U.S. patent application number 10/751465 was filed with the patent office on 2004-08-19 for method and apparatus for transmitting data.
This patent application is currently assigned to Inmarsat, Ltd.. Invention is credited to Kawai, Nobuyuki, McTiffin, Michael John, Randall, David, Wong, Hok Shuen.
Application Number | 20040163024 10/751465 |
Document ID | / |
Family ID | 10797774 |
Filed Date | 2004-08-19 |
United States Patent
Application |
20040163024 |
Kind Code |
A1 |
Kawai, Nobuyuki ; et
al. |
August 19, 2004 |
Method and apparatus for transmitting data
Abstract
Data relating to the status of users 12 in a mobile
communications system is broadcast via satellite 8 from a network
register 4 to local registers 10 in an HDLC format. Each of the
local registers 10 requests retransmission of any HDLC frames which
are incorrectly received. The network register 4 records the
earliest transmitted frame not yet acknowledged by all of the local
registers and inhibits transmission of new frames if they fall
outside a transmission window relative to the earliest transmitted
frame. The network register 4 polls the local registers 10 for
retransmission requests and the local registers 10 also send
unsolicited requests to the network register 4. The network
register 4 only retransmits a frame once if multiple requests for
that frame are transmitted within a predetermined period. When a
new local register 10 enters the broadcast reception group, the
network register 4 informs the new local register 10 which new
frame will next be transmitted. The number of bits used for frame
sequence numbers is greater than that defined in the HDLC
protocols.
Inventors: |
Kawai, Nobuyuki; (Tokyo,
JP) ; Wong, Hok Shuen; (Surrey, GB) ;
McTiffin, Michael John; (Hants, GB) ; Randall,
David; (Hants, GB) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Assignee: |
Inmarsat, Ltd.
London
GB
|
Family ID: |
10797774 |
Appl. No.: |
10/751465 |
Filed: |
January 6, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10751465 |
Jan 6, 2004 |
|
|
|
08904312 |
Jul 31, 1997 |
|
|
|
Current U.S.
Class: |
714/748 |
Current CPC
Class: |
H04B 7/18523 20130101;
H04L 1/187 20130101; H04L 1/1809 20130101; H04L 2001/0093 20130101;
H04L 1/1685 20130101; H04L 1/1887 20130101; H04B 7/18582
20130101 |
Class at
Publication: |
714/748 |
International
Class: |
H04L 001/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 1996 |
GB |
9616042.9 |
Claims
1. Apparatus for transmitting data relating to the status of user
terminals in a mobile communications system from a central station
having a database for storing said data to a plurality of local
stations each having a local database for storing said data, the
apparatus comprising means for broadcasting said data in a common
channel receivable by each of said local stations; means for
receiving error correction request signals from each of said local
stations; and means for sending error correction signals to each of
said local stations in response to said error correction request
signals.
2. Apparatus as claimed in claim 1, wherein said data is broadcast
in a plurality of frames, said error correction request signals
indicate selected ones of said frames, and said means for sending
error correction signals is responsive to said error correction
request signals to retransmit the selected frames.
3. A method of transmitting data relating to the status of user
terminals in a mobile communications system from a central station
having a database for storing said data to a plurality of local
stations each having a database for storing said data, the method
comprising broadcasting said data in a common channel receivable by
each of said local stations; receiving error correction request
signals from each of said local stations; and sending error
correction signals to each of said local stations in response to
said error correction request signals.
4. A method as claimed in claim 3, wherein said data is broadcast
in a plurality of frames, said error correction request signals
indicate selected ones of said frames, and the step of sending
error correction signals comprises retransmitting said selected
frames.
5. Apparatus for transmitting data to a plurality of data receiving
stations, comprising: means for transmitting said data in a common
channel receivable by each of said receiving stations in a format
comprising a plurality of frames; means for receiving error
correction request signals indicating selected ones of said frames
from each of said receiving stations; and means for retransmitting
said selected frames to each of said receiving stations in response
to said error correction request signals; wherein said means for
retransmitting is operable, if a plural number of said error
correction request signals indicating the same selected frame are
received within a predetermined period, to retransmit said same
selected frame less than said plural number of times.
