Page 121 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Exposure to Computer Disciplines

Notes
Figure 6.6: An ISA Modem Manufactured to Conform to the V.34 Protocol

Today, the ITU standard V.34 represents the culmination of the joint efforts. It employs the most
powerful coding techniques including channel encoding and shape encoding. From the mere
4 bits per symbol (9.6 kbit/s), the new standards used the functional equivalent of 6 to 10 bits
per symbol, plus increasing baud rates from 2,400 to 3,429, to create 14.4, 28.8, and 33.6 kbit/s
modems. This rate is near the theoretical Shannon limit. When calculated, the Shannon capacity
of a narrowband line is, with the (linear) signal-to-noise ratio. Narrowband phone lines have a
bandwidth from 300-4000 Hz, so using (SNR = 30dB): capacity is approximately 35 kbit/s.
Without the discovery and eventual application of trellis modulation, maximum telephone
rates using voice-bandwidth channels would have been limited to 3,429 baud * 4 bit/symbol ==
approximately 14 kbit/s using traditional QAM. (DSL makes use of the bandwidth of traditional
copper-wire twisted pairs between subscriber and the central office, which far exceeds that of
analog voice circuitry.)

6.3.1.7 V.61/V.70 Analog/Digital Simultaneous Voice and Data
The V.61 Standard introduced Analog Simultaneous Voice and Data (ASVD). This technology
allowed users of v.61 modems to engage in point-to-point voice conversations with each other
while their respective modems communicated.

In 1995, the first DSVD (Digital Simultaneous Voice and Data) modems became available to
consumers, and the standard was ratified as v.70 by the International Telecommunication Union
(ITU) in 1996.
Two DSVD modems can establish a completely digital link between each other over standard phone
lines. Sometimes referred to as “the poor man’s ISDN,” and employing a similar technology, v.70
compatible modems allow for a maximum speed of 33.6 kbps between peers. By using a majority
of the bandwidth for data and reserving part for voice transmission, DSVD modems allow users
to pick up a telephone handset interfaced with the modem, and initiate a call to the other peer.
One practical use for this technology was realized by early two player video gamers, who could
hold voice communication with each other while in game over the PSTN.
Advocates of DSVD envisioned whiteboard sharing and other practical applications for the
standard, however, with advent of cheaper 56kbps analog modems intended for Internet
connectivity, peer-to-peer data transmission over the PSTN became quickly irrelevant. Also, the

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