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NASA Technical Reports Server (NTRS) 19940018344: System services and architecture of the TMI satellite mobile data system PDF

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J N94-22817 System Services and Architecture of the TMI Satellite Mobile Data System D. Gokhale, A. Agarwal COMS AT Laboratories 22300 COMSAT Dr, Clarksburg, Md 20871 (301) 428 4220 (Tel) (301) 428 7747 (Fax) A. Guibord Telesat Mobile Incorporated ABSTRACT via this system include Remote Data Base Access and Entry, Fleet Management, Supervisory Control and The North American Mobile Satellite Service Data Access (SCADA), and multicast data, as well as (MSS) system being developed by AMSCfI'MI and messaging. Potential customers of this system include scheduled to go into service in early 1995, will include transportation (trucking, taxicab) companies, field the provision for real time packet switched services sales, field services, public services agencies (police, (Mobile Data Service - MDS) and circuit switched ambulance, fire), oil companies, utilities, as well as the services (Mobile Telephony Service - MTS). These general category of mobile professionals. services will utilize geostationary satellites which MDS GROUND SEGMENT provide access to Mobile Terminals (MTs) through L- band beams. The MDS system utilizes a star topology ARCHITECTURE with a centralized Data Hub (DIt), and will support a large number of mobile terminals. The DH, which The ground segment architecture of the MDS accesses the satellite via a single Ku band beam, is system is shown in Figure 1. The MDS ground responsible for satellite resource management, for segment elements consist of aData Hub (DH), Mobile providing mobile users with access to public and Terminals, and Remote Monitoring Stations. The private data networks, and for comprehensive network MDS system can be configured to operate either in an management of the system. This paper describes the integrated manner within the MSS system, or as a various MDS services available to the users, the standalone system. In an integrated system, the DH ground segment elements involved in the provisioning interfaces with the MSS Network Operations Center of these services, and a summary description of the (NOC) for allocation of system resources, and with the channel types, protocol architecture and network Network Control Center (NCC) for supporting circuit management capabilities provided within the system. switched services to integrated voice data MTs. In an integrated system, the DH RF Equipment may be INTRODUCTION shared with the NCC or FeederLink Earth Station (FES) RF Equipment. In a standalone configuration, The North American Mobile Satellite Service the MDS system operates with a pre-allocated set of (MSS) system [1] being developed by AMSCfI'MI resources from the NOC operated by the MSS service and scheduled to go into service in early 1995, will provider. A brief description of the MDS ground include the provision for real time packet switched segment elements is provided next: services (Mobile Data Service -MDS) and circuit switched services (Mobile Telephony Se_ice -MTS) Data Hub Terminal Equipment (DH - TE) -The The MDS system uses packet switching techniques to DH-TE provides packet switched communication provide for the dynamic sharing of satellite resources services to MTs. On the terrestrial side the Dtt between a large number of mobile users. The interfaces to public and private data networks as wall architecture of the MDS system is similar to packet as customer host computers. It provides for the switched VSAT systems, wherein a centralized Data dynamic assignment of MDS capacity to MTs on a Hub 0014) communicates with a large mir/il_Jr of-- demand basis. It also provides for the overall remote units. In contrast to currently dei3ioyed larid management and control of the MDS network. The mobile data systems such as INMARSAT Standard C, DH interfaces with the Network Management System which primarily support store and forward messaging, (NMS), which includes the Customer Management the MDS system provides a flexible architecture which Information System (CMIS), to obtain customer is capable of supporting a wide variety of application configuration information and to provide network types. Potential applications which can be supported usage information. Network status and configuration 479 informatioanre also communicated between the DH As shown in the figure, the DH can directly interface and Network/Systems Engineering functions within with the fixed DTE, or the interconnection can be via the NMS. an intermediate public or private data network. This service is compliant with the 1988 and 1984 versions of the CCITI" Recommendations X.25. The X.25 Mobile Terminal (Mr) -The MT provides user access to the packet switched services provided by the service provides a flexible mechanism for the deployment of value added services. An MDS service MDS system. Four MT types are supported by the MDS: full duplex, half duplex, receive only, and provider can easily provide such services by integrated voice/data. The half duplex MT, which integrating off-the-shelf hardware and/or software does not require an RF diplexer provides a low cost, such as protocol gateways (e.g SNA packet assemblers/disassemblers). A number of