on coax, but gambled that fiber would prove durable enough
for outside installation. Bell Labs was in the midst of testing
whether fiber hardware could withstand field conditions in
ducts under a parking lot at its Norcross, Ga., plant.
With diode-laser reliability still questionable, Hi-OVIS
engineers chose LED transmitters. That, in turn, led to a choice
of fibers that may seem surprising today—step-index multimode
plastic-clad silica with a 150-µm core, 350-µm cladding, and a
coating that raised its outer diameter to 700 µm. Attenuation
was less than 10 dB/km at the 830 nm LED wavelength.
That technology was less advanced than Bell Labs had chosen for the interoffice trunk system it was developing—
graded-index multimode fiber with 50-µm core and 3 dB/km loss at
850 nm. Bell preferred gallium-arsenide diode laser transmitters but reserved LEDs as a backup. The cutting-edge of fiber
transmission was moving to longer wavelengths. In 1976,
Masaharu Horiguchi of Nippon Telegraph and Telephone and
Hiroshi Osani of Fujikura Cable measured loss of 0.47 dB/km
at 1. 2 µm and then discovered the lower-loss 1. 55 µm window.
However, Hi-OVIS did not need cutting-edge technology.
Bell’s goal was transmitting 45 megabits per second through
10 km of fiber. The longest link in Hi-OVIS was to a broadcast
television antenna 4 km from the operation center; terminals
ranged from 370 m to 2. 27 km away, with the average fiber
link 700 to 800 m. The required bandwidth was only 10 megahertz, with each downstream fiber carrying only one video
and one audio channel in a 6-MHz band, and upstream fibers
carrying the same service upstream, plus a 200 bit-per-second
channel at 6. 6 MHz for customer requests and control signals.
The Hi-OVIS network was a switched star configuration,
with two fibers running between the operation center and each
terminal. Each household selected one of the available program
channels, which the operation center switched to the fiber serving that household. Branch cables strung overhead on poles
that each contained 36 fibers and that split into subscriber
cables with two fibers in them.
The switched-star configuration allowed Hi-OVIS to send
any one of 30 input signals available at the control center
Select for ordinary TV broadcasting
Program code indicator
10 key indicator
Select for news
Switch: set at “MON” for video
projected on camera at home
terminal; set at “LINE” for video
transmitted from Center
Select to advance (FWD) or reverse
[ Hi-OVIS keyboard ]
NHK
24 6
8 10 12
NHK
20
HCT
MON
LINE
4 5
F WD
BWD
Q/A SEND
REQ
9 21 34 36 55
6 22 23 24 25
25
0
VTR VTR VTR
1 23
C
Select for programs of local origin
Select for UHF TV programs
Select for video programs
transmitted from the Center
Select to reserve a program
Select to end ongoing programs
Select to answer questions posed
during local origin program services
