10/25/95
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--.' TME INTRODUCTION OF PACK-T SATELLITE COMMUNICATIONS
Robert E. Kahn
Advanced Research
Abstract
The preparations which led to the SATNET
xeriment are discussed in this paper. Various
packet satellite tariff considerations and
architectural issues are presented along with a
smmary of future plans for use of the technology.
1. Introduction
This paper reviews the process' which led to the
introduction of packet stellite technology in the
970s, The development of this technology was
undertaken by the Advanced Research Projects
Agency (ARPA) in order to evaluate its utility for
efficient long haul computer communications with a
potentially large number of geographically
distributed users. TBis effort was undertaken in
conjumctioq with participatimg organizations in
the O.K. and.Norway, but does not necessarily
reflect their ¾1ewa on this subject.
The%most notable example of this technology is the
Atlantic Packet atellite Network, known as
SATNET, which has been in operation on the
Atlantic Intelsat IV satellite since late 1975 and
which currently serves a community of researchers
in the U.S, the O.K. and Nory. Underlying the
SATNET technologF is the basic packet switching
technology which wa first introduced during the
late 1960's. The November 1978 IEœ Proceedings
contains a comprehensive treatment of packet
communications technology and includes a paper on
General Purpose acket Satellite Networks which
provides a good introduction to the subject [I,2].'
SATNœT consists of a__i.ng_e bad%ast channel
shared by multiple er%h tions w'nlch use Time
Di¾ieion Multiple Access (TDMA) and emit packets
according to a channel access protocol. Tese
earth stations may be connected to one or more
aubsoribem nctwor.. Eeh earth station contains
& prorable satellite processor (a controller
and related electronics) ich implements the
satellite channel protocols and interfaces. The
system provides complete connectivity between all
the participatin& earth stations and allows
dynamic allocation of the satellite channel amens
them. Different priority levels ma be supported
efficiently on the same channel without
unnecessary preallocation or preemption of
Projects Aency
resources. Tc broadcast property of the channel
enables a transmission from one earth station to
be received by 811 the others including itself.
Both conferencing and delivery of multiple address
packet ca be achieved efficiently as a result.
The Arpanet was the first example oF a packet
switched network which used point-to-point
terrestrial lines (across the U.S.) in a store anJ
forward sFstem [3,]. Following the installation
of the first Arpanet nodes, a number of papers
appeared in the literature on the application of
packet switching to multiple access radio'channels
[5,6,7,8]. The ARPA-sponsored effort at the
University of Hawaii wes the first to emonstrate
burst transmission oF packeLs by radio for
computer access by terminals within line of sight
of the computer center. In this system, called
the ALOM system [9,10], packets were simply
transmitted when they were ready to send - at
random instants of time. No explicit control of
the radio channel was invoked. Rather, o
occasion, packets would collide in the air,
destroying each other and would be retransmitted
t a later random time. The multiple access
nature of this system resemble a packet satellite
net, except that the terminals were much closer to
and quite unequally spaced from the computer
center which (like a satellite) formed the hub of
the system. The Hawaii researchers extended the
concept of radio packets to satellite
communication directly, and experimentally
verified the concept using test packets over
NASA's ATS-I satellite between Hawaii and
NASA-Ames. The technique of operating a Packet
Satellite Net in an uncontrolled fashion became
kno aS "Pure Aloha".
A significant body of theoretical wor on the
analysis of Aloha Systems appeared in te
literature in the early 1970's and various
improvements on the original random transmission
technique were proposed and evaluated [11,12]. In
the Slotted Aloha technique, first introduced by
Roberts, the time axis at the satellite is divided
into equally spaced intervals called time slots
which hold a single packet [133. Under the
Slotted Aloha regime, packet an only be
transmitted startin& at the beinning of a slot.
For fixed length packets and PoXseen traffic
arrivals, the capacity of the slotted system is
twice that of the unslotted system due to the
reduced number of collisions at light traffic
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loads, In oth the slotted and the nslotted
ease, Ome form of stability cntrol is eeded
(1,15]. For efficient use of a packet satellite
channel, where the cne-way-ranslt time is much
of satellite channel allooatlon strategy i
appropriate [ 1O]. A priomlty oriented demand
s].1oea%lon of oafcity and ls orently In aily
use In STNET
tb simulation and analysis were used extensively
d effectively in vestigai ese mnd other
cbnel access sches. -ve, thim r s
able to deal effectively on a purely theoretical
associated with develoent of e tecnnoloEy.
