ISDN Connectivity by Jeff Newman and David Willis Problems With ISDN Although ISDN is a handy communication standard, it has its problems--the most prevalent is availability. Traditionally, ISDN has been very difficult to obtain because it requires newer hardware and software within local telephone switches before it can be delivered to your business or house. Some areas are lucky and aren't stuck with older switch technology that was implemented many years ago by the telephone companies. Some switches have been around for a long time and are fine for supporting normal POTS communication. Before that, the phone companies didn't need to fix what wasn't broken. Now the phone companies are faced with the fact that a new technology such as ISDN is requiring them to pay big bucks to upgrade the hardware and software used to dri ve their vast communications networks. Even if ISDN is available to those switches in your area, you must be within 2,000 feet of your central office for ISDN to work, as all signals traveling across copper wire diminish over distances. For the distance and equipment reasons and because many technicians need to be educated to be able to install and maintain it, ISDN is not yet available in many areas. This has been the downfall of ISDN for many years. Although voice calls can be made from the ISDN connection to anywhere, data calls can only be made to other ISDN connections, which is the reason that most people want it. So an ISDN connection is not very useful unless your destination site, such as a branch office or Internet service provider (ISP), can also be equipped with ISDN. Despite their slow start, tone providers across the continent are aggressively deploying ISDN. Many have greater than 30 percent deployment currently and plan to have more than 50 percent deployment by 1997. Another problem with ISDN is that the cost of equipment has traditionally been high. This woe is quickly diminishing, as the number of vendors jumping on the ISDN bandwagon is growing rapidly. Some equipment is starting to rival that of newer POTS equipment such as modems. Another traditional problem with ISDN is its complexity. Many different configurations of BRI ISDN and services are offered. You must order the ISDN connection that will match the requirements of the equipment that you are placing to the ISDN line. This problem is also quickly fading as standards organizations, in cooperation with vendors, are establishing ISDN ordering codes, provided by a particular vendor for their equipment that customers can give to the telephone company when ordering their lines that will handle all of the provisioning. Most vendors will give you these codes, but it does mean that you need to decide on your equipment and vendor before you purchase your line. You only need to know a few codes called service profile IDs (SPIDs), needed by the equipment to communicate with the switch to identify its required services. SPIDs are provided by the telephone company when the connection is established. Some more innovative companies, such as Adtran, (www.adtran.com)are coming out with ISDN products today that can even auto-detect SPIDs, further reducing the effort of implementation. Another problem with ISDN is that while you can get ISDN, your central office may not yet have the software needed to implement a full 64 Kbps on each B channel. Some of the older signaling software requires the signaling to be handled in-band (over the B channels) instead of the out-of-band signaling that the D channel provides. The 16 Kbps required to manage and carry information relative to the connection must be allocated from the 64 Kbps available to B channels, which is 8 Kbps per channel. Thus, many central offices are limited to 56 Kbps on each B channel or 112 Kbps aggregated bandwidth for a connection. This limitation also creates many 112-Kbps bridges across the country, so even though you might have 128-Kbps ISDN locally, you might only get a 112-Kbps connection when calling over long distances to some remote sites. Still, this is an attractive bandwidth when you know that you will get at least that much and it will be a reliable connection. The phone companies are quickly solving this in-band signaling problem as well. Still more woes. Even though ISDN is ideal for small office and home office (SOHO) solutions, it doesn't provide power to its equipment like POTS does to its telephones. If power is lost from the utility outlets in an office, that office will not have voice or data communication unless some type of UPS or generator is used to supply power to the critical components of the ISDN connection, so you must take care to supply power to important applications using ISDN. May 16, 1996 Print This Page E-mail this URL