6. Apparatus as claimed in claim 5, wherein said means for
retransmitting is operable to retransmit each selected frame only
if said selected frame has not previously been transmitted within
said predetermined period.
7. A method of transmitting data to a plurality of data receiving
stations, comprising: transmitting said data in a common channel
receivable by each of said receiving stations in a format
comprising a plurality of frames, receiving error correction
request signals indicating selected ones of said frames from one or
more of said receiving stations, and retransmitting said selected
frames to said receiving stations; wherein, if a plural number of
said error correction request signals indicating the same selected
frame are received within a predetermined period, the step of
retransmitting said selected frames comprises retransmitting said
same selected frame less than said plural number of times.
8. A method as claimed in claim 7, wherein said retransmitting step
comprises retransmitting each selected frame only if that selected
frame has not previously been transmitted within said predetermined
period.
9. Apparatus for transmitting data to a plurality of data receiving
stations, comprising: means for transmitting said data in a common
channel receivable by each of said receiving stations in a format
comprising a plurality of frames; means for receiving error
correction request signals indicating selected ones of said frames
from each of said receiving stations; means for transmitting said
selected frames to each of said receiving stations in response to
said error correction request signals and means for receiving from
each of said receiving stations acknowledgement signals indicating
the earliest of said frames which has not been received by that
station, wherein the means for transmitting is operable to
broadcast a new frame which has not been previously broadcast only
if the sequential order of said new frame is less than a
predetermined number greater than the earliest of said frames which
has not been received by any one of said receiving stations.
10. A method of transmitting data to a plurality of data receiving
stations, comprising: transmitting said data in a common channel
receivable by each of said receiving stations in a format
comprising a plurality of frames; receiving error correction
request signals indicating selected ones of said frames from one or
more of said receiving stations; retransmitting said selected
frames to said receiving stations; and receiving from each of said
local stations acknowledgement signals indicating the earliest in
sequence of said frames which has not been received by that local
station, wherein a new frame which has not previously been
broadcast is broadcast only if the sequential order of said new
frame is less than a predetermined number greater than the earliest
of said frames which has not been received by any one of said local
stations.
11. Apparatus for transmitting data to a plurality of data
receiving stations, comprising: means for transmitting said data in
a common channel receivable by each of said receiving stations in a
format comprising a plurality of frames; means for receiving error
correction request signals indicating selected ones of said frames
from each of said receiving stations; and means for transmitting
said selected frames to each of said receiving stations in response
to said error correction request signals; wherein the frames are
broadcast in a format including frame sequence information
indicating the sequence of each frame, but not including receive
state information indicating the sequence of any frames received
from any of the receive stations.
12. Apparatus as claimed in claim 11, wherein the frames are
broadcast in a format complying with the standard ISO/IEC 7809,
option 10, except that some or all of the receive state variable
field as defined in that standard is occupied by the send state
variable field.
13. Apparatus as claimed in claim 12, wherein the send state
variable field is eleven bits in length.
14. A method of transmitting data to a plurality of data receiving
stations, comprising: transmitting said data in a common channel
receivable by each of said receiving stations in a format
comprising a plurality of frames; receiving error correction
request signals indicating selected ones of said frames from one or
more of said receiving stations; and retransmitting said selected
frames to said receiving stations; wherein the frames are
transmitted in a format including frame sequence information
indicating the sequence of each frame, but not including receive
state information indicating the sequence of any frames received
from any of the local stations.
15. A method as claimed in claim 14, wherein the frames are
transmitted in a format complying with the standard ISO/IEC 7809,
option 10, except that some of all of the receive state variable
field as defined in that standard is occupied by the send state
variable field.
16. A method as claimed in claim 15, wherein the send state
variable field is eleven bits in length.
17. Apparatus for transmitting data to a plurality of data
receiving stations, comprising: means for transmitting said data in
a common channel receivable by each of said receiving stations in a
format comprising a plurality of frames; means for receiving error
correction request signals indicating selected ones of said frames
from each of said receiving stations; means for transmitting said
selected frames to each of said receiving stations in response to
said error correction request signals; means for receiving a link
request signal from an additional receiving station; and means for
transmitting to the additional receiving station in response to
said link request signal information indicating the sequence number
of the latest transmitted frame.
