The Mobile Satellite (MSAT) Mobile Data System (MDS) will offer a wide range of packet switched data services. The characteristics and requirements of the services are briefly examined. A proposed architecture to implement these services is presented along with its connectivity requirements. A description of the inbound and outbound channels is provided which are based upon the signalling for the circuit switched services. Additionally, the duties of the Network Management System are examined

Kerr, R. W.

Skerry, B.

English

NASA Technical Reports Server

N92-24i23
An Architecture for the MSAT Mobile
Data System
R.W. Kerr and B. Skerry
Telesat Mobile Inc.
P.O. Box 7800 Ottawa, Ontario
K1L 8E4, Canada
Phone: (613) 746-5601
Fax: (613) 746-2277
ABSTRACT
The MSAT MDS will offer a wide range of
packet switched data services. The characteristics
and requirements of the services are briefly exam-
ined: A p.roposed architecture to implement these
services is presented along with its connectivity
requirements. A description of the inbound and
outbound channels is provided which are based
upon the signalling for the circuit switched ser-
vices. Also, the duties of the Network
Management System are examined.
INTRODUCTION
The Mobile Data System (MDS) for MSAT
will provide packet switched data to mobile users
anywhere in North America. Due to the harsh
nature of the land mobile environment, the system
will have to be very robust in order to provide a
virtual 'error-free' connection between the Mobile
Earth Terminal (MET) and the Data Hub (DH).
The Data Hub will be the central point at which all
METs interconnect to other public and private
data networks. Smaller Regional Data Hubs will
provide service to METs in limited areas (one or
two beams) and will possibly allow for a reduc-
tion in backhaul charges.
Central to the design philosophy behind the
METs is the concept of an integrated voice and
data terminal. All voice and data services will be
available on the same MET, although the MDS
will operate independently of the Mobile
Telephone System (MTS) and the Mobile Radio
System (MRS). However, it is anticipated that
with the advent of ISDN in the Public Switched
Telephone Network some of the services offered
on the MDS could operate within the framework
of the MTS.
Given the dual nature of the basic MET, it is
foreseen that significant cost savings can be
achieved by adopting as much as is feasible for
the MDS from the signalling for the circuit
switched services (MRS/MTS). Thus the same
modulation, coding, interleaving etc. that is used
for MRS/MTS signalling will be used as the basis
for the packet switched data services [1 ]. As the
MRS/MTS signalling packet sizes have not been
optimized for data throughput, some changes will
obviously have to be made.
Unlike some currently proposed mobile
satellite systems [2,31 the MSAT MDS will not
operate on the same inbound and outbound
signalling channels used for the circuit switched
services. This makes it possible to guarantee
access to the system for MRS/MTS regardless of
any congestion that would result from the MDS
traffic. Also, this allows the data service to
expand and change in a modular fashion indepen-
dent of the MRS/MTS. Different service
providers can decide to offer their own packet or
circuit switched services autonomously.
Service Offerings
The MDS will provide packet switched data
services to both integrated voice/data METs as
well as dedicated data terminals. Many of these
services are currently being offered [4[, but the
MSAT MDS will allow for faster, cheaper, and
more flexible service. Some of these services are
described below, although the list is not
exhaustive.
Vehicle Location. Automatic Vehicle
Location (AVL) will allow a central dispatcher to
International Mobile Satellite Conference. Ottawa, 1990
323
https://ntrs.nasa.gov/search.jsp?R=19920014880 2020-03-17T12:29:31+00:00Z
monitor the position of a fleet of vehicles, with
periodic updates. This implies some sort of
navigation device on the vehicle, such as Loran-C
or GPS. This can be coupled with navigation
services to allow for optimum routing of a
vehicle.
Two-way general messaging. Short
messages can be sent between a MET and public
or private data networks. An electronic mail box
would be a value added service to store messages
if a MET is unavailable.
File transfer. This would provide for the
MET to exchange relatively long files with the
DH. There would be a tradeoff as to whether a
file should go as packets on the MDS or demand
a circuit via the MRS/MTS.
Interactive data. In order to allow for
interactive data sessions, the MDS will have the
capability of guaranteeing a specified response
time. This could be done either on a priority
basis or by assigning a TDMA slot for the MET.
Monitoring and control. Supervisory
Control and Data Acquisition (SCADA) will
allow for METs to monitor and control METs on
a real-time basis.
Virtual circuits. A MET will be able to
request a virtual circuit of variable throughput in
order to establish a 'virtual circuit'. This would
give a user a TDMA slot into which they can put
whatever data they wish. From the DH to a
