1.2 UHF DAMA Satellite Communications Network

UHF DAMA SATELLITE COMMUNICATIONS
CAPABILITIES, ARCHITECTURE,
NETWORK CONTROL STATION:
AND PERFORMANCE
Lee E, Taylor
Frank Ganaden
ViaSat, Inc.
Carlsbad, California
ABSTRACT
To establish context, this paper briefly describes the
histoiy of DAMA on UHF military satellites, beginning
with experimental systems built during the 1970’s. The
paper goes on to describe the technical features common
to present-day 5-kHz and 25-kHz DAMA systems, and
proceeds to take an in-depth look at capabilities unique to
each of the DAMA waveforms, which were designed
independently and took rather different philosophical
design approaches. The variety of services which are
offered (e. g., global and local voice, data, and packetized
messages) are described, as are the techniques used to
allow automatic adaptation to variations in load,
individual terminal link qualities, and trafic priorities. In
addition the various types of network configurations
supported are described. The hardware and so~are
architectures for the currently-fielded Network Control
Station sites are then presented. Recently an automated
software tool set that can characterize DAMA network
pe~ormance in terms of call setup times and channel
usage eficiency has been developed. Pe~ormance data
are presented, demonstrating the general quality of service
that can be delivered. Finally the paper describes
evolutionary enhancements planned for the DAMA
waveforms and overall system, as additional use warrants
and finding becomes available.
I. INTRODUCTION
The combined techniques of Time Division and Demand
Assigned Multiple Access (TDMA/DAMA) offer the
promise of significantly more efficient usage of
narrowband UHF satellite channels for all of the military
services than the original, dedicated network per channel
mode of operation. This paper on the subject of UHF
DAMA controllers is particularly timely, since a 5-kHz
network control capability was fielded and went
operational in late 1997, and the controller upgrade to add
25-kHz DAMA capability will be reaching the same
operational state early in 1999. Performance data for the
operational system is hence just now becoming available
and is the primary focus of this paper although a
significant amount of system description information is
included for the purpose of establishing context.
Initial proposals to apply TDMA/DAMA concepts to
narrowband military UHF satellite channels originated in
the mid- 1970s. MIT Lincoln Laboratory built i~nd
demonstrated a prototype DAMA fuily-automated
controller and network terminal for use on 25-kHz
channels in 1978 (Reference 1). Shortly thereafter, the U.
S. Navy contracted with Motorola to develop a fieldable
25-kHz TDMA controller, which resulted in the TD-1271
multiplexer. In the ensuing years, many terminals
consisting of TD- 1271s and E-Systems’ WSC-3
transceivers have been deployed, and TDMA has become
the regular day-to-day operating mode for Navy 25-kHz
UHF satellite communications. Recently the Navy
installed the DAMA Semi-Automated Controller, a
prototype channel controller developed at NRaD which
replaces the manual marker-boards used until now to
manage access to TDMA slots with a real-time perscmal
computer application.
The concepts for DAMA on 5-kHz UHF transponder
channels evolved about a decade behind 25-kHz. Among
the early 5-kHz DAMA development programs were the
UHF Satellite Terminal System (USTS) and Mini-DAMA
projects in the 1980s. These were followed by a
competitive development of prototype, automated 5-kHz
network controllers by Titan Systems and ViaSat for the
Air Force in 1993 – 1995. The second-generation ViaSat
Network Control Station (NCS) is the system presently
fielded world-wide and being upgraded to add 25-kHz
control capability.
For purposes of ensuring interoperability among different
manufacturers’ terminals, standard waveforms have been
specified in MIL-STD-188-182 for 5-kHz bandwidth
channels and in MIL-STD-1 88-183 for 25-kHz channels.
Though the two standards were developed at different
times, and with different military services being most
involved, they share many common concepts and
approaches; these are the focus of Section II which
describes at a top level how DAMA “works”. Section III
addresses the unique features of the individual waveforms,
taking on a strong 5-kHz DAMA flavor since that standard
specifies protocols in much more detail and was develclped
more recently. Section IV describes the DAMA controller
arGhitGGture,starting with the world-wide placement of the
primary and alternate channel controllers, and a discussion
of the concepts for transfer of control. The architecture
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section delves into the next lower level of detail,
describing how the functions at each site are distributed
across hardware elements plus indicating the size,
language, and operating system for each processing
element. Section V, Performance, discusses quantitatively
how well the DAMA channels function under traffic load.
In the present era, before sizeable numbers of DAMA
terminals are deployed, these data can only be derived
using the Traffic Generator, an emulation tool that resides
in the channel controller computer, along with the NCS
application. The performance data are of course required
in order for ViaSat to demonstrate compliance with
specifications. But when more operationally realistic
scenarios are used, the performance data can also help
direct the DAMA development community towards the
high-leverage improvements to the system in the future. In
addition the DAMA operations planning community can
use the same tools to predict performance under various
network configurations they might consider for live
missions. This paper closes with a short section describing
presently planned improvements to the military standards,
and the Navy’s near-term plans for adding capability to the
DAMA Network Control Stations,
master time base to which all users on the channel
synchronize, and time slots that contain burst
transmissions originating from those individual channel
controller and network terminals using the channel. The
slots must include “guard time” before and after each
terminal’s bursts, to accommodate timing drift between
instances when the terminal re-synchronizes. The use of
periodic slots for communicating inherently continuous
data streams requires that the data be buffered at the
originating terminal, resulting in end-to-end transit delays
that do not occur in continuous transmission systems.
