This article originally appeared in TidBITS on 1997-09-25 at 12:00 p.m.
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Question: Why Aren't I Seeing Full ISDN Speeds?

by Glenn Fleishman

Question: Why Aren't I Seeing Full ISDN Speeds? I'm using two-channel ISDN, and I should see nearly 128 Kbps. Instead, I'm only getting about 70 Kbps at the fastest. What's going on?

Answer: When you order ISDN in the U.S., you typically receive two-channel Basic Rate Interface (BRI) ISDN. The two channels each operate like separate phone lines. Each channel can carry 56 Kbps or 64 Kbps - the actual number depends on how each phone company has implemented ISDN - but the two channels are independent unless you use the right protocol.

That's why you're maxing out at 60 to 70 Kbps for each of your two open connections. Since there's no way to control which channel the data traverses, you might end up with one sitting idle while the other is clogged.

MPPP (Multi-link Point-to-Point Protocol) "binds" the two separate channels into one virtual channel, so you can take advantage of the full 112 or 128 Kbps available. MPPP must be built into the ISDN device on your end of the connection and at the service provider or company on the other end. Your computers shouldn't have to be reconfigured to make MPPP work.

If you use an ISDN device plugged into your computer's serial port, you might encounter a different problem. Many serial ports can't support speeds faster than 115 Kbps. Unfortunately, the only solution is to upgrade. If you use a PC, you can buy a special ISA serial card; if you use a Mac, you need a more recent model (Quadra 660AV and newer, generally) that uses the 230 Kbps GeoPort-based serial port technology.