Bridge - an explanation

A device that connects two or more physical network components and forwards frames which have source and destination addresses on different network components.Today’s access router or brouter supports everything from asynchronous modem dial-up to X.25, leased line, ISDN, frame relay and increasingly SMDS (Switched Multimegabit Data Services) and ATM (Asynchronous Transfer Mode).

However, because the data connection is rarely required to be up all day - it may indeed be active for only a couple of seconds at a time to transfer e-mail messages - ISDN is often the preferred solution for Internet connectivity.

Basic Rate ISDN (BRI) actually comprises two 64Kbps (or 56Kbps) lines (the "B" - or "Bearer" - channels) for voice, image and data communications, plus a separate 16k "D" - or "Data" - channel. Apart from the obvious improvement in bandwidth over standard modem connections, ISDN also offers extremely fast call set-up times of just a second or two (and often less). This is a great boon for e-mail transfer, when the amount of time to set up a call using modems can be far greater than the actual time needed to transfer the data.

The 128Kbps from the combined B-channels is not the end of the story however. With many of the products available it is possible to aggregate multiple ISDN lines, so achieving bandwidths of 640Kbps and more. However, from a cost point of view, you need to be careful when establishing whether Basic Rate (2x64Kbps connections) or Primary Rate (30x64Kbps connections) ISDN is what you need.

Currently the cross-over point between the two occurs at about five Basic Rate lines. So once above this requirement it is more cost effective to install a Primary Rate service whether you need all 30 channels or not. Many PTO’s now provide primary rate ISDN in smaller "packs" of 6, 8, 10, 15 or 25 channels at a time, and will often allow you to increase your bandwidth in single 64Kbps channel increments.

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