ISDN vs. modems

A single ISDN line can support a wire speed up to 128K bit/sec, which is more than four times faster than a V.34 modem running at its highest possible speed. Compression can double this to approximately 250K bit/sec for many files. ISDN does this with much higher reliability and much faster connection times than a modem.

ISDN supports 64K or 56K bit/sec synchronous connections. The serial interfaces on most personal computers are not capable of this. Most PCs use asynchronous communications and cannot support high-speed synchronous connections.

The International Telecommunication Union's telecommunications standards division has defined rate adaptation protocols, including V.120, to support both synchronous and asynchronous data over an ISDN connection. V.120 takes the data from the PC and formats it into High-Level Data Link Control (HDLC) packets and transmits them across the ISDN line. It supports flow control so the interface speeds on the two sides of the connection do not have to be the same. V.120 is commonly used to make a direct ISDN connection to host computers and bulletin boards.

Analog modems cannot compete with ISDN performance. When V.42 error control is not used, start and stop bits are sent with each character. Thus, an eight-bit character would take 10 bits to transmit. Top-speed V.34 modems use V.42 error control. When V.42 error control is used, no start or stop bits are sent over the line. The asynchronous bytes are stored until a standard packet size is reached or a timeout elapses. Then theyre wrapped in V.42 - or Link Access Protocol for Modems - headers and sent synchronously.

A very similar scheme called V.120 is used with ISDN. The main difference is that V.120 overhead might be a few bytes greater. These bytes carry information to emulate an analog connection, such as whether the carrier is present or if the break has been sent. These two schemes have the following advantages:

Throughput enhancement over modems by a ratio of 10-to-8 (for eight-bit payloads) because start and stop bits don't go down the wire.

Error detection, since the HDLC framing gives a 2-byte cyclic redundancy check (CRC).

Error correction, via a mechanism for block retransmission.

They also have a couple of disadvantages:

Latency - When keystroking, the modem may wait to see if more characters are forthcoming before sending or may just have slow store-and-forward code.

Overhead - Each block, which may potentially be as small as a single character of payload, may be surrounded by leading flag, address byte, command byte, two CRC bytes and trailing flag for big expansion. Obviously, file transfer blocks aren't each expanded, but if the V.42 block size is 256 bytes, you still have six overhead bytes for each 256 bytes.

The next step for modem technology appears to be V.34bis, which may increase the wire speed to 33.6K bit/sec. We wonder how effective this will be when many V.34 connections don't achieve a full 28.8K bit/sec capability on typical phone lines today.

Another important consideration in comparing modems and ISDN is the use of compression. This time, modems have the edge. V.34 modems include V.42bis compression, which can improve throughput by a factor of two on many files. The ISDN products in this test did not include compression, but vendors are planning on adding this capability. IBM, for instance, is scheduled to offer V.42bis compression over V.120 for the IBM WaveRunner by the end of this month.

Readers can get these telecommunications specs for free. Gopher to info.itu.ch, or send E-mail to itudoc@itu.ch containing the single word "help" in the body.

By Bob Larribeau and Lawrence Marks