Telebit

Telebit Corporation was a US-based modem manufacturer, known for their TrailBlazer series of high-speed modems. One of the first modems to routinely exceed 9600 bit/s speeds, the TrailBlazer used a proprietary modulation scheme that proved highly resilient to interference, earning the product an almost legendary reputation for reliability despite mediocre (or worse) line quality. They were particularly common in Unix installations in the 1980s and 1990s.

The high price of the Telebit modems was initially not a concern as their performance was equally high compared to other systems. However, as new designs using V.32 and V.32bis began to arrive in the early 1990s, Telebit's price/performance ratio was seriously eroded. A series of new designs followed, but these never regained their performance lead. By the mid-1990s the company had been part of a series of mergers and eventually disappeared in 1998 after being acquired by Digi International.

Startup

Telebit was founded by Paul Baran, one of the inventors of the packet switching networking concept. Baran had recently started a networking company known as Packet Technologies on Bubb Road in Cupertino, California, which was working on systems for interactive television. While working there, he hit on the idea for a new way to implement high-speed modems, and started Telebit across the street. Packet Technologies was a major beta customer for Telebit in late 1985. Packet Technologies later failed, and several of their employees were folded into Telebit, while most of the others formed StrataCom, makers of the first Asynchronous Transfer Mode (ATM) switches.

PEP and the TrailBlazer

In contrast to then-existing ITU Telecommunication Standardization Sector (ITU-T) V-series protocols, such as the common 2400 bit/s V.22bis, the TrailBlazers' used a proprietary modulation system known as Packetized Ensemble Protocol (PEP), based on orthogonal frequency-division multiplexing (OFDM). It 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 in steps of 10 bps with decreasing line quality.

Using a large number of carriers spread across the phone network's bandwidth meant that the chance that any one carrier would be subject to a problem was high. In order to correct for the unavoidable errors this would cause, the Trailblazers were one of the earlier implementations of the MNP error-correcting protocols. Although these protocols added overhead, and errors caused further overhead, the combination of all of these features still provided much higher throughput than conventional designs running on the same lines. In contrast, something like a 2400 bit/s modem might suffer from a smaller number of errors in a given time, but those errors would require longer to fix through re-transmission at the slower speed. If those errors were being caused by a constant source of noise on those frequencies, PEP would simply turn those carriers off, while the 2400 bit/s modem could do nothing about this.

Most modems of the era were set up with both channels with equal speed (full duplex), or, in the case where data was primarily sent in one direction, with a single high-speed channel ( half duplex). The TrailBlazer instead allowed any one of its 512 channels to be assigned to transfer data in either direction, a technique they termed "adaptive duplex". 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 digital signal processors (DSP) take turns using the bandwidth tended to 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 from the receiver ("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 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 protocols to talk to the remote modem. In general, spoofing worked well with any protocol that used small packets, and thus generated many messages; the TrailBlazers initially supported UUCP, and support for XMODEM, YMODEM, SDLC and Kermit followed.

Support for these features did not come cheaply; the TrailBlazer Plus, for instance, used a Texas Instruments TMS32010 DSP processor for the actual modulation and demodulation functions, and a Motorola 68000 for control. This meant that the TrailBlazers were significantly more expensive than most other modems. However, their speed and spoofing ability made the TrailBlazer modems extremely popular in the Unix world, as they could dramatically improve UUCP throughput, even at low connection speeds on very noisy lines. Improvements of over seven 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. The original TrailBlazer, T1000 and T2000 were backwards-compatible with the 2400 bit/s V.22bis standard, allowing them to connect with what was then the most common modem speed when talking to other brands of modems.

NetBlazer

Another Telebit product was the first on-demand Internet dialup router, the NetBlazer. The product was developed by a team led by Mike Ballard, formerly of Packet, who eventually became Telebit's CEO in 1992.

The original NetBlazer was a standard PC with a serial card connected to user supplied external modems by serial cables. Later smaller versions, the PN and STi, were offered which consisted of a small-form-factor PC combined with custom software and one of a variety of modems or other connection systems (ISDN, etc.) combined into a large modem-like box. Administrators connected to it via Ethernet, which was also used for maintenance commands and setup.

In its first release the NetBlazer software supported TCP/IP using SLIP, but a later upgrade added Point-to-Point Protocol (PPP) and support for IPX and AppleTalk.
The protocol stack was a commercially licensed and heavily modified version of KA9Q. A later low-end model, the NetBlzer LS, switched from the Intel 80386

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