dial-up experiences (was Re: question aaout ISP's)

Eric Smith eric at brouhaha.com
Fri Aug 8 14:17:36 CDT 2008


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.



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