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

6 Pages·0.45 MB·English

by  NASA Technical Reports Server (NTRS)

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
[phone] (Tel)
[phone] (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
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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
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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
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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