Each controlled DAMA channel has four fundamental
types of slots. The first type is “forward” orderwires
originated by the NCS and containing control information
such as slot assignments for the population of network
terminals. Slot type two is ranging slots that are
transmitted and received by individual terminals and are
used to measure signal propagation time from the terminal
to the satellite. The third type is “reverse” orderwires
originated by the network terminals and containing control
information such as service requests for the NCS. Finally,
the fourth category of slots contains communications
bursts among pairs of users or network members. Both
waveforms offer users a basic type of service; a
synchronous, half-duplex circuit with destination(s)
Simple
restricted to the “local” satellite footprint.
depictions of the frame formats for 5- and 25-kHz DAMA
are included in Figure 1.
II. COMMON 5- AND 25-KHZ CAPABILITIES
UHF narrowband channels are themselves derived through
frequency division multiplexing across the allocated band
and time division is the multiple access technique chosen
for handling multiple users per channel. TDMA requires a
o
5-kHz
Frame
●
“
8.96 sec
FOW
●
~ Contention
.
RNG
4
j Contention
;
ROW
&
~ Assigned
: RNG + ROW
~
~
COM
4
0
Generic
Frame
. . .
Forward
Orderwire
Frame
~
:
Range
Burst
! Reverse
~ Orderwire
~
!
User
Communications
1.386 sec
o v
● ●
”
CCOW
[ C:M
: RNG
; ;;:
[RCCOW;
C:M
Figure 1: UHF DAMA Frame Formats
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duration
●
A
25-kHz
Frame
●
:C:M
●
.*
**
*O
With multiple network terminals using the. channel, and
some reverse orderwire slots allocated to “random access”
(use by more than one terminal) there is a possibility that
two terminals will transmit in the same slot, The result is
contention (mutual interference) such that neither
terminal’s burst can be successfully received on the
satellite downlink. Both military standards include
provisions for handling this situation, consisting of retransmission protocols that will ensure that the contention
is ultimately resolved.
Data conveyed in the various types of slots can be
protected against the effects of transmission errors by the
application of forward error correction codes. In addition,
cyclic redundancy check coding on selected slots provides
detection of the rare errors that cannot be corrected.
Various symbol burst rates can be selected; higher rates
being most efficient and lowest rates providing additional
robustness for terminals with small antennas. In general,
the orderwires are operated at the lowest burst rates and
with strongest coding, in order to guarantee the best
possible reception by the most disadvantaged terminals.
Finally, in the context of signal processing, the waveforms
both use phase modulation, whose constant-envelope
properties are suited to the non-linear satellite
transponders. Orderwire data flowing in both directions are
typically encrypted to prevent traffic analysis.
Just as in the public telephone system, a DAMA user
needs a destination number in order to place a call. The
DAMA address space has 216elements, which can signify
terminal port addresses, network addresses, or “guard”
addresses (which can indicate an individual person,
function, or mission). Calls may be placed point-to-point,
multi-party conference calls may be established without
preplanning, or preplanned multi-party networks may be
activated, Requests for service include a numeric
precedence level in the range 1 -5, so that when the
overall load exceeds channel capacity, lower precedence
traffic may be preempted. Each individual DAMA channel
has its own forward and reverse orderwire slots, so a given
terminal will typically remain on the channel it started out
on – its “home” channel. In the event of high load on one
channel, however, the NCS can send one or more
terminals to a different DAMA channel; they can even be
commanded to move from a 25-kHz to a 5-kHz channel
(or vice versa). Finally, if very high priority users require a
channel without TDMA buffering delays, the NCS can
assign them temporarily to “dedicated” channels that
operate full-duty-cycle in accordance with MIL-STD-188181.
In addition to a centralized time base generator, UHF
DAMA employs a centralized channel controller, which
“assigns” individual users and networks to time slots, Each
channel controller has one or more alternates, typically
located at a different geographical site. Control may
transfer from primary to alternate under a number of
situations, including an unplanned equipment outage at the
primary site, a primary site operator request to hand over
control to the alternate, or an alternate site operator request
to take control,
Parameter
Frame Duration
Basic Time Unit
I/O Rates
Burst Rates
Guard Band Size
Modulation
FEC Code Rates
5-kHz
(MIL-STD-182)
8.96 sec
8.75 rnillisec
“building block”
75-2400 bps
600-3000 SDS
25.208 millisec
SOQPSK
%
25-kHz
(MIL-STD-183)
1.3867 sec
52 microsec
“chip”
75-16000 bps
9.6-16 kst)s +
1.25 rnillisec
BPSK
‘/2,%
Figure 2: Comparative UHF DAMA Numerics
111.UNIQUE FEATURES OF EACH WAVEFORM
Philosophically, the 5-kHz DAMA standard exerts much
tighter control over the population of network terminals
and is intended to be much more dynamic in adapting to
changing conditions. Terminals are required to log in with
the channel controller immediately upon acquiring the
downlink and uplink timing whereas in 25-kHz DAMA a
terminal may passively monitor the channel and make its
presence known to the channel controller much later, using
a variety of reverse orderwire types. Typically this will
occur upon a status change such as putting an I/O port in
service or changing the port configuration of baseband
equipment.
