Page 123 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Exposure to Computer Disciplines

Notes In such situations a small amount of memory in the modem, a buffer, is used to hold the data
while it is being compressed and sent across the phone line, but in order to prevent overflow
of the buffer, it sometimes becomes necessary to tell the computer to pause the data stream.
This is accomplished through hardware flow control using extra lines on the modem-computer
connection. The computer is then set to supply the modem at some higher rate, such as 320 kbit/s,
and the modem will tell the computer when to start or stop sending data.

6.3.1.10 Compression by the ISP

As telephone-based 56k modems began losing popularity, some Internet service providers such
as Netzero and Juno started using pre-compression to increase the throughput and maintain their
customer base. As example, the Netscape ISP uses a compression program that squeezes images,
text, and other objects at the modem server, just prior to sending them across the phone line.
Certain content using lossy compression (e.g., images) may be recompressed (transcoded) using
different parameters to the compression algorithm, making the transmitted content smaller but
of lower quality. The server-side compression operates much more efficiently than the on-the-
fly compression of V.44-enabled modems due to the fact that V.44 is a generalized compression
algorithm whereas other compression techniques are application-specific (JPEG, MPEG,
Vorbis, etc.). Typically Website text is compacted to 4% thus increasing effective throughput to
approximately 1,300 kbit/s. The accelerator also pre-compresses Flash executables and images
to approximately 30% and 12%, respectively.
The drawback of this approach is a loss in quality, where the GIF and JPEG images are lossy compressed,
which causes the content to become pixelated and smeared. However the speed is dramatically
improved such that Web pages load in less than 5 seconds, and the user can manually choose to
view the uncompressed images at any time. The ISPs employing this approach advertise it as “surf
5× faster” or simply “accelerated dial-up”.

6.3.1.11 List of Dialup Speeds
Note that the values given are maximum values, and actual values may be slower under certain
conditions (for example, noisy phone lines). For a complete list see the companion article list of
device bandwidths. A baud is one symbol per second; each symbol may encode one or more
data bits.
Connection Bitrate (kbit/s) Year Released

110 baud Bell 101 modem 0.1 1958
300 baud (Bell 103 or V.21) 0.3 1962

1200 modem (1200 baud) (Bell 202) 1.2
1200 Modem (600 baud) (Bell 212A or V.22) 1.2

2400 Modem (600 baud) (V.22bis) 2.4
2400 Modem (1200 baud) (V.26bis) 2.4

4800 Modem (1600 baud) (V.27ter) 4.8
9600 Modem (2400 baud) (V.32) 9.6

14.4k Modem (2400 baud) (V.32bis) 14.4
28.8k Modem (3200 baud) (V.34) 28.8

116 LOVELY PROFESSIONAL UNIVERSITY