Communication protocol for satellite data processing

Details Communication protocol for satellite data processing Communication protocol for satellite data processing

1996-05-28

2003-02-18

Rao, Seema S. (Department: 2661)

Multiplex communications

Communication over free space

Portable address responsive receiver

C370S321000, C370S436000, C370S445000, C370S468000, C370S471000, C370S432000, C370S524000

Reexamination Certificate

active

06522635

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to an improved communication protocol for satellite messaging, and more particularly, to improvements to data protocols, such as the Standard-C communication protocol, for sending and receiving data messages using a satellite.
BACKGROUND ART
One example of a currently successful and operational satellite based data service is INMARSAT's Standard-C Communications System. The following description of the Standard-C System has been reprinted, with permission, from “INMARSAT Standard-C Communications System” by Kevin Phillips, IEEE International Conference on Communications '88: Digital Technology—Spanning the Universe, Jun. 12-15, 1988, pp. 1117-1122 (Available from IEEE Service Cent (cat. #CH2538-7/88)) ©1988 IEEE.
FIG. 1
shows the major elements of the Standard-C system architecture which consists of coast earth stations (CESs), a network coordination station (NCS) in each ocean region and ship earth stations (SESs). Each CES serves as a gateway between the terrestrial network and the INMARSAT Standard-C communications system. The types of interface provided at the CES are decided by the coast earth station operator, however, Telex, EGC message processing and distress message handling are mandatory.
All SESs that are active in an ocean region are required to register with the NCS and a copy of the list of all registered SESs is held at each CES and used as a basis for accepting or rejecting shore originating calls. In addition, the location of every registered SES is held at the CES so that calls received at the CES for a SES that is in another ocean region may be redirected and the call not lost. CESs may operate their traffic channels in a demand assigned mode. If traffic and satellite power considerations call for this mode of operation to be used, the NCS allocates on a need basis, CES TDM channels, SES Signalling Channels and SES Message Channels.
Three NCSs are provided in the Standard-C system, one in each ocean region, managing central resources such as traffic channels when demand assigned operation is used and coordinating signalling and control traffic. Each NCS transmits a NCS Common Channel which is received by all SESs when they are not involved in message transfer. The common Channel is used to announce shore-to-ship messages waiting at CESS, for broadcasting EGC messages and at various stages for protocol signalling packet transfer.
The NCS is responsible for synchronizing access to SESs by CESs and maintains a database containing the state of each commissioned SES including an indication whether a particular SES is currently engaged in message transfer with a CES. Statistics concerning every processed call are transmitted by CESs to the NCS. This and other data held by the NCS is used for network management and monitoring purposes. A SES is the mobile earth station used by mobile subscribers. Below is a summary of some of the primary characteristics of the standard-C SES:
(a)
G/T (5° elevation):
−23 dB/K
(b)
minimum EIRP:
+12 dBW
(c)
modulation:
BPSK
(d)
data rate
600 bit/s
i) shore-to-ship:
ii) ship-to-shore
current satellites:
300 bits/s
future satellites:
600 bit/s
(e)
operational bandwidth
Transmit:
1626.5-1646.5 MHz
Receive:
1530.0-1545.0 MHz
(f)
tuning increment:
5 kHz
The system allows the use of very low G/T receivers at the ship earth station (SES). Standard-C uses all digital transmission techniques for both signalling and message data. SES equipment may therefore take advantage of the low cost trend of digital technology. Standard-C SESs do not require the use of a dedicated receiver for receiving signalling information.
FIG. 2
shows a block diagram of the SES. It consists of a DCE (data circuit terminating equipment) providing the interface to the satellite network, and a DTE (data terminal equipment, for example a personal computer) which provides the user interface. For ship-to-shore message transfer, a message is formatted in the DTE and then transferred to the DCE for transmission. In the shore-to-ship direction, the DCE receives the complete message from the radio channel before passing it to the DTE for the attention of the user. The SES may be equipped for reception of Enhanced Group Calls (EGC) or a separate Receive Only Terminal for EGC reception may be used. When idle, every SES tunes to and receives a TDM channel transmitted by the NCS; the NCS Common Channel. The channel is used to transmit announcements to SESs that shore-to-ship calls are ready at a CES for transmission to the SES. Enhanced Group Calls are also transmitted on this channel.
The requirement for a very low G/T message based mobile earth station, resulted in the following major design features:
(a) the use of a very low gain antenna requires robust modulation and coding scheme had to be employed to alleviate the effects of multipath propagation. Rate 1/2, constraint length
7
convolutional coding and frame interleaving are used to ensure that the packet error probability is met for the design link budget;
(b) shore-to-ship TDM channels require a relatively high satellite EIRP of 21 dBW and the capability of operating in a demand assigned mode is provided to save satellite power if network conditions require it;
(c) the packet data transmission medium is completely transparent to the type of data to be passed over the traffic channels;
(d) the need to cater for future new services and applications requires considerable flexibility to be provided in the design of the access control and signalling protocols;
(e) operation in a spot beam environment is provided by the ship earth station automatically identifying the appropriate satellite spot beam. This ensures that the Standard-C system will be compatible with future generations of satellites which may use spot beams;
(f) the store-and-forward nature of Standard-C allows the system to make use of off-peak satellite capacity.
The initial services to be offered by Standard-C CESs to users will be:
(i) access to the International Telex network;
(ii) a wide range of Enhanced Group Call Services; and
(iii) access to rescue Coordination Centers (RCCs).
Additional optional services are available within the system and may be offered by CESs. These services include access to electronic mail and message handling services (X.400 for example), individual, group and area directed polling, and a data reporting service. The polling and data reporting services are primarily aimed at providing a service to remotely operated and land mobile terminals.
A comprehensive message addressing system has been adopted so that access to various terrestrial data networks and a variety of specialized services can readily be provided. A number of different types of channels are used in the Standard-C system. In addition to those channels required for shore-to-ship and ship-to-shore communications and signalling via the satellite, there are also inter-station links on the shore side for network monitoring and control purposes.
The NCS common channel is a TDM carrier transmitted continuously by the NCS to all SESs in the ocean region. SESs tune to the NCS common channel when they are idle. The channel operates at 1200 symbols/s with a fixed length frame of 8.64 s. The information is scrambled, half rate convolutionally encoded and interleaved on a frame by frame basis. The data rate is therefore 600 bit/s and all message and signalling information is conveyed in packets. In each frame, 639 bytes are available for packets. The first packet in every frame is always the bulletin board packet. This packet is followed by a number of signalling channel descriptor packets used to transfer information concerning SES usage of the signalling channels associated with that TDM.
The CES TDM channel is used for the forward link when the CES is communicating with a SES. Its structure is identical to that of the NCS common channel described above, and is used for carrying call set-up signalling, shore-to-ship message, acknowledgements, and call clear down signalling. A CES may operate more

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