MET, the user can be allocated one of the TDM
slots on a demand-assigned basis.
ARCHITECTURE
The main Data Hub will be the central point in
a double star network configuration. It will
manage communications between a large number
of MSAT data terminals and various public or
private data networks. In the overall MSAT
network, the Data Hub will communicate with a
Group Controller (GC) to request channels for
the operation of the packet data network (see
Figure 1). The GC is responsible for managing
power and bandwidth resources. Note that this
architecture has many similarities with existing
VSAT networks [5].
It is possible as that traffic in certain areas or
beams will require the addition of a regional data
hub. A regional DH may only need to operate in
one or two beams and would handle a subset of
the MDS customers (see Figure 2). The main DH
will communicate in all beams and can assign
METs to a regional data hub. Once assigned to
the regional DH, that DH will control a MET.
J _lnterfaceJI
( Packet _'_
_MET )-_ Pro&ssor _ /m_Tfo_)
;7 - .
[I£_f;ce) (Applicatio_
 Processor)
Figure 1. Data Hub Architecture
Network operation will be managed on the
assigned channels by the Network Management
System (NMS) at the Data Hub. The NMS
carries out all of the functions which are required
for the efficient operation of the network. A
description of the NMS capabilities are discussed
below. This structure reduces the GC processing
load and the need for combined packet data and
signalling channels for the GC, as the DH has the
responsibility of managing the data channel
implementation and user assignments.
The NMS has direct control over the Packet
Processor, which is responsible for the operation
of the communication links and all of the opera-
tions required to convert satellite packets to a
form of data usable by the destination network
and routing it to that network (and vice versa).
The protocols used on the other networks are well
defined standards. In the initial implementation,
the protocol that will be used will be X.25. Other
protocols may be implemented at a later date,
depending on customer demand.
On the satellite side of the Packet Processor,
the communication link protocols between the
MET and the DH can not be well established
protocols. This is because the propagation envi-
ronment for Mobile Satellite communications can
result in bursts of errors. The proposed proto-
324
International Mobile Satellite Conference, Ottawa. 1990
cols for the MDS satellite communication links
are described in a later section.
The MSAT MET will convert the data for the
User terminal from the satellite protocol to a
standard protocol to allow for an asynchronous
connection between the MSAT MET and User
terminal. This terminal could take the form of a
hand-held keyboard/display, or possibly a
personal computer.
The MDS will be able to offer a wide variety of
additional features through the addition of an
application processor. This processor handles
value added services, which enhance the service
seen by the customer and are not a function of a
communications network. For example, these
features could include a mailbox, a paging
service, or broadcast services where a message is
periodically repeated.
Channel Descriptions
As indicated previously the MDS is based
upon the signalling for the MRS/MTS [1] which
in turn draws heavily upon other proposed
systems [2,3,6]. However, the MDS has been
designed to allow greater data throughput and
more flexibility while at the same time trying to
reduce the complexity of both the METs and the
network infrastructure.
Signalling unit structure. The basic
signalling unit (SU) structure is 96 bits long.
Note that although the term signalling unit is
used, the packet may contain data and/or
signalling (but of course only for the MDS). The
96 bit SU includes a CCITT recommended 16 bit
Cyclic Redundancy Check (CRC) for error
checking.
In order to gain greater data throughput, SUs
can be 'chained' together either in the same frame
or in subsequent frames. This allows an initial
SU (ISU) to be followed by one or more subse-
quent SUs (SSU) as in other systems [6]. Thus
the addressing and other overhead needs to be
sent only in the ISU.
Modulation and coding. The proposed
initial form of modulation to be used on the MDS
is Aviation Binary Phase Shift Keying (A-BPSK)
at a transmission rate of 2400 sps (the same as the
MRS/MTS signalling). Rate 1/2 coding (k=7)
will be used which gives an information rate of
1200 bps.
Future expansion of the MDS could lead to the
inclusion of trellis coded modulation (TCM) at
4800 bps [7]. This may be fairly easy to imple-
ment because TCM may be used as one of the
modulations for the voice channel. Others could
be added as well. Thus a fully developed MDS
could have a wide range of modulation and
coding techniques as in the aeronautical system
[3]. This would provide service to a variety of
METs with different characteristics and under
various propagation environments.
DH-P channel. This is the outbound TDM
channel from the Data Hub to the METs. Its
frame structure is very similar to the signalling
