Coax Opens Wideband Market.
Prior to the 1960's there was very little work being done on millimeter-wave frequencies, except for some limited experiments conducted by Bell Laboratories and others, and some short-haul communications experiments by some radio amateurs. During CN's first decade advances in technology, as well as new FCC allocations, opened up the millimeter-wave portion of the spectrum to exciting new broadband communications possibilities.
A breakthrough came in 1974 when United Aircraft's Norden Division announced the development of a radio transceiver for 22 and 39 GHz. The communication capacity of a band is determined by its bandwidth in hertz, and the number of hertz available in a given percentage of bandwidth increase directly with carrier frequency. As a result, a 10-percent band centered at 100 GHz (wavelength equal to 3 millimeters) has as much communication potential as the entire radio spectrum up to 10 GHz. Thus, millimeter waves allow for interference-resistance modulation techniques that require broad bandwidths. Additional channels are also provided by multiple antenna beams and orthogonal polarization.
The problem with millimeter waves has been the transmission losses, which always increase with frequency. These losses are significant when millimeter waves are propagated through the earth's atmosphere. Because of the shorter wavelengths, the size of most system components must be reduced. Actually, this can be an advantage in some cases, because narrow-beam, high-gain, millimeter-wave antennas, for example, are quite small and more practical than at lower frequencies, and allow for "searchlight beam" distribution of signals within cities and for communication via satellites.
The FCC in 1974 encouraged development of millimeter wave equipment with the allocation of frequencies 18 GHz for local distribution of signals by common carriers in the Domestic Point-to-Point Microwave Radio Service.
Norden's early millimeter-wave radio was intended for point-to-point transmission of digitally formatted information. The radio transmits and receives digitized voice and data simultaneously. It weighs less than 30 pounds and is 21 inches in diameter and 21 inches in depth. It can be mounted on a 3-inch-diameter mast atop any building without strengthening the structure or building huge towers. Its error rate was extremely low, it required little maintenance, and it was constructed withstand adverse weather, including extremes of heat and cold. Alarms were provided for radio and power supply. Power may be supplied from a station battery or from 115 VAC.
Both GTE and Bell Labs conducted early experiments with millimeter waves, the latter actually constructing an eight-mile trial section of millimeter waveguide in Morris County, New Jersey, for testing in 1974 and 1975. This single tube, 2-1/2 inches in diameter, was able to carry about 230,000 calls at a time . . . enough capacity to transmit, letter-by-letter, a full 24-volume set encyclopedia in 0.1 second. Voice, data, and facsimile signals were transmitted by pulse code modulation technique through the copper-lined steel pipe. The system had an ultimate capacity of nearly 500,000 telephone conversations. Millimeter-wave IMPATT diodes which, under controlled conditions, have emitted a continuous wave at 150 GHz became the power source for waveguide transmission, emitting frequencies as high as 110 GHz with 100 milliwatts of power.
Altho millimeter waveguide land lines have pretty much lost out to fiber optics for wideband transmission, the potential for high frequency transmission is currently being exploited by DTS . . . Digital Termination Systems . . . which are being used in the new Digital Electronic Message Service.
The service makes use of wideband local loops. Capable of carrying data at rates to 1.544 Mbps, the DTs loops will be suitable for applications such as high-speed document distribution videoconferencing as well as high-speed data communications. For the Dems service, the DTA links will be interconnected via satellite or terrestrial inter-city channels.
CN Data Communications Editor Morris Edwards points out that "DTS is a legacy of Xten, the wideband digital communications service proposed, and later dropped, by Xerox. In filing for the Xten service, Xerox asked the FCC to re-allocate a little-used portion of the frequency spectrum in the band from 10.55 to 10.68 GHz for intracity communications. Xerox planned to use the new frequencies for a digital microwave radio link between rooftop antennas within a city, or between the rooftop antennas and a nearby satellite nearth station. In January, 1981, the FCC approved the request and, although Xerox withdrew, several other firms filed to offer digital electronic message service based on the use of DTS local loops."
Each licensee for extended service is allocated two 5-MHz channels: one for transmission from the central station to subscribers, and the other for use by subscribers sending information and data to the central station. In addition, two 2.5-MHz point-to-point channels are allocated for communications between central stations. Limited service carriers are assigned half the channel width: two 2.5-MHz channels for central-subscriber operations, and two 1.25-MHz channels for point-to-point communications between central stations.
In July, 1982, the FCC authorized five firms to offer extrnded service: Isacomm of Atlanta; MCT Telecommunications of Washington; Contemporary Digital Services of New Rochelle, New York; Tymnet of Cupertino, California; and Satellite Business systems (SBS) of McLean, Virginia. Since then, close to 50 firms have applied to offer digital electronic message service.
With the advent of the communications satellite, large amounts of information can be easily sent from one city to another. The satellite carriers have made a significant investment in earth stations for intercity distribution of information. The problem lies in distribution of the information to the local user . . . and it is in this vital area that DTS is doing battle with fiber optics, broadband cable, and even cellular radio!
