U.S. patent application number 09/865460 was filed with the patent office on 2002-06-20 for data communication method and apparatus.
Invention is credited to Feldman, Howard Ray, Kawai, Nobuyuki, Remael, Francois-Arnaud, Smith, Richard Douglas Lane.
Application Number | 20020075826 09/865460 |
Document ID | / |
Family ID | 10772328 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020075826 |
Kind Code |
A1 |
Feldman, Howard Ray ; et
al. |
June 20, 2002 |
Data communication method and apparatus
Abstract
An interface between a data terminal and a digital
communications link implements protocols and frame formats designed
to reduce delays in a data communication with a remote terminal.
The interface overcomes the need to send or receive an HDLC SABME
or UA control signal, and allows data to be sent over the digital
communications link as soon as the communication parameters are
established. In a non-ARQ (error correction) mode, the interface
sends control signals and data over the communications link in
frames subdivided into many small subframes of fixed length, each
subframe having a length code. The interface, when arranged for
connection to the data terminal through a telephone network,
encodes call progress signals from the telephone network for
sending over the digital communications link. The interface encodes
and decodes interrupt signals for sending between the data terminal
and the digital communications link when an interrupt signal is
detected.
Inventors: |
Feldman, Howard Ray;
(Kenton, GB) ; Kawai, Nobuyuki; (Tokyo, JP)
; Smith, Richard Douglas Lane; (Malvern, GB) ;
Remael, Francois-Arnaud; (Pleumeur Bodou, FR) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Family ID: |
10772328 |
Appl. No.: |
09/865460 |
Filed: |
May 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09865460 |
May 29, 2001 |
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08930037 |
Dec 22, 1997 |
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6278696 |
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Current U.S.
Class: |
370/329 ;
370/463 |
Current CPC
Class: |
H04L 69/08 20130101;
H04L 49/90 20130101; H04L 67/14 20130101; H04L 69/324 20130101;
H04L 49/9078 20130101; H04B 7/18532 20130101; H04B 7/1858 20130101;
H04L 9/40 20220501; H04L 12/5601 20130101; H04M 11/06 20130101;
H04L 1/18 20130101 |
Class at
Publication: |
370/329 ;
370/463 |
International
Class: |
H04L 012/66 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 1995 |
GB |
9506759.1 |
Claims
1. A method of operating a data communications interface connected
between a first data terminal and a digital radio frequency
communications link connectable to a second data terminal;
comprising: receiving from the digital communications link a
connection control signal which sets the parameters to be used for
data communication between the first and second terminals; and
sending data received from the data terminal to the digital
communications link in response to receipt of said connection
control signal.
2. A method of operating a data communications interface connected
between a second data terminal and a digital radio frequency
communications link connectable to a first data terminal;
comprising: (a) sending to the digital communications link a
connection control signal which sets the parameters to be used for
data communications between the first and second data terminals;
and (b) sending data received from the second data terminal to the
digital communications link in response to the completion of step
(a).
3. A method as claimed in claim 2, comprising: repeating steps (a)
and (b) until a confirmation signal, which indicates receipt of the
connection control signal by remote equipment, is received from the
digital communications link.
4. A method as claimed in any preceding claim, wherein the data
communications interface is operable in HDLC asynchronous balanced
mode for communication over the digital communications link.
5. A method of operating a data communications between a data
terminal and a digital radio frequency communications link,
comprising: transmitting a signal to the digital communications
link in a format comprising one or more frames each of a constant
frame length, each said frame including a plurality of subframes
each of a constant subframe length and including variable length
information indicative of the length of valid information in that
subframe.
6. A method of operating a data communications interface between a
first data terminal and a digital radio frequency communications
link for connection to a second data terminal, comprising:
receiving a signal from the digital communications link in a format
comprising one or more frames, each of a constant frame length,
each said frame including a plurality of subframes each of a
constant subframe length and including variable length information
indicative of the length of valid information in that subframe,
wherein one of said subframes includes a plurality of data sent by
said second data terminal, and sending the first of said data
received to the first data terminal before the last of said data is
received from the digital communications link.
7. A method of operating a communications interface connected
between a telephone network and a digital communications link,
comprising: receiving a call progress signal from the telephone
network and sending call progress information over the digital
communications link in responses thereto, wherein said call
progress information includes information relating to the frequency
of said call progress signal.
8. A method as claimed in claim 7, wherein the call progress
information includes information relating to the modulation of said
call progress signal.
9. A method of operating a communications interface between a
telephone network and a digital communications link, comprising:
receiving a call progress signal from the telephone network,
selecting one of a predetermined set of call progress codes
according to the type of the call progress signal and sending the
selected call progress code over the digital communications
link.
10. A method as claimed in claim 9, wherein said set of codes
correspond to a ringing, busy and an unobtainable type of call
progress signal respectively.
11. A method of operating a data communications interface between a
data terminal and a digital radio frequency communications link,
said interface having a buffer for storing data for transmission
from the data terminal to the digital communications link,
comprising: receiving an interrupt indication from the data
terminal; and clearing said buffer and sending an interrupt signal
to the digital communications link in response to the receipt of
the interrupt indication.