outbound channel for the GC (see figure 3). Data
packets may be grouped in 3 or 4 SUs (ISU and
2 or 3 SSUs) in one frame to increase the data
throughput to a MET. The DH configuration will
be broadcast periodically to ensure every has the
current parameters and channel assignments.
MET-R channel. This inbound random
access channel will be similar to the signalling
access channels. The difference is that short
messages will be allowed on these channels (see
Figure 4). Messages of up to 3 packets will be
able to be sent. Initial SU with the address _nd
header and up to two Subsequent SUs which
contain a reference to the ISU, data and CRC,
hence increasing the amount of data that can be
sent in a packet.
The MET-R channel will use Sloppy Slotted
Aloha [8] in order to minimize the guard times.
Sloppy Slotted Aloha is a form of Slotted Aloha
in which METs are allowed to 'spill over' into
adjacent slots on their first access, but will have
their timing corrected for subsequent accesses.
The retransmission interval of the MET-R channel
will vary dynamically in accordance with the
traffic.
MET-T channel. The ir_hotmd TDMA
channel will have very short guard times like the
MET-R channel because any MET using the
MET-T channel is assumed to have its tinting
corrected. It may have larger packets that MET-R
channel, and/or the capability to assign multiple
adjacent slots in every frame to increase the data
throughput.
International Mobile Satellite Conference, Ottawa, 1990
325
Network Management System. One of
the duties of the NMS is scheduling long-term
accesses on MET-T channels. A few of the
expected applications require allocation of
capacity for periodic reporting. The reservation
may be needed in one of two ways, either by a
customer selected reporting interval or at a
specific time. For a given interval, the NMS
assigns the MET to a slot that is a given time
period from the first transmission where the MET
requested the service. The specific time reserva-
tion the NMS assigns capacity within a minute of
the customer selected time. The NMS re-assigns
METs' accesses when it is necessary.
It is expected that short term assignments to the
MET-T channel, such as reservations for multi-
packet transmissions will be sent on a separate
MET-T channel to avoid conflicts between long
and short term reservations.
The NMS also exercises control on all of the
METs accessing the DH. This is required to
prevent malfunctioning METs from interfering
with other communications, by either corrective
actions, such as adjustments to the MET's
frequency or timing, or a complete shutdown of
the MET. Control of the MET is also a require-
ment of the network to accommodate pre-emption
requirements for AMSS.
SUMMARY
The Mobile Data System on MSAT will
provide for a wide variety of services for users
across North America. By adapting as much as
possible from the circuit switched services, the
cost of both the network infrastructure and the
METs can be significantly reduced. At the same
time, by offering the MDS on separate channels
from MRS and MTS signalling, independence of
the services is assured, which enhances the
system's flexibility and modularity.
The concept of a regional Data Hub offers
further possibilities for gradual system expansion
to meet specific market needs. By offering inter-
connectivity with public and private data
networks as well as many value added services,
the MDS will be able to provide a comprehensive
set of data services.
REFERENCES
[1] "A Network Control and Signalling System
for the North American MSS", CCIR SG8
Final Meeting, November 1989.
[2] Mobilesat System Description, Aussat
Mobilesat, September 1989.
[3] Aviation Satellite Communications (SatCom)
_, Project Paper 741, AEEC, March
1988.
[4] M. Shariatmadar, K. Gordon, B. Skerry, H.
EI-Damhougy, D. Bossler, "System
Architecture for the Canadian Interim Mobile
Satellite System", Proceedings of the Mobile
Satellite Conference, pp 163-169, May
1988.
[51 J. Stratigos, M. Rakesh, "Packet Switch
Architectures and User Protocol Interfaces
for VSAT Networks", I E E E
Communications Magazine, Vol. 26, No.7,
pp 39-47, July 1988.
[6] Standard-M System Definition Manual,
INMARSAT, September 1989.
[7] R. Kerr, P.J. McLane, "Coherent Detection
of Interleaved Trellis Encoded CPFSK on
Shadowed Mobile Satellite Channels", 1EEE
GlobeCom '89, November 1989.
[8] S. Crozier, "Sloppy-Slotted Aloha", Second
International Mobile Satellite Conference,
published in these proceedings, June 1990.
326
International Mobile Satellite Conference, Ottawa, 1990
Data
Hub
Regional
Data llub
Figure 2. Network Topology
q 2400
Symbols
IFH] 12 Packets. interleaved and coded 1_. Flush
] ['_ 12 symbols
84 [ 2304 symbols I}
sYmt ,,,. ...J/
Frame Header _
Unique Word
12 SUs of 96 bits each
Figure 3. DH-P Channel Format
_. 2400
Symbols
Slot Slot Slot
1 2 3
204 symbols I
inc. 12tqushb,ts [ l
240 symbols
Figure 4. MET-R Channel Burst Format
Slot
10
International Mobile Satellite Conference, Ottawa, 1990
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An architecture for the MSAT mobile data system

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