Also there is a 5-kHz DAMA protocol to
-ensure that the controller’s version of each- terminal’s
guard list (all of the addresses associated with each I/O
port) remains consistent with the terminal’s version. When
discrepancies are detected the protocol automatically
brings the lists back into a consistent state, accepting either
the channel controller or terminal version, under NCS
operator control for each terminal. The 5-kHz channel
controller reconstructs the frame format and rebroadcasts
slot assignments every frame ! This allows more or less of
the frame, as demand requires, to be allocated to forward
and reverse orderwire traffic. It also allows immediate
preemption of lower precedence user traffic to
accommodate more important calls and for immediate
restoration of the preempted service once the higher
precedence call terminates. Basically, if a 5-kHz terminal
does not receive its slot assignment in a frame, it ceases
transmitting; when the slot assignments start appearing
again, the terminal resumes usage of the slots. Finally, the
continuous broadcast of service assignments allows
terminals that log in late to immediately join established
networks. In contrast, the 25-kHz waveform sends a slot
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connect in a single frame and then does not repeat it. Thus
the channel controller must originate a new copy of the
slot assignment in order to bring a late entering terminal
into a network; and the controller must send a slot
disconnect orderwire message in order to force a call off
the air. As a result of its more fixed frame format, the 25kHz waveform offers precisely one forward orderwire
message and one reverse orderwire message slot per
frame. Similarly segments of the 25-kHz frame devoted to
user communications take on a relatively fixed format;
they can change but generally not from one frame to the
next. So, if the current 25-kHz DAMA frame format is
structured for, say, nine 1200 bps slots and one 75 bps slot
(the format designated “9, B, A“) and some of these slots
are assigned to active calls, the channel controller may
require several frames to change the format in order to
accommodate a 2400 bps call. The 5-kHz waveform is
sufficiently dynamic that the controller can immediately
compress the existing services into the end of the frame
and create a 2400 bps slot.
The 25-kHz DAMA waveform provides an end-end link
quality measurement capability, At the request of either
the channel controller operator or a terminal operator, the
terminal can be assigned the single link test slot (see
Figure 1) for enough frames to accumulate statistically
significant bit error rate data. The terminal itself originates
the uplink and receives the downlink, so this constitutes a
true end-to-end link test. The burst rate is variable over the
options available for communications services, so the data
may be related directly to performance in the
communications slots. Once the link test is completed, bit
error rate results are displayed at the terminal and reported
to the channel controller using a reverse orderwire
message. At present the channel controller does not
automatically use the link test data in selecting a
communications slot with appropriate burst and code rates,
since bit error rates rather than carrier to noise density
ratios are reported. The capability to report C/No is a
feature of the next-generation 25-kHz standard (“183A”)
and will ultimately be used by the 25-kHz controller. In
contrast, 5-kHz DAMA network terminals derive link
quality information by processing forward orderwire bursts
over many frames. This technique has the limitations of
providing information only on the terminal’s downlink and
only at the very robust 800 sps rate 1/2 coded mode.
Nevertheless, results are periodically sent to the channel
controller in reverse orderwire status reports, and the
controller uses the results to automatically select burst and
code rates which will ensure adequate performance for the
weakest terminal in the service. The 5-kHz controller
strives to achieve 10-3 bit error rate (BER) for voice
services and 10-5 BER for data services.
In addition to the generic half-duplex, synchronous, localfootprint circuit service type offered by both waveforms,
5-kHz DAMA distinguishes between voice circuits and
data circuits for BER purposes. Also, MIL-STD-182
includes the structure for handling start, stop, and fill bits
necessary for operation of asynchronous data I/O. Beyond
circuit services, the 5-kHz waveform includes packetized,
simplex “message” services. When the length of a data
communication is precisely known a priori, such as in the
case of a computer-to-computer file transfer, the message
can be divided into small packets and sent at a variable I/O
rate per frame, in the background relative to the circuit
services. Exactly enough packets are allocated to just fill
up each frame, after all active circuit services have been
accommodated. This feature allows highly efficient usage
of the available channel capacity. At present message
length is limited to 114688 bits because of possible
cumulative timing drift between source and destination.
Point-to-point
message
services
include
acknowledgements of each burst and of the entire
message, providing an extremely reliable form of
communication. Another service-oriented capability
unique to 5-kHz DAMA is global connectivity, which
applies to all point-to-point (vs. network) calls. The worldwide complement of channel controllers intercommunicate
and have the intelligence to find a user anywhere globally
and to automatically relay traffic through multiple satellite
hops until it reaches the proper destination.
5-kHz DAMA terminrds are required to specify in each
service request whether the traffic is voice or data and
whether the I/O is synchronous or asynchronous. A quite
different approach is used with 25-kHz DAMA, where the
terminal originating the service specifies one of 99
“configuration codes”, the specific values of which are left
open to be operationally assigned. Each code will
correspond to a complete string of I/O equipment, so in
addition to voice/data and sync/asynchronous it conveys
whether the port is attached to an encryption unit, if so
what model, and what variety of I/O device is currently
attached.
A unique feature of the 25-kHz DAMA waveform is the
ability to assign slots in a second channel. This is termed a
“frequency-switched slot connect”, and requires the
participating terminals to tune their radios twice per frame,
such that they access orderwires on the home channel and
handle communications bursts on the other channel. A
second unique 25-kHz DAMA capability is a call waiting
alert similar to that offered in commercial telephony. If a
user is denied connection to a second party because the
second party is busy with higher precedence traffic, the
originating user may manually send a “paging” reverse
orderwire and the channel controller will originate a “call
waiting” forward orderwire which may be displayed at the
busy party’s terminal.
One noteworthy unique feature of 5-kHz DAMA is the
ability for users to notify the channel controller that their
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use of a slot is completed, using an in-band signal rather
than taking the time and bandwidth to send a reverse
orderwire. The terminal simply inserts a special 12-bit
“tear-down” code in the circuit burst kind (CBK) field of
the last transmitted burst. Since the channel controller
monitors usage of every slot, upon receipt of the teardown
CBK, the controller broadcasts a teardown forward
orderwire and immediately puts the slot back into the
available pool. Another unique 5-kHz concept is the
“future forward orderwire”. The first few fields of a Future
FOW are well defined so that the message can be
recognized by old terminals and ignored, whereas
terminals fielded since the remainder of the Future FOW
fields were defined can go on to process the rest of the
message and hence offer advanced capabilities. This
feature greatly facilitates interoperability among old and
new terminals; the channel controller must, of course, keep
records of which terminals support which versions of the
military standard.