such X.25 limited feature, alternative to the basic full duplex MT. gateway products are commercially available on the The receive only MT supports only unicast services market from a large number of vendors. Applications and can be used for applications such aspaging and such as store and forward messaging can also be easily multicast data reception. The integrated voice data supported via off-the-shelf communications software MT (IVDM) combines the capabilities of a full duplex MDS MT, and acircuit switched MT. packages on personal computers and workstations. The second service in the Basic Services Remote Monitor Station (RMS) -The RMS provides category is the Asynchronous Service which provides the capability to monitor the L Band RF spectrum and for the interconnection of an asynchronous DTE transmission performance in a specific L-band beam. An RMS is located in each L band beam and interfaces connected to the MT with apacket mode DTE connected to the DH. The Asynchronous Service uses with the DH via a satellite or terrestrial link. procedures compliant with the 1988 and 1984 versions of the CCrlT recommendations X.3, X.28 and X.29 MDS SERVICES protocols. Figure 3shows the Asynchronous Service architecture. As shown in the figure, the In designing MDS services, two key Asynchronous Service also provides the capability for requirements were taken into account - a) emphasis the asynchronous DTE attached to the MT to was placed on providing application independent core communicate with a fixed asynchronous DTE through networking services and b) supporting service the use of an external Packet Assembler/Disassembler. offerings that are compliant with international standards. Emphasis on the core networking services, The primary advantage associated with tim which provide for basic packet switched circuit setup, basic services: X.25 and Asynchronous is that they data transfer and circuit takedown, allows the system provide the flexibility to support value added services to support user specific applications in a very flexible without any impact on the core MDS network. manner. Adherence to international Standards makes it However a number of attributes associated with a possible to use commercial off-the-shelf landmobile system cannot be fully utilized by these communications software for providing value added services. To provide a mechanism for utilizing these services and also reduces the cost associated with attributes, the MDS system also supports two implementing the MDS ground segment. The Specialized Service categories: Reliable Transaction standards compatible core networking services Service (RTS) and Unacknowledged Data delivery category is referred to as Basic Services in the MDS Service (UDS). RTS provides the capabilitylo system. Since some MDS capabilities (such as efficiendy complete a short transaction multicast data, and TDMA bandwidth reservation) (request/response) between mobile and fixed users. could not be optimally accessed via the core This service is especially efficient when the fixed user networking services, an additional service category connected to the DH originates the transaction request, referred to Specialized services is also supported by since the DH can allocate space on theinboun_d- the MDS system. reservation channel for the response. For applicati_s (e.g cargo monitoring, location tracking) which require As described earlier, the Basic services periodic polling, _thisservice is much more e_ffi_Cj_¢nt category provide for transparent communications than usin-g ilSeX.25 _r,Ti6e, since a) fewer me_s_ages services between data terminal equipment (DTE) are generated and b) aprioiri capacity can be assigned connected to the mobile terminal and DTEs connected on th_inbounffchannel. to the data hub. Two types of Basic Services are provided within the MDS system: X.25 and The second S_cialized service is the Asynchronous. The X.25 Service provides for the Unacknowledged Data delivery Service (UDS) which establishment of virtual circuits between an X.25 DTE provides the capability to transmit non-assured data. attached to the MT and a fixed X.25 DTE connected to More importantly, UDS provides the capability of the DH. Figure 2shows the X.25 Service architecture. multieasting data to MT User Groups. Both the RTS 480 and UDS services are offered to end users via a channel framing consist of fixed size slots (108.15 specialized services access function within the DH and msec, and 42.88 msec respectively). The transmission the MT. Message primitives and formats for of data packets into these slots is done in accordance with the slotted Aloha scheme. The MT-DT channel applications to communicate with the specialized service access function have also been standardized in uses variable length framing. The access to the MT- the MDS system. DT channel is done in aTime Division Multiple Access (TDMA) manner under the coordination of the In addition to packet data services, MDS also DH. Two types of MT-DT frames (types A & B) are supports the provisioning of an integrated voice/data used. Type A frames which provide for two Service to MTs. This service allows a class of Mrs information fields, are used to support the termed Integrated Voice/Data MTs OVDMs) to use piggybacking of requests for additional capacity on the the circuit switched system (MTS) for