The protocol structure of the international aeronautical mobile satellite service (AMSS) is reviewed with emphasis on those aspects of protocol performance, validation, and conformance which are peculiar to mobile services. This is in part an analysis of what can be learned from the AMSS experience with protocols which is relevant to the design of other mobile satellite data networks, e.g., land mobile

Moher, Michael

English

NASA Technical Reports Server

N92=24197
OSI-Compatible Protocols
for Mobile-Satellite Communications:
The AMSS Experience
Michael Moher
Communications Research Centre
Ottawa, Canada
ABSTRACT. The protocol structure of the
international aeronautzcal mobile satellite service
(AMSS) zs reviewed with emphasis on those aspects
of protocol performance, validatwn, and
conformance which are peculiar to mobile servzces.
This zs in part an analys2s of what can be learned
from the AMSS experience with protocols which is
relevant to the design of other mob_le-satelhte data
networks, e.g., land-mobzle.
1.0INTRODUCTION
This paper reviews material presented to the
Aeronautical Mobile Satellite Services (AMSS) Panel
of the International Civil Aviation Organization
(ICAO) regarding the protocols for the planned
international aeronautical-mobile data service.
Particular emphasis is placed on the datalink
protocols and some of the work being done at the
Communications Research Centre (CRC) in this
area.
2.0 OVERVIEW OF AMSS AND OSI
One underlying premise of the AMSS is that it
is constructed in manner which is consistent with
Open Systems Interconnect (OSI) principles and in
particular that it uses the seven-layer protocol stack
illustrated in Figure 1. The work of the AMSS panel
is concerned with the bottom three layers of this
model: the physical, the datalink and the network
layers. However, the AMSS is viewed as
representing a satellite subnetwork within a much
larger Aeronautical Telecommunications Network
(ATN), which includes numerous other subnetworks
among which are VHF datalink networks, radar
communication networks and fixed data networks.
One conception of the ATN is illustrated in Figure 2.
The prime motivation for adopting the OSI model as
a guide in developing the AMSS is this view of
AMSS as part of a much larger ATN.
Among the attractions of the OSI model are
the belief that it leads to more interoperable systems
both intranetwork and internetwork, and the
standard protocols available for the different
protocol layers: protocols which have been to some
extent validated and thoroughly tested. A fmther
attraction of this approach is that tile
communication system is kept indei)endent of the
application, implying a much easier maintenance
and upgrading of application software.
The drawback of the OSI model is that it is
inherently designed for medium and wideband data
networks. In mobile systems, power constraints
imply that data rates range from hundreds of bits to
a few kilobits per second, which is one to two orders
of magnitude lower than the typical minimum data
rate one would find in fixed data networks.
Consequently, the overhead due to the use of seven
layers of protocols can use proportionately more of
an already scarce resource. Minimizing the message
length at the application layer only eases this
problem slightly, because at some point the
contribution of the protocol overhead to the total
message length becomes dominant.
The single example of Automatic Dependent
Surveillance (ADS) messages illustrates many of
these aspects of an OSI system which are importanl
in mobile applications. ADS reports are aircraft
position information derived fl'onl on-board
navigational aids and are a potentially important air
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International Mobile Satellite Conference, Ottawa, t990
https://ntrs.nasa.gov/search.jsp?R=19920014954 2020-03-17T12:12:01+00:00Z
traffic control application of AMSS. At the
application layer most of the redundancy is removed
from these messages leaving a standard message
length of 88 bits. However, after passing through
the top five OSI layers one calculation [1] shows that
the message length presented to the data link layer is
248 bits; the majority of this additional overhead is
due to the 128-bit Network Service Access Point
(NSAP) address standard for the ATN (128 bits) [2].
However, the greatest overhead (most of which is not
attributable to OSI) occurs when this message is
converted to the 1368 bits which are transmitted
over the channel. This latter figure includes the bits
needed for a burst preamble and synchronization,
and also includes the rate 1/2 coding applied to all
information bits. On the other hand, designing to
OSI principles means that changes in the content or
the length of the ADS report can be made without
requiring any change in the delivery system. The
interconnectability offered by the ATN means that
the ADS report can be automatically delivered over
the best of a number of communications
alternatives, e.g. satellite, VHF data link, or possibly
Mode S surveillance radar.
-7-
A_
This example clearly illustrates the ilnportant
consideration that must be given to the protocols in
the design of a mobile data network where there is
limited bandwidth available at the physical layer.
Opportunities for improving performance exist at all
the different layers. For example, at the network
layer an NSAP address of 128 bits[2], in principle,
allows over 1038 destinations to be directly
addressed, which is an extremely large amount of
flexibility. It would be very advantageous if the
network-datalink protocol convergence function
limited the amount of flexibility through the use of
some default addressing or other forms of address
compression.
On the other hand there may be areas where
the potential improvements are limited. For
example, at the physical layer the very nature of the
mobile network implies a necessity for some form of
random access strategy and its inherent inefficiency.
Ideally one would minimize the use of a random
