Zenith Cable Modem - A Nascent Market - Quality Perceptions and Reality

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.)


Modem Hardware:
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.)


RF:
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.

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