18. A method of transmitting data to a plurality of data receiving
stations, comprising: transmitting said data in a common channel
receivable by each of said receiving stations in a format
comprising a plurality of frames; receiving error correction
request signals indicating selected ones of said frames from one or
more of said receiving stations; and retransmitting said selected
frames to said receiving stations, the method further comprising
receiving a link request signal from an additional receiving
station, and transmitting to the additional receiving station in
response thereto information indicating the sequence number of the
latest transmitted frame.
19. Apparatus for receiving data from a broadcast station,
comprising means for receiving said data and means for transmitting
to the broadcast station at predetermined intervals an error status
signal which indicates whether error correction information is
required from the central station.
20. Apparatus as claimed in claim 19, wherein the means for
transmitting is additionally responsive to a polling signal from
the central station to transmit said error status signal.
21. Apparatus as claimed in claim 19 or 20, wherein said data is
broadcast in a plurality of frames, and wherein said error status
signal comprises either an error correction request signal
indicating selected ones of said frames which were not correctly
received, or a signal indicating that no error correction is
required.
22. A method of receiving data from a broadcast station, comprising
receiving said data and transmitting to the broadcast station at
predetermined intervals an error status signal which indicates
whether error correction information is required from said central
station.
23. A method as claimed in claim 22, further comprising
additionally transmitting said error status signal in response to a
polling signal from said broadcast station.
24. A method as claimed in claim 22 or 23, wherein said data is
broadcast in a plurality of frames and said error status signal
comprises either an error correction request signal indicating
selected ones of said frames which were not correctly received, or
a signal indicating that no error correction is required.
25. Apparatus for receiving data from a broadcast station,
comprising: means for transmitting to said broadcast station a link
request signal; means for receiving from said broadcast station
information indicating a current broadcast sequence number; and
means for receiving said data in a format comprising a sequence of
frames.
26. A method of receiving data from a broadcast station,
comprising: transmitting a link request signal to said broadcast
station; receiving from said broadcast station information
indicating a current frame number for said data; and receiving said
data in a format comprising a sequence of frames.
27. Apparatus for receiving data from a broadcast station,
comprising: means for receiving said data in a format comprising a
sequence of frames; and means for transmitting signals to said
broadcast station in a format including receive state information
indicating the sequence number of the last in sequence of the
received frames, but not including transmit state information
indicating the sequence of any frames transmitted to the broadcast
station.
28. Apparatus as claimed in claim 27, wherein the frames are
transmitted in a format complying with the standard ISO/IEC 7809,
option 10, except that some or all of the send state variable field
as defined in that standard is occupied by the receive state
variable field.
29. A method of receiving data from a broadcast station,
comprising: receiving said data in a format comprising a sequence
of frames; and transmitting signals to said broadcast station in a
format including receive state information indicating the sequence
number of the last in sequence of the received frames, but not
including transmit state information indicating the sequence of any
frames transmitted to the broadcast station.
30. A method as claimed in claim 29, wherein the frames are
transmitted in a format complying with the standard ISO/IEC 7809,
option 10, except that some or all of the send state variable field
as defined in that standard is occupied by the receive state
variable field.
31. Apparatus as claimed in any one of claims 1, 2, 5, 6, 9, 11,
12, 13, 17, 19, 20, 21, 25, 27 and 28, wherein the data is
broadcast via satellite.
32. A method as claimed in any one of claims 3, 4, 7, 8, 10, 14,
15, 16, 18, 22, 23, 24, 29 and 30, wherein the data is broadcast
via satellite.
33. Apparatus as claimed in any one of claims 5, 6, 9, 11, 12, 13,
17, 19, 20, 21, 25, 27 and 28, wherein the data relates to the
status of user terminals in a mobile communications system and each
of the receiving stations has associated therewith a database for
storing said data.
34. A method as claimed in any one of claims 7, 8, 10, 14, 15, 16,
18, 22, 23, 24, 29 and 30, wherein the data relates to the status
of user terminals in a mobile communications system and each of the
receiving stations has associated therewith a database for storing
said data.