access scheme once the mobile is logged on the
system, performing subsequent accesses using some
form of controlled access. However, in an OSI
APPUCATION
PRESENTATION
SESSION
","2 _ _#:,_
_TRANSPORT:
Network Servlce Access Points
(NSAP=)
SNDCP SNDCPSNDCP SNDCPSNDCP SNDCP
DATA UNK DATA UNK DATA DATA UNK DATA DATA LINK
PHYSICAL PHYSICAL PHYSICAL PHYSICAL PHYSICAL PHYSICAL
[ i L J l 1
Avionic= Subnetwork /dr/Ground Subnetwork Ground Subnetwork
Figure 1. The ATN protocol architecture[2]. (Legend: IP. lnternetwork Protocol;
SNDCP - Subnetwork Dependent Convergence Protocol; SNAcP - Subnetwork Access Protocol)
International Mobile Satellite Conference, Ottawa, 't990
A2t
j b
Ground Subnohvo_ A_/Ground Subnohvo_
o.o. NA_N, _ e.g. _ell#o, VHF
Av_nlcs $ubnehNo_
e.g. L_N, A_nc 429
Figure 2. Conception of ATN composed of a number of interconnected subnetworks [2].
system the communications cannot be tailored to the
application and as a result some inefficiencies are
irreducible. However, there is ongoing research in
the area of improving random access schemes [3],
and other areas where the transmission media can be
used more efficiently.
The OSI layer which is most directly concerned
with the physical communications is the datalink
layer, and it is at this layer that there is the most
opportunity to optimize the performance of the
limited physical resources. This is the layer that the
remainder of this paper will concentrate on.
3.0 DATALINK PROTOCOLS
There are a number of services which the
datalink layer must provide[4], and from the
datMink protocol proposed by INMARSAT for the
AMSS the most important of these relevant to
mobile systems are:
-- priority queuing of messages,
-- message segmentation and re-assembly, and
I error detection and re-transmission.
The combination of priorities and message
segmentation, breaking long messages up into short
packets of uniform length, prevent a long low
priority message from hogging the narrow bandwidth
datalink and causing excesssive delay to higher
priority messages; and it also allows low priority
messages to be interrupted by high priority messages
and then to resume without the need to be
completely re-transmitted. Message segmentation
also allows the selective re-transmission of those
portions of a message which were corrupted or lost, a
situation which is not uncommon due the generally
relatively poorer quality of mobile data links.
In the case of AMSS, considerable time and
effort has been spent by INMARSAT to develop a
datalink protocol which attempts to optimize tile
physical resources available. The underlying
assumption being that with sufficient optimization
at the datalink layer, standard 1SO protocols such as
ISO 8208 (X.25), ISO 8473, and ISO 8073/8602 can
be used at the network, internetwork, and transport
layers, respectively[2].
However, even within OSI there are interlayer
conflicts. For example, the priorities used by the
datalink layer are derived from the standard
priorities attached to all aeronautical
communications by the Radio Regulations of the
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International Mobile Satellite Conference, Ottawa, t990
CCIR. However, priority is not a standard Quality
of Service (QOS) parameter for an OSI datalink
layer, in particular, it is not a standard QOS
parameter passed by the standard X.25 network
protocol. Priorities could be passed as part of the
facilities field in the X.25 protocol, but this reduces
to some extent the benefits of using a standard
network protocol.
A detailed analysis of the datalink protocol for
the AMSS will not be performed here but we will
concentrate on some system level concerns about the
protocol which are shared with land-mobile
networks.
4.0 ASPECTS OF DATALINK PROTOCOLS
4.1 Performance
The main performance criteria placed on a data
communications system are average message delay,
maximum message delay, and reliability, as a
function of message priority. The verdict is still out
on the what performance is expected to be provided
by the AMSS. One conclusion is clear, initial service
will be slow because of the low data rates available
and the large overheads associated with the data.
However, this is acceptable in initial AMSS which
serves oceanic and low-density areas [5] where the
response time is, for the most part, not critical.
However, there have been suggestions that AMSS
may be used for air traffic control applications in en
route areas, where the required response time is
significantly shorter. In this latter case a detailed
simulation of the protocols will be necessary to
determine if the performance requirements can be
met.
4.2 Protocol specification and validation
As with the introduction of any new protocol
there is a need to carefully specify and validate the
protocol to insure that it performs the required
functions, and that it does so efficiently and without
error. Protocol validation is not a new problem, but
it becomes increasingly important problem as the
size and complexity of a communication network
grows. Although protocol validation is not a new
problem, there does not appear to be a well defined
solution.
The approach taken with the AMSS datalink
protocol is to specify it using the standardized
formal description language SDL. Languages of this
type are designed to allow the user to express all the
details needed to specify an implementation[@ In
that sense they are not a minimum description of
the protocol required to prove correctness and insure
interoperability. While these languages are in a
sense more complete, it has been our experience that
at times they can also be ambiguous.
Among the several approaches that can be
taken to the problem of validating a protocol are the
following [6]:
-- formal verification methods
-- implementation of the protocol and testing the
implementation,
-- simulation of the protocol, and
--in a few cases, it is possible to construct
automated design procedures that can be proven
to produce correct designs, but this an area of
