Zenith Cable Modem was one of the first proprietary cable modems. The two basic models are one operating at 500 kilobits per second (Kbit/s), and the other at four megabits per second (Mbit/s) with BPSK and approximately a 25% alpha.
History The Zenith Cable Modem was originally developed for a mid-split cable television network in the mid-1980s. It was used as an 8-bit full-height PC/AT-type card containing an Intel 80186 dedicated CPU, connected to an external white box about 2" x 12" x 6". Other similar products were made by Ungermann-Bass (UB) under the 10BROAD36 standard and Vital ink. UB had models supporting RS-232 and Ethernet outputs, as well as a re-modulating frequency translator. In late 1993, Zenith Electronics and Prodigy provided 12 modified 500 Kbit "white modems" to Cox Communications in San Diego, including two with IBM Microchannel support. These modified modems were intended to support the Prodigy Cable Modem trial, which began on a 1500 homes-passed fiber node in El Cajon, CA. The modification allowed sub-split operation, with a fixed upstream frequency and a downstream at 74.75 MHz, within the 4 MHz space between analog channels four and five. The initial trial consisted of a Prodigy server in the El Cajon headend, connected via the Microchannel-based cable modem to an Olsen Frequency Translator. This basic network supported the 1500 home passed fiber nodes, with six "subscribers" including one employee of Cox, who was also the head-end manager. One card was installed in rackmount PC in the Federal headend, another in the El Cajon headend. 1500 high pass filters were installed to eliminate any ingress from the drops. Service was reasonably reliable. Zenith updated their white modems to a matte black case, adding the Homework name and marketing to cable operators at the $350 price range for 250 units. The new design used the same size external case, added LEDs to indicate power/TX/RX/activity and replaced the full sized 8-bit ISA card with a smaller 16-bit version. The new design dropped the onboard CPU. The modem connected to the card used a 15-pin D-shell connector—which exactly matched the PC's game connector. Both models were powered by the PC. The new modem was frequency agile, with a configuration utility that ran on the PC to set up US and DS frequencies. The option to configure the card's MAC address was soon dropped. However, the card's MAC address was not printed on the outside of the board, and thus was invisible once installed into the PC. Nor was the MAC address printed in a machine readable (bar code) format. Cox added these to the CM prior to installation, and tracked the subscriber to modem MAC address in an Excel spreadsheet, as the MAC address contained too many digits to fit into any fields within the customer billing system.
Product versions White modem, from 1980s based design Black internal modem, Homework External modem, one Ethernet jack External modem, two Ethernet jacks HW revision A: first black case modem HW revision B: volume production of an estimated 750–1000 units for shipment to 8–10 cable operators HW revision C: introduced a two board model and 4" tall case that would allow use with a traditional Ethernet 10BASE-T connection instead of an internal PC card. The external models used an RJ-14 style phone jack for serial control, configuration, and management using a Zenith supplied utility application. HW revision D: eliminated the susceptibility to RF interference from nearby cell phones. The fault caused the transmit side phased locked loop (PLL) to fail. Resolution required a power cycle, fault was non-detectable in software other than no connectivity. HW revision E: introduced dual 10BaseT interface for University of California, San Diego, and other customers. Added modem MAC address to external CM, and in a bar code readable format. All Zenith modems contained a small switch controlling the second coax port. The modem could either operate on one coax (built in diplex filter) or two (Headend use, dual plant networks, or external diplex filter.) The initial Zenith "Headend" design was to use a residential cable modem inside a server, and a frequency translator to convert the upstream transmit frequency to the downstream receive frequency. This design did not scale to more than a few networks, so the Channelizer was introduced—a 2.5" tall rack mountable device with separate US and DS RF ports, the diplex filter switch, and an Ethernet port. The first large scale Zenith headend was constructed in Cox San Diego using Zenith's booth demonstration hardware from the 1994 Western Cable Show. Three Frequency Translators, plus four Channelizers were installed in El Cajon. These plus three other Channelizers were interconnected to an 8 port Cisco Catalyst 1200 Ethernet switch with single mode FDDI uplink for a 4 device, 140 km FDDI ring around San Diego county. The Prodigy server obtained a traditional Ethernet card. The seven frequency translators plus seven Channelizers supported 65,000 homes passed on 23 fiber nodes, and occupied a full rack.
Technology demonstration to initial deployment
Prodigy was deployed over cable modem to 200 users in time for the 1994 Western Cable Show. To grow the system from 16 users in three fiber nodes, to 200 required adding what became 64,000 homes passed, making the Cox San Diego–Prodigy cable modem field trial the world's largest cable modem deployment (by service area) at the time. The acceptance criteria were too strict to achieve the 65% homes passed high speed internet penetration which is common today:
Prodigy Subscriber greater than nine months Cable Subscriber Live in 2-way activated area Open slot in 80386 based PC or greater Willingness to participate, when called between December 15, 1994, and January 3, 1995 The result was a population of 1/4 of 1% of the homes passed, or 64,000 homes passed for 150 users. At the time two-way communication over this service area was assumed reliable given that two-way Impulse Pay Per View cable boxes were deployed to these same places. The Pioneer Set Top Boxes used FSK, retransmitted up to 32 times over three days, and worked for up to five purchases without a return path.
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