IV. DAMA ARCHITECTURE
The DAMA “system” is comprised of the entire suite of
NCS equipment at all four sites worldwide: Norfolk,
Naples, Guam, and Hawaii. Each site is termed an NCS
“segment” and has both an east side and a west side for
controlling channels on satellites in the two directions. For
5-kHz global traffic relaying purposes, the east and west
sides are connected by a local area network. The systemlevel architecture is depicted in Fig. 3, which indicates
CONUS, Atlantic, Pacific, and Indian Ocean satellites plus
two Network Terminals.
CONUS
PAC
The combination of NCS segments, links, and satellites
presently form an eight-channel ring backbone network.
Individual network terminals connect into the ring using a
single channel on their local satellite. Once terminals have
logged-in with the channel controller, the system
automatically provides them service accesses on demand.
Typically, each satellite channel will be configured with a
“primary” channel controller (PCC) and an “alternate”
(ACC) located at the other NCS site within view of the
same satellite. Geographically dispersed alternates provide
considerable robustness in the face of natural or military
disasters. An ACC can take over channel control in the
event of planned or unplanned PCC outages. In the
planned case, the PCC operator can manually initiate
handover to the alternate; similarly, the ACC operator has
the ability to request channel takeover, but this is allowed
only with approval by the current PCC operator. Once
control has been handed or taken over, the “old” PCC
equipment can be placed off line for maintenance or other
activities. In the event of unplanned PCC failure, and if an
ACC is actively logged onto the channel, that ACC will
automatically take over control of the channel and begin
transmitting forward orderwires after missing the forward
orderwire bursts from the PCC for four frames. The
nominal configuration of PCC and ACC assignments at the
four NCS sites is shown in Fig. 3, although the situation
will be dynamic.
Each NCS segment has been installed with eight fullduplex processing “threads” (i. e., a ViaSat MD-1324
modem and an E-Systems RT- 1771 radio) per side; a
straightforward growth path to 16 channels per side has
been demonstrated, but not yet fielded. Each side of an
NCS segment consists of three basic elements as shown in
Fig. 4: an antenna array, a set of unattended racks
containing radio-frequency and modem equipment, and an
operator workstation with personal computers (PC’s),
monitors, and I/O equipment.
)-
4
Ak
AL
Ab
Antenna
Array
Ah
}
RF
, , 1 1 1 1 I
1 1 R/T’s
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Modems 1
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10
Figure 3: UHF DAMA World-wide Architecture
Modems t
Pcc
LAN
LANT
RF
1 I , 1 I 1 I
I I R/T’s
1 I 1 t #
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}
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Work
station
Figure 4: NCS Segment Architecture
The antenna array for the current eight-channel
configuration consists of two transmit and one receive
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antennas per side. A photograph of the Hawaii antenna
array is included as Figure 5. The photograph shows all
three west side antennas, the east side receive antenna, and
one of the east side transmit antennas which is shown in
the foreground. In situations with multiple closely spaced
satellites, the NCS antennas are typically pointed to access
all spacecraft, rather than being optimized for a particular
satellite.
Figure 6: NCS Equipment Racks
Figure 5: NCS Antenna Array
Figure 6 is a photograph of the four NCS racks installed at
the Hawaii site. Doors on the two east side racks are open,
showing the modems (gray), transceivers (black), and
other RF and AC power equipment front panels. A
Rubidium frequency standard provides 10-11accuracy to
facilitate synchronization of network terminals to the
overall DAMA system. As an option, each side of arI NCS
may be configured with a 5-kHz hot standby modem-radio
“thread”. Then, in the event of the failure of a single PCC
modem or radio, that side of the NCS will switch in the
standby thread if the channel PCC fails to receive three of
its own forward orderwires. This avoids takeover by the
ACC that would occur after the fourth missed FOW,
effectively keeping channel control at the local site. The
“cost” of activating the standby thread is that one less
channel can be actively controlled in this configuration.
NCS operators are provided a DAMA control personal
computer (PC) and workstation for each side of the
segment, through which they can configure the system,
make manual service assignments, monitor system usage,
and be notified of any failures detected in the rackmounted equipment. The 5-kHz and 25-kHz applications
execute simultaneously on the same PC. Additionally, the
NCS operators have three I/O devices at their workstations
that may be used to communicate with the population of
network terminal operators: a voice terminal, a “red” (with
encryption) data terminal, and a “black” (without
encryption) data terminal. Each is attached to a specific
modem, but “in-site routing” over a local 1553 bus
automatically allows any I/O device to communicate using
any one of the channels being controlled. The voice
terminal is a KY-99 MinTerm, which implements standard
2.4 kbps Advanced Narrowband Digital Voice Terminal
(ANDVT) speech coding and encryption. The data
terminals consist of a software application on a desk-top
personal computer. Black and red data terminals use
separate PCs, and the red PC is interfaced to two KIV-7
data encryption units, providing full-duplex capability
using KG-84 encryption techniques. The data terminal
application is called the Integrated Data Terminal (IDT)
and operates similarly for both red and black data, though
the screen background colors are different in order to
remind the operators of the sensitivity of the red data.
Operators may either compose and send “notes” from the
keyboard/monitor, or they can load files from diskettes
into the PC and then send entire files. This application
includes an end-to-end error detection code, so
transmission errors are flagged to the receiving operators.
The Hawaii East workstation is shown in Figure 7. The
larger monitor supports the channel controller computer,
while the smaller monitor is shared between the two data
terminal computers. The classified encryption units are
behind the paper covers in the photograph.
Figure 7: NCS Workstation
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DAMA Network Control Station application software
consists of five computer software configuration items,
resident in the Channel Controller, Relay Bus Controller,
and Integrated Data Terminal personal computers and the
modem. The modem firmware CSCI consists of a
“Common” component and a separate component for each
UHF MIL-STD waveform (181, 182 and 183) only one of
which is resident in memory and operating at any given
time. The orderwire encryption CSCI resides on a separate
processor on a circuit card assembly within the modem.
