Zenith Cable Modem
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Zenith Cable Modem is one of the first proprietary cable modems deployed at limited scale. There are two basic models, one operating at 500 kbit/s, the other at four Mbit/s using BPSK and about a 25% alpha.
- 1 History
- 2 Product versions
- 3 Technology demonstration to initial deployment
- 4 The Zenith MAC layer
- 5 Provisioning a Zenith cable modem
- 6 A nascent market - Quality perceptions and reality
- 7 RF requirements for a reliable Zenith network
- 8 "State of the art" RF plant maintenance in 1994/1995
- 9 First Zenith Network Network Operations Center (NOC)
- 10 Videoconferencing over cable modems in 1994/1995
- 11 Zenith at Cogeco in Canada
- 12 References
- 13 External links
The Zenith Cable Modem was originally developed for a mid-split cable network in the mid-1980s, and used as an eight-bit full height PC/AT type card containing an Intel 80186 dedicated CPU, connected to an external white box about 2"x12"x6". Other similar products were made by Ungermann-Bass under the 10BROAD36 standard, and Vitalink. UB had models supporting RS-232 and Ethernet outputs, as well as a remodulating frequency translator.
In late 1993, Zenith and Prodigy provided about 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 begun on a 1500 homes passed fiber node in El Cajon, CA. The modification allowed subsplit operation, with a fixed upstream frequency, and the downstream at 74.75 MHz, within the 4 MHz space between analog channels 4 & 5.
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 1500 home passed fiber nodes, with six "subscribers" including one employee of Cox who was also the headend manager. One card was installed in rack mount 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, and a newspaper article appeared in the Los Angeles Times.
Zenith updated their white modems to a matte black case, adding the Homeworx name, and marketing to cable operators in 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 eight-bit ISA card with a smaller 16 bit version. The new design dropped the onboard CPU. The modem connected to the card using 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 in 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.
White modem, from 1980's based design
Black internal modem, Homeworx
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 10BaseT 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.
Clearly this design does not scale to more than a few networks, so the Channelmizer 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. 3 Frequency Translators, plus 4 Channelmizers were loaded into a car and driven to El Cajon for installation Christmas Eve. These plus 3 other Channelmizers 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 7 frequency translators + 7 Channelmizers supported 65,000 homes passed on 23 fiber nodes, and occupied a full rack.
Technology demonstration to initial deployment
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A decision was made to deploy Prodigy over cable modem to 200 users in time for the 1994 Western Cable Show. To grow the system from 16 users in 3 fiber nodes, to 200 required adding what became 64,000 homes passed, making the Cox San Diego / Prodigy cable modem field trial the worlds largest cable modem deployment (by service area) at the time.
The acceptance criteria was simply too strict to achieve the 65% homes passed high speed internet penetration which is common today:
- Prodigy Subscriber > 9 months
- Cable Subscriber
- lived 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 being deployed to these same places. The Pioneer Set Top Boxes used FSK, retransmitted up to 32 times over 3 days, and worked just fine for up to 5 purchases without a return path.
The Zenith MAC layer
CSMA/CD, or Ethernet "like" was expanded as follows: Data rate reduced to 500 kbit/s for up to 50 (100?) miles plant radius Data rate reduced to 4 Mbit/s for up to 25 miles (40 km) plant radius The transmitting device would first listen for activity on the wire, if there was activity, it would activate the "A" LED, and wait. Activity is defined as any RF energy, intelligible or not, at an absolute amplitude of -25 dBmV into the device receive port within a 1 MHz or 6 MHz passband (500kbit/s or 4 Mbit/s respectively.)
Upon detection of no activity, and appropriate backoff timers (as per Ethernet CSMA/CD standard), the CM would begin transmitting a preamble, then the start of the Ethernet frame. Simultaneously, the transmitting CM would begin listening for its own burst. If the CM were able to decode its own burst, and correctly compare a checksum of the first 17 bytes of the packet, it would declare no-collision, and continue. If there was a checksum error, it would declare a collision, and stop transmitting after the 19th byte. This was easily and directly measured using Novel LANalyzer software on a dedicated Windows 3.1 for Workgroups PC.
The card kept some locally accessible counters on excessive and late collisions, but there was no driver or management of any kind.
By November 20, 1996 Zenith released SNMP MIBs and management for their Channelmizer under software release 9.32, under the title "Node Data Controller SNMP"
Provisioning a Zenith cable modem
The physical possession of a properly installed modem connected the subscriber to the network, and the use of this modem with a packet sniffer would enable full viewing of all transmitted and received packets from every user sharing that frequency translator.