12. A method of operating a data communications interface between a
data terminal and a digital radio frequency communications link,
said interface having a buffer for storing data received from the
digital communications link for sending to the data terminal,
comprising: receiving an interrupt signal from the digital
communications link; and clearing said buffer and sending an
interrupt indication to the data terminal in response to receipt of
the interrupt signal.
13. A method as claimed in any preceding claim, wherein said
digital communications link comprises a satellite link.
14. Data communications interface apparatus for connection between
a first data terminal and a digital radio frequency communications
link connectable to a second data terminal, comprising: means for
receiving data from the first data terminal; means for detecting
receipt from the digital communications link of a connection
control signal for setting the parameters to be used for data
communication between the first and second data terminals; and
means for sending said received data to the digital communications
link in response to the detection of said connection control
signal.
15. Data communications interface apparatus for connection between
a second data terminal and a digital radio frequency communications
link for communication with a first data terminal, comprising:
means for sending to the digital communication s link a connection
control signal for setting the parameters to be used for data
communications between the first and second data terminals; means
for receiving data from the second data terminal; and means
arranged to send said received data to the digital communications
link in response to the sending of the connection control
signal.
16. Apparatus as claimed in claim 14 or 15, further comprising
means for detecting receipt of a confirmation signal from the
digital communications link, wherein said means for sending the
connection control signal is arranged to repeat the sending of the
connection control signal, and the means arranged to send the
received data is arranged to repeat the sending of the received
data, until receipt of said confirmation signal is detected, said
confirmation signal being indicative of receipt of the connection
control signal by remote apparatus connected to the digital
communications link.
17. Apparatus as claimed in any one of claims 14 to 16, including
an interface operable in HDLC asynchronous balanced mode for
communication over the digital communications link.
18. Data communications interface apparatus for connection between
a data terminal and a digital radio frequency communications link,
comprising: means arranged to send information to the digital
communications link in a format comprising: one or more frames each
of a constant frame length, each said frame including a plurality
of subframes each of a constant subframe length and including
variable length information indicative of the length of valid
information in that subframe.
19. Data communications interface apparatus for connection between
a first data terminal and a digital radio frequency communications
link connectable to a second data terminal, comprising: means for
receiving a signal from the digital communications link in a format
comprising: one or more frames, each of a constant frame length,
each said frame including a plurality of subframes each of a
constant subframe length and including variable length information
indicative of the length of valid information in that subframe,
wherein one of said subframes includes a plurality of data sent by
said second data terminal, and means arranged to send the first of
said data to the first data terminal before the last of said data
is received from the digital communications link.
20. Communications interface apparatus for connection between a
telephone network and a digital communications link, comprising:
means for receiving a call progress signal from the telephone
network and means responsive to said receipt to send call progress
information over the digital communications link, wherein said call
progress information includes information relating to the frequency
of said call progress signal.
21. Apparatus as claimed in claim 20, wherein the call progress
information includes information relating to the modulation of said
call progress signal.
22. Communications interface apparatus for connection between a
telephone network and a digital communications link, comprising:
means for receiving a call progress signal from the telephone
network and means responsive to said receipt to select one of a
predetermined set of codes according to the type of said call
progress signal and to send said selected call progress code over
the digital communications link.
23. Apparatus as claimed in claim 22, wherein said predetermined
set of codes corresponds to ringing, busy and unobtainable call
progress signals respectively.
24. Data communications interface apparatus for connection between
a data terminal and a digital radio frequency communications link,
comprising a buffer for storing data for transmission from the data
terminal to the digital communications link, means for detecting an
interrupt indication from the data terminal; means arranged to
clear said buffer in response to said detection; and means arranged
to send an interrupt indication to the digital communications link
in response to said detection.
25. Data communications interface apparatus for connection between
a data terminal and a digital radio frequency communications link,
comprising a buffer for storing data transmission from the digital
communication link to the data terminal; means for detecting an
interrupt indication from the digital communications link; means
arranged to clear said buffer in response to said detection; and
means arranged to send an interrupt signal to the data terminal in
response to said detection.
26. A satellite earth station including apparatus as claimed in any
one of claims 14 to 25.
27. Data terminal equipment including apparatus as claimed in any
one of claims 14 to 25.
28. A communication system including apparatus as claimed in any
one of claims 14 to 25.
Description
TECHNICAL FIELD
[0001] 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
[0002] 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 (.TM.) or INMARSAT-M (.TM.)
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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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
[0011] An example of data communication between the mobile DTE 2
and the fixed DTE 18 using the INMARSAT-M (.TM.) system will now be
described with reference to FIG. 4.
[0012] Call Set-Up
[0013] In this example, the mobile DTE 2 initiates a call by
sending the code ATD (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.
[0014] Training
[0015] 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:
[0016] (i) satellite/PSTN ARQ or non-ARQ (Automatic Repeat
Request)
[0017] (ii) Maximum data rate
[0018] (iii) 7 or 8 bit data
[0019] (iv) 1 or 2 stop bits
[0020] (v) Odd, even or no parity
[0021] 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.
[0022] 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.
[0023] 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.