With a suitable antenna, radio and modem controller, the
modem can function as a stand-alone network terminal.
Figure 8 provides the characteristics of the NCS software
elements, including size in terms of source lines of code
;LOC).
CSCI
Operating
Language
Size
System
(SLOC)
Channel
Windows NT
Ada
106000
Controller
Relay Bus
DOS
c
1800
Controller
Integrated
c
Windows 95
4800
Data Term’1
Modem
Custom
c
78000
Real-time Exec
Orderwire
Custom
Ada
5700
Encryption
Real-time Exec
Figure 8: NCS Software Characteristics
V. PERFORMANCE
The NCS Traffic Generator is one of two fundamental
tools used to determine 5- and 25-kHz DAMA network
performance and quantify the NCS ability to control
multiple channels under heavy traffic conditions. The
Traffic Generator is file-driven and runs as a software
application on the same channel control computer as the
NCS software. Written in Ada, the Traffic Generator
reads an input data file to determine the traffic parameters
to be used for generation of service request and service
termination messages. The input data allows specification
of uniform random or exponential distributed message or
circuit point-to-point service requests and specification of
uniform random or exponential distributed service
durations. The input data provides the Traffic Generator
valid terminal addresses to be used in generating service
request messages and provides all terminal configuration
data, including I/O data rate and baseband equipment type,
for the required orderwire messages, Furthermore, the
Traffic Generator inputs allow for a percentage of the total
requests originated by a source address to be directed to a
specific destination address. Only one active service
request from an address is allowed at any time. However,
for 25-kHz DAMA the Traffic Generator does allow
requests to be generated while the address is active in a
service. This accommodates terminals requesting retransmission of the slot assignment or requesting another
user to be added to an existing service. The Traffic
Generator implements the service request re-transrnission
protocols described by the 25-kHz DAMA standard.
Running concurrent with the NCS application, the Traffic
Generator accesses the NCS service request queue to
determine whether a generated service request was
received by the NCS. If the request was not received, the
Traffic Generator calculates the backoff interval for the
request, re-transmitting the request at the designated time.
A future upgrade of the Traffic Generator will implement
the re-transrnission protocols described by the 5-kHz
DAMA standard based on the number of ROW random
access slots provided in a frame.
The channel controller application maintains an “Event
Log”, which records all forward and reverse orderwire
traffic, with time-of-day tags, in a text file, During
extended running periods, the Event Log can fill the
allocated PC memory, at which point it is automatically
archived onto the NCS hard disk. The Event Log is
naturally useful to NCS developers and operators since it
details “what actually happened”. It can also be postprocessed by our Performance Analysis Tool (PAT), an
application written in the C language which tracks each
service request from “life to death”, noting: origination
time, number of request retransmissions, time placed in the
NCS service queue, time removed from the service queue
(i. e., assigned a slot on the channel), and time completed.
As it processes an archived Event Log file, the PAT
outputs a cumulative networking performance summary
report record at the conclusion of each minute of NCS run
time. This record includes true call arrival rate, reverse
orderwire contention delay, service queue delay, true
service duration, and blockage rate. The mean value,
variance, and maximum value for each parameter is
derived. In addition the 95%-tile service queue delay is
reported.
This performance investigation starts by defining, in as
comparable a way as possible given waveform and Traffic
Generator limitations, a priori “operating points” for
individual 5-kHz and 25-kHz channels. Ideally these two
operating points should relate to a worldwide population of
network terminals and satellites and should constitute a
relatively heavy, but theoretically supportable, traffic load.
Step two is to drive the actual Network Control Station
software application with the Traffic Generator configured
to generate the desired load, and to determine network
performance under the operating point load, using the
following parameters: queueing delay and average channel
use efficiency, (Other standard networking performance
measures including reverse orderwire contention delay and
call blockage rate were considered, but are presently not
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modeled precisely in our Traffic Generator.) After
interpreting the performance data for each operating point,
we will adjust call rates, in an attempt to determine the
effect on the pair of performance parameters. Constituents
of the load are: the number of active terminals, the average
call rate per terminal (Poisson distributed), and the average
call duration (Exponentially distributed) per terminal, for
various types of services (i. e., voice, data, messages).
With their global, reliable message service capability, 5kHz DAMA channels might be considered most valuable
for users that originate primarily data traffic with
occasional voice communications requirements. Similarly
25-kHz DAMA channels, with their relatively quick voice
turn-around performance (due to the short frames), are in
some sense best suited for users who contribute a sizeable
volume of voice traffic. So for this investigation we will
split terminals into “data” and “voice” categories,
assigning the former to 5-kHz home channels and the latter
to 25-kHz home channels.
Consider a time in the intermediate future, when 16000
two-port DAMA terminals (5090 of the total available
address space) are deployed world wide, comprised of
6500 Army, 6500 Air Force and 3000 Navy units. And
assume that the Army and Air Force terminals split among
data and voice categories at the ratio 5500/1000. Navy
terminals split with datahoice ratio 500/2500. Assuming
uniform geographical distribution, that 25% of the
terminals will be active at any given time, and that 8
satellites are in orbit, the result would be 360 data
terminals and 140 voice terminals active simultaneously
on any single satellite.
Each of the UHF Follow-on (UFO) satellites has 38
channels: 17 “wideband” (25-kHz) and 21 “narrowband”
(5-kHz). We will assume that three of each channel type
are placed into the channel controller’s (MIL-STD-188181) single-access pool. If the communications planners
are successful in distributing the population of active
terminals evenly across the remaining DAMA channels,
the result will be approximately 20 terminals per 5-kHz
channel and 10 terminals per 25-kHz channel.
