Bank Links Operations Center to HQ with Its Private Microwave System.
Fleet National Bank, one of New England's largest commercial banks, faced a communications problem similar to that of other decentralized businesses. The bank has its headquarters in the city and branches and service facilities in the surrounding region. It could not satisfy its communications needs with a single exchange because of this decentralization.
As a result, Fleet National Bank installed a private telephone system that has been upgraded to keep pace with its expansion. The upgrade required vast increases in the number of connecting circuits between switch locations at its main office and operations center.
After evaluating several alternatives, the bank decided to install a line-of-sight microwave link between the buildings capable of transmitting 24 million bits of information per second. It decided against leasing equivalent data and voice-grade lines from the telephone company to perform this service, on the grounds of high leasing costs and anticipated installation delays. The bank also looked at a fiber-optic link and found that approach to be impractical, after considering the prohibitive cost of obtaining right-of-way and burying fiber-optic cable in Providence.
Fleet National Bank selected an M/A-Com 23DR 23-GHz digital microwave system to be integrated into its existing Rolm CBXII private telephone system. The M/A-Com 23DR is a product of M/A-Com MAC, located in Billerica, Massachusetts, which specializes in low-density microwave communications. This private microwave link can sustain up to 384 simultaneous conversations, as well as provide for computer data transmission.
The microwave system has the capacity of four telephone company T2 lines, the equivalent of 16 telephone company T1 (DS-1) lines, each capable of transmitting at the rate of 1.544 Mb/s. The T2 data rate is 6.312 Mb/s. Transmission and reception is by two four-foot-diameter parabolic antenna dishes. One is located on the headquarters building in Providence, positioned to remain as unobtrusive as possible on the architecturally striking building. The other antenna is in direct line-of-sight on the operations center roof.
Economical Means of Obtaining Data
The microwave system provides the most economical means for obtaining the required voice and computer data capacity. Payback on the investment will be slightly over one year, based on savings of lease charges for the equivalent telco cables and the cost of necessary peripheral equipment. The installation was completed in June of this year, and has been operating with 100 percent uptime since then, the bank reports.
The private microwave link operating at 23 GHz could not have been installed four years ago. The FCC has not opened that part of the microwave frequency spectrum for private telecommunications applications and system hardware capable of handling the traffic was not available.
Private fiber-optic links are capable of handling the traffic and the costs for the transmission and receiving equipment are comparable to those of a microwave link. However, the requirement that private fiber-optic links be buried within a city proves to be an insurmountable handicap for this approach. From a technical point-of-view, fiber-optic links are better suited for longer-distance, higher-density communications, regardless of the cost of burying fiber-optic cable.
The use of telephone company cable was also viable technically, but there are delays in obtaining these cables, in addition to the high cost of leasing. Moreover, the leased cable is subject to periodic rate increases and the cable approach lacks the versatility and expansion flexibility of the microwave link.
In this installation, the microwave link is user-friendly. The bank's staff members need only follow routine procedures for making internal calls with the Rolm telephone system. No special procedures are needed to make calls between the two buildings. However, users have reported that the voice quality of the calls going over the microwave link is superior. There are no hums, clicks or interference.
23-GHz Microwave Frequency Best
The 23 GHz microwave frequency region is well suited for private, low-density telecommunications. The equipment is generally smaller and lighter than that for the lower microwave frequencies. In the band from 21.2 to 23.6 GHz, small two to four-foot antennas may be employed and transmission up to 15 miles is attainable with low-power, solid-state transmitters. Repeaters operating in the same band can double or triple that range.
Each end of Fleet National Bank's duplex microwave link consists of transmitter and receiver antenna assemblies mounted outside. Four sets of rack-mounted digital interface units with power supplies and baseband circuits are located in the buildings at each end of the microwave link. Interconnection between the indoor and outdoor units is made by means of standard coaxial cables and connectors. This eliminates the high cost of waveguide "plumbing."
Solid-state Gunn-diode transmitters provide typical 66-milliwatt (mW) output in each of four channels. The digital system is frequency modulated, making it less susceptible to noise than an amplitude-modulated system.
Four-foot antennas were selected for the Fleet National Bank system to accommodate the load. These antennas provide 45-dBi gain as compared with the 40-dBi gain of the two-foot antennas, also standard microwave system components.
Heavy Rain Affects Reception
Heavy rain affects the reception in the 23-GHz microwave region. However, studies show that the rain must be heavy and sustained, typically in excess of four inches per hour, to cause an outage. In most parts of the country this is likely to occur only a few hours per year. As a result, systems operating at these frequencies can expect to have 99.999 percent link availability.
In additional to engineering reasons for wanting the higher gain of the four-foot antennas in the Fleet National Bank microwave link, the larger antenna provides a narrower 0.9-degree beam as compared with the 1.8-degree beam of the two-foot antenna. The narrower beam results in a higher concentration of energy at the receiving antenna, further reducing stray microwave energy. The narrower beam makes it more difficult for outsiders to intercept the communications and it also reduces the susceptibility of the system to interference from existing or future microwave links operating in the vicinity.
Fleet National Bank is but one example of a recent M/A-Com MAC 12 to 16 T1 installation. Others include those for TWA at New York's Kennedy Airport and for General Motors in Detroit.
Other applications for microwave links include their use in private security systems and for teleconferencing with high-resolution, closed-circuit TV. A promising application is the transmission of video signals from costly fixed medical diagnostic instruments located in special buildings at hospitals to remote terminals for more convenient viewing.
Images Sent to Doctors via Microwave
For example, signals for the generation of scanned video images from MRI (magnetic resonance imaging) machines can be transmitted from the imaging center where the patient is being examined to the offices of attending physicians, surgeons and other specialists. This means that only the patient need go to the center and the medical professionals need not waste time by commuting to the site. This simplifies scheduling and permits more-efficient workloading of the apparatus.
Frequency bands in the 23-GHz region are available to qualified users for the formation of private telecommunications links. The FCC requires that the user specify the intended mode of operation and this is indicated on the license. A single user may be licensed for more than one frequency band, to permit the creation of a non-interfering private-area network.
The 18-GHz band has also been opened for private telecommunications. Since both the 18-GHz and 23-GHz bands have similar characteristics, the cost of equipment for operating in both bands is similar. The path lengths at these frequencies are short so that the same bands can be licensed to many different users througout the country. However, the FCC makes sure that they are adequately separated to eliminate interference.
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|Date:||Oct 1, 1985|
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