[0024] If training is successful, the LES MW 12 sends to the MES
MIU 4 a Connect LCM 80, which indicates the data rate at which
training was successful and confirms satellite/PSTN ARQ/non-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 the mobile
DTE 2.
[0025] Then the LES MIU 12 sends an HDLC SABME (Set Asynchronous
Balanced Mode Extended) signal 84, which is required under the HDLC
protocol to establish HDLC Asynchronous Balanced Mode, to the MES
MIU 4. The MES MIU 4 responds with an HDLC UA (Unnumbered
Acknowledge) signal 86, to indicate that the HDLC SABME signal 84
has been received and data transfer may now take place.
[0026] Data Transfer
[0027] Any data which has already been sent by the mobile DTE 2 in
response to the connect indication 82 is buffered in the MES MIU 4
until the HDLC UA signal 86 has been sent, and any data already
sent by the fixed DTE 18 is buffered in the LES MIU 12 until the
HDLC UA signal 86 has been received. Data transfer 88 occurs
between the MES DTE 2 and the MES MIU 4 through the RS232 link.
Data transfer 90 occurs between the MES MIU 4 and the LES MIU 12
through the satellite 8 as 8-bit data, with start, stop and parity
bits having been removed by the local MIU.
[0028] In 7-bit mode, an extra zero is inserted to fill out each
byte to 8 bits over the satellite link, and is removed by the MIU
which receives the data.
[0029] Data 92 is sent over the PSTN 14 and corresponding data 94
is exchanged between the modem 16 and the fixed DTE 18.
[0030] Data is buffered in the MIUs to accommodate differences in
data rates and non-synchronous operation between the MES DTE 2 and
the fixed DTE 18.
[0031] Call Clearing
[0032] At the end of a data call, the MES DTE 2 sends an ATH (Hang
Up) signal 96 to the MES MIU 4, which in turn sends a call clearing
signal 98 to the MES ACSE 30. The MES ACSE 30 signals channel
release 100 to the LES 10 and the channel is cleared (not shown) by
the network control station. A clearing indication 102 is sent by
the LES ACSE 44 to the LES MIU 12, which sends a call-clearing
indication 104 to the modem 16. Finally, the modem 16 sends a
clearing indication 106 to the fixed DTE, and goes on-hook.
[0033] Fixed Originated Calls
[0034] In a call originated by the fixed DTE 18, as shown in FIG.
5, the flow of signals shown in FIG. 4 is substantially reversed.
Corresponding reversed signals are given the same references as in
FIG. 4, but are dashed. Corresponding non-reversed signals are
given the same references as in FIG. 4.
[0035] Call Set-Up
[0036] Fixed DTE 18 initiates the call by sending an ATD signal 46'
to the modem 16, which sets up a call at 108 to the LES MIU 12. A
call set up indication 48' is sent from the LES MIU 12 to the LES
ACSE 44 and a satellite channel is set up at 50'. Once a data mode
has been set up on the satellite channel, a ringing signal 110 is
sent from the MES ACSE 30 to the MES MIU 4, which sends a ringing
indication 56' to the mobile DTE, and a ringing signal 112 is sent
from the LES ACSE 44 to the LES MIU 12, which sends a ringing tone
114 to the modem 16, causing the modem 16 to send a ringing
indication 116 to the fixed DTE 18.
[0037] In response to the ringing indication 56', the mobile DTE 2
sends an ATA signal 66' to the MES MIU 4, which sends an off-hook
indication 118 to the MES ACSE 30. A connect signal 120 is sent
from the MES ACSE 30 to the LES ACSE 44, which sends a connect
signal 122 to the LES MIU 12.
[0038] Training
[0039] After sending the off-hook indication 118, the MES MIU 4
sends an Establish LCM 64 to the LES MIU 12, which signal is
similar to the Establish LCM 64 in the example shown in FIG. 4.
[0040] In response to receipt of the Establish LCM 64, the LES MIU
12 trains the modem 16 at 76, as in the example of FIG. 4.
[0041] When training is complete, the modem 16 sends a connect
indication 78 to the fixed DTE 18 and the LES MIU 12 sends a
connected indication 70 to the LES ACSE 44, followed by the Connect
LCM 80 to the MES MIU 4, to indicate the rate at which training was
successful. In response to receipt of the Connect LCM 80, the MES
MIU 4 sends a connect indication 82 to the mobile DTE 2. To
complete the training sequence, the LES MIU 12 sends the HDLC SABME
signal 84 to the MES MIU 4, which responds with the HDLC UA 86, as
in the FIG. 4 example.
[0042] Data Transfer
[0043] Data transfer 90 may now take place over the satellite link.
Data transfer 88, 92 and 94 may already have begun, in which case
the data will be buffered at the local MIU.
[0044] Call Clearing
[0045] When data transfer is complete, the fixed DTE 18 sends an
ATH signal 96' to the modem 16 and call clearing proceeds in the
reverse direction to that shown in FIG. 4, except that the MES MIU
4 signals the end of the call by indicating "NO CARRIER" 124 to the
mobile DTE 2.
[0046] Signal Formats
[0047] The format of signals exchanged between the MES MIU 4 and
the LES MIU 12 will now be explained with reference to FIG. 6.
