Page 120 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 6: Data Communication

6.3.1.4 Error Correction and Compression Notes
Operations at these speeds pushed the limits of the phone lines, resulting in high error rates. This
led to the introduction of error-correction systems built into the modems, made most famous with
Microcom’s MNP systems. A string of MNP standards came out in the 1980s, each increasing the
effective data rate by minimizing overhead, from about 75% theoretical maximum in MNP 1, to
95% in MNP 4. The new method called MNP 5 took this a step further, adding data compression
to the system, thereby increasing the data rate above the modem’s rating. Generally the user could
expect an MNP5 modem to transfer at about 130% the normal data rate of the modem. Details of
MNP were later released and became popular on a series of 2,400-bit/s modems, and ultimately
led to the development of V.42 and V.42bis ITU standards. V.42 and V.42 bis were non-compatible
with MNP but were similar in concept: Error correction and compression.

Another common feature of these high-speed modems was the concept of fallback, or speed
hunting, allowing them to talk to less-capable modems. During the call initiation the modem
would play a series of signals into the line and wait for the remote modem to respond to them.
They would start at high speeds and progressively get slower and slower until they heard an
answer. Thus, two USR modems would be able to connect at 9,600 bit/s, but, when a user with a
2,400-bit/s modem called in, the USR would fallback to the common 2,400-bit/s speed. This would
also happen if a V.32 modem and a HST modem were connected. Because they used a different
standard at 9,600 bit/s, they would fall back to their highest commonly supported standard at
2,400 bit/s. The same applies to V.32 bis and 14,400 bit/s HST modem, which would still be able
to communicate with each other at only 2,400 bit/s.
6.3.1.5 Breaking the 9.6k Barrier

In 1980, Gottfried Ungerboeck from IBM Zurich Research Laboratory applied powerful channel
coding techniques to search for new ways to increase the speed of modems. His results were
astonishing but only conveyed to a few colleagues. Finally in 1982, he agreed to publish what is
now a landmark paper in the theory of information coding. By applying powerful parity check
coding to the bits in each symbol, and mapping the encoded bits into a two-dimensional diamond
pattern, Ungerboeck showed that it was possible to increase the speed by a factor of two with the
same error rate. The new technique was called mapping by set partitions (now known as trellis
modulation).

Error correcting codes, which encode code words (sets of bits) in such a way that they are far from
each other, so that in case of error they are still closest to the original word (and not confused with
another) can be thought of as analogous to sphere packing or packing pennies on a surface: the
further two bit sequences are from one another, the easier it is to correct minor errors.
V.32bis was so successful that the older high-speed standards had little to recommend them.
USR fought back with a 16,800 bit/s version of HST, while AT&T introduced a one-off 19,200
bit/s method they referred to as V.32ter (also known as V.32 terbo or tertiary), but neither non-
standard modem sold well.
6.3.1.6 V.34/28.8k and 33.6k

Any interest in these systems was destroyed during the lengthy introduction of the 28,800 bit/s
V.34 standard. While waiting, several companies decided to release hardware and introduced
modems they referred to as V.FAST. In order to guarantee compatibility with V.34 modems once
the standard was ratified (1994), the manufacturers were forced to use more flexible parts, generally
a DSP and microcontroller, as opposed to purpose-designed ASIC modem chips.

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