Finally, the complete load input scenario must specify
average call rates and durations for each type of traffic.
Our scenario postulates that circuit services require 30
seconds channel time on average and that (5-kHz) message
services average 3.6x104 bits. Simple analysis (as
summarized in Figure 9, below) shows that if each “data”
terminal (those assigned to 5-kHz channels) originates 1
voice call, 2 data circuit calls and 2 message services per
hour, and if each terminal has a nominal link quality of 40
dB-Hz, the combined offered communications load (i. e.,
excluding orderwires) should on average total 534 building
blocks per frame. Since each frame consists of 1024
building blocks (bb), and if we assume the orderwire
occupies a constant 223 building blocks as shown in the
table, then the 534 bb comprises a 66V0load on the 801
total bb available for communications. Each circuit service
requires one request and one teardown reverse orderwire
transmission and each message service requires only a
single request orderwire transmission; thus the average
total reverse orderwire load for 20 terminals is 160
transactions per hour or 0.2 usages for each of the assumed
two reverse orderwire slots per frame. The actual number
of reverse orderwire slots in the 5-kHz DAMA frame
varies automatically from frame to frame. Only in the rare
instance when the frame is almost entirely packed with
communications bursts will the number of reverse
orderwire slots be as low as two.
Service
Type
Voice
Data
Msg
Total
COM
FOW
RNG
ROW
Total
Ow
Call
Rate
I/trm/hr
2/trm/hr
2/trm/hr
Call
Length
30 sec
30 sec
36 Kb
Burst
(bb)
832
526
99
Total
bblhr
66560
84160
63360
100/hr
125
32
17
160
ROW/hr
Grand
Total
Avg
bbffr
166
210
158
534
125
64
34
223
757
Figure 9: 5-kHz Channel Load Analysis
Theory tells us that a slotted Aloha system remains stable
at loads below 0.37 per slot occurrence. Hence reverse
orderwire contention with this scenario should be minimal.
Similar analysis, as summarized in Figure 10, can be
conducted for 25-kHz DAMA channels assuming that
Frame Format {5, 5, 3} is in force, offering four 2.4 kbps
and two 1.2 kbps slots. Each “voice” terminal originates
30 voice calls and 15 data circuit calls per hour. Each
individual service requires two reverse orderwire
transmissions: a two-party call and a call complete. The
total average communications load for 10 terminals
actively using the channel is 37.5 minutes of voice traffic
per slot per hour, 37.5 minutes of data traffic per slot per
hour and 0.35 reverse orderwire usages per frame. The
resulting 62% channel occupancy is comparable to the
66% loading on the 5-kHz channels, but the total reverse
orderwire load on the single RCCOW slot per frame is
very close to the theoretical maximum and would result in
significant orderwire contention (not currently modeled
precisely in our Traffic Generator).
Since the scenario was defined on a per-channel basis the
Traffic Generator was run on a single channel at a time,
separately for 5-kHz and 25-kHz DAMA. The initial 5-
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I
Service
Type
Voice
Data
Total
COM
Total
Call
Call
I
Rate
Length
30/term/hr
30 sec
15/terrn/hr
30 sec
I
Total
I
/slot/hr
37.5 minutes
37.5 minutes
450 calls/hr
900
RCCOW/hr
Ow
Figure 10: 25-kHz Channel Load Analysis
kHz performance measurement was taken over a period of
13 hours and resulted in 1292 service requests. This total is
slightly low compared to the expected value of 1312
requests for that period, and differs as a result of a finite
population effect; the Traffic Generator creates no new
service requests from a given 110 port while that port is
still busy with earlier traffic. Circuit service duration
averaged 38 seconds overall. This is higher than the 30
second specified value because of two phenomena. First,
each service must fill an integer number of frames, so the
resultant rounding up adds a half frame (4.5 seconds) to
the average. Second, contention in reverse orderwire slots
delays some of the teardown messages; the associated
service is extended until the teardown is finally received
by the channel controller. Average queueing delay for our
kHz frame format have thus far precluded deriving a
purely analytical result against which to compare the 37
second experimental queueing delay value. We have
analytical results for the less complex 25-kHz case,
however (see below). Recall that the 5-kHz operating point
was designed to generate approximately 6670 average
channel usage. For one frame during each minute of the
scenario, the Performance Analysis Tool tallies the number
of building blocks assigned to communications services
and the actual number of building blocks available for
assignments after accommodating the FOW and contention
ranging slots. Percent channel usage was derived by
averaging this ratio over a period of 100 minutes
beginning three minutes into the scenario. This is
sufficient time for the channel loading to reach a steady
state. Data for this run converged well to a 70V0 usage
value with 100 samples. The extended service durations
due to frame-filling and orderwire contention mentioned
above are believed to be the primary reason why measured
usage exceeded the predicted value.
To assess the affect of load variation on our two
performance measures, we conducted additional 5-kHz
runs at 25, 50, 75 and 12590 of the operating point load.
Running at 125% was feasible since loading on the reverse
orderwire at the 1009o operating point was well under the
slotted Aloha theoretical maximum. The results for both
0,8
0.7
006
a01
a
2
z
z
w
Cm
w
z
0.2
0.1
●
o0
.
20
40
60
80
100
Average Load (calls per hour)
120
1
u.
o
20
40
Average
100
60
80
Load (calls per hour)
120
1 o
Figure 11: 5-kHz NCS DAMA Queueing Performance
Figure 12: 5-kHz NCS DAMA Slot Usage Efficiency
initial operating point was 37 seconds as depicted in
Figure 11, Since all calls were of the same precedence
there could be no preemption, so some calls had to wait
until preyioufi uwm finifih~d on th~ Qhannvl b~fvr~ they
could be assigned channel time. Complexities with the
composite message and circuit traffic plus the dynamic 5-
the initial operating point and the load excursions are
plotted in Figures 11 and 12. Queueing delays should
theoretically tend towards zero as load diminishes. Our
expaimrzmd data rdlums th~ implemvntr+tkm, howev=,
tending towards a half frame (4.5 seconds) because
requests are presented to the channel controller at times
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throughout the frame yet assignments can only be made in
the FOW slot, at the start of each frame. At the other
extreme, queueing delay increases beyond proportion as
load increases. This conforms to theory, as will be shown
more explicitly with the 25-kHz data. Channel usage tends
towards zero, in theory and practice, as load diminishes.