Access to an exrentl system s essential to
address topics such as fault etection and
iolation, system stat monitorln and debin&,
terfacl to terretri netr and atewas,
ftware structure d rfoanoe. It was
feasible to oarr out a test of the tecnnolo&y on
e of several existin stellites a it apare
as if exlstl grod stations could be ed  a
cet mode of operation th only
mifications to provide external on/off control
of the carrier by a prorable satellite
processor at each earth station. e cket
satellite technology s also seen as a
tentlally useful lon& term adjunct to exittin&
netrk tecnolo6y for lon ha applications
vclvint oonfarencin&, mti-destiation
broadcasting d escially to provide
connectivity between a lar&e nr of sites (each
with low duty cycle traffic) usin& a 11
fraction of %he leased channel banddth that a
fully co.acted actwork of polut-to-t olrouits
uld have required-
In the 1973-1g? te frame, the only viable
choices for such a test (fr the U.S. point of
view) were the Intelsat satellites, the NASA
rental satellite and one of the several
military satellites. The Intelsat syat s
preferred choice for this activity ecae It
uld be made available most easily and had the
tential for supporting the reultl tecology
on s ceial basis upon cpetlon, if it
proved to be econic. The military satellites
re less appropriate ohoices as tere s not yet
a stated requirement for cket satelll service
in the military. At$.te, international
terest in packet switCi s growi
significantly, and ssible requirements for
interconnection of domestic cket networks in the
different co.tries were identified. In 197B, %he
PANET ad Just been extended to orway and the
U.K., and expertentel use of the APANET was
provint to e quite worthile for resesrc
purposes.
is is the context in which the subect of an
exrental prr on cket satellite
tecbnolo6y was first raised with the British PoSt
office, with the Communications tellite
rration (Comsat), and subsequently with
Norwegian Telecommunication Administration (NTA)
-end .Noulan Defense eaearch Stablisment
,,(DBE)...Jm the follong section,
preparations for te SAET exrent arc
tlin al with the approval proess icb s
2. Preparln for te SET rent
In 97, &he U.E.-Poa Office ree o support
the SATNET exrent by contributing the O.K.
half of te satellite link and by providing access
 the necessary eart.mtalon eqent in
gland. A progrle mtelltte processor was
installed ab te Goonhilly earth station and
ected back tca Eatery at the ARPANET node on
te praises of University 11ege ndon (OCL)
with a 8 Kbps oicatlon line. UCL was
prepared to accept the main research
resnsiility for the O.K. prticipation l te
SATNET progr, an subsequently di o.
Also in 197, sat agreed to U.S. participation
In suc  expertentel activity, ut only if
We carried out under te auspices of one of the
several U.. International Record arriers (IRCs)
icb historically have played the le of
inteediary in ringlng lnternatin data
services to the end customer. omsat is the U.S.
presentative to Intelsat. en te SATNET
project was bei formulated, Comsat also operated
otb the space segment er contract to Intelsat
d the .S. earth stations for the conrti of
U.S. oers. Intelsat itself now orates
space segment.
The only generic classes of service wlch could be
offered by Comsat were those specifically approved
(tariffed) by Intelsat. Clearly, e cket
satellite service was not onE the. After
rlod of iscussion within telsat lasting
saver monts, an Intelsat tariff for a
multi-station service was approved In late t97.
e SA progr was initiated in ptber 1975
with one Intelsat stdar A ( meters) earth
station at Eta, West Virginia and a silar one
at Gnbilly s, gland. Within Noy, the
interactions wit the NTA were handled entirely by
te Norwegian fense esearcb Estblisent.
ile Norgtan partiolpation In the SATN
progr had started with the first meeting of
searchers In 1975, their active rticipation on
the channel began  late 1977 usi the Nordic
earth station at Tan, Sweden. Shortly
tereafter, msat Laboratories mae preparstlons
to rtlcipate actively with a small nattendeO
rtb Terminal (ET) at larksburg, ryland for
system diandais and evaltion. e PET differed
fr the three standarO A earth stations In that
it bad only a 10 meter antenna and could only
ceive at 16 Kilobits/second lle the other
stations could receive at 6 Kilobits/second. All
four stations can transmit at 6 Kiloite/second,
but the large stations must duce their
transmission rate to 16 Kilobits/second to talk to
te T.