Signals are transmitted on an RF channel with a single channel per
carrier (SCPC). The signal commences with a header portion P,
followed by a variable number of fixed-length SCPC frames SM.sub.1,
SM.sub.2 . . . SM.sub.n. The end of the signal is indicated by an
end portion E.
[0048] Each SCPC frame SM is subdivided into four sections, each
containing a header H.sub.1, H.sub.2, H.sub.3, H.sub.4, a data
field D.sub.1, D.sub.2, D.sub.3, D.sub.4, and dummy bits (shaded).
The data fields D.sub.1 and D.sub.2 together form one or more HDLC
frame, which is repeated in the data fields D.sub.3 and D.sub.4, to
increase the energy per bit. The contents of each HDLC frame depend
on whether data or control information is being sent.
[0049] If data is being sent, the HDLC frame has an information (I)
format shown in FIG. 7, formed from the concatenated data fields
D.sub.1 and D.sub.2. The data is headed by a packet length byte L,
which indicates the length of valid data in the HDLC frame.
[0050] There then follows an address byte A, two control bytes C
and data I to the length indicated by the packet length byte L. The
HDLC frame ends with a cyclic redundancy check CRC. Any unused
bytes following the CRC are filled with random data. The data I is
only accepted by the receiving MIU if the CRC is valid. In ARQ
mode, the receiving MIU requests re-sending of invalid HDLC frames,
while in non-ARQ modes the invalid HDLC frames are discarded.
[0051] The control bytes C include acknowledgement and frame number
information indicating the sequence number of the transmitted frame
and the sequence number of the last frame received correctly.
[0052] Line control messages are sent as unnumbered information
(UI) HDLC frames, of a format shown in FIG. 8. Each unnumbered
information (UI) frame consists of a packet length byte L, an
address byte A, a control byte C, an optional information field I
and a two-byte CRC. Further HDLC frames may follow, with the last
HDLC frame being terminated by hex FF and the remaining available
bytes being filled with random bits. The line control parameters
are encoded in the control byte C and optional information
field.
[0053] A supervisory (S) HDLC frame format is also used for flow
control messages, but this format is not relevant as background to
the present invention.
[0054] However, the above data communications system has not been
implemented and the inventors have identified the following
problems in the implementation of data communications systems of
the type exemplified above.
[0055] Delay
[0056] As can be seen from FIGS. 4 and 5, a delay occurs between
the receipt of the connect indication 82 and the receipt of data at
88, by the MES DTE 2. The expected data may already have been sent
by the fixed DTE 18, but would be buffered at the LES MIU 12.
Likewise, there may be a considerable delay between receipt of the
connect indication 78 by the fixed DTE 18 and the receipt of data
at 94 by the fixed DTE. The expected data may already have been
sent by the mobile DTE 2, but will be buffered at the MES MIU 4
until the HDLC UA 86 has been sent.
[0057] In many protocols operated in DTEs, a timer is set upon
receipt of a connect indication and, if there is a long delay
before data is received, the timer may time out and terminate the
call. One such protocol is PPP, which is used for Internet dial-in
services. These protocols cannot usually be modified by the user.
Therefore, such protocols appear unsuitable for use with a
communications link of the type described above.
[0058] In the protocol described above, it is assumed that the
Connect LCM 80 is correctly received by the MES MIU 4. If it is
not, the call will fail.
[0059] Considerable delays may occur in the link between the MES
MIU 4 and the LES MIU 12. The data generated by a DTE must be
formatted by the local MIU into the frame formats described above,
which incurs a delay because the data for a whole frame must be
received before the length and CRC information can be calculated
and the frame transmitted. The delay is equal to one frame
duration, which is 240 ms in the above example. The receiving MIU
cannot begin to pass the data on until a complete SCPC frame is
received and the CRC and length information can be checked. This
incurs a further delay of one frame duration. The formatting and
unformatting therefore incur approximately 1/2 second delay in
either direction. These delays are in addition to processing delays
at the MIUs and propagation delays in the satellite link, and
result in a significant reduction of data throughput under
protocols typically used by DTEs, particularly non-windowing or
stop and wait protocols, such as X-protocols.
[0060] Mobile User Acceptance
[0061] The mobile user hears a ringing tone from the MES MIU 4 when
the channel is set up in the example shown in FIG. 5, or the
ringing LCM 60 is received. However, this ringing tone is generated
locally by the MES MIU 4 and may not resemble the ringing tone
which would be heard from the PSTN 14 if the mobile DTE 2 were
instead connected by a conventional modem over the PSTN to the
modem 16 and fixed DTE 18.
[0062] User acceptance of new communications systems is hampered by
perceived differences from the system with which a user is
familiar, so that the mobile user may doubt whether a call has been
set up if the ringing tone is locally generated and does not vary
according to the location of the fixed DTE 18.
[0063] Interrupt Signals
[0064] An interrupt signal, such as "break" or "Ctrl-C" is often
used during connection to an on-line database, for example to
interrupt a long undesired listing from the database. However, no
means are provided in the protocols implemented by the MES MIU 4
and the LES MIU 12 in the above example for identifying an
interrupt request over the satellite link, or for controlling data
flow in that situation.