Removing a device for non-payment required physically retrieving the device, and preventing the availability of used or stolen modems. There was no ability to disable a modem remotely.
A nascent market - Quality perceptions and reality
While not directly competing against any other existing product—residential ISDN penetration was sub 0.1%, the technology had severe growing pains:
19 Independent causes of two errors—CT-9 and CT-16, "A communications failure has occurred"
Driver / Software:
1) IPX driver not loaded when launching windows
2) IDNX 56 kbit/s link from local Prodigy Server in El Cajon Headend to Yorktown NY was down
3) Prodigy Server in El Cajon Headend experienced driver wedge due to excessive noise in return path, and inability to transmit packets, call Prodigy and request server reset
4) Duplicate CM MAC address
5) Lost modem frequency configuration.
6) Traffic Storm / ARP Storm, excessive traffic of any kind, or an ARP storm would shut down the network. Fastest resolution was often to take the network down for a few minutes, and reboot the affected systems. (Often required calling the customer on the phone and asking them to do so.)
7) Cell phone used near the cable modem, PLL unlocked, power cycle to resolve.
8) Channelmizer lock up, cell phone used near headend cable modem, PLL unlocked. Power Cycle to resolve.
9) Frequency translator drift, resolution: replace the Olsen unit with model made by Wavecom (now Vecima Networks, who now makes upconverters for the Cisco DOCSIS compliant CMTS platforms)
10) diplex filter switch got "bumped"
11) External modem module disconnected or plugged into PC game port instead of modem card. (Less of an issue with black modem as LEDs would go dark, but impossible to verify on white modem without LEDs.)
12) LRC STS-75 end of line terminator causing Common Path Distortion (CPD) A mechanical design defect occurred when a 22 gauge piece of stainless steel wire was press fit into an aluminum housing and compressed to become "watertight." Unfortunately, corrosion developed anyway, and the resultant diode from dissimilar metals caused the CPD. Resolution: replace 10,000 units at $4/part + $25/labor with gold anodized model.
13) Return path alignment and headend combining: Independently develop the "X-point" return path alignment and amplitude compensation process. Due to the Amplitude Modulation (AM) format of the return path FP lasers, a +25 dBmV CW tone injected at the fiber node status monitoring module would not arrive at the headend with constant amplitude. Pad inputs to frequency translator to ensure +5 dBmV input on this tone for each fiber node (individually).
14) Microwave Filter Corp brick wall filter on the output path to the frequency translator was too sharp, introducing excessive group delay
15) PCS Cable Telephony trial precision CW clock distribution tone at 73.25 MHz jamming subscriber transmitters, causing "A" light on solid, and 100% collisions
16) Poorly chosen upstream frequency for white modems, resolved for deployment of black modems
17) Too many homes passed per frequency translator. We got it working reliably with 8500 to 10,000, but it worked much better at about 6500 homes passed.
18) Speaker modulation. When the coax connector on a nearby television set is loose, and the audio is really loud, the mechanical vibration of the television set alters the physical connection of the coax, allowing ingress and other interference.
19) Missing return path amplifier. During the early days of the trial, few repair trucks carried return path capable amplifiers. One amplifier was replaced with a one way amplifier and improperly noted. The customer's pregnant wife watched the service tech try for hours to resolve the fault. Their 4 month son watched the resolution after a Compaq luggable 386 based system with an Internal modem was connected to figure out why this one home was so problematic.
RF requirements for a reliable Zenith network
Carrier/anything of at least 25 dB at the receive port of all modems. To support the time varying nature of an HFC network, 35 dB C/anything was required at the input to the frequency translator in the headend.
Carrier/anything is not a technical term, but is intended to include: C/Noise, C/Interference, and distortion products.
Downstream input to each cable modem of +5 dBmV +/- 2 dB. Due to the non-existence of any real input AGC, the lack of a re-modulating frequency translator, and lack of upstream transmit power control, the tight receive level range accommodated upstream path variation.
"State of the art" RF plant maintenance in 1994/1995
An HP 8590L (low cost version of 8591c), video out connected to a Cadco channel modulator operating above 550 MHz, with a Radio Shack black and white TV (picture tube) tuned to UHF channels to see the picture. Add to that a comb generator with 4-6 CW tones, an HP Calan 1776 "portable" spectrum analyzer, which is about the size of an igloo cooler for a weekend car camping trip. Not all amplifiers contained a good return path test point, so the repair lab was asked to modify some line extender diplex filters to pass the forward, and divert the return path to an F connector. A pad socket was also converted as well. Any signal entering an amplifier with an amplitude of above -45 dBmV was located and fixed. CLI was reduced to under 5 uV/m. 2 years later, Milo Medin CTO of @Home was reported to have said, "Please show me the fiber receivers that are connected to the outside plant, these don't have enough noise, so they are certainly not connected or properly aligned." Later, he was quoted as saying "Cox San Diego return plant is so clean you could eat off it." San Diego never had serious RF issues when it came to return path after 1995.
