Making The Connection With ISDN Routers By Jeff Newman   ISDN is finally available in your area, and your peers, who refer to the technology as "It Still Does Nothing," are avoiding you now because your ISDN Basic Rate Interface (BRI) or Primary Rate Interface (PRI) lines are scheduled for installation. With a confident grin, you're moving ahead with deployment. Now you just need to figure out how to migrate your small office/home office (SOHO) users from legacy single-line, analog, peer-to-peer connections to this digital, multichanneled, measured rate, high-speed medium--while keeping down the costs of operation. Fortunately, you have many equipment choices--terminal adapters (TAs), bridges and routers--and ways to combine each to establish dial-up networks. ISDN TAs are inexpensive and wo rk most like old analog modems. This similarity helps reduce the learning curve for your users. Unfortunately, in most areas, ISDN is a measured rate service: There's no "free" calling area as with plain old telephone system (POTS) service. With POTS service and analog modems, you don't worry about connecting, disconnecting and managing your own connection because local calls are free. With ISDN, you can't ignore these because the cost of calls can eat you alive. ISDN bridges operate transparently to the user. However, they don't provide any real method of optimizing connect time. Bridges, for the most part, can't analyze specific protocols to spoof unwanted connections or perform the level of packet filtering needed for efficient use of your links. ISDN routers, like other routers, let you build anything from complex switched-circuit networks to simple peer-to-peer dial-in connections. Additionally, ISDN routers let you fine-tune your line usage, which can reduce charges. ISDN routers are y our most likely choice because of these benefits. To deploy a cost-effective ISDN dial-up network and still enable your users to take advantage of ISDN's performance, what are the essentials any ISDN router must have? At the Remote End For SOHOs, a low-cost ISDN router provides a lot of functionality. Many ISDN routers are equipped with some combination of a standard Ethernet port, multiple POTS ports and a built-in fax modem. Some even have a built-in Ethernet hub. This bundling obviates the need for extra analog lines for the user's office and additional equipment. POTS ports and Ethernet hubs are the latest additions to ISDN routers. POTS, in general, gives the user two analog phone numbers that they can use to make and answer calls, transfer data and send or receive faxes on the fly. Any two of these transactions can happen simultaneously, and all are transparent to the user. In addition to the frosted-coating of built-in POTS ports and hubs, routers must provide the vitamins and min erals essential for a well-balanced and cost-effective home office. Assuming your equipment is compatible with your switch type and ISDN parameters--and most are--adherence to standards is the most important thing to look for in ISDN routers. These standards let your hardware interoperate with equipment residing at an Internet service provider (ISP) or at other corporate sites. Adherence to standards also may help save on cost of operation. Setting the Standard The Multilink Point-to-Point Protocol (PPP) lets two connecting units negotiate and combine multiple channels (in ISDN's case, the two "B" channels) to serve as one larger pipe with expanded bandwidth. Thus, two 64-Kbps ISDN channels can be combined to give two connecting locations an aggregate bandwidth of 128 Kbps. Multilink PPP also lets your equipment add and drop channels on the fly to avoid extra costs when usage is low and extra channels are unneeded. There are proprietary link implementations on the market, but support for Multil ink PPP ensures interoperability with all carrier equipment. Multilink PPP is a must, and most ISDN routers support it. The Bandwidth Allocation Control Protocol (BACP), a standard to look for in routers, controls the addition and subtraction of channels from the link. Although Multilink PPP is a more advanced method of managing bandwidth, BACP determines how channels should be added or subtracted to change the bandwidth of multichannel sessions between peer equipment. To download an Adobe Acrobat .pdf format version of the ISDN Routers Buyer's Guide charts, click here.

Updated May 12, 1997