PEP and The TrailBlazer
In contrast to then-existing ITU V-series protocols, notably the common 2400 bit/s V.22bis, the TrailBlazers' proprietary PEP (Packetized Ensemble Protocol) modulation employed a large number (initially up to 512) of closely spaced carrier frequencies, each modulated at 6 baud, encoding 0, 2, 4 or 6 bits per interval. Under favorable conditions, the devices could reach data rates of 6 baud x 6 bits-per-baud x 512 carriers = 18432 bits per second. If a particular carrier was distorted, attenuated or interfered with, it could be turned off, allowing the data rate to degrade gracefully with decreasing line quality.
The Trailblazer's high data rate mode was available in one direction only. At a time when modems were actually pretty simple devices and used modulation techniques labelled either "full duplex" or "half duplex", the Trailblazer was termed internally an "adaptive duplex modem". The modem was designed to use most of the bandwidth in a single direction, with a relatively low speed reverse channel. The modems at the two ends of the connection would negotiate line turnarounds, reversing the directions of the high-speed and low-speed channels, based on the amount of data queued for transmission in each modem.
While this adaptive duplex scheme was able to send large files quickly, for users accustomed to having the distant computer echo characters, the delay associated with having the DSPs take turns using the bandwidth tended make interactive typing difficult, as there could be as much as a second and a half delay for a single character echo. This also caused problems for file transfer protocols, e.g., UUCP 'g' or Kermit, where a small packet of data was sent by one computer, followed by a wait for acknowledgment ("send and wait").
The TrailBlazer addressed this problem through a technique known as "protocol spoofing". When the local computer sent a packet to the modem for transmission, the modem's controller immediately sent an ACK message generated locally. This fooled the computer into thinking the packet had already reached the far end, prompting it to send another packet. The error correction normally being applied in the protocol was instead handled using a proprietary replacement protocol operating on top of the MNP error-correcting protocols to talk to the remote modem. In general, spoofing worked well with any protocol that used small packets, and thus generated lots of ACK messages; support for XModem and Kermit followed.
Support for these features did not come easily; the TrailBlazer Plus, for instance, used a Motorola 68000 to drive its electronics. This meant that the TrailBlazers were generally very expensive. However, its spoofing ability made the TrailBlazer modems extremely popular in the Unix world, as it could dramatically improve UUCP throughput, even at low connection speeds on very noisy lines. Improvements of over 7 times faster than a 2400 bit/s modem were not uncommon. Sites that required long-distance telephone calls to exchange UUCP mail could pay for the price of a TrailBlazer in long-distance savings fairly quickly.
The Trailblazers also introduced an extensive set of commands for setting up its various options. While most of the simple commands were based on the Hayes command set, like dialing a number or hanging up a phone, their proprietary capabilities were supported by proprietary commands and syntax. Most of these took the form of register=value pairs, leading to extremely long and almost undecipherable setup strings.
In 1988 Telebit added the T1000, essentially a TrailBlazer limited to a lower-speed 9600 bit/s version of PEP, remaining compatible at that speed with existing TrailBlazers. The T2000 added support for synchronous communications, typically used between mainframe computers.
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