SUMMARY OF THE INVENTION
[0065] According to one aspect of the present invention, there is
provided interface apparatus for connection between a data terminal
and a communications link which sends data received from the data
terminal to the communications link under an HDLC protocol when it
has received or sent a connection signal which establishes the
parameters of a call, without sending an HDLC SABME command or an
HDLC UA response. In this way, the delay between connection to a
remote data terminal and receipt of the data by the remote terminal
is reduced.
[0066] According to another aspect of the present invention, there
is provided interface apparatus for connection between a data
terminal and a communications link, which interface is arranged to
send a connect command which establishes the parameters of a call,
and which requires a response, to the communications link, and to
send data received from the data terminal to the communications
link before a response to the connect command is received. In this
way, data is available at a remote interface as soon as it connects
to a remote data terminal in response to the connect command.
[0067] According to another aspect of the present invention, there
is provided interface apparatus for connection between a data
terminal and a communications link, which is arranged to send data
from the data terminal to the communications link in a format
comprising a frame having a data field including a plurality of
subframes each of which comprises only length information and data.
In this way, the interface need only delay transmission by one
subframe duration, in order to calculate the length information,
instead of by a whole SCPC frame duration. In addition, the remote
interface can start sending data to a remote data terminal as soon
as it is received from the communications link.
[0068] According to another aspect of the present invention, there
is provided interface apparatus for connection between a first and
a second communications link, which is arranged to receive a
ringing signal from the first communications link, to discriminate
either the type or the frequency and modulation of the ringing
signal, and to send to the second communications link a selected
one of a predetermined set of codes in accordance with the
discrimination. As a result the ringing tone may be reproduced or
indicated at equipment connected to the second communications
link.
[0069] According to another aspect of the present invention, there
is provided an interface between a first communications link and a
second communications link, which is arranged to encode an
interrupt signal from the first communications link, to send the
encoded interrupt signal over the second communications link and to
clear data from a data buffer in response to receipt of the
interrupt signal.
[0070] In this way, interrupt signals may be sent without delay
between communications links.
BRIEF DESCRIPTION OF THE DRAWINGS
[0071] Specific embodiments of the present invention will now be
described with reference to the accompanying drawings in which:
[0072] FIG. 1 shows a link between data terminals over a satellite
link and a PSTN;
[0073] FIG. 2 is a functional block diagram of a mobile earth
station and its associated interface to a data terminal;
[0074] FIG. 3 is a functional block diagram of a fixed earth
station and its associated interface to a PSTN;
[0075] FIG. 4 is a protocol diagram showing a data call originated
from a mobile earth station;
[0076] FIG. 5 is a protocol diagram showing a data call originated
from a fixed earth station;
[0077] FIG. 6 is a diagram of the format of an SCPC signal sent
over the satellite link;
[0078] FIG. 7 is a diagram of the format of an HDLC information
frame;
[0079] FIG. 8 is a diagram of the format of HDLC unnumbered
frames;
[0080] FIG. 9 is a protocol diagram showing a modification to the
protocols shown in FIGS. 4 and 5 in a first embodiment of the
present invention;
[0081] FIG. 10 is a flowchart of the operation of the MES MIU 4 in
FIG. 9;
[0082] FIG. 11 is a flowchart of the operation of the LES MIU 12 in
FIG. 9;
[0083] FIG. 12 is a diagram of the format of mini-frames used in a
non-ARQ mode in the first embodiment;
[0084] FIG. 13 is a diagram of the format of the ringing LCM 60 in
the first embodiment;
[0085] FIG. 14 is a diagram of the format of an SCPC signal in a
second embodiment of the present invention;
[0086] FIG. 15 is a diagram of the format of the HDLC information
frame used in ARQ mode in the second embodiment;
[0087] FIG. 16 is a diagram of the format of HDLC unnumbered frames
used in ARQ mode;
[0088] FIG. 17 is a diagram of the format of mini-frames used in
non-ARQ mode.
MODES FOR CARRYING OUT THE INVENTION
First Embodiment
[0089] An embodiment of the present invention, which is a
modification of the protocol shown in FIGS. 4 and 5, is shown in
FIG. 9. The diagram of FIG. 9 replaces the protocol exchange, in
the section from the Establish LCM 64 to the data transfer 88, 90,
92 and 94 in FIGS. 4 and 5. The operation of the MES MIU 4 is
described with reference to FIG. 10, and the LES MIU 12 with
reference to FIG. 11. As soon as the satellite channel has been set
up at 50 or 50' (step 200) the MES MIU 4 begins to send the
Establish LCM 64 (step 202).
[0090] If the MES DTE 2 originated the call (step 222), as in FIG.
