Communication method and apparatus

Details Communication method and apparatus Communication method and apparatus

1997-12-22

2001-08-21

Ngo, Ricky (Department: 2664)

Multiplex communications

Duplex

Communication over free space

C370S315000, C370S338000

Reexamination Certificate

active

06278696

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a data communication method and apparatus for use in radio frequency communications and in particular, but not exclusively, to an interface for connecting a data terminal, either directly or indirectly, to a digital radio frequency communications link.
BACKGROUND ART
One example of a radio-frequency communications link for use in connecting data terminal equipment (DTE) is an asynchronous data service proposed for the INMARSAT-B™ or INMARSAT-M™ satellite communications system, as described for example in Chapters 12 and 14 of “Satellite Communications: Principles and Applications” by Calcutt and Tetley, 1st edition, published by Edward Arnold.
The overall layout of the satellite communications system, when used for data communications, is shown in
FIG. 1. A
mobile DTE
2
is connected via an RS232C interface to a modem interface unit (MIU)
4
. The MIU
4
simulates a Hayes-compatible modem and is able to decode Hayes-type commands from the mobile DTE
2
, so that off-the-shelf communications software may be used in the mobile DTE
2
. The MIU
4
does not perform modulation or demodulation in this case, since it is not connected to an analog line. Instead, the MIU
4
provides an interface to a mobile earth station (MES)
6
which allows communication via a satellite
8
to a fixed or land earth station (LES)
10
. The LES
10
is connected to an LES MIU
12
which interfaces the satellite link to a public switched telephone network (PSTN)
14
and therefore functions as a modem to convert analog signals on the PSTN
14
to digital signals on the satellite link, and vice versa. A fixed DTE
18
is connected to the PSTN
14
through a modem
16
of standard type. Alternatively, the LES MIU
12
may be connected to the fixed DTE via an ISDN and an ISDN adapter, or via another type of network allowing data to be sent in another format.
FIG. 2
shows the MES MIU
4
and the MES
6
in greater detail. The MES MIU
4
comprises a DTE interface
20
, which provides an RS232 physical interface and emulates an AT.PCCA type modem, i.e. it complies with the minimum functional specification for data transmission systems published by the Portable Computer and Communications Association (PCCA), including the use of the AT command set and responses.
Data received by the DTE interface
20
is sent to a buffer
22
, which is in turn connected to an MES interface
24
. The MES interface
24
implements, in ARQ (automatic repeat request) mode, a variant of the HDLC (High Level Data Link Control) protocol, as defined in ISO recommendations ISO/IEC 3309, ISO/IEC 4335: 1993 and ISO/IEC 7809: 1993. The particular version employed is ISO HDLC BAC 3.2, 4, 8, 10, 12 as defined in ISO 7809: 1993 (synchronous, two-way simultaneous, duplex, non-switched). A controller
26
controls the operation of the interfaces
20
and
24
and the flow of data through the buffer
22
.
The MES includes an RF modulator/demodulator
27
, connected to an antenna
28
, for RF modulating the output of the MES interface
24
and transmitting the output through the antenna
28
to the satellite
8
, and for RF demodulating RF signals received from the satellite
8
through the antenna
28
and sending the demodulated signals to the MES interface
24
. The MES
6
also includes access control and signalling equipment (ACSE)
30
, for setting up and clearing the satellite link, which exchanges data with the controller
26
of the mobile MIU
4
.
The MES ACSE
30
communicates with a network control station (NCS) which allocates communications channels and supervises communications traffic through the satellite
8
and communicates with further ACSE at the LES.
The mobile MIU
4
, MES
6
and ACSE
30
may be integrated in a mobile unit and the antenna
28
may be integrated or connected externally with the mobile unit.
FIG. 3
shows the LES
10
and the LES MIU
12
in greater detail. The LES MIU
12
includes a modem
31
for demodulating analog signals from the PSTN
14
and modulating digital signals for the PSTN
14
, and a modem interface
32
which supports modem protocols such as V.42 error correction, for communication with the modem
16
. If the PSTN
14
is a digital network, a suitable interface is used instead of the modem
30
.
The modem interface
32
is connected through a buffer
34
to an LES interface
36
, which implements protocols compatible with the MES interface
24
, so that data can be exchanged between the LES MIU
12
and the MES MIU
4
. A controller
38
supervises the operation of the modem interface
32
, buffer
34
and LES interface
36
. The LES interface
36
is connected to an RF modulator/demodulator
40
which modulates signals for transmission to the satellite
8
through an antenna
42
, and demodulates signals received from the satellite
8
though the antenna
42
. Call set-up and clearing are controlled by an LES ACSE
44
within the LES
10
which exchanges signals with the LES MIU
12
, the MES ACSE
30
, and the network control station (NCS).
EXAMPLE
An example of data communication between the mobile DTE
2
and the fixed DTE
18
using the INMARSAT-M™ system will now be described with reference to FIG.
4
.
Call Set-up
In this example, the mobile DTE
2
initiates a call by sending the code AND (dial)
46
to the MES MIU
4
, which sends a dialling indication
48
to the MES ACSE
30
. A call is then set up during the period
50
by exchanging call set-up signals between the MES ACSE
30
, the LES ACSE
44
and the network control station. When the call has been set up on the satellite link, a dialling signal
52
is sent from the LES ACSE
44
to the LES MIU
12
, which dials the requested number using a dialling sequence
54
over the PSTN
14
. The modem
16
sends a ringing indication
56
to the fixed DTE
18
, and the PSTN
14
sends a ringing tone
58
to the LES MIU
12
. The LES MIU
12
sends a Ringing line control message (LCM)
60
to the MES MIU
4
, which in turn sends a ringing indication
62
to the mobile DTE
2
to indicate that the call has been successfully set up.
A line control message (LCM) typically requires a response from the MIU that receives it. The response may either be another LCM or an echo of the original LCM, if no response is available.
Training
The parameters of the call must now be established. The MES MIU
4
sends an establish line control message (LCM)
64
, which requests options to be supported, to the LES MIU
12
. The parameters requested in the Establish LCM
64
are:
(i) satellite/PSTN ARQ or non-ARQ (Automatic Repeat Request)
(ii) Maximum data rate
(iii) 7 or 8 bit data
(iv) 1 or 2 stop bits
(v) Odd, even or no parity
Options (iii) to (v) relate to the data format to be used in the link between the mobile DTE
2
and the MES MIU
4
and between the LES MIU
12
and the fixed DTE
18
. ARQ mode can be set independently in (i) for the satellite link and PSTN link.
The fixed DTE
18
responds to the ringing indication
56
with an ATA (answer) signal
66
. The modem
16
sends an answer signal
68
to the LES MIU
12
, which causes a connect signal
70
to be sent to the LES ACSE
44
, the MES ACSE
30
and the MES MIU
4
.
Next, the LES MIU
12
attempts to establish a reliable data rate for communication with the fixed DTE
18
by means of a training sequence
76
. For example, the LES MIU
12
sends a test signal at 2400 bit/s and detects whether the test signal is confirmed by the modem
16
. If it is not confirmed, a test signal is sent at 1200 bit/s and the LES MIU
12
awaits confirmation from the modem
16
. If no confirmation is received, training is unsuccessful and the call cannot proceed.
If training is successful, the LES MIU
12
sends to the MES MIU
4
a Connect LCM
80
, which indicates the data rate at which training was successful and confirms satellize/PSTN ARQ
on-ARQ modes, and the modem
16
sends a connect indication
78
to the fixed DTE
18
. In response to the Connect LCM
80
, the MES MIU
4
sends a connect indication
82
, including the call parameters, to

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