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The technical aspects of the SATNET experiment are
not addressed in this paper. Other companion
papers address both system level and experimental
aspects of the program ['19,20,2]. In the
remainder of this paper, the relevant tariff
considerstigma will be diacused and two key
architectural issues will also be considered.
3. Intelsat Tariffs
The new Intelsat tariff which was approved in late
1974 was for a new kind of service known as
Multi-Destination Half-Duplex (MDHD). Simply
stated, MDHD allows one r more members of
Intelsat to Jointly share a eon channel on any
of the Intelsat Satellites for a modest MOHD
payment to Intelsat. The normal leased service
offerings from Intelsat to its members are a
point-to-point service between two earth stations
and a broadcast Service from one prospecified (but
fixed) earth station to at least two others. The
point-to-point service can be either half-duplex
(one way) or full-duplex (two-way). The broadcast
service utilizes only one channel, as a reverse
path is not included.
The MDND capability may be viewed as an extension
of the broadcast service %o allow moe than one
prospecified earth station to transmit. MDHD
allows all participating earth stations to
transmit using their own channel access protocol '
to resolve contentions.
To any member country already participating in an
MDHD service, the added cost is nominally zero
allow additional earth stations to share the MDHD
channel. This assumes that capacity limitations
'are not exceeded and that coordination among a
larger number of sites costs the same. Mwever, a
payment must be made to Intelsat by each member
country which chooses to Join (share) an existing
FHD channel, so the total payment received by
Intelsat for MDHD service grows linearly with the
number of countries. The easons for a choice of
tariff in which the cost per country is
independent of the number of participants depends,
in part, upon the olitical structure of Intelsat.
The subject of PTT tariffs to the end customer,
although not specifically discussed in this paper,
would generally include earth station
terrestrial charges, as well as space segment
charges.
If we assume Intelsat'normally charges amember C
per one-way channel of a certain capacity for a
total of 2C counting bth ends, then the same
revenue would be gathered if each of the
participating members in an HD channel wre
charged 2C/N apiece (assuming N participants).
The members, in turn, could base charges to their
customers on these costs plus the added costs of
ground station euipment and terrestrial
interconnection. This kind of formals in which
the space segment charge is independent of the
number Of earth stations appears well-suited to
domestic services where all the earth stations are
owned by one authority. However, this formula
poses several problems when applied to the
,international situation,.where the earth stations
are separately owned and operated.
First, the eats ase ½r aeh participating
ountry ould fluctuate as ß function of the
number of participating countrieS, making
fi&ncial management nd planning awkward and
unpPedictable at est. .Second and mute
imprant, .le voing rights f eaah member
country in Intelsat are a function of its total
ayments to Intelsat. Primarily, for that rason,
the Intelsat Mp tariff ws fixed to be a
costant ½ per channel per country.
Te Intelsat broadcast tariff illustrated in Fig.
1(a) shows one transmitter which is charged C and
four receivers each of which is charged C/2 for a
total of 3C. Since at least two receivers oust be
present for a broadcast service, the minimum.
charge is ZC (wich is identical to the half
duplex in-to-plnt tariff between two
The revenue produced by the broadcast tariff
increases linearly with the addition of more
ground stations at an increment of C/ per added
receiving station.
The MDND tariff illustratcd in Fig. l(b) shows
each participating country being charged C for the
right to receive add transmit on the same channel.
The net payment to Intelsat, 5C, is almost double
the charge for the smple broadcast case.
However, the value received for this added cost is
full N-way cOnneCtivity since anX of the earth
stations can transmit to the others at anF time
according to the chosen channel access protocols.
The MDHD tariff is also considerably cheaper than
that for N distinct broadcast channels to
implement N-way connectivity (NC va. INC 
N(N-1)C/2]). Along with the initial hiher cost
of N boadcast channels would nome N tes the
capacity, however, eEardless of wether It ess be
ud effeotively or not.
With these existing tariffs, the cost per country
normalized by total number of channels of network
capacity available o the N earth stations is C
for the I'HP case and [C + (N-1)C/]/N = (N+lJC/2
for the case of N broadcast channels. If existing
MDHD tariffs are extended to channels wit a
higher bandwidth using a 'linear extrapolation" of
the current tariff, the charges for obtaining the
added capacity with multiple lower capacity
broadcast channels will be half as much as the
single MDHD channel as the number of participating
earth stations becomes large. Since this ratio
eflects only the current tariff structure, the
ratio could be changed (e.g., becoe closer to
unity) with a non-linear tariff revision
applicable to higher bandwidth channels.
