISDN component needs an NT1 to operate. There are advantages and disadvantages to having a built-in NT1 unit. The built-in NT1s are usually a tad cheaper than external ones, and the user doesn't have to deal with finding a place to put it. The simplest external NT1s are usually about two-thirds the size of a standard modem, and they cost around $100 to $150.

The disadvantages of internal NT1s is that you will have to leave the equipment powered up to use the other devices connected to the NT1 device. This often involves leaving your entire computer running, so it can power the TA card, and the TA card can power its NT1, so your phone connected to the NT1 can operate. This is often unacceptable to companies trying to conserve on electricity bills and equipment wear. Most vendors offer their equipment with or without NT1 units, so you will have a choice.

ISDN TAs and NICs

ISDN TAs work essentially the same as POTS modems. They look the same physically as well as to the application using them for data transfer. Like POTS modems, there are all types of TAs: internal, external and PC Cards (PCMCIA). It doesn't matter which one you choose to implement as long as it fits your needs.

You must consider one thing, however, when deciding to go with an external TA, even though it isn't as much of a problem as it used to be. The user equipment must have the serial chip set that can handle the higher speeds of ISDN. Nothing less than a 16550 UART will do if you want full ISDN speeds via external TAs. If possible, having an advanced serial card and driver is ideal to eliminate any problems and take full advantage of the ISDN speeds. However, most 486 and above PCs and 68040 and above Macs will handle the serial speeds without any problems.

When choosing a modem, make sure that the modem supports 2B channel operation and the vendor either supplies drivers that will allocate the B channels or has support from your brand of operating system to allocate the channels so that you can take advantage of ISDN's channel allocation features. Some older modems won't even let you use the second B channel.

ISDN NICs carry many of the same attributes as TAs, but function as regular LAN cards would on a 10BASE-T network. Similar to how a TA appears as just another modem to the application, an ISDN NIC fools its application into believing that it is talking to a regular NIC. The vendor usually provides special NDIS or ODI drivers for the ISDN NIC that replace traditional ones used for Ethernet. These drivers handle the ISDN functions of the card as well as the translations from that network to the next layer of protocols such as IPX and IP. ISDN NICs are essentially an interface to another network, ISDN.

Which one is better? That naturally depends on your needs and implementation. TAs can be made to perform the same functions through softw are and can serve many other simple roles such as plain terminal connections as well. NICs are often but not always more transparent to the user when initiating and terminating connections and can function as a NIC. TAs are typically more popular than ISDN NICs.

ISDN Bridges

ISDN bridges do just that. They bridge data from one LAN to a remote LAN transparently to the user just as though he or she were on the destination LAN. Like TAs, ISDN bridges come in internal and external fashions and offer built-in NT-1 unit options. They are essentially TAs or NICs with bridging firmware that will transfer LAN traffic regardless of protocol between remote sites. Bridges are a good choice for sites that run many protocols and operate over relatively short distances. Bridges offer some packet filtering to help reduce traffic, but do not filter out most of the packets generated by routers and servers that could normally keep an expensive long distance link up.

You must plan carefully before deciding to go with a bridged solution as opposed to routing and choosing which product to use. Many bridges take full advantage of standard ISDN channel allocation protocols and compression algorithms to help manage the connection and minimize the usage of the ISDN line, reducing the cost of operation. It is important to find out which product supports the filtering and line management features that you need for your network.

ISDN Routers

Among the more complex of the ISDN devices, routers allow remote sites to take full advantage of an ISDN link and the traffic moving between the various locations. Complex LAN configurations can be made simple and effective using ISDN routers. Packets can be routed and filtered according to protocol and packet type, and can be intelligently routed quickly to various sites using either a single BRI connection, multiple BRIs or PRI connections.

The router vendors have been hard pressed over the past t wo or so years to abide by standards for remote access and LAN-to-LAN routing connections and link management. Standards such as Multilink PPP and Stac compression are among the most popular issues among router solutions. These standards define common protocols for ISDN devices to establish an ISDN link, route specific protocols, allocate bandwidth on demand and compress data between different vendors equipment. ISDN interoperability among various vendors has proven to be a nightmare in the past. This is important when scalability is a factor in choosing a routing solutions or when third parties are to be integrated into your network solution such as ISPs.

Many of the routers can link remote dial-in users as well as linking LANs to LANs. Many also offer bridging capability for the protocols that they don't route. There are standards for each. As a result of the standards effort, many ISDN routers now offer the most cost-effective solution to linking branch offices that need the bandwidth and low cost. Beware: Many still use proprietary solutions and still don't do the trick. Good ISDN routers will tend to cost more than any of the other ISDN devices mentioned in this text, but will soon pay for themselves in cost of operation when linking central LANs and remote LANs.

Aside from single BRI routers, higher-end solutions are available that work with these lower-end BRI routers. Some vendors offer routers with multiple BRI interfaces with the option to add more as you scale. If you are already planning to start big, weigh the cost of multiple BRI connections against a PRI and related equipment. Large companies with many remote users and ISPs typically use PRI circuits and routers to best manage many users dialing in from many destinations. PRI is very expensive, but ideal in these situations. For the rest of us, BRI can be scaleable and cost effective if planned right.

For more on PRI routers see "ISDN: PRIme For The Enterpr ise" (Network Computing, May 15, 1996).

May 16, 1996