voice, stream TDMA channel. This technique enables the MT to data, and facsimile, and the packet switched system pipeline data requests and have the capability to transfer data at rates close to the inbound channel (MDS) for packet data services. The circuit switched information rate. service provisioning is supported via co-ordination between the DH and the Network Control Center (NCC) which allocates the satellite circuit resources MDS PROTOCOL ARCHITECTURE for the mobile telephony service. The call setup signalling between the NCC and the telephony call The internal protocols designed for the MDS control function at the MT is done over the MDS system are required to provide sufficient functionality channels by utilizing the RTS Services. A FES for supporting the Basic and Specialized service selected by the NCC for completing the call provides categories, while taking into account the unique the interconnection of the circuit with the public characteristics associated with the land mobile switched telephony network (PSTN). environment,. Efficiency was an extremely important criteria in designing these protocols, given the high MDS CHANNEL ARCHITECTURE cost associated with the MSS satellite resources. Efficient recovery of errored packets due to the fading Four channel types (one outbound, three and shadowing conditions was another important inbound) are defined at the physical layer to provide requirement. Unlike typical VSAT systems, where the the connectivity between the MTs and the DH. The remote nodes are designed to be operational at all outbound (DH to MT direction) channel termed DH- times, the design of the MDS protocols also needed to D, operates as a Time Division Multiplex (TDM) take into account the requirements for frequent channel with a data rate of 6750 bps using resynchronization of the protocol state machines given differentially encoded QPSK modulation and rate 3/4 the operational nature of the MTs. convolutional coding. The frame structure utilized over the DH-D channel is shown inFigure 4. Fixed The internal protocol architecture used within size frames which carry variable data segments are the MDS network is shown in figure 5. A layered transmitted over this channel which operates at a protocol architecture consistent with the Open System nominal information rate of 5062 bps. The DH-D Interconnect (OSI) model is used in designing the frame structure provides the common timing reference MDS protocols. The access to the MDS physical for the synchronization of inbound channel frames. channels (DH-D, MT-DRd, MT-DRr, and MT-DT) is controlled by the Channel Access and Control (CAC) Three types of inbound channels: MT-DRr, protocol. The CAC protocol provides different MT-DRd, and MT-DT are used in the MDS system. functionality at the MT and the DH. At the DH, the These channels employ differentially encoded QPSK CAC protocol isresponsible for formatting of packets modulation at a transmission rate of 6750 bps. Data is within the DH-D frames, and for allocating TDMA coded using rate 1/3, constraint length K = 7 capacity on the MET-DT channel, in response to convolutional coding. The MT-DRd is a contention requests from the MT or upper layer protocols at the type channel (slotted aloha) which is used to transmit DH. At the MT, the CAC controls access to the short packets for interactive applications. The MT- inbound channels. It implements the retransmission DRr is also a contention type channel, except that it is backoff algorithm for accessing the slotted Aloha only used for making capacity requests for the MT-DT (MT-DRr, and MT-DRd) channels. It is also channel. The MT-DT channel is a reservation responsible for monitoring the inbound packet queues (assigned) channel, the access to which is controlled and for requesting MT-DT channel capacity. The by the DH. CAC provides for the prioritized processing of packets within the MDS network. It also implements The frame structures for the three channels are algorithms for congestion control of the MT-DRd, MT-DRr, and MT-DT channels. shown in Figure 4. The MT-DRd and MT-DRr 481 MDS NETWORK MANAGEMENT ARCHITECTURE The Basic Service Categories (X.25 and Asynchronous) in the MDS system arc supported by the MDS Packet Layer Protocol (MPLP) and MDS The MDS system provides for a Data Link Protocol (MDLP). The MDS Packet Layer comprehensive set of network management Protocol (MPLP) provides procedures for the setup, procedures. All five functional areas specified within data transfer, and clearing of multiple virtual circuits the OSI network management framework: between the MT and the DH. ISO 8208 which Configuration, Fault, Performance, Security, and supports symmetric X.25 packet intcrcounection is Accounting are covered within the system. Key items used as the baseline protocol for MPLP. MPLP within each of these functional areas are briefly incorporates small enhancements to ISO 8208 for summarized below: more efficient operation in the MDS environment. For instance, astrategy which reduces the number of Configuration Management - Procedures are Receiver Ready 0LR) packets is incorporated into provided for automated commission_mg of new MTs, MPLP. performance verification of MTs, updating of MT parameters, system parameter distribution, and The MDS Data Link Protocol (MDLP) maintenance of several databases that