8 Aug
2008
8 Aug
'08
12:17 p.m.
Chuck Guzis wrote:
The Trailblazers at one point were *the* modem to have
if you were
doing UUCP-type transfers between systems. Since it used a
proprietary protocol, you had to go Telebit-to-Telebit. I had
friends at Telebit, and vaguely remember that they used multiple
carrier frequencies, but not too much other than that.
Although the Trailblazer transferred about 14 to 16 Kbps uncompressed,
it was a 7 baud modem! [*] It used slightly fewer than 512 carriers,
each transmitting at a very low rate.
The system was referred to as Packetized Ensemble Protocole (PEP),
although technically that described the patented method of distributing
the data across the carriers rather than the actual modulation. It was
the predecessor of DMT (Discrete Multi Tone) modulation used on most
ADSL loops, and OFDM (Orthogonal Frequency Domain Multiplexing) used by
802.11a, .11g, .11n, and many other high-bandwidth wireless systems. At
one time Telebit management thought they might be able to get royalties
on the PEP patent from the companies making ADSL equipment, but AFAIK
that never happened.
Since PEP was half-duplex, with only a very slow reverse channel, it had
to do line turnarounds to handle full-duplex traffic. UUCP g protocol
had acknowledgement frames that were large enough to cause a line
turnaround, which would dramatically lower the throughput for an
otherwise unidirectional transfer. Telebit put special "spoofing" for
the UUCP g protocol into the model, so that it could avoid line
turnarounds while still having end-to-end acknowledgements. That made
it a great modem for UUCP.
When the CCITT (now ITU-T) was working on the next high-speed PSTN modem
standard beyond V.32, referred to as V.fast while in development,
Telebit proposed a full-duplex version of PEP, but it was not ultimately
chosen to be the V.34 standard.
I worked at Telebit from 1991 to 1995, but mostly on the NetBlazer
router products rather than modems. From time to time the lab had to be
cleaned up, and one time my manager gave me an enormous wire-wrapped
board (perhaps 24" by 30") which he couldn't bear to see thrown away.
He said it was the first PEP modem prototype, and he knew that I was
interested in computer history, so he wanted me to take it. A few years
ago I met Paul Baran, founder of Telebit (and inventor of packet
switching), and he authenticated it. I donated it to the Computer
History Museum.
Eric
[*] A "baud" is a symbol (signal change) per second, not a bit per
second. They are equal only if each symbol conveys a single bit.
However, all dialup modems at 2400 bps or higher, and full-duplex dialup
modems at 1200 bps, use modulations with multiple bits per symbol. For
instance, V.32 9600 bps modulation is actually 2400 baud with four bits
per symbol.