Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-12-10
2004-02-10
Bost, Dwayne (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S439000
Reexamination Certificate
active
06690935
ABSTRACT:
The invention concerns a telecommunications method and system, in particular ones using packet mode, in which it is necessary to transfer calls simultaneously from one path to another.
BACKGROUND OF THE INVENTION
The transfer of calls from one path to another is sometimes referred to as “handover” or “hand-off”.
In some communications systems it is necessary for signals to be handed over from one path to another during a call or connection.
A first example of this is a mobile telephone system currently under development, usually referred to as the Universal Mobile Telephone System (UMTS) and in which each mobile station is connected to the network by a fixed base transceiver station radiating in a particular geographical area. The call must be handled by another base transceiver station when the mobile station moves away from that area. The two base transceiver stations are connected on their upstream side to the same switch. Accordingly, in this case, a call between a terminal and a switch must be handed over from a first path via a first base transceiver station to a second path via a second base transceiver station.
A second example is a telecommunications system using a constellation of non-geostationary satellites in low or medium orbit. The orbits are chosen so that practically all of the surface of the Earth is covered, in other words so that at least one satellite can be seen at any time from any point on the Earth (sometimes with the exception of polar areas). Because the satellites are not geostationary satellites, each point on the Earth sees the same satellite for only a limited time, in the order of 15 minutes at most. The telecommunications system is therefore organized so that, from the point of view of a terrestrial user, when one satellite leaves the area of visibility there is another satellite ready to take over the call. In each area, having a diameter of several hundred kilometers, for example, a connection station connects each terminal to the network, and the station and the terminal communicate via a satellite. In this case, the call between a terminal and the connection station is effected initially via a first satellite (first path) and subsequently via a second satellite (second path).
In all such systems, information is transmitted in digital form in cells or packets. The cells comprise a particular number of bits, for example, as in the ATM (Asynchronous Transfer Mode) standard. The packet length can vary.
Because the transmission times are generally different on the two paths, when a call between two particular points, for example a terminal and a connection station, is handed over from one path to another, a cell or packet on the second path can arrive before an earlier cell or packet on the first path, and it is necessary to retransmit the packets or cells in the correct order. Also, handover must not cause any cells or packets to be lost.
In circuit mode, the problem of the order of the cells or packets is solved by dating the transport frame or transmission medium. In other words, each cell or packet corresponds to a given (dated) section of the transmission medium.
This dating solution entails transmitting additional data. Also, it cannot be applied in packet mode because, in this case, the data can belong to different streams of information which are asynchronous and may not correspond to dates of the transmission medium. For example, the same transmission may convey information of different kinds with different bit rates and possibly different priorities.
To solve the problem of the order of the cells or packets at the receiver, the last cell on the first path can be transmitted with a mark, and two receivers can be provided, one for each path, each having a demodulator and a buffer. The buffer of the second path receiver delays the cells received until the last cell of the first path, which is identified by a mark, has been received and processed. Once the last cell has been received, the buffer of the second path receiver is released.
That prior art technique has the drawback of necessitating a receiver with two demodulators and two buffers, one for each path. That disadvantage is particularly serious in the case of a terminal for consumer applications, which must be of low cost. Also, marking the last cell on the first path constitutes signaling that mobilizes limited communications resources.
OBJECTS AND SUMMARY OF THE INVENTION
The invention addresses those drawbacks.
In the transmitter cells or packets intended for the second path are transmitted only after the last cell or packet intended for the first path has been transmitted, and in the transmitter and/or in the receiver cells or packets on the first or second path are delayed so that cells or packets on the second path arrive after cells or packets on the first path.
It is therefore unnecessary to transmit signaling for the last cell on the first path.
Also, the cells or packets on the second path arrive after those on the first path, and it is therefore unnecessary to provide two demodulators. A single demodulator is sufficient.
In an embodiment of the invention, the transmission times on the first and second paths are made equal. To this end, buffers in the transmitter and/or the receiver delay the cells or packets for long enough to obtain equal transmission times, for example.
The buffers are the same as those used to construct queues, for example. They can equally well be separate from those queue memories.
The invention concerns not only a telecommunications method and system but also a transmitter and a receiver for implementing the method.
The transmitter includes two buffers, one for each path, and it includes means for preventing transmission of data at the output of the second buffer and said means are inhibited after transmission of the last cell or packet on the first path.
In one embodiment of the invention, transmitting the last cell or packet on the first path unblocks transmission of cells or packets from the second buffer. Alternatively, the transmission of cells or packets from the second buffer is blocked for a particular time period following the handover command sufficient for the first buffer allocated to the first path to be emptied by transmitting all the cells or packets on the first path.
A receiver in accordance with the invention includes only one demodulator.
The method, the transmitter, and the receiver of the invention can be applied equally well to handing over a single connection or a plurality of connections. However, in the latter case (handing over a plurality of connections), the invention has additional particular features that address the drawbacks of prior art solutions.
Prior art solutions to the problem of packet mode handover from one path to another also have the disadvantage that they take some considerable time to complete handover if there is a multiplicity of connections. This is because:
The cells or packets to be transmitted are held in buffers and there is a buffer for each particular type of waiting data. For example, one buffer (queue) is provided for cells or packets corresponding to telephone conversations, another for electronic mail, and another for image data.
The various queues generally have different rates or speeds. Because the command to hand over or transfer cells or packets is executed on the upstream side of the buffers, the time to transmit the cells or packets to the output of the buffers varies from one buffer to another, i.e. from one connection to another. The time at which each cell or packet is transmitted cannot be predicted, because it depends on the content of each of the buffers and the grade of service each of them is allocated.
Given the above conditions, the time period during which the data is handed over from one path to another cannot be minimized, and this lack of synchronization complicates the transmission of the data.
The invention therefore also provides a method of transmitting non-dated or non-numbered digital data in packet mode between a remote transmitter and a remote receiver in which
Al-Abedeen Tarif Zein
Bousquet Jacques
Calot Guillaume
Denis Xavier
Duros Bertrand
Alcatel
Bost Dwayne
Gelin Jean A
Sughrue & Mion, PLLC
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