contain network provides for the reliable sequenced delivery of configuration information. Keeping in mind the packets. In terms of functionality, the MDLP is similar mobility of the users as well as the evolving nature of to the Link Access Protocol - Balanced (LAP-B) the service, MDS provides for the dynamic defined in CCITT Recommendation X.25. However, reconfiguration of all system and MT specific unlike LAP-B, MDLP incorporates a number of parameters via automated system procedures (Mr features that provide for efficient operation in the parameter update and bulletin board distribution). landmobile environment. These include a selective repeat error recovery scheme to recover from lost Fault Management - The DH incorporates several packets, and a rapid synchronization of the protocol procedures to detect network failures and implement state machine in response to frequent MT on/off appropriate restoral actions. Failure modes that are conditions. detected include malfunctioning channel units, failure of terrestrial links, failure of RF link, rain fade, and The MDS Specialized Services Protocol lack of connectivity with the NOC. The DH is also (MSSP) provide for the multiplexing of application required to implement procedures for the detection of messages over the reliable transaction and malfunctioning MTs. unacknowledged data delivery services provided by MDS Transaction Protocol (MTP) and MDS Performance Management -The DH maintains, and Unacknowledged Link Protocol (MULP), respectively. makes available to the network operator, management MTP supports Wansactions involving short messages information variables related to overall MDS between the MT and the DH and vice versa. MTP is performance. These include traffic loads, congestion especially efficient when the transactions are indicators, error indicators, and protocol statistics. originated by fixed users, since capacity for the The MT is also required to maintain performance response can be allocated over the reservation channel statistics for each internal MDS protocol layer (MPLP, (MT-DT). MULP provides for the transmission and MDLP, _, CAC, Physical), Messages have also reception of unacknowledged data packets to and from been defined to transfer these statistics back to the DH MTs. More importandy it provides for the and make them available tonetwork and systems multicasting of data from the DH to MT user groups. engineering personnel. Finally the Bulletin Board protocol (BBP) Security Management -Given the large potential for provides for the dissemination of system information fraudulent access in a mobile environment, a number from the DH to all MTs. The bulletin board pages are of access authentication procedures have teen organized in a manner that reduces the system incorporated into the MDS system. At the simplest overhead, while providing for significant flexibility in level, two separate terminal identification numbers are making incremental changes and incorporation of used when communicating with the MT. A Forward additional system parameters. Terminal Identification Number (VIIN) is used for outbound messages from the DH to the MT, while a Reverse Terminal Identification Number (RTIN) is used for inbound messages. More sophisticated security mechanisms are also provided for access authentication, during virtual circuit setup, and via a periodic polled challenge issued by the DH. An 482 additional user facility has been defmed within the made available to the CMIS operated by the service X.25 call request and call accepted packets to carry the provider, which is responsible for customer billing. authentication information. REFERENCES Accounting Management -The DH maintains [1] J. Lunsford, R. Thome, D. Gokhale, W. Garner, accounting records for both the basic, as well as G. Davies, "The AMSC/TMI Mobile Satellite Services specialized services. The Basic Services records are (MSS) System Ground Segment Architecture", AIAA collected by the off-the-shelf packet switch (Terrestrial 14th international Communications Satellite Interface Subsystem). The accounting records are Conference, Washington, D.C. March 1992 Satellite Beams Network Management System !ii!(i!:!iliDillil¸I Fixed Users Figure 1.MDS Ground Segment Architecture X.25 DTE r D X.25 DTE Figure 2. X.25 Service X._ biDS Figure 3. Asynchronous Data Service 483 DH-D [_ 1608 bits, 238.22 ms @ 6750 bps v [ 32 bits 196 bytes (after rate 3/4 E coding) 8bits MT-DRr 1_., 298 bits, 42.88 ms @ 6750 bps __I 48 bits 32 bits 32 bits 21 -rate 1/3 coded bytes 18 bits MT-DRd I_., 730 bits, 108.15 ms @ 6750 bps ,.._I I-" "-f I .L ri_o_ I Data 48 bits 32 bits 32 bits 75-rate 1/3 coded bytes 18bits NT-DT ('rye-A) I_ <= 7552 bits, <= 1118.8 ms @ 6750 bps _ I I-" "-I [ CW I Taltn_ UW IFixed length data ! Flush I Variable length data IFlush i 48 bits 32bits 32 bits 24coded bytes 18bits 24-900 coded bytes 18bits MT-DT (Type-B) I_. <= 7360bits, <= 1090.4 ms @ 6750bp,s ...._! I- - I 48 bits 32 bits 32bits 48- 900 coded bytes 18bits Figure 4. MDS Channel Frame Formats DH MET I i ,,.L. I MPLP I M_P I BBPI MDLPI I MULPI MTP BBP[ MDLP [ MULP [ MTP I ChannelAccessandControl(CAC) ChannelAccessandControl(CAC) I I ! SatellitePhysicalLayer SatellitePhysical Layer MET-DRr/ MET-DRd DH-D MET-DT Figure 5. MDS Protocol Architecture 484

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