4, the LES MIU 12 responds to the channel being set up (step 220)
by sending the ringing LCM 60 to the MES MIU 4 (step 224). The LES
MIU 12 then awaits receipt of the Establish LCM 64 (step 226)
before beginning to train the modem 16 (step 228). Once training 76
is successful, the LES MIU 12 begins to send the Connect LCM 80
(step 230). Meanwhile, the modem 16 sends the connect indication 78
to the fixed DTE 18, which begins to send data 94, 92 to the LES
MIU 12. After the LES MIU 12 has sent the Connect LCM 80, the
remainder of the SCPC frame SM is vacant. The LES MIU 12 detects
whether data has been received over the PSTN 14 (step 210) and, if
so, fills the remainder of the SCPC frame SM with the data 90. The
MES MIU 4 receives the Connect LCM 80 (step 204) in response to the
Establish LCM 64, echoes the Connect LCM' 80 to the LES MIU 12
(Step 206) and sends the connect indication 82 to the mobile DTE 2
(step 208). The MES MIU 4 then receives the data 90 (step 210) and
sends it as data 88 to the mobile DTE 2 (step 212). The mobile DTE
2 responds to the connect indication 82, by sending data 88' to the
MES MIU 4 (step 214), which is sent to the LES MIU 12 (step 216)
immediately after the echo 80' to the Connect LCM 80.
[0091] The LES MIU 12 continues to repeat the connection LCM 80
(step 230) in every SCPC frame SM, including the data 90 (steps
232, 234), until the echo 80' is received (step 236). Subsequently,
the data 90' is sent as data 92' to the modem 16 and as data 94' to
the fixed DTE 18. No SABME 84 is sent by the LES MIU 12 or awaited
by the MES MIU 4 before data can be sent. The SABME 84 can be
dispensed with because it was decided that the MES MIU 4 and LES
MIU 12 would always operate in asynchronous balanced mode and
therefore there is no longer a need to send the SABME/UA signal and
response during call set up, which are required by the HDLC
protocols. Data can be received by the DTEs as soon as they receive
a connect indication, so that the risk of certain DTE protocols
timing out is substantially reduced.
[0092] The SABME/UA exchange may, however, be used to reset the
satellite data link.
[0093] Mini-Frames
[0094] A frame format used in this embodiment will now be described
with reference to FIGS. 12 to 14.
[0095] In a non-ARQ mode, no error recovery procedures are used on
the satellite link and it is therefore possible to depart from the
HDLC frame format in order to overcome the problems of throughput
delay. The HDLC frame format used in non-ARQ mode is shown in FIG.
12.
[0096] The data field D.sub.1 comprises six mini-frames m.sub.1 to
m.sub.6 and the data field D.sub.2 comprises six further
mini-frames m.sub.7 to m.sub.12. The mini-frames are repeated in
data fields D.sub.3 and D.sub.4. Each mini-frame m comprises 6
bytes, consisting of a length byte L and five data bytes b.sub.1 to
b.sub.5. The contents of each mini-frame m are as shown in Table 1
below.
1TABLE 1 Type L b1 b2 b3 b4 b5 I FA 50 * 5F * * 93 * * * A0 * * * *
6C * * * * * UI 05 Address Control Info CRC CRC
[0097] An information (I) type mini-frame m may include from zero
to five bytes of valid data, and the number of valid bytes is
indicated by the relevant hex code for the length byte L shown in
Table 1. No CRC, address or control bytes are included. The I type
mini-frame is used to carry user data.
[0098] An unnumbered information (UI) type mini-frame m has the
length byte L set at "05", and includes an address byte b.sub.1, a
control byte b.sub.2, an information byte b.sub.3 and two CRC bytes
b.sub.4 and b.sub.5. The UI type mini-frame is used to send line
control messages (LCMs), which are encoded in the information byte
b.sub.3, as shown in Table 2 below.
2TABLE 2 Ack bit bit bit bit bit bit bit bit Command/Response type
7 6 5 4 3 2 1 0 Connection at 1200 bps 2 1 1 1 0 0 0 0 0 without
ARQ (from LES) Connection at 2400 bps 2 1 1 1 1 0 0 0 0 without ARQ
(from LES) ringing (from LES) 3 0 0 0 0 0 0 1 1 Break (both) 3 0 0
0 0 0 1 0 0
[0099] The heading "Ack Type" in Table 2 above refers to the type
of acknowledgement required for that LCM. A Type 2 LCM is repeated
until an echo is received or until a timer times out; the Connect
LCM 80 is an example of this. A Type 3 LCM is sent only once and is
echoed by the remote MIU. A Type 1 LCM (not shown in Table 2) is
repeated until it is acknowledged by a different LCM; the Establish
LCM 64 is an example of this. The Establish LCM 64 is not encoded
in a mini-frame, since it is sent before non-ARQ mode can be
established.
[0100] When encoding data into mini-frames, an MIU need only
assemble enough data for one mini-frame, in order to calculate the
length code L, before sending that mini-frame, in contrast to the
normal HDLC information frame for which 66 bytes must be assembled
and the CRC calculated before the data can be sent. I and UI
mini-frames can be accommodated within the same SCPC frame. In the
protocol shown in FIG. 9, this feature allows data 90 to be sent
immediately after the Connect LCM 80.
[0101] If less than five bytes of data are available for formatting
at an MIU, the number of bytes available will be formatted in a
mini-frame and the length byte L will be set accordingly. The
remaining bytes are filled with random bits.
