ATM switching architecture for a wireless telecommunications...

Details ATM switching architecture for a wireless telecommunications... ATM switching architecture for a wireless telecommunications...

1998-08-13

2001-05-29

Olms, Douglas (Department: 2661)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S397000

Reexamination Certificate

active

06240078

ABSTRACT:

TECHNICAL FIELD
The invention relates to an ATM switching architecture for a wireless telecommunications network, comprising a plurality of ports for a plurality of access points of a wireless cellular communication system and means for performing switching operations and mobility control.
PRIOR ART
In order to provide mobile communication services, modern telecommunications networks include switching stations (switching centers) each handling a plurality of access points (base stations) for establishing radio links to mobile terminals. One single switching station not only establishes the connection between the mobile terminal and any requested other terminal in the network, but it also deals with the mobility control for the mobile terminal. This control manages the handover if the radio link deteriorates during the ongoing connection and if a new radio link has to be established. The new radio link may be established to the access point of a neighbouring cell or to that one in the same cell.
There is a recent need for a flexible and powerful data transfer over the telecommunications network. This need may be met by ATM (Asynchronous Transfer Mode). ATM is based on the concept of virtual paths (VP) and virtual channels (VC). An ATM-architecture is known e.g. from U.S. Pat. No. 5,633, 868.
U.S. Pat. No. 5,633,868 discloses several methods for performing so-called hand-offs (which is a process of changing the route of information flow to and/or from the wireless terminal to a fixed point e.g. a radio port). There are hard and soft hand-offs (both fast and slow). A hard hand-off sets up a new route and almost concurrently tears down the old route; it is used by wireless access technologies that do not use diverse air routes. A soft hand-off, on the other hand, is employed by systems with diverse air routes. The fast hand-off is accomplished by either completely or partly pre-establishing the VP/VCs between a given Packet Handler and all of its associated radio ports and by only activating those resources that are actually needed at a given time. However, if there is no pre-established VP/VC, the hand-off is slow.
ATM operates connection oriented, i.e. a connection is established before ATM cells are transmitted. Establishing and releasing connections is done via signalling operation (via signalling channels). In an ATM-system, there is one or a plurality of VCs emanating simultaneously from one terminal. In the ATM switch, these VCs enter through a single port of the switch, but may end at different destination ports in the switch. This does not put any problems in a fixed network which does not require the switching of the VCs from one port to another port during an on-going connection. However, a cellular communications network has to deal with handovers and therefore with switching between several physical ports. The switching operation has to take place as quickly as possible in order to minimize the loss of data (or to minimize the buffer capacity required for a so called lossless or seamless handover).
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a switching architecture in a telecommunications network, wherein the hardware is able to make quick handovers for ATM connections.
The object is achieved by the features defined in claim
1
. According to the invention, the means for performing switching operations and mobility control is separated into two units with different functionality. The first unit is performing the mobility related switching (mobility control), wherein a virtual path which contains all virtual channels of a mobile terminal is switched from one port that is associated with an access point to a port that is connected to the second unit. The second unit is performing the call related switching operation (e.g. switching of permanent VCs, switching of signaled VCs), wherein the virtual channels of the input port are distributed to different destination (or target) ports in the unit.
If a handover is required, it is only necessary that the first unit performs a redirection of all VCs to an alternate port. All VCs can be handled as if they were in one “pipe”, so that said “pipe” can be directed as a whole to a new port. The switching unit that handles the VP/VC connections has nothing to do with the handover, it can work as if there was no handover at all. That means that it may deal in a conventional manner with setting up the connection and with allocating the VCs (e.g. user channel, signalling channel) to the different ports.
The proposed solution has the following advantages:
The ATM switch does not have to switch each of the VCs individually (which may each join to different ports or which may terminate in the switch, as is the case for the signalling channel).
It is not necessary to have a switch architecture that is able to perform seamless switching of connections. Nevertheless, the handover does not require the implementation of shifting VP cross-connections.
The problem of having to do a CAC (connection admission control) recalculation for each individual VC (in order to assure that the new port has sufficient bandwidth to accept the VCs) can be avoided.
The two units may be realized within the same hardware but in separate functional units (a hybrid VP/VC switch). The control program defines whether the unit is for mobility control or for VC switching. This minimizes the costs of the hardware. It is not necessary to develop and to produce different components. The two units of the hardware component are connected in series by means of a fixed (external or internal) loopback link. One particular port of the mobility control unit is therefore connected with one particular port of the connection related switching unit.
There may be two physically separate dedicated hardware components. It is an advantage of the present ATM switch architecture that it can be used to modify (i.e. extend the operation of) existing ATM switches by simply inserting a front-end module between the access points (that serve for wireless communication links) and the conventional ATM-switch.
The intra-switch handover is very simple. If the existing radio link between the access point and the mobile terminal undergoes degradation or breaks down, the mobility control unit selects the new access point (which may be determined either by the mobile terminal or by the mobility control) and reassigns the Virtual Path Identifier (VPI) to the corresponding new port. All VCs are therefore shifted to the new port by one operation.
The VP/VC connection related switching unit may be of a conventional type. As it is known in the art, an ATM switch contains a signalling unit that is connected to the mobile terminal by a virtual channel for VP/VC connection related signalling (e.g. SETUP, CONNECT, RELEASE; see e.g. The ATM Forum: ATM USER-NETWORK INTERFACE SPECIFICATION Version 3.1, Prentice Hall, 1995). This means that there are at least two VCs that terminate at different destination entities in the ATM switch. In many cases, there will even be a plurality of user VCs that branch off to different destinations.
According to a preferred embodiment, a handover buffer is placed (inserted) in the fixed link between the two functional units. The buffer may be a FIFO unit that always contains copies of a certain number of preceding ATM-cells. If cells get lost in the radio channel because of a handover, they may be transmitted again by simply recalling the buffered cells and transmitting them via the newly established radio link.
Further details of the invention as well as different combinations of preferred features will become evident on the basis of the following description.


REFERENCES:
patent: 5787077 (1998-07-01), Kuehnel
patent: 5875185 (1999-02-01), Wang et al.
patent: 5889769 (1999-03-01), Mizuno
patent: 6023461 (2000-02-01), Raychaudhuri et al.
patent: 0679042A2 (1995-10-01), None
patent: 0749261A2 (1996-12-01), None
patent: WO 94/28645A1 (1994-12-01), None
patent: WO 97/29605A1 (1997-08-01), None

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