As the load increases, channel usage efficiency generally
improves although the rate of improvement decreases as
more and more blockage occurs. In summary for our 5kHz operating point and excursions, queueing delay and
channel usage performance were generally as expected.
The initial 25-kHz performance measurement was taken
over a period of 4 hours and resulted in 1386 total service
requests, This total is notably low compared to the
expected value of 1770 requests for that period, the
difference again a result of the finite network terminal
population. Recall that the total population is only ten
terminals per channel so the effect should be more
pronounced than in the twenty terminals per 5-kHz
channel case. In any event, the effect is immaterial, if the
actual load is used for data analysis and plotting. Service
duration averaged 31 seconds overall. This is higher than
the 30 second ~pecified value because of the same-two
Using the standard “M/M/l” queueing model (Reference
2) with parameters 1. = 0.098 calls/second and p = 33.5
seconds per call gives a theoretical corresponding
queueing delay of 6.7 seconds. The difference can be
attributed to framing necessary for a TDMA system and to
various implementation losses. Recall that the 25-kHz
operating point was designed to generate approximately
62% average channel usage. For one frame during each
minute of the scenario, the Performance Analysis Tool
tallies the aggregate assigned baseband data rate. For
example if two 2400 bps and one 1200 bps slots were
assigned, the aggregate would be 6000. Percent channel
usage was derived by taking the ratio of aggregate
assigned rate to 12000. Data for this run converged well to
a 48% value with 100 samples. The deficit in loading
(1386 actual vs. 1770 specified calls) is believed to be the
primary reason why measured usage differed from the
predicted value.
In order to assess the affect of load variation on our two
performance measures, we conducted additional 25-kHz
runs at 25, 50, 70 and 90% of the operating point load.
Running beyond 100% is not feasible because of the
reverse orderwire limitation. The results for both the initial
●
4
●
0
0
100
Average
200
,
300
I
400
5(
100
200
Average
Load (calls per hour)
Figure 13: 25-kHz NCS DAMA Queueing Performance
phenomena as in the 5-kHz case. Filling slots completely
adds a half frame (0.7 seconds) to the average and
contention among call complete messages in reverse
orderwire slots extends a small number of services. The
pipeline for transferring CCOWS from the channel
controller to the modem should add an additional two
frames. Average queueing delay for our initial 25-kHz
operating point was 8.4 seconds as shown in Figure 13.
Load (calls
300
400
5( )
per hour)
Figure 14: 25-kHz NCS DAMA Slot Usage Efficiency
operating point and the load excursions are plotted in
Figures 13 and 14. Experimental data are represented with
diamonds and theoretical data with the solid curve.
Queueing delays should theoretically tend towards zero as
load diminishes. Our experimental data reflects the
implementation, tending towards 2.5 frames (3.5 seconds)
because of the CCOW pipeline. This effect overshadows
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the half-frame average wait for the forward orderwire slot
that dominated in the 5-kHz case. Towards the other
extreme, queueing delay increases with increased load,
measured data differing from theoretical by roughly 1.5
seconds. Channel usage tends towards zero, in theory and
practice, as load diminishes. As the load increases, channel
usage efficiency generally improves although the rate of
improvement diminishes as more and more blockage
occurs. Once again, for our 25-kHz operating point and
excursions,
queueing
delay and channel usage
performance were generally as expected.
In practice, selecting an operating point will require
network planners to trade-off between channel usage
efficiency and queueing delay. In all likelihood, 30+
second queueing delays as observed with the initial 5-kHz
operating point would be unacceptable to the using
community, so the planners would need to back off the
total offered load, for example to the 50% point. This
would result in approximately 10 seconds mean queueing
delay and 40 percent channel usage efficiency. In
determining the precise optimum operating point
communications planners would also want to consider the
90* or 95* percentile queueing delay, and they would want
to enable per-call precedence rankings so that preemption
could be used to minimize delays for important individual
users. The Traffic Generator and Performance Analysis
Tools can handle these more complex scenarios, which
may be the subject of additional work.
VI. FUTURE DIRECTION
We begin this section by mentioning upgrades to MILSTD-188-18 1, which governs single access (i. e., nonTDMA/DAMA) use of the UHF transponders, since we
anticipate certain advanced technical features from MILSTD-181 will ultimately be used in the DAMA standards
also. In particular, the “B” version release of MIL-STD181 aims at significantly higher channel throughput by
using multi-h continuous phase modulation (Reference 3).
Early experimental results show and the new standard
reflects that 9.6 kbps is achievable over 5-kHz channels
and that up to 12 kbps is even possible. Similarly for 25kHz channels the new standard specifies 48 kbps and
includes options for up to 64 kbps. For the long term,
multi-h modulation can be expected to be included in both
5-kHz and 25-kHz DAMA standards.
The first major revision to MIL-STD-188-182, termed
“182A”, was approved on 31 March 1997. Any entirely
new network terminal developments commencing after
that date are required to conform to the A version. The
channel controller software is being updated to support
both versions of the standard and will be delivered to the
operational sites in early 1999, along with the 25-kHz
DAMA capability.