[0102] In normal operation, when connection is achieved at 2400
bit/s, the satellite link, the link between the MES DTE 2 and the
MES MIU 4 and the link between the LES MIU 12 and the modem 16
operate at nominally the same rate. However, the links are in fact
plesiochronous, since there is no means provided for synchronising
them.
[0103] If the data input rate to an MIU from the local DTE exceeds
the output rate of the MIU to the satellite link, the excess data
will be buffered until the buffer 22 or 34 is full, whereupon flow
control signals are sent back to the local DTE. If the data rate
from the local DTE is less than the output rate to the satellite
link, mini-frames will be sent with less than five bytes per
mini-frame.
[0104] Call Progress Indication
[0105] The format of the ringing LCM 60 will now be described with
reference to FIG. 13. In this example, the LES MIU 12 has not yet
received the Establish LCM 64 and ARQ or non-ARQ mode are not
established so that standard HDLC frames or mini-frames are not
used.
[0106] FIG. 13 shows a sequence of SCPC frames sent by the LES MIU
12 from the point at which a data mode channel is set up. The SCPC
frames are subdivided into UI ringing frames r which indicate
whether a ringing tone is present on the PSTN 14, and are
terminated by a hex FF byte. Ring on frames r.sub.1 indicate
ringing, whilst ring off frames r.sub.0 indicate no ringing. Thus,
in frame SM.sub.2, the frames begin as r.sub.0, change to r.sub.1
at point R.sub.1 where ringing begins and revert to r.sub.0 at
point R.sub.0 where ringing ends.
[0107] If the ringing tone from the PSTN 14 is repeated, the frames
r.sub.1 are again transmitted by the LES MIU 12 while the tone is
present. The LES MIU 12 continues to sending ring off frames
r.sub.0 during receipt of the answer signal 68 and modem training
76, which is completed at point T.sub.c. From the beginning of the
next SCPC frame SM.sub.n+1, at T.sub.0, the LES MIU 12 uses
standard HDLC frames in ARQ mode or mini-frames in non-ARQ mode,
depending on the error recovery mode established by the Establish
LCM 64.
[0108] In ARQ mode, the frame SM.sub.n+1 comprises a standard HDLC
unnumbered frame containing the Connect LCM 80 and optionally one
or more I frames, while in non-ARQ mode the frame SM.sub.n+1
comprises mini-frames m.sub.1 to m.sub.12 including the Connect LCM
80 and optionally one or more I frames containing any available
data.
[0109] The MES MIU 4 receives the ringing frames r and signals the
ringing indication 62 to the mobile DTE 2 in response to onset of
the ring on frames r.sub.1. The MES MIU 4 includes an audible tone
generator which is activated by the ring on frames r.sub.1 and
therefore reproduces the cadence of the ringing signal from the
PSTN 14. This reassures the user that the ringing tones actually
represent ringing at the PSTN 14.
[0110] In one embodiment, the LES MIU 12 analyses the frequency and
tone modulation of the ringing tone 58 and encodes these in the
ring on frames r.sub.1. The MES MIU 4 decodes the frequency and
modulation and activates the audible tone generator to reproduce
the ringing tone 58 accurately.
[0111] In another embodiment, as well as encoding ringing signals,
the LES MIU 12 also encodes "busy" and "unobtainable" signals
received from the PSTN 14, so that the mobile user gains more
complete information on call progress at the PSTN 14.
[0112] The LES MIU 12 compares the signals from the PSTN 14 with
known "ringing", "busy" and "unobtainable" signals and selects a
corresponding code for sending the MES MIU 4. The code is sent in a
UI frame, in the same way as the ring on frames r.sub.1.
[0113] Alternatively, if the PSTN 14 is of the type which generates
sub-band call progress signals instead of call progress tones, the
LES MIU 12 may generate a corresponding code directly in response
to the sub-band call progress signals without detecting their
duration or frequency.
[0114] An additional advantage of encoding the type of the call
progress signals is that the MES DTE 2 may display the call
progress status, so that the user does not have to recognise what
status is meant by the reproduced tones.
[0115] Interrupt Signals
[0116] A DTE user wishing to interrupt the progress of a data call
may enter a "break" command at the DTE. It is important that the
"break" command should reach the remote DTE as soon as possible,
since it usually indicates that unwanted data is being received or
the user wishes to terminate the call abruptly. Therefore, when the
LES MIU 12 receives a "break" command from the fixed DTE 18 over
the PSTN 14, or the MES MIU 4 receives a "break" command from the
mobile DTE 2, it discards any buffered data awaiting transmission
over the satellite link and sends a break signal in the next frame
as an LCM. The format of a mini-frame break LCM is shown in Tables
1 and 2 above, for sending in non-ARQ mode.
[0117] In ARQ mode, the break signal is sent in an unnumbered HDLC
frame. The receiving MIU receives the break signal over the
satellite link and transmits a break command to the local DTE
immediately, discarding any buffered data to be sent to the local
DTE. In this way, break commands can be sent rapidly to the remote
DTE. As well as "break" commands, other interrupt commands such as
"Ctrl-C" may be encoded and the buffers cleared in the same
way.