Description
[0001] The present invention relates to a method and apparatus for
transmitting data, and particularly but not exclusively to a method
and apparatus for updating a plurality of location registers from a
central location register.
[0002] In a mobile communication system, it is necessary to
maintain a database of mobile users, including information on their
last known locations so that calls may be routed to them
efficiently. For example, in the GSM terrestrial cellular system,
the location information is stored in a home location register
(HLR) which is updated as a mobile terminal moves from one area to
another. In addition, location information is stored in a visitor
location register (VLR) associated with each mobile switching
centre (MSC). The VLR duplicates parts of the information in the
HLR and allows the MSC to obtain relevant information without
contacting the HLR.
[0003] In the Inmarsat.TM. mobile satellite communication system,
signals are relayed via geostationary satellites which have a large
coverage area, known as an ocean region. It has hitherto been
necessary for a calling party to know in which ocean region the
called party is located. The calling party calls a different number
depending on which ocean region is being called.
[0004] However, as the number of regions served by a satellite
communication system increases, it becomes difficult for the
calling party to determine in which region the called party is
located.
[0005] The document U.S. Pat. No. 5,303,286 discloses a combined
satellite and terrestrial cellular communication system which has a
database of roaming users in the satellite service area. Changes to
this database are communicated to terrestrial cellular user
databases and satellite databases, so that these other databases
can be updated accordingly. The database update information is
communicated over a packet switched network to the terrestrial
cellular databases and via a satellite uplink to the satellite.
[0006] According to the present invention, there is provided an
apparatus and a method for propagating data on the status of user
terminals in a mobile communication system from a central register
to a plurality of local registers, in which the status data is
broadcast over a common channel from the central register to all of
the local registers and return channels are provided from each of
the local registers to the central register through which error
correction request signals can be sent. In this way, information
which is needed by all of the local registers can be transmitted
efficiently, while overcoming any errors involved in receiving the
broadcast.
[0007] Preferably, the error correction request signals are
selective request signals which request correction of only selected
portions of the broadcast information which were received with
errors. While this system is inherently more complicated than other
error correction systems, such as go-back-N, it allows a high
broadcast throughput for a given permissible bit error rate.
[0008] Preferably, the central location register may, for a
predetermined period after transmission of correction information,
prevent retransmission of the same information so as to avoid
unnecessary repetition when requests for the same correction
information are received from different local registers with
different timings.
[0009] Advantageously, the information may be broadcast via a
satellite.
[0010] According to another aspect of the present invention, there
is provided a method and apparatus for point-to-multipoint
communication using HDLC protocols, in which a non-standard control
word format is used. A broadcasting station sends only the frame
send sequence number, while each receiving station returns only the
frame receive sequence number to the broadcasting station. Extended
frame sequence numbering is provided, so that a greater number of
bits is used for frame sequence numbering than is defined in the
HDLC protocols. Preferably, eleven bits are used to specify the
frame sequence numbering.
[0011] According to another aspect of the present invention, there
is provided a method and apparatus for point-to-multipoint
communications, in which each receiving station periodically sends
unsolicited responses to the broadcast station so as to inform the
broadcast station of any errors in the received data and of the
last frame received in a consecutive sequence. In this way, more
efficient use may be made of the return channel or channels.
[0012] According to another aspect of the present invention, there
is provided a point-to-multipoint communication system, in which
each receiving station is able to send retransmission requests
relating to data previously broadcast by the broadcast station. On
receipt of a retransmission request, the broadcast station only
retransmits the requested data if it has not previously been
transmitted within a predetermined period of time.
[0013] According to another aspect of the present invention, there
is provided a point-to-multipoint communication system using HDLC
protocols, in which a lower window variable is updated past a frame
number only when all of the receiving stations have acknowledged
receipt of that frame number.
[0014] According to another aspect of the present invention, there
is provided a method and apparatus for point-to-multipoint
communications using an HDLC protocol, in which a receive station
may be added to a group of receive stations during a
point-to-multipoint communication. The transmit station sends a
current transmit sequence frame number to the new receive station
and the receive station stores that sequence number as the start
frame sequence number. In this way, receive stations may be added
to a point-to-multipoint transmission without affecting the frame
sequence numbering of receiving stations already taking part in the
transmission.