From an architectural point of view, the use of
multiple roadcast channels has both positive and
negative features which are identified in section
6. However, in most applications, it is doubtful
whether initial network-wide traffic will be large
004
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005
enough tc justify commencing service with more
than a single MDD channel.
4. COMSAT and IRC Filings in the
The U.S., U.K. and Norway participation
has been on an experimental basis and a service
has not yet been offered to Customer in any of
eae countries. In the U.S., aa filed a
rff with the FOC in 1975 %o offer -
exrimental acke% atelllte cability
 its desinateO =oatmatters via one of the
ICs. The service sat offend was based On the
 servie obtaln fr Intelsat, d
aented as requid with the DroErable
satellite pocessor at the eart s%ion. In its
filing, sa also eferred to its
service as HD.
sat bought or leased all te necessary Kroud
tation equileat to provide the exrental
service as for a nodal oeroisl
offering. In a competitive selection, Western
Union International (WUI) was cnose b the
service to the ARA proEr. WUI, in tn, filed
obtained fr Comsat, which they amented witD a
terrestrial circuit before supplying it 5o the
6over. eat.
goVeret's request for proposals was umual in
that it did not dire any specific destination or
custe location ahead. Rather, 1% sply asked
for  HD channel from the U.S. o an
pecified int in the U.K. and sated taC all
of the U.K. costs re to  as$ed by the
British Pot Off lea. A lnt of contact In
Post Office was identified. e request also
stated that additional mpecified destinations
mh% ave to e connected subsequently, as Nervy
eventually was.
To validate te initial delivery of the service
and to verify meatoral of service in he event
outage, only a loopback test fr the
cuzter site (ich was speclfle to  the
iic Data alysis nter tn Alexandria,
Virginia) to he satellite and ack wa require.
The paten% to WUI was not cndent on the
partioilon (or rfoanae) of y other
co.try (or its equ..owver, the  and
COIAT tariffs both included a small charge
ortional to the nr of participating sites
for cochinatica.
A diar of the HD paent flow during the
experiment Is sho in Fi. .
5. SATET rent
The SATNET ent was conducted nlnally
dui the period from ptemr 1975 through
ptember 1978 and involved researchers fr each
of the three rticiti countries. The basic
physical architecture of SAET was dictated by
many programmatic considerations (e.g. ue of
existing ground stations and satellites) so the
actual hardware configuration merely reflects what
%rag available for use in the experiment. However,
tDe logical architecture of the system has been a
auJeot for research and bas constantly evolved
during the coUrse of the program. Neither the
software architecture nor the system protocols
were prescribed in advance and the aSh-hardware
parts of the system interfaces were also allowed
to evolve, which they did. Each was e major
subject for investigation and exploratory
development during te CoLirma Of the project. The
resulting logical architecture wll be ¾ery useful
in designing a more advanced follow-on system. In
addition, an effort ws undertakem to develop and
demonstrate a high prformance digital burst modem
and error control unit for possible operational
use wit SATNET after the experiment.
A major decision in the program was to separate
the SATNET development and testing from the
closely related internerring research activities
which were Just getting underway. It was decided
to pursue the internerring research using a
separate minicomputer gateway in each country
simultaneously connected as a Host on SATNET and
as a Host on the Arpanet [22,23]. This
arrangement left enough flexibility to pursue
gateway related research without resulting
software changes (in rel~time) to SATNET or
Arpanet. The gateway software could have been
incorporated within the physical confines of
either SATNET or Arpanet, or split between them.
owever, keeping it separate for the purposes of
the experimental program provided maximum
flexibility to the internettln s researchers, many
of whom were also working on SATNET, Arpanet or
other ongoin networ related pro, rams without
unnecessarily distracting those SATNET researchers
who did not need to be deeply involved in the
internerring work at that time.
Technical direction of the program beginning in
September 1975 was the responsibility of Linksbit
Corporation, San Diego, allfornia who prepared a
comprehensive test plan to guide the conduct
the experimental program. Major participants were
CO. sat, Bolt Beranek and Newman, University of
California at Los Angeles (UCLA) and the Defense
Communications Agency in the U.S., University
College LOndon and the Post Office in the
and NDRE and NTA in Norway.