Less than a decade ago, fiber optics was an emerging technology. Today lightwave transmission has become the system of choice . . . worldwide. And, in the decade to come, giogabit transmission rates over lightwave lines may give "wideband" a whole new meaning!
Only 10 years ago, AT&T Bell Laboratories was experimenting with light-carrying fibers. Now, there are more than 80 AT&T-installed lightwave routes in service, and over 30 others in construction.
Down in Florida a company called Microtel is building a $55 million underground fiber-optic cable link called "Laser Net." Scheduled for completion by the end of 1986, Laser Net will be used by corporations high-speed data, teleconferencing, facsimile and video communications requirements as well as voice. Centel is one of the backers of the Microtel system and Wilco-Centel is handling the construction, using Anaconda Wire & Cable fiber optics.
An experimental lightwave system has been built that can transmit one billion bits of information (the equivalent of 20 digital TV channels, 14,000 telephone conversations or 100 average novels) each second. In the experiment conducted at AT&T Bell Labroatories in Holmdel New Jersey, laser light pulsing one billion times a second (one gigiabit rate) traveled through 75 miles of glass fiber. Reportedly a record for unboosted transmission of one-giabit signals, the experiment demonstrates the capability of the sensitive light detector used to detect the light signals at the end of the 75-mile span.
Last year, the first leg of the largest lightwave system began operating when AT&T beamed voice and data communications between New York and Washington over lightguide cables. This was the start of a 776-mile "Northeast Corridor" system to link Boston, New York, Philadelphia, Washington and Richmond.
Miami's fast-growing commercial district has been wired with glass to handle the growth in telecommunications . . . both voice and data . . . among brokerage houses, banks and high-rise condominiums. "If we hadn't gone to half-inch fiber cables for this project, we would have had to install 20 copper cables, each of them the diameter of a fist, just to meet today's demands," explains Al Naranjo, engineering manager at Southern Bell. "And to keep pace with the growth we expect, we would have to install two more cables each year for the next three years. Then we'd have had to dig up the streets to make room for more cable in duts and manholes."
In an experiment that was performed late last summer, laser light pulsing hundreds of millions of times each second traveled unboosted through 100 miles of hair-thin glass fiber. These results may lead to future generations of practical, high-capacity lightwave communications systems that could carry huge amounts of voice, data, video and graphics over lightguide fiber across continents and under oceans. At the 420 Mb/s rate of the experiment, so much information is transmitted so quickly that the entire text of 40 full-length novels could be sent in one second.
the first commercial single-mode fiber-optic transmission system in the western world was commercially inaugurated last fall by ITT Telecom Network Systems and Continental Tel of New York. The 90-megabit FTS-3C system links two of Continental's digital central offices over a span of 23 miles and is capable of 1,344 two-way telephone channels. This first-of-a-kind fiber-optic system is carrying live traffic over interoffice trunks between Continental's exchanges in Norwich and Sidney, New York, located about 50 miles southeast of Syracuse.
The laser was readily regarded as a would-be carrier of wideband information since its development in the 1960s. The difficulty of modulating gas, chemical and solid lasers however has delayed this promise.
A laser said to offer dramatic implications for future generations of lightwave communications systems has been announced by Bell Laboratories. Called the "cleaved coupled-cavity" (C.sup.3.) laser, the semiconductor device has properties that promise significant improvements in lightwave system capacities, longer unboosted transmission distances, ultrapure long wavelength output ideally matched to the transparency of the glass fibers that carry the light signals, and ease of manufacture. It is the first practical communications laser whose output can be tuned electronically from one ultrapure single-frequency to another.
Philip Rubin, lightwave systems expect for Bell Labs, says: "Activity in optical-fiber technology is impressive, increasing at an everfaster rate throughout the world. The coincidence of lightwave development with conversion to digital switching accelerated the process. What the future holds for this new technology is probably beyond anybody's ability to accurately forecast.
"There are such interesting prospects as the introduction of high-definition video systems with stereophonic sound. Or videotex systems linked to regional, national, or even international data bases. Or computerized monitoring of residences and businesses by fire, police, and utility offices. Certainly, the wideband capabilities of lightwave technology will be a major impetus to opening those Information Age windows."
Bandwidth is the goal . . . and, be it by oint radio, broadband coaxial cable, or fiber optic lightwaves, innovative communications engineers are right on target.
One advisor to the Federal Communications Commission has estimated that all telephone exchanges in the United States will be interconnected by optical fibers by the mid-1990s. Others see the personal computer revolution as escalating demands imposed on loop facilities to the point where the only viable option would be the installation of local fiber systems. Trouble-prone analog loops will give way to reliable wideband digital links.
|Printer friendly Cite/link Email Feedback|
|Date:||Sep 1, 1984|
|Previous Article:||Everything Coming Up Digital.|
|Next Article:||Breakup of the Bell System.|
|SMC ACQUIRES WIDEBAND LICENSE.|
|ULTRA-WIDEBAND GROWTH PREDICTED TO EXPLODE.|
|ENTROPIC SHIPS FIVE MILLIONTH MIPS-BASED LINK CHIPSET.|
|Ultra-wideband communications systems; multiband OFDM approach.|