[0015] The above point-to-multipoint communication systems are
advantageously applied for transmitting user status data from a
central user database to local user databases in a mobile
communication system. Preferably, the information is broadcast from
the transmit station to the receive stations via satellite.
[0016] Specific embodiments of the present invention will now be
described with reference to the accompanying drawings in which:
[0017] FIG. 1 is schematic diagram showing a point-to-multipoint
communication between a network location register and a plurality
of LES location registers in a satellite communication system
according to an embodiment of the present invention;
[0018] FIG. 2 is a more detailed schematic diagram of the network
location register and control station of FIG. 1;
[0019] FIG. 3 is a more detailed schematic diagram of one of the
LESs and its associated location register of FIG. 1;
[0020] FIG. 4 is a protocol diagram showing a communication between
the network location register and one of the LES location registers
of FIG. 1;
[0021] FIG. 5 is a diagram of the contents of an SREJ frame in the
protocol exchange shown in FIG. 1; and
[0022] FIG. 6 is a protocol diagram showing link set-up procedure
between the network location register and a new LES location
register.
[0023] As shown in FIG. 1, a network control station (NCS) 2 is
connected to a network global location register (GLR-N) 4, which
stores information relating to the current location of mobile users
logged on to a mobile satellite communication system. The network
control station communicates with a plurality of land earth
stations (LES) 6a, 6b, 6c via a satellite 8. The satellite may, for
example, be an Inmarsat-3 geostationary satellite.
[0024] Associated with each LES 6 is a corresponding LES global
location register (GLR-L) 10. Each GLR-L 10 comprises a database
which stores a copy of the data stored in the GLR-N 4.
[0025] A mobile earth station (MES) 12 is located within the
coverage area of the satellite 8. When the MES 12 is switched on,
it sends a log-on signal which is received by the satellite 8 and
relayed to the NCS 2. The NSC 2 then registers log-on information
in the GLR-N 4, comprising the identity of the MES 12 and a time
stamp registering the log-on time. Each GLR-L 10 holds a copy of
all the log-on information stored in the GLR-N 4. Any changes to
the contents of the GLR-N 4 are communicated to all of the LESs 6
so that calls routed through any LES 6 to an MES may be sent via
the satellite 8 if the MES 12 is in the corresponding ocean region,
or routed to other LESs (not shown) serving other ocean regions,
according to the location information for that MES stored in the
GLR-L 10.
[0026] An example of the arrangement of the GLR-N 4 and the NCS 2
is shown in FIG. 2. An NCS controller 32 is connected to a storage
means 36 for storing the GLR-N database. The storage means may
comprise one or more hard disc drives, and/or random access
memory.
[0027] The NCS controller 32 sends signals to and receives signals
from an RF modulator/demodulator 38 connected to an antenna 40
directed towards the satellite 8.
[0028] The NCS controller 32 is connected by a terrestrial link 34
to a network operations centre (NOC, not shown) and to other NCS
controllers of other NCSs, one of which is provided for each ocean
region.
[0029] Thus, MES status information from MESs in the corresponding
ocean region is obtained by the NCS controller 32 from the RF
modulator/demodulator 38 as MESs log onto the NCS 2, and status
information on MESs from other ocean regions is received from the
terrestrial link 34. The NOC coordinates channel assignments
between the ocean regions.
[0030] An example of the arrangement of one of the LESs 6 and its
associated GLR-L 10 is shown in FIG. 3. A mobile switching centre
(MSC) 42 is connected to a terrestrial link 46, which may be
connected to a public service telephone network (PSTN) 47, an
integrated services digital network (ISDN) or other network. The
MSC 42 is also connected via a further terrestrial link to other
MSCs of other LESs, to allow calls to be routed to other LESs
either in the same or in a different ocean region.
[0031] The MSC 42 derives the correct routing for a call from the
PSTN 47 from a storage means 44, which stores the GLR-L database.
Calls routed to the corresponding LES 6 are connected to an LES
controller 48 which communicates via an RF modulator/demodulator
50, an antenna 52 and the satellite 8 to the called MES.