The most notable new features
provided by MIL-STD-182A include subframing, global
message services, and punctured codes. Subframing is a
technique which allows communications bursts to begin
and end anywhere within the assigned time slot. Use of
this technique means that terminals no longer must
accumulate a full frame of data before starting a
transmission; as a result end-to-end throughput delays are
significantly reduced, a gain of particular benefit to voice
users. With subframing, average round-trip delays are
reduced from roughly 25 seconds to 4 seconds. Global
messaging is a straightforward extension of the global
circuit services already offered in MIL-STD- 182. Finally,
new rate Y4 and 7/s punctured codes are derived by
“throwing away” (i. e., not transmitting) channel bits from
the standard MIL-STD-182 rate % code. Performance is
somewhat degraded but that can be tolerated by terminals
with strong link quality. The result is greater channel use
efficiency, since shorter time slots are required.
The “A” version of MIL-STD-188-183 was approved in
March 1998. In this instance, the major enhancements are
5-kHz slave channels, 8-bit channel addressing, and
ephemeris orderwire. The concept of slave channels puts
all controlling orderwire on a 25-kHz DAMA home
channel, but permits the terminals to frequency-switch
every frame to a time slot on a 5-kHz channel whose frame
timing is synchronized to the home channel. This is
analogous to a frequency-switched slot connect (described
previously) across multiple 254cHz DAMA channels. The
benefit is letting terminals on a busy 25-kHz channel use
slots on 5-kHz channels without sustaining the delay of
permanently moving the terminals to a 5-kHz DAMA
channel, The very earliest 25-kHz DAMA design used 6bit channel addressing, limiting the channel set to 64
elements. In the interim, many additional new satellites
have been put in place, and the present standard frequency
plan uses 8-bit addressing. This important enhancement
will allow 25-kHz DAMA to operate on any of the extant
channels. Finally, MIL-STD- 183A defines the formats of
several new forward orderwires, which collectively will
allow the channel controller to disseminate the complete
set of orbital parameters for the satellites. This feature will
support “passive ranging” by terminals with orbital
calculation software; these terminals will be able to precalculate range to the satellite rather than sending a probe
burst and measuring range.
The 5-kHz DAMA and the first-generation 25-kHz
DAMA Network Control Stations were developed and
installed under Air Force sponsorship, The Navy’s long
term role as UHF satellite communications system
developer, however, was reflected in that all NCS
and
Navy
Computing
installations
are
at
Telecommunications Area Master Station (NCTAMS)
sites. The Navy is presently directing the evolutionary
Joint (UHF) Milsatcom Network Integrated
(JMINI)
Control System program whose charter is to upgrade both
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5-kHz and 25-kHz DAMA. Two significant early JMINI
improvements will be inclusion of a Network Management
System
(NMS),
and
achievement
of
channel
synchronization. The NMS will be a communications
planning tool, instances of which will reside at Joint Chiefs
of Staff, various commanders in chief (CINCS) who “own”
individual satellite channels, and the NCS sites, all
interconnected through a secure network. It will be
through the NMS that all decisions concerning
apportionment of the channels and ranking of individual
terminal’s and network’s importance will be made and
disseminated. The ultimate destination of the “rules” for
assignment of capacity will be downloaded into the NCS
computers and implemented in real-time as requests for
service arrive. Presently, frame timing for DAMA
channels is synchronized on a per site basis and at the time
of transmission from the ground. Maximally useful system
synchronization requires alignment at the satellite, and for
every channel (both 5-kHz and 25-kHz) world-wide.
Benefits are numerous and major, the prime ones being
allowing cross-channel scheduling and precluding frame
timing jumps upon transfer of control to an alternate
station. This will be a big step towards “seamless” 5 and
25-kHz DAMA wherein users will not need to concern
themselves with implementation details; they would
simply request services and be assigned an appropriate
slot.
After more than 20 years of evolution and development,
network controllers for both 5-kHz and 25-kHz standard
UHF DAMA waveforms have been recently fielded. Large
numbers of network terminals designed to the standard
waveforms are just now going operational, and
commanders are expected to convert more and more
channels to the DAMA mode as the user population
warrants. This paper addressed performance of the overall
DAMA system. The approach was to use a discrete event
simulator to impose traffic loads close to the theoretical
maximum on the actual channel controller software, and to
measure key networking performance parameters, It was
demonstrated that the system provides good quality service
under heavy loads.
ACKNOWLEDGEMENTS
The authors are most indebted to Dr. John Bacigalupi, who
extended the Traffic Generator capability to include 25kHz DAMA and who conducted all of the simulation runs.
Development and fielding of the DAMA Network Control
Station were funded by the U. S. Air Force Electronics
Systems Center, Maj. Brian Casey, Program Manager.
Enhancements to channel controllers are presently being
conducted under the auspices of the U. S. Navy Space and
Electronic Warfare (SPAWAR) center, Mr. James Parsons,
Program Manager.
REFERENCES
1. L. E. Taylor and S. L. Bernstein, “TACS – A Demand
Assignment System for FLEETSAT,” IEEE Transactions on
Communications, Vol. COM-’27,No. 10, October 1979.
2.
D. Gross and C. M. Harris, “Fundamentals of Queueing
Theory,” Wiley and Sons, 1974.
3,
I. Sasase and S. Mori, “Multi-h Phase-Coded Modulation,”
IEEE Communications Vol. 29, No. 12, December 1991.
3.
for
Standard
“Interoperability
MIL-STD-188-181,
Dedicated
5-kHz
and 25 kHz
UHF
Satellite
Communications Channels,” 18 September 1992.
4.
MIL-STD-1 88-182, “Interoperability Standard for 5-kHz
UHF DAMA Terminal Waveform,” 18 September 1992.
5.
MIL-STD-188-183, “Interoperability Standard for 25-kHz
UHF TDMA./DAMA Terminal Waveform,” 18 September
1992.
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