ISDN has a latency
of about 10 ms. Its throughput may be twice that of a modem, but its
latency is ten times better, and that's the key reason why browsing
the Web over an ISDN link feels faster than over a modem.
One reason standard
modems have such poor latency is that they don't know what you're doing
with your computer, or why. An external modem is usually connected through
a serial port, and all it sees is an unstructured stream of bytes coming
down the serial port.
much-maligned Apple GeoPort Telecom Adapter may solve this problem.
The Apple GeoPort Telecom Adapter connects your computer to a telephone
line, but it's not a modem. Instead, all modem functions are performed
by software running on the Mac. The main reason for the criticism is
that this extra software takes up memory and slows down the Mac, but
in theory it "could" offer an advantage no external modem
could match. When you use the GeoPort Telecom Adapter, the modem software
is running on the same CPU as your TCP/IP software and your Web browser,
so it could know exactly what you are doing. When your Web browser sends
a TCP packet, the GeoPort modem software doesn't have to mimic the behaviour
of current modems. It could take that packet, encode it, and send it
over the telephone line immediately, with almost zero latency.
Sending 36 bytes
of data, a typical game-sized packet, over an Apple GeoPort Telecom
Adapter running at 28.8 Kbps could take as little as 10 ms, making it
as fast as ISDN, and ten times faster than the best modem you can buy
today. For less than the price of a typical modem, the GeoPort Telecom
Adapter could give you Web browsing performance close to that of ISDN.
Even better, people who already own Apple GeoPort Telecom Adapters would
need only a software upgrade.