[0032] In order for this routing to work effectively, the contents
of each GLR-L 10 should be accurate and up-to-date and therefore
the changes to the contents of the GLR-N 4 should be communicated
substantially without errors to each GLR-L 10. Protocols by which
this is achieved will now be described.
[0033] Channel Types
[0034] For communication of MES information from the NCS 2 to each
LES 6, a broadcast channel is used. The broadcast channel may be
time division multiplexed with other channels. Data is broadcast in
an HDLC format conforming generally to ISO standards ISO/IEC 3309,
ISO/IEC 4335 and ISO/IEC 7809. Within ISO/IEC 7809, options 3
(single frame retransmission), 4 (unnumbered information), and 8
(command I frames only) are adopted. The NCS 2 acts as the primary
station and the LESs 6 act as secondary stations.
[0035] In the transmission link, the HDLC frames are not aligned
with the TDM slots but are packed contiguously into the TDM slots
as a continuous bit pipe.
[0036] HDLC frames containing messages from each of the LESs 6 to
the NCS 2 occupy a corresponding slot of a TDMA frame of a return
channel so that each LES 6 has an individual slot assigned to
it.
[0037] HDLC Messages
[0038] Both data and commands are transmitted by the GLR-N 4 but
only responses are transmitted by each GLR-L 10 and no MES
information is sent in the return direction.
[0039] The I (Information) frames transmitted by the GLR-N 4
include a sixteen bit control field and an eleven octet information
field.
[0040] As defined in standard ISO/IEC 7809, option 10, (extended
sequence numbering), the control field of the information frame has
the format shown below in Table 1.
1TABLE 1 Standard Control Field Format for Information Frames Bits
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Contents 0 N (S) P/F N
(R)
[0041] The variable N(S) is the serial number of the transmitted
Information frame and the variable N(R) is the serial number of the
next frame expected to be received in sequence by the transmitting
station. Thus, if a frame with N(S)=3 is received by a station
after a frame with N(S)=1, but no frame with N(S)=2 has yet been
received, N(R) for the frames transmitted by that station will
remain at 2 until a frame with N(S)=2 is received, in which case
N(R) will be updated to 4. At each station there is stored a send
state variable V(S), which corresponds to the sequence number of
the next frame to be transmitted, and a receive state variable
V(R), which determines the current value of N(R).
[0042] In the above standard frame numbering system, seven bits are
provided for representing N(S) and N(R) and therefore the frame
numbers are modulo 128. For each station, a window size K is
defined. A transmitting station will cease to transmit new frames
if the current transmit state variable V(S) is more than K greater
than the N(R) of the last received frame. Therefore K defines the
maximum number of unacknowledged frames which may be sent. With the
standard 7-bit numbering system, the maximum value for K is 63
(half the maximum frame number) to avoid confusing the reception of
a new frame with the reception of a retransmitted frame from a
previous modulo 128 cycle.
[0043] The HDLC frame numbering system is specially adapted for the
system according to the embodiment of the present invention. The
GLR-N 4 stores an acknowledge state variable V(A).sub.x
corresponding to the last frame acknowledged by each GLR-L
10.sub.x. For example
V(A).sub.x=N(R).sub.x-1
[0044] where N(R).sub.x is the latest N(R) received from GLR-L
10.sub.x. The GLR-N 4 also stores a lowest acknowledge state
variable V(A) corresponding to the lowest acknowledge state
variable V(A).sub.x of any GLR-L 10.sub.x which has established a
link to the GLR-N 4.
i.e. V(A)=MIN (V(A).sub.x).
[0045] The GLR-N 4 stores a transmit state variable V(S) which
defines the sequence number of the next frame to be transmitted. A
transmit window is defined by the variables V(S), V(A) and K, the
window size, such that new I frames will only be transmitted if
V(S).ltoreq.V(A)+K.
[0046] The GLR-N 4 is therefore limited in the number of new I
frames which can be transmitted by the performance of the poorest
quality link to any of the GLR-L 10.
[0047] Therefore, any GLR-L 10 is disconnected if the GLR-N 4 does
not receive any frames from it within a predetermined period.
