Page 122 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 6: Data Communication

standard was never expanded to allow for the making or receiving of arbitrary phone calls while Notes
the modem was in use, due to the cost of infrastructure upgrades to telephone companies, and
the advent of ISDN and DSL technologies which effectively accomplished the same goal.
Today, Multi-Tech is the only known company to continue to support a v.70 compatible modem.
While their device also offers v.92 at 56kbps, it remains significantly more expensive than
comparable modems sans v.70 support.
6.3.1.8 Using Digital Lines and PCM (V.90/92)

In the late 1990s Rockwell/Lucent and U.S. Robotics introduced new competing technologies
based upon the digital transmission used in modern telephony networks. The standard digital
transmission in modern networks is 64 kbit/s but some networks use a part of the bandwidth
for remote office signaling (e.g., to hang up the phone), limiting the effective rate to 56 kbit/s
DS0. This new technology was adopted into ITU standards V.90 and is common in modern
computers. The 56 kbit/s rate is only possible from the central office to the user site (downlink).
In the United States, government regulation limits the maximum power output, resulting in a
maximum data rate of 53.3 kbit/s. The uplink (from the user to the central office) still uses V.34
technology at 33.6 kbit/s.

Figure 6.7: Modem Bank at an ISP

Later in V.92, the digital PCM technique was applied to increase the upload speed to a maximum
of 48 kbit/s, but at the expense of download rates. For example a 48 kbit/s upstream rate would
reduce the downstream as low as 40 kbit/s, due to echo on the telephone line. To avoid this
problem, V.92 modems offer the option to turn off the digital upstream and instead use a 33.6
kbit/s analog connection, in order to maintain a high digital downstream of 50 kbit/s or higher.
V.92 also adds two other features. The first is the ability for users who have call waiting to put
their dial-up Internet connection on hold for extended periods of time while they answer a call.
The second feature is the ability to quickly connect to one’s ISP. This is achieved by remembering
the analog and digital characteristics of the telephone line, and using this saved information to
reconnect at a fast pace.

6.3.1.9 Using Compression to Exceed 56k
Today’s V.42, V.42bis and V.44 standards allow the modem to transmit data faster than its basic
rate would imply. For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3*6 ==
320 kbit/s using pure text. However, the compression ratio tends to vary due to noise on the
line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG
video). At some points the modem will be sending compressed files at approximately 50 kbit/s,
uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between.

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