Modem Speeds
A modem can be judged by the
speed to transfer data per second. Today's recommended speed of a modem is
56k (56,600bps), although older modems use 33K (33,600bps). Remember the
faster the modem the less time spent waiting for Web pages to load, and of
course will also reduce your telephone bill! There are faster MODEM
connections like ISDN a dual line system that can give you speeds up to
112bps but currently these are more expensive to install and rent.
Often, one modem in a
connection is capable of sending data much faster than the other can
receive. Flow control allows the receiving modem to tell the other to pause
while it catches up. Flow control exists as either software, or XON/XOFF,
flow control, or hardware (RTS/CTS) flow control. With software flow
control, when a modem needs to tell the other to pause, it sends a certain
character, usually Control-S. When it is ready to resume, it sends a
different character, such as Control-Q. Software flow control's only
advantage is that it can use a serial cable with only three wires. Since
software flow control regulates transmissions by sending certain characters,
line noise could generate the character commanding a pause, thus hanging the
transfer until the proper character (such as Control-Q) is sent. Also,
binary files must never be sent using software flow control, as binary files
can contain the control characters. Hardware, or RTS/CTS, flow control uses
wires in the modem cable or, in the case of internal modems, hardware in the
modem. This is faster and much more reliable than software flow control.
Traditional modem standards
assume that both ends of a modem session have an analog connection to the
public switched telephone network. Data signals are converted from digital
to analog and back again. Limiting transmission speeds to 33.6Kbps (Thousand
Bits Per Second) with current V.34 modems, and due to limitations of the
public telephone network the theoretical maximum is 35Kb
Now with V.90 technology a
different assumption is made: that one end of the modem session has a
pure-digital connection to the phone network (which Internet Service
Providers and corporations already use for remote access) and takes
advantage of that high speed digital connection.
By viewing the public switched
telephone network as a digital network, V.90 technology is able to
accelerate data downstream from the Internet to your computer at speeds of
up to 56Kbps *. In this way, V.90 technology is different from other
standards of today, because it digitally encodes downstream data instead of
modulating it as analog modems do. The data transfer is an asymmetrical
method, so upstream transmissions (mostly keystroke and mouse commands from
your computer to the central site, which require less bandwidth) continue to
flow at the conventional rates of up to 33.6Kbps. That is upstream data
(Data sent from your modem) is sent as an analog transmission that mirrors
the V.34 Standard. Only the down stream data transfer takes advantage of the
high-speed V.90 rates
V.90 technology is ideal for
Internet users, because you really need the 56Kbps speed for downloading of
Web pages with sound, video and other large files. All that is needed is for
your V.90 modem to be connected to an ISP or corporate site using V.90
technology over their digital lines to the network.
Modems can receive data at
speeds of up to 56Kbps however due to FCC (Federal Communications
Commission) rulings on maximum permissible transmit power levels during down
load transmissions; speeds of 54Kbps are the maximum. Actual data speeds
received will vary depending on line conditions.
V.90 also know as V.PCM (Pulse
Coded Modulation) does not make the assumption of the V.34 and other early
analog standards. V.90 assumes there is only 1 analog portion of the
downstream transmission path (the upstream data conforms to the V.34
standard). This assumption of only 1 analog segment is good for most areas
where +80% of the locations connected have a digital connection to the
network. Most all ISP’s (Internet Service Providers) and Corporate Accounts
are digitally connected to the network. In this way the entire data path is
operating at a 64Kbps (8 bit samples @ 8000 Samples per Second) data stream
and is only throttled back at your local analog loop that runs from your
house to your local Central Office (CO). If we view this local loop going
from your house as a "Noisy" Digital path, then we can enjoy the high data
rates of V.90. |