[0118] The above embodiments have been described with reference to
the Inmarsat-M (.TM.) asynchronous data service. However, the
embodiments may be applied to the Inmarsat-B (.TM.) asynchronous
data service, with certain modifications described below. The
Inmarsat-B (.TM.) system is capable of data rates of up to 9600
bit/s and therefore different signal formats, frame lengths and
parameter codes are needed.
Second Embodiment
[0119] FIG. 14 shows the format of an Inmarsat-B (.TM.) signal on a
single RF channel. The signal begins with a header portion P,
followed by SCPC frames SB.sub.1 to SB.sub.n and terminated by an
end signal E.
[0120] Each SCPC frame contains 1872 bits and has a duration of 80
ms; it is subdivided into four data fields D.sub.1 to D.sub.4 each
preceded by a sub-band signalling field S.sub.1 to S.sub.4. Data
fields D.sub.3 and D.sub.4 do not repeat data fields D.sub.1 and
D.sub.2, but contain additional data.
[0121] FIG. 15 shows the contents of the data fields D.sub.1 to
D.sub.4 concatenated to form an HDLC information (I) frame,
comprising a length byte L, address byte A, control byte C,
variable length data I up to a maximum of 98 bytes, and a two-byte
CRC. The length byte L indicates the position of the CRC and any
remaining bytes after the CRC are filled with random bits.
[0122] In the format shown in FIG. 16, the contents of the data
fields D.sub.1 to D.sub.4 are concatenated at the receiving MIU to
form one or more unnumbered information (UI) HDLC frames U.sub.1 to
U.sub.n. Each UI frame comprises a length byte L, an address byte
A, a control byte C, an optional information field I and two CRC
bytes. After the last frame U.sub.n, a byte is set to hex FF and
the remaining available bytes are filled with random bits. Each UI
frame may carry an LCM.
[0123] FIG. 17 shows the format of mini-frames m used for
Inmarsat-B (.TM.). Each data field D contains two mini-frames of 13
bytes each, with the last two bits of the data field D being
unused. The format of each mini-frame m depends on the type of the
mini-frame, as shown in Table 3 below.
3TABLE 3 Type L b.sub.1 b.sub.2 b.sub.3 b.sub.4 b.sub.5 b.sub.6
b.sub.7 b.sub.8 b.sub.9 b.sub.10 b.sub.11 b.sub.12 I AF I 3A * I CA
* * I F6 * * * I F9 * * * * I C6 * * * * * I F5 * * * * * * I OA *
* * * * * * I 39 * * * * * * * * I 9A * * * * * * * * * I E2 * * *
* * * * * * * I FC * * * * * * * * * * * I 7B * * * * * * * * * * *
* UI 05 A C I CRC CRC FF random random random random random
random
[0124] An information (I) type mini-frame has one length byte L and
twelve data bytes b.sub.1 to b.sub.12. The length byte L indicates
the number of valid data bytes b, as in the first embodiment, with
any unused bytes containing random bits. An unnumbered information
(UI) type mini-frame uses only bytes b.sub.1 to b.sub.5. Byte
b.sub.6 contains hex FF, while bytes b.sub.7 to b.sub.12 contain
random bits. The information I carried in byte b.sub.3 is shown in
Table 4 below.
4TABLE 4 Ack bit bit bit bit bit bit bit bit Command/Response type
7 6 5 4 3 2 1 0 Connection at 1200 bps 2 1 1 1 0 0 0 0 0 without
ARQ (from the CES) Connection at 2400 bps 2 1 1 1 1 0 0 0 0 without
ARQ (from the CES) Connection at 4800 bps 2 1 1 1 1 1 0 0 0 without
ARQ (from the CES) Connection at 9600 bps (or 2 1 1 1 1 1 1 0 0
greater) without ARQ (from CES) Break (both) 3 0 0 0 0 0 1 0 0
[0125] In the Connect LCM 80, data rates of 1200, 2400, 4800 or
9600 bit/s may be indicated, which are the data rates supported by
Inmarsat-B (.TM.). The acknowledge type corresponds to the type
described above with reference to Table 2.
[0126] In the above description, the modem interface units may
either be separate units from the DTEs and earth stations or may be
integrated with their respective earth stations. Furthermore, the
mobile DTE 2, the MES MIU 4 and the MES 6 may all be incorporated
in a single mobile unit.
[0127] The MES MIU 4 and the MES 6 may be a fixed installation and
may serve a local network which connects many DTEs to the MES MIU
4. The PSTN 14 may be replaced by a local network. These and other
variants are well-known to the skilled person.
[0128] The present invention is not limited to data service systems
of the Inmarsat-M (.TM.), Inmarsat mini-M (.TM.) or Inmarsat-B
(.TM.) type. Instead, the skilled person will recognise that the
protocols and formats described may be applied to other
communications systems having interfaces between communications
links conforming to different standards, and in particular systems
in which delay may be incurred by the protocols or formats
implemented by such interfaces or in which it is desirable to
emulate call progress tones from conventional single links in such
systems or in which break signals must be handled without
delay.
[0129] The present invention may advantageously be applied to a
communications system including a satellite link, but is also
applicable to terrestrial cellular communications systems, and
other systems including a terrestrial radio frequency link or other
types of link.
* * * * *