telephone wires is 33,600 bps. (Fast modems are designed to automatically
step downto the transmit and receive at the fastest rate available across the
entire connection from your home or office to the phone company’s switch-
ing office, and then across the city or country or around the world to another
local switching office, and from there on the local loopto the destination
home or office.)One large advantage of telephone modems is that they are dial-updevices.
That means that you can call any other computer that has a modem, or make
your choice among various connection points for ISPs(Internet service
providers). That also makes them highly portable; a dial-up modem in your
laptop can connect to any standard analog telephone line while you are on
the road, and dial up to your home office or to a connection point for an ISP.Consumer modems are asynchronousdevices, meaning that the stream of
data is not dependent on timing. Words of computer data are surrounded by
additional start and stop bits that identify them. By contrast, specialized
modems used in certain direct connections or over leased lines may be
synchronous,meaning that data marches to the beat of a clock and therefore
does not require some of the extra bits used in consumer models.Be careful before you attach a standard dial-up modem to an office or hotel
phone system. Within a modern structure, the phone system may be digital
or use a nonstandard design up to the point where it connects to the POTS
and the rest of the world. If you have any doubt about the design of a phone
system, ask before you plug in. A mismatch could result in frying your modem
and possibly causing damage to your laptop.Cable modems ....................................................................................
Today, for many users the best way to get past the limitations of a telephone
modem is to instead use a cable modem. The beauty of this design lies in the
fact that it does not in any way have to work around Bell’s old copper wires.228 Part IV: Failing to Communicate
Web enabler
Adding in necessary bits to mark the begin-
ning and end of 8-bit computer words plus
an error checking bit, the net throughput of a
300 bps modem might be equivalent to about 27bytes per second. If modems had not advanced
beyond that point, the World Wide Web would
never have been possible.