First Zenith Network Network Operations Center (NOC)
Modeled after the University of California San Diego Telecommunications NOC at the Central Utilities Building: Novel LANalyzer software w/ internal cable modem, and somebody to manually move the test connection between frequency translators. Only one frequency translator could be measured at a time. HP Openview (HPOV) for Windows, version 3.1, and a 24 port HP Advancestack Ethernet HUB with AUI "uplink." This combination allowed the Advancestack hub to send out IPX diagnostic packets to the NDIS driver in each PC. Assuming the PC was not rebooted, and running the driver at all times, the HP openview application would display the Netware device ID – 00000,mac-address as there was no real Netware server / router. HPOV displayed each device in purple if active, and blue if discovered and non-responsive. The management software would generate NDIS "diagnostic pings" from the hub, and report the number of successful responses. Rebooting the PC, loading / unloading the driver, or experiencing any number of RF faults would cause each cable modem to transition from purple to blue hundreds of times per day.
HPOV could export the log file to a Paradox Database for later analysis and trending. Eliminating the PC reloads / driver reloads from the RF faults, and correlating RF faults on a single frequency translator vs. the entire flat network proved beyond the capabilities of the Zenith Network. These methods became the justification and fundamental research for what was later patented by others: Patent Numbers: 6,032,019, 5,943,604 Echo device method for locating upstream ingress noise gaps at cable television head ends. Eventually, this technique became mandatory in DOCSIS 3.0, although it was rejected and ignored for 3 years as a "proprietary extension to DOCSIS" by @Home, and copied by every DOCSIS CMTS vendor.
By September 1994, the NOC was expanded with the introduction of 2 @ Cisco Catalyst 1200 Ethernet to FDDI switches, and 2 @ Cabletron MMAC-3 FNB hubs (6 Ethernet + 2 FDDI), constructing a reliable path from the Federal Headend to the El Cajon headend, and eliminating one unreliable path between the Advancestack hub and the various Channelmizers / frequency translators.
Videoconferencing over cable modems in 1994/1995
CUseeME and Apple QuickTime Conferencing 1.0 beta  worked perfectly over the Zenith and Lancity proprietary cable modems when the RF path was sufficiently clean. These modems were able to interoperate on the same RF fiber nodes using Frequency Division Multiplexing, and an Ethernet switch to join the otherwise incompatible networks.
Late in the beta, 160x120 pixel video was increased to 320x240, and the audio quality greatly improved on the Apple PowerMacintosh 7100 AV.
Zenith at Cogeco in Canada
Cogeco in Canada launched with the 4 Mbit/s, external Rev D cable modem in roughly 1996, and by August 1997 had 2500 subscribers. The network was failing for 3 reasons:
- The Zenith network is a pure bridged CSMA/CD Ethernet. It has no understanding of the Layer 3 ARP protocol. Thus ARP storms through the layer 2 switches used in the first headend designs would clog the network.
- The layer 2 switches and layer 3 routers of the time did perform any QOS or rate limiting, thus a 4 Mbit/s input port on the Channelmizer could be flooded with up to 10 Mbit/s of traffic bursts.
- The Cogeco network contained an unusual reverse path amplifier as part of the retrofit to accommodate return path. This particular amplifier contained a design defect whereby the pad and equalizer plug in modules would literally fall off the circuit board when the device was opened. A thermal incompatibility existed causing the holes for the parts to increase in diameter, while the pins would elongate and become smaller. This fault was documented and proven as a result of Cogeco's first DOCSIS deployment with the Cisco uBR7246, MC11-FPGA linecard, and IOS 11.3(4)NA, which contained the "flap list."
- [Novel Netware LANalyzer for Windows, P/N 136-001692-002 Rev A Rel 2.0 Copyright 1983-1993]
- See excellent analysis by Francis Edginton in 2003 article. Francis wrote many of the test algorithms in the HP/Agilent 8591c Cable TV Personality for FCC performance testing.
- QuickTime Conferencing Press Release
- David Waks recalls 1993-1994 launch Article
- Modern CMTS performance measurements Article
- Patent Search Patent Numbers: 6,032,019, 5,943,604 Echo device method for locating upstream ingress noise gaps at cable television head ends