current research.
Formal verification methods refer to the specification
of the protocol as a transition system or the
equivalent of communicating finite state machines.
Then the state space of such system is exhaustively
searched for undesired properties such as unmatched
communication events, deadlocks, and infinite loops.
Although such an analysis can be automated, the
state space of such a system may grow so large with
the number of messages and the number of machines
present that such a verification becomes infeasible,
although protocol validation for systems containing
up 107 states have been proposed in the literature[7].
For example, the X.25 protocol has been partially
verified by such an analysis.
By choosing to specify the AMSS datalink
protocol in SDL, simulation/implementation appears
to be the only the available method of protocol
validation, until a transition system description is
available. There are several levels on which the
protocols can be validated by
simulation/implementation. Current work being
performed at CRC is validating the performance of
the datalink protocols using a minimum subset of
the system, one AES and one GES, together with
simulated channel conditions. Validation of the
protocols in a more complete system is a desirable
second step in this process, but it is not clear if this
can be done without either performing an in-service
validation, or simplifying the simulation and
possibly missing some of the protocol interactions.
International Mobile Satellite Conference, Ottawa, 4990
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4.3 Conformance testing
After a protocol has been validated, there
remains the problem of insuring that all
manufacturers conform to the standard. In theory,
one would like to access to all interfaces between the
different OSI layers, however, cost and
manufacturer's design usually make this impractical.
Furthermore, the goal of a standard is
interoperability of different manufacturers'
equipments rather than explicit specification of an
implementation. This has the consequence that there
are a limited number of standard test points
available for testing protocol conformance. For
example in the ICAO standard for AMSS, the only
test points available for testing conformance are the
interfaces at the network layer and the interface at
the physical layer, that is, the signal-in-space as
shown in Figure 3. As a consequence, the approach
to testing equipment conformance will in general be
very different from validating the protocol. In the
latter, one can isolate the different OSI layers and
validate each independently; while with the former,
end-to-end conformance may be the only test
available. Since the number of variations can
increase exponentially with the number of layers
combined, the latter may perform a far greater
testing problem, and the only approach would
appear to be insertion of the equipment in a
simulation test suite where a equipment conformance
can be tested over a wide range of standard
scenarios[8]. The one saving grace is that this type
of test only needs to done once for each
implementation, and that there are a limited
number of manufacturers.
A potential future problem is the correction of
problems found in the protocol after the system has
gone into service. There may be questions as to
whether implementations must go through a formal
test procedure with each upgrade or whether in-
service testing can be sufficient. Another
consideration is that, because of its size and the
number of users, the complete system will not be
upgraded simultaneously and thus each upgrade
should be backwardly compatible.
5.0 CONCLUSIONS
In this paper several observations have been
made about the development of the AMSS
communication protocols and their implementation,
emphasizing those areas which are relevant to other
mobile systems. The OSI approach offers great
flexibility and interconnectability to data
communications with the penalty of significant
protocol overhead. The implication is that in a
narrowband system, such as mobile-satellite
networks, great care should be taken to minimize
these overheads at all layers. At the datalink layer,
in particular, there is the opportunity to optimize
the use of the available physical resources. However,
it is clear that protocol validation should be
performed as early as possible in the design process,
and that each subsequent design change should be
validated; in that way, the cost of correcting errors is
minimized.
DATA UNK DATAUNK
Lower
PHYSICAL Tet1'er PHYSICAL
t l
Satolllte Unk
S_gnal-ln-Space
Figure 3. Hlustration of test points available for protocol conformance test.
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International Mobile Satellite Conference, Ottawa, _990
REFERENCES
[1] V.Foose et al, "Preliminary estimates of data link
transmission overhead for ADS messages", WP-36,
3rd Panel Meeting of ICAO AMSS, February 1990.
[2] F.Colliver Jr., "The Aeronautical
Telecommunications Network", WP-37, 3rd Panel
Meeting of ICAO AMSS, February 1990.
[3] S.N.Crozier, "Sloppy Slotted ALOHA", IMSC'90,
Ottawa, Canada, June 1990.
[4] "Reference Model for Open Systems
Interconnection for CCITT Applications", CCITT
Rec. X.200, 1984.
[5] Report of the Special Committee on Future Air
Navigation Systems of ICAO, FANS/4 Report,
Montreal, Canada, May 1988.
[6] B.Pehrson, "Protocol Verification for OSI',
Computer Networks and ISDN Systems, vol. 18,
pp.185-201, 1990.
[7] G.J.Holzmann, "On Limits and Possibilities of
Automated Protocol Analysis", IFIP Int. Conf. ,on
Protocol Spec., Testing and Verif. VII, Zurich (North-
Holland, Amsterdam, 1988).
[8] R.J.Linn, "Conformance Testing for OSI
Protocols", Computer Networks and ISDN Systems,
vol.18, pp.203-219, 1990.
international Mobile Satellite Conference, Ottawa, t990
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OSI-compatible protocols for mobile-satellite communications: The AMSS experience

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