Coordinating a program involving participants fro
multiple countries as an important challenge that
was met at several different levels. Ouarterly
review meetings were held (otated among the
different locations) and attended by all the
participants. Technical progress was reviewed at
these meetin&s, technical issues were discussed
and resolved and plans for each succeeding usrter
were refined. Research issues end results were
docented and circulated in a series of informal
working group notes. The ARPANET played a
particularly important role in executing the
effort as well as in coordinating it. It provided
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006
ira means by which %he satellite processors were
down-line loaded and debugged, and the meas by
aich SATNET itself was cTtolled and monitored
as i% was being developed. The message pasir
capability of the bo3ts on the ARPANET were used
to keep all participants informed of technical
progress, system status, often by direct reporting
from tl%e programmable satellite processors in
SATNET, and to resolve questions and culminate
experiments on a day-by-ay basis. Without much a
capability, it is doubtful that the overall
experimental program could have been carried out
successfully.
The main results of the exriment are bainõ
documented by Linksbit Corporation (with inputs
frm all the participants) in a final report to be
available shortlyl A Smmary of the findings show
that the SATNET experiment demonstrated the
feasibility of the packet satellite technology,
llluminaed many of the most iportant technical
and non-technical issues and provided a system
that can support advanced co-purer comunication
research applications. lthouh the subject of
packet voie has not been emphasized in this
article, it played an integral ole in the SATET
design. SATNET is the only operatin lone haul
packet switched network in the world that has been
demigned to handle both packet switched voice and
data.
6. Architectural Issues
Two architectural issues arose durin the Course
of %his project which are appropriate to single
out for mention. The first issue is selecting the
functionality that ought to reside in the
proccssor which is cOldCared with the rest of the
earth station equipment and the functionality that
ought to reside at the terrestrial interface {to
the earth station) which might be located some
distance from it. The second major issue concerns
the means of increasing the overall traffic
handlin E capacity of the system. Each of these
issues are briefly mentioned below.
a. Functionality of the Earth Station and its
Terrestrial Interface
Although not all the functions mplemented in
SATNET need to resi__-.h earth station, a
minimu set of functions must be located there to
control timin and access to (and transmission on)
the satellite channel. Barts of the functionality
might he moved to a terrestrial location distant
from, but connected to, the earth statio by a
communication line. One attempt at the definition
of the functionality is iven in [2]. In
particular, certain aspects of the functionality
which deal with multiplexE traffic r many
'uers into a comslte stream to the earth station
could proably be relocated without penalty in
erformance provided delay or unrelitbility is not
added outside the earth station. Accountin and
other administrative functions could also be
remo%ed from the earth station without penalty.
b. Expansion of Network Capacity
Although a single 64 Kilobits/second channel is
:learl uld be nufficient for many
applications. The capacity of a SATNET system
could be expanded l several ways. First, it
could be simply scaled up in data ate. The
ability of s packet switch to handle.
ultl-eabit/econd data as been demonstrated
[]. Ibis would suffice for an expansion of one
or two orders of mEnitude. A transponder can
typically handle upwards of 6 4eEbits/second ,
however, nd the newer smtellite systems are
epected to support higher data rates still.
Multi-Droeemsor systems see capable of supportin
these hi,her data rates on a single shared
satellite channel without either increased delay
in bufferin or processir. owever, the number
of processors must grow linearly With capacity and
special attention must be paid to communication
between processors and with external devices.
A second alternative, whic baches attractive
wen the overall netrk traffic is high enough,
is to corpoate dedicated upliks using
Frequency Dlvfslon Multiple Access (FA).
this scheme, which is illustrate  FiE. 3, a
separate Drocesso at each eart station uld
dolink and would pa=s alons %o a concenta%or
only hose packets elned fo i earth
Te capaelty of the concenao ould %hen be
sized  he thoushpu inSended fo ha site
wlch 6sably d be h les than th total
network traffic. In this scheme, a mification
would be required at each exlatin E Eund station
fo each new addition to the net, which is a ma3om
isadantage. However, it s highly modular and
shoed be easy to upErmde in an orational
system.
The use of multiple FA broadcast channels, one
per site, reUuoes the earth ttion prccessinE
requirements but it also does not provide the
flexibility that comes from the dyaic
of pacity in a MA system. A thir alternative
is a hybrid of cases one a two above in which
e of the uplin may be BD channels (using
TA) ile the rest may be badcast channels
each from a sidle source.