Disconnection is performed by the GLR-N 4 transmitting a DISC
command addressed to the relevant GLR-L 10. If a maximum window
size of 63 is used, and a typical I-frame transmission rate of 11
per second is assumed, the maximum window will be exhausted after
2.8 seconds. Thus, any GLR-L 10 would be disconnected if it does
not acknowledge within a 2.8 second period. This would lead to an
unacceptably high rate of disconnection.
[0048] In order to overcome this problem, the I frame sent from the
GLR-N 4 to each GLR-L 10 in the broadcast channel has the format
shown below in Table 2.
2TABLE 2 Control Field Format for Information Frames Bits 1 2 3 4 5
6 7 8 1 2 3 4 5 6 7 8 Contents 0 SPARE N (S)
[0049] The variable N(S) is the serial number of the Information
frame. The variable N(R) is omitted, since no information is sent
from each GLR-L 10 to the GLR-N 4 and it is therefore not necessary
for the GLR-N 4 to broadcast N(R). The transmit sequence number
N(S) is represented by 11 bits, giving a maximum window size of
1023. Therefore, at a transmission rate of 11 I frames per second,
the maximum window size will be used up in 46.5 seconds. This
allows sufficient time for a link to any GLR-L 10 to be
reestablished in the event of failure.
[0050] The I frame information contents include an MES identity
code identifying a specific MES, and location and status
information for that MES.
[0051] The format of the control field in RR frames transmitted by
any GLR-L 10 are shown below in Table 3.
3TABLE 3 Control Field Format for RR Frames Bits 1 2 3 4 5 6 7 8 1
2 3 4 5 6 7 8 Content 1 0 0 0 F N (R)
[0052] N(S) is omitted from the control field, since no data is
transmitted by GLR-L 10. Instead, an 11 bit receive sequence number
N(R) is used. This enables a maximum window size of 1023 as
described above. The significance of the final bit F will be
discussed in the next section.
[0053] Selective Request
[0054] The forward link is susceptible to noise both in the uplink
from the NCS 2 to the satellite 8 and each downlink from the
satellite 8 to each LES 6. Uplink noise will affect the quality of
all the downlinks equally, whereas downlink noise only affects the
relevant downlink. Therefore, some transmission errors will be
common to all the LES 6, while some will be specific to one or more
LES 6. An error correction protocol, is used which maximises the
broadcast throughput for a given bit error rate under these
conditions.
[0055] FIG. 4 shows a protocol exchange between the GLR-N 4 and one
of the GLR-L 10. The GLR-N 4 sends a series of information frames I
to the GLR-L 10 over the broadcast channel. Periodically, the GLR-L
10 responds with a response signal R.sub.0, which may be a receive
ready (RR) frame or a selective request frame (SREJ). The RR frame
indicates that no retransmission of frames is needed, while the
SREJ frames specifies which information frames should be
repeated.
[0056] Periodically, the GLR-N 4 sends an RR or I frame with the
poll bit P set, shown in bold in FIG. 4. The GLR-L 10 responds with
a frame R.sub.1, which is either an RR or SREJ frame, with the
final bit F set. The polling by the GLR-N 4 is provided in addition
to the unsolicited response R.sub.o, by the GLR-L 10. This enables
the system to recover from failure by the GLR-N 4 to receive
periodic unsolicited SREJ frames from any GLR-L 10 and failure by
the GLR-L 10 to receive retransmitted I frames.
[0057] The content of the information field of an SREJ frame from
the GLR-L 10 to the GLR-N 4 is shown in FIG. 5.
[0058] The information field is 48 bits long and can hold up to
four receive sequence numbers N(R). The first N(R) is located in
the control field, while the remaining three N(R) are located in
the information field. Each receive sequence number N(R) indicates
a frame number which has not been received. The validity bit V
indicates whether the following N(R) is valid, so that less than 4
frames can be indicated, with the validity bit V set to zero for
the N(R) fields which are not used.
[0059] If the GLR-L 10 requires retransmission of more than 4
frames, the current four lowest-numbered frames which have not yet
been received correctly are requested.
