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Microwave Links Used for Direct Interconnection of Broadband LANs.

The emergence of broadband local-area network (LAN) technology as a highly efficient means for intra-facility two-way data, voice and video communications is being realized and implemented at a much faster pace than initially anticipated. Having proven their usefulness and feasibility over small premises, LANs are increasingly being considered to cover larger areas; that is, to integrate several widely dispersed facilities into one large network.

To provide a larger user base under an integrated LAN, it becomes necessary to extend the so-called "branches" of the LAN tree to reach every building or facility in the network. It is this requirement for interconnecting widely dispersed locations by coaxial cables (which is an expensive, if not impossible, task) that has curtailed the full use of the LAN potentials to date.

But now, thanks to broadband microwave radios, it is possible to economically interconnect all the dispersed LAN sites in a city by microwave links. This effectively turns several LANs into a metropolitan-area network (MAN), providing full access to a set of common resources to all the users in the system.

The basic concept is based on the heterodyning principle where any given carrier can be up or down-converted to another frequency when properly mixed with a locally generated signal. This process is basically independent of the modulation content of the desired carrier; analog amplitude modulated (AM), frequency modulated (FM), digital frequency shift keying (FSK) or quadrature phase shift keying (QPSK).

Figure 1 shows the simplified block diagram of a one-way microwave link. In the transmit direction, the desired VHF carrer (C.sub.d.) in the frequency range of 54 to 440 MHz and at the proper level is injected into the transmitter. This signal is taken to an up-converter circuit, where it is mixed with a locally generated microwave carrier called the local oscillator (LO) signal (fo) in the microwave frequency range. The mixer combines the two signals and produces the sum (fo+C.sub.d.) and the difference (fo-C.sub.d.) at its output. A bandpass filter after the mixer selects only one of these new signals (fo+C.sub.d.) and passes it to an output amplifier. This final microwave carrier is amplified and fed to the antenna for radiation toward the distant receiver. At the receive site, the microwave energy is picked up by the antenna and passed to the receiver. After passing through a bandpass filter, the received microwave carrier is taken to a mixer where it is mixed with a locally generated signal that is identical in frequency to that of the downconverted very-high frequency (VHF) carrier is made available at the output of the receiver at the same VHF frequency and with the same modulation as the transmitter input.

Handling Traffic in Opposite Direction

For a two-way communication system, a second transmitter-receiver pair handles the flow of the traffic in the opposite direction. In such cases, some equipment (such as the antennas) are not duplicated but used by both the transmitter and the receiver at the same location. Figure 2 shows a more detailed block diagram of a transmit-receive terminal.

With LAN systems, even when we have perfect transmission channels, the actual transmission delay experienced by the carriers in going from the farthest user terminal to the head-end translator and back will affect the overall LAN system throughput. So the location of the head-end translator and the maximum length of the LAN tree branches becomes critical in the context of the LAN performance constraints. It is interesting to note that the transmission delays introduced by broadband systems such as the Hughes AML microwave link are a fraction of that of a coaxial cable of comparable length.

Every microwave-LAN interconnect system must be carefully tailored to meet the performance requirements of a given LAN or point-to-point system, if true transparency on the part of the AML equipment is to be achieved.

Figure 3 shows the simplified block diagram of a typical microwave LAN interconnect where two independent single-cable LANs are to be integrated into one system. In this example, all the users on LAN 2 are to become part of the population of LAN 1.

To implement such an integration, the coax cable of LAN 1 is tapped through a standard directional tap at a convenient point closest to the location where the microwave equipment is to be housed. The choice of this location is dictated by the need to have line-of-sight transmission between the two sites of LAN 1 and LAN 2. At the other end, the microwave radio equipment is directly interfaced with the LAN 2 coax cable at its head point.

With the connections so made, the coax cable of LAN 2 becomes a major "branch" of the system "tree" under LAN 1 and the two-way communications among the users of LAN 2 themselves and the users on LAN 1 take place as if all the users were connected by a single network. The flow of signals to and from LAN 2 take place in the following manner:

The channel carriers within a given LAN spectrum, transmitted by the cable modems of the users on LAN 2, will arrive at the microwave equipment. They first pass through a duplexer filter that separates the forward and return frequency bands in order to be compatible with the two-way single-cable system. The carriers then go to an amplifier where their level is adjusted to the optimum value required by the transmitter (TX) unit. The TX unit up-converts the carriers to the designated microwave band for onward transmission. At the other end, the microwave signals are picked up by the antenna and passed to the microwave receiver unit (RX), where they are down-converted to their original VHF frequencies. They will then pass through an amplifier that sets their level to the optimum value for injection into the LAN 1 coax cable.

The carriers, now being on LAN 1 cable, will be treated like the other in-house carriers. That is, they will be received by the LAN 1 head-end translator, converted to the forward frequencies, and injected back into the cable to go to all users on LAN 1 cable. To go to LAN 2 users, the carriers will enter the microwave radio equipment through the directional tap and undergo a process similar to their path in the reverse direction. In this manner, the LAN 2 system will be fully integrated into LAN 1. The coverage range of the broadband microwave radio equipment basically depends on the operating microwave frequency, the terrain and climatic conditions of the area and, of course, the availability of line-of-sight propagation between the two LAN sites to be interconnected. However, for many applications such distances could range up to twenty miles. Fully protected broadband microwave systems are generally required for LAN interconnect systems.
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Sarraf, J.; Rabowsky, I.
Publication:Communications News
Date:Oct 1, 1985
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