7- Future Plans
SATNET currently sees as the backbone for a
r of innovative research applications d has
bece the prima packet transrtation vehicle
ten the U.S. and rope for cputer
lcations and command and control research.
SinCe  1979, AANET access fr the U.K. has
r lmost exclusively via SA on a provisional
sis. It Is planned to continue the use of
SATNET as the primary link between ARPA end its
search partners in rope. e ARPAN link
London (via Norway) is scheduled to be taken
during the last quarter of 1979 aer which
only available ARPANET access fr the U.K. will
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10/25/95 09:23 '703 620 0913
 007
b via SATNET. NDHE will utilize SATNET for
research purposes; %he only planned use of the
rmanir pOin-to-point ARPANT link fro the
U.S. to Norway will be foletrieval of seismic
data, which was the o/6inal function of that linc
prior bo its incorporation in the ARPANST in 1973.
WithiD the U.S., ARPA plans %0 use the SATNET
satelite =hannel operatir a 3 Mbps with 5
anteana initially at Lincoln Laboratory,. i
xtton, saachuetts d USC/ISI in rina
ey, Califomma. AdditOn sites In the n
Fansieo aea and Washi.tun, D.C. aea will
also be adoe. A o uae fo %he
channel is o exploe ne use. of the ATNET
techn01oy %o supp% mul:i-use lnte6ated packet
voice nd Oatm emlCation. ly with the
increseed anwidth will a test of this concept
ssile usin multiple aee an dea sources
including a mix oF 2.4 Kilobits/second to 64
facsimile and nodal cpu%e %o upu%eF
The fense iationa eny also plans
utilize tRis tecolosy alon with ARPA for
advanced re.arch and develoet on D
tegrated sta/voice networks of the ture.
 the international soene, packet satellite
technology may be ueful fo a wie variety of
ten%ial applications. e such possibility
ich is bein E offered as a seviee by the
and the US Postal Sewlee is Intelst. This
an innovative new facsile service ich is
Dlan evolve, inividual point-to-point links
oory. A cke stelite y could support
=he initial Intelpot raffie %h oly oe shaed
channel, wit consideal less oal aellic
owidh th multiple point-tit link would
quie and without noticeable de6radaion of
Acowledmenta
Tis effort would not have been ssible without
e eooera%on and support of e BPiih
Office an the Norwegian Teleeiatlons
Admiustration;'  orgai:ios layed a ery
central le In he Drr. lt ranek &
Nean (RtN), COMSAT, an LinCbit rporation
played signifil,mr _zz!n evepi th pak
satellite technology. CT speahesded the
sppval priest. UCL, NDRE, UC, and
with the assistance of BN, caied out the
decDire %he lapse ea9hic distan=es fF SATNET
and each other which might otherwise have been a
deterrent. The success of the progr was due
no mall measure to e technical direction
provided by Linksbit rporation.
He f orenoes
[1] Special Issue on packet Comunications, IEEE
Proceedings, Nov, 1978
[]
I.M. Jacobs etal, "General Purpose Packet
Satellite Networks," IEEœ Pc., pp. 1418 -
1467, Nov. 1978
[]]
L.O. Roberts and B. U. Wessler, "Computer
Network Development to AChieve ReSource
Sharing," AFIPS Conf. Proc., $JCC, pp.
[]
R.E. Kahn, "Resource Sharing Cxxuputer
Communication Networks," IEEE Poc., pp. 1397
- 107, Nov. 1972
N. Abramson, "The Aloha System - Another
Alternative for Computer Cmunicattons,"
AFIPS Conf. Proc., FJCC, pp. 695 - 70Z, 1970
L.O. Roberts, "Extensions of Packet
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551-560, 1975
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2 2
SiTE 1 IC/2 SITE I
Fig. 1. (a) Broadcast Tariff. {b} HDHD Tariff
NOIVAY
Fi. 2. MDHD Payment Flow
BROADCAST DOWNLINKS
BROADCAST
UPLINK 1 2 3 N
P--'-----'=-:'--' t CONCENTRATOR Et INTERFACE
TO TERRESTRIAL NETWORK
Fig..
Station Configuration for Multiple Broadcast
Charmelm in a High Capacity System
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