Multiplex communications – Pathfinding or routing – Combined circuit switching and packet switching
Reexamination Certificate
2000-01-31
2004-10-05
Pezzlo, John (Department: 2662)
Multiplex communications
Pathfinding or routing
Combined circuit switching and packet switching
C370S329000, C370S437000, C370S432000
Reexamination Certificate
active
06801524
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to the field of wireless communication. More specifically, the present invention relates to the field of wireless simplex packet communication.
BACKGROUND OF THE INVENTION
There is a considerable need for dispatch communications, i.e., simplex bi-directional communications between a dispatcher and remote (field) units. This need is conventionally filled by specialized equipment operating over dedicated frequencies. Examples of this type of equipment are the dispatch radios used by police, fire, ambulance, taxi, and delivery services. In dispatch systems, a single dispatch unit typically uses one frequency (frequency “A”) for transmission and another frequency (frequency “B”) for reception, with all field units using frequency “B” for transmission and frequency “A” for reception.
Dispatch radios share many problems with other simplex systems, e.g., construction-site walkie-talkie radios, personal-service radios, and other business radios. In such systems, all units typically use a single frequency for both transmission and reception. By necessity, the number of units in such systems is severely limited.
Such communication systems are often simplex. That is, a given unit may only transmit or receive at one time, but not both. This limitation is both a weakness and a strength of such systems. Since only one unit of a communicating pair may be transmitting at one time, interruptions are impossible, regardless of the urgency involved. On the other hand, the equipment need not have the complexity and expense of full duplex communication equipment. Because of their similarities, dispatching and single-frequency systems may be generally classed as push-to-talk (PTT) systems for the purposes of this discussion.
PTT systems suffer from a significant number of problems. A major one of these problems is that PTT systems are typically proprietary. That is, the equipment for a given system is often made by a single manufacturer. This obliges the user/owner to deal with this single manufacturer. The equipment is therefore often more expensive than similar equipment for other services, even though that other equipment may be more sophisticated than the needed equipment. The reasons for this are complex, involving the scale of production as well as the lack of competition.
Similarly, such equipment often must be serviced by specially trained and licensed personnel. Again, being a small market, a given area will often have only a small pool of qualified service agencies/personnel. Such an agency is typically licensed or certified by the manufacturer. This again leaves the user/owner at the mercy of the manufacturer through the service personnel, resulting in a decrease in competition and an increase in service expenses.
Because such PTT equipment is often manufactured and serviced by a single company, the user/owner may well be left without support of any kind should that manufacturer cease to do business. Alternatively, the user/owner of the equipment may be faced with a considerable difficulty should the local service agency of the equipment manufacturer cease to represent that manufacturer. This often necessitates that the equipment be returned to the manufacturer for servicing, thereby effecting unreasonable delays.
PTT systems are typically manufactured to fulfill specific and unique requirements. That is, while the PTT dispatch system used by a taxicab company is similar in design and function to that used by a fire department, they are designed to operate at different frequencies and are not interchangeable. This non-interchangeability extends beyond physical constraints and into the areas of licensing and legislation. Therefore, a small rural volunteer fire department on a tight budget is constrained from using donated taxi dispatching systems. The systems and their components are not interchangeable.
Because of this incompatibility of hardware and operating frequencies, two different PTT systems cannot readily intercommunicate. For example, in an emergency situation it may be desirable to coordinate police, fire, and medical field units from a single dispatching unit. This is not normally feasible without a special cross-service dispatching unit and/or multiple dispatching units in the same location. Overcoming such incompatibilities increases the expense of each of the systems while being an inefficient compromise at best. Additionally, the use of such a centralized and complex dispatching center often necessitates the use of a highly skilled and specially trained dispatcher (operator). This, too, increases system expense.
PTT systems typically operate within specific frequency bands by law. These bands have limited capabilities, thus creating a problem when many services must use the same band. Since each PTT system providing a given class of service, e.g., taxicab dispatching, must share the same band while simultaneously utilizing different channels (frequency allocations with the band), such channels are often at a premium in large metropolitan areas. Occupation of all available channels in a given area would prohibit the assignment of another channel in that area. Therefore, a potential new user may be inhibited from receiving a needed license.
Likewise, since a shortage of channels may produce a waiting list for licenses, the loss of a license for a given channel, however briefly and for whatever reason, may result in the assignment of that specific channel to a new licensee, thereby effectively driving the former license holder out of business.
PTT systems also have coverage problems. Not only does the specific equipment have an operating range limited by design, the operating range is also limited by geography. For example, operation is typically limited to “line-of-sight” for the frequencies and signals involved. Shadows may thereby be cast by natural and artificial geography. In a typical scenario, for example, a taxicab dispatching service may lose contact with any cab in an area shadowed by a hill. Similarly, a messenger service may have only intermittent and unpredictable contact with messengers in a downtown area due to a large number of steel and concrete buildings. Both problems derive from the very structure of a PTT dispatching system. That is, all mobile field units must communicate with a fixed dispatching unit via an electromagnetic line-of-sight. Therefore, if the geophysical relationship between the field unit and the dispatching unit is such as to inhibit transmission and/or reception, then communication is lost.
Dispatching systems make up a significant portion of PTT systems in use. PTT dispatching systems typically have a single dispatching unit and a plurality of field units. As previously mentioned, the dispatching unit may transmit on frequency “A” and receive on frequency “B,” while the field units transmit on frequency “B” and receive on frequency “A.” This means that a PTT dispatching system has an assigned dispatching unit that differs in kind as well as operation from the field units.
The centralized dispatching unit of a PTT dispatch system typically transmits to all field units simultaneously. That is, a typical two-frequency PTT dispatching system cannot readily communicate to only a subset of the assigned field units. There are systems in which selective dispatching is implemented, but all such systems are expensive and inefficient. For example, each field unit may have an address affixed to the beginning of each dispatch intended exclusively therefore. The use of such an address header therefore allows private messages to be dispatched. However, this increases radically in complexity when multiple (but not all) field units are to be addressed.
In an alternative dispatching scheme, the centralized dispatching unit may have multiple transmission frequencies. This allows normal dispatches (i.e., those intended for all field units) to be transmitted on a first frequency with selective dispatches being transmitted on a second frequency. In this scheme, the dispatcher would instruct the a
Cooley & Godward LLP
Pezzlo John
Sonim Technologies, Inc.
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