DECT/GSM external handover

Details DECT/GSM external handover DECT/GSM external handover

2000-06-21

2001-08-21

Chang, Vivian (Department: 2682)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C455S439000

Reexamination Certificate

active

06278880

ABSTRACT:

FIELD OF INVENTION
This invention relates to external hardover during DECT/GSM interworking, and especially to preventing loss of data during external handover.
BACKGROUND OF INVENTION
FIG. 1
illustrates two DECT (digital European cordless telephone) communication systems interworking with a GSM (global system for mobiles) communication system. The DECT systems each comprise a fixed part
1
,
2
. Each fixed part comprises a cluster control fixed part (CCFP)
3
,
4
and radio fixed parts (RFP)
5
,
6
,
7
,
8
, each of which is connected to the respective CCFP. The CCFPs are connected over an A-Interface
9
to a GSM mobile service centre (MSC)
10
. DECT portable part (PP) handsets
11
,
12
can communicate wirelessly with the RFPs when they are in range. When a portable part moves from the range of one RFP (the “old” RFP) to the range of another RFP (the “new” RFP) there is a need for handover of the wireless communications with the portable part. When the handover is between two RFPs that are under the control of the same CCFP (handover
13
in
FIG. 1
) this can be handled by the DECT system in a procedure called internal handover, with data loss protection. When the handover is between two RFPs that are under the control of different CCFPs anchored by the MSC (handover
14
in
FIG. 1
) the procedure is known as external handover.
Document [14] cited below defines the implementation of bearer services. This takes as its basis that in the DECT/GSM interface the GSM error correcting protocol RLP is terminated to the interworking unit (IWU) of the DECT fixed part, and the DECT air interface (based on the C data profile) provides its own error correction over the DECT air interface by re-transmitting the MAC (medium access control) layer protocol and by the DLC (data link control) layer re-transmitting the LAPU protocol. This protocol model is illustrated in
FIG. 2
, where block
15
represents the DECT portable part, block
16
represents the DECT fixed part and block
17
represents the GSM mobile switch centre interworking unit. Thus, in this standard implementation the RLP and LAPU protocols function independently; the RLP protocol operating over the A-interface (
18
in
FIG. 2
) between GSM and DECT and the LAPU protocol operating over the DECT air interface (
19
in FIG.
2
). The LAPU protocol runs all the way from the CCFP to the PP, so situations where data might be lost in DECT internal handover can be taken care of either by the DECT MAC layer error correction or by the LAPU. However, error correction in the event of external handover is unclear.
In this document the protocol used for the provision of GSM non-transparent service is referred to as the GSM A-interface LAP protocol. A current example of this protocol is RLP but other protocols (such as V.120) could be used in the future. In some situations in this document the RLP has been used as an example.
In the past, specifically for DECT/GSM interworking have not taken care of external handover. However, there is a possibility that data could be lost if external handover takes place whilst data is being transferred. This is because the RLP and LAPU systems function independently. The MSC receives acknowledgement (sent by the DECT fixed part) of a sent data frame before the frame has reached its final destination (the portable part). If external hardover has occurred and the frame has been sent to the wrong CCFP then it cannot be sent to the portable part.
FIG. 3
illustrates the situation where in a data service—in which the system carries signals representing data that has been provided to the system in a digital form by a user of the DECT portable part, or that is to be provided by the system to that user in a digital form—data frames (I-frames) are being transferred in the mobile terminated direction (i.e. in the direction from the MSC to FP) and then, using the DECT LAPU, over the DECT air interface. The first I-frame is passed at
20
from the interworking function of the MSC
21
to the first fixed part (FP
1
)
22
. FP
1
acknowledges the received frame at
23
and then forwards the data at
24
using DECT LAPU to the portable part
25
. The PP acknowledges the data at
26
. If an external handover of the PP from FP
1
to FP
2
(at
27
) then occurs the FP
1
will acknowledge (at
28
) the next frame to the MSC but will not be able to send it to the PP. And FP
2
cannot resend the data to the PP because it does not have the frame. In addition, in the IWF the GSM LAP send window has been incremented after successful I-frame acknowledgement and the data has been lost from the RLP buffers.
FIG. 4
illustrates the situation of mobile originated transmission. In this case no data loss can occur because the PP can re-transmit the data (at
29
) to FP
2
. However, the PP has to be able to continue the same LAPU connection over the new U-plane link, so the interworking unit of FP
2
has to be able to know the status of the state variables (the send and receive counters) of the old link.
Thus the greater problem is encountered when data is being sent towards the portable part (PP) during external handover.
One aim of the invention is to address this problem.
SUMMARY OF THE INVENTION
According to the present invention from one aspect there is provided a method for controlling handover of a DECT portable part from a first DECT fixed part to a second DECT fixed part during provision of a data service, both fixed parts communicating with a GSM mobile switch centre, the method comprising the first fixed part detecting that the handover has been requested, and in response modifying its control of the link between itself and the mobile switch centre to inhibit acknowledgement to the mobile switch centre of data not received by the portable part from the first fixed part.
Preferably the first fixed part may detect that handover has been requested by detecting a request from the portable part (e.g. an {MM-INFO-REQUEST}) or a command from the mobile switch centre (e.g. a {HANDOVER COMMAND}).
Upon handover of the portable part to the second fixed part the second fixed part preferably applies a reset to the link with the mobile switch centre. This reset preferably resets the RLP entity over the link. The link is suitably reset by means of a SABM message.
In response to detecting that the handover has been requested the first fixed part preferably flow controls the link between itself and the mobile switch centre. The first fixed part may flow control the U-plane and/or the RLP and/or data flow. The flow control may be exercised by using bit X. Upon handover of the portable part to the second fixed part the second fixed part preferably releases flow control on the link with the mobile switch centre.
Preferably an interworking unit of the fixed part performs the said modification of control.
As is described in more detail below, one preferred method uses one or more of the following steps:
upon detection of the external handover procedure the interworking unit of the “old” fixed part flow controls the RLP and/or the data flow using the bit X and sends the remaining data in its buffers to the portable part over the LAPU link;
data can then be sent to the MSC direction as long as the U-plane link is present to empty the FP
1
IWU buffers;
a new RLP entity is created in the “new” fixed part IWU when the link between it and the MSC is established after the DECT U-plane LAPU link has been initiated;
upon establishment of the link between the “new” fixed part IWU and the MSC IWF the “new” FP IWU resets the RLP link by issuing a SABM command and releases the flow control if it has not yet been released;
then the data traffic via the “new” fixed part IWU proceeds as normal.
According to the present invention from a second aspect there is provided a method for controlling handover of a DECT portable part from a first DECT fixed part to a second DECT fixed part during provision of a data service, both fixed parts communicating with a GSM mobile switch centre, the method comprising the first fixed part rela

Affiliated with

Also associated with

Rating

DECT/GSM external handover does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with DECT/GSM external handover, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and DECT/GSM external handover will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2530589