[0060] Since the number of bits available in the SREJ frame is
limited by the TDMA slot length, there is a trade-off between the
number of bits used to represent frame sequence numbers N(S) and
N(R) and the number of receive sequence numbers N(R) which can be
fitted into SREJ frame. Increasing the number of bits used to
represent frame numbers increases the maximum window size, but
reduces the number of frames which can be requested in a SREJ
frame. An 11-bit sequence number provides a balance which is suited
to the noise characteristics of the system shown in FIG. 1.
[0061] Retransmission
[0062] After receipt of an SREJ, the GLR-N 4 determines whether
each of the requested frames has previously been scheduled for
retransmission within a predetermined preceding period T3. If the
frame has not been scheduled for retransmission within that
preceding period, that frame is scheduled for retransmission and is
duly retransmitted by the GLR-N 4 after any other frames with
higher priority have been transmitted.
[0063] If the frame number of a frame requested for retransmission
is not within the current transmit window (V(A) to V(A)+K) the
request is ignored.
[0064] Since some of the information frames I are likely to be
incorrectly received by more than one GLR-L 10, there may be
multiple retransmission requests for the same information frame
from different GLR-L 10, at varying times due to lack of
synchronisation between the different GLR-L 10. The period T3 is
set so that multiple requests for retransmission of the same frame
will all be received within the period T3. In this way, unnecessary
repeat retransmissions are avoided.
[0065] Alternatively, the GLR-N 4 stores the sequence numbers of
all requested frames within the period T3 and at the end of the
period schedules all the requested frames for retransmission. All
the requested frames are transmitted only once in response to
requests received during the period T3, regardless of how many
times they are requested.
[0066] Link Recovery
[0067] When a new GLR-L 10 enters into service, or an existing
GLR-L 10 recovers after a disconnection communication failure or a
system crash, it is necessary to reestablish a data link between
that GLR-L 10 and the GLR-N 4. In a standard HDLC protocol, the
send and receive state variables V(S) and V(R) of each station are
set to zero when establishing a data link. However, in a
point-to-multipoint system as shown in FIG. 1, this would require
all the other GLR-L 10 to reset their state variable V(R) as
well.
[0068] In the present system, an alternative protocol is used to
connect or reconnect a GLR-L 10 to the GLR-N 4, as shown in FIG. 6.
The GLR-N 4 broadcasts information I or receive ready RR frames to
all the GLR-L 10 (step 16). The GLR-L 10 initiates establishment of
a data link (step 18) and sends an unnumbered information (UI)
frame indicating that a data link is requested (step 20).
[0069] If the GLR-N 4 receives the data link request 20, it outputs
its current send state variable V(S) in an unnumbered information
(UI) frame, which is transmitted as a data link response at step
24. At step 26, the GLR-L 10 receives the data link response and
sets its receive state variable V(R) as the current send state
variable V(S) received in the data link response 24. At step 28,
the GLR-L 10 sends an unnumbered acknowledged (UA) signal, which is
received by the GLR-N 4 at step 30 and the link set-up is complete.
Thereafter, the GLR-N 4 stores an acknowledge state variable
V(A).sub.x for that GLR-L 10.
[0070] At each transmission, the transmitting station sets a timer.
If an expected response is not received, the transmitting station
retransmits the signal after a period T1, but terminates the
procedure if no response is received after N2 repetitions of the
signal.
[0071] Embodiments of the present invention use a
point-to-multipoint transmission system to broadcast information
relating to status of user terminals in a mobile satellite system.
However, it will readily be appreciated that aspects of the
invention may be applied to any other mobile communication system,
whether using satellite or terrestrial cellular links, in which it
is necessary to update multiple copies of a database of the status
of users, such as the GSM system.
[0072] Furthermore, embodiments of the present invention provide
advantageous protocols for implementing a point-to-multipoint
transmission system. Aspects of the present invention may therefore
be applied in other contexts than the transmission of mobile user
status information, and can be applied to many other types of
point-to-multipoint transmission system, particularly wireless
transmission systems which are susceptible to noise.
[0073] References to mobile user terminals will be understood to
include wireless terminals which are not in fact mobile, by reason
of being installed in temporary or permanent immobile
installations.
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