Transcript of VSAT Presentation
No Slide TitleWhat is a VSAT ?
Demonstration of Equipment
Sales, service and support offices worldwide
Traded on NASDAQ (GILTF) since 1993
Revenues in 2001: $389M
Three Regional Headquarters:
Gilat Asia, Pacific Rim and Africa – Petach Tikva, Israel
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Satellite-based Wide Area Network (WAN), with centrally managed
hub
Remote site: less than 1.2m dish antenna
Multi-service platform: Data, telephony and multimedia
communications
Optimal for continent-wide networks of hundreds or thousands of
units
Small networks integrated in shared hub service
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Cost savings over terrestrial lines
Nationwide reach, distance-independent
Quick deployment, network flexibility
Increased network availability
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Different Data Steams can be sent simultaneously to many
users
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Unicast, Multicast, Broadcast
Broadcast Packets are sent to all users in the Network
Simultaneously
Broadcasts are Not Acknowledged
VSAT Networks can use Reliable Broadcast Protocols and appl
ications that are based on NACK’s, not ACK’s
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The name of the game is THROUGHPUT !
A 56K Modem will typically connect at speeds of only 43Kbps
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Telephony
Corporate: Telephony/Data infrastructure
Intranet and IP infrastructure for the enterprise
IP multicast-based services
~24 Hour Period
Single Satellite theoretically can provide up to 42% Earth
Coverage
Large, expensive, difficult to launch
Located approximately every 2o above the equator
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Altitude
No (Works in a Constellation)
(*Single LEO Satellites must be constantly tracked and suffer from
‘Doppler Effect’)
Size
Launch
Multiple; Can be “Piggybacked”
Qualcomm based CDMA
48 satellite constellation (8 planes x 6 ea. + 4 spares)
52 now in orbit !
80% Earth coverage (+/- 68 degrees)
LEO orbit (1414 km)
Qualcomm GSP1600
Inbound Return Channel via Dial-Up Modem
Can be used with existing infrastructure
Example: Harmonic’s CyberStreamTM
Satellite
Router
Internet
RFT
Network is Independent of Existing Infrastructure
VSAT Antenna Size dependent upon Power and Gain of Hub
Antenna
Also Upon Inbound Bitrate, ODU Power and Satellite Footprint
Contention Based Access – Usually TDMA or FTDMA
Typical Ping Times Approximately 650-700ms
Hub
VSAT
VSAT
VSAT
VSAT
VSAT
VSAT
VSAT
VSAT
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Some systems require initial signaling via the Hub
Larger Antennas, Higher Power required at the VSAT
Smaller Antenna, Lower Power required at the Hub
Used extensively in Telephony Networks
Delay minimized on VSAT to VSAT Calls
VSAT
VSAT
VSAT
VSAT
VSAT
VSAT
Typical Hub Configuration
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VSAT Networks use Geostationary Satellites (GEO)
All located directly above the equator, at an altitude of ~36,000
km and spaced approximately every 2 degrees
Band
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Space Segment
VSAT Networks lease space segment from the Fixed Satellite Service
(FSS) Provider
Price is mainly determined by Bandwidth and Power
Geosynchronous Satellites frequencies consist of an Uplink and
Dowlink, each covering a 500 MHz bandwidth
The many transponders operating within this range typically extend
from 36-72 MHz each
Each Transponder has a finite power level that is shared amongst
the users
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# of bits transmitted with each symbol
If more bits can be sent with each symbol, then the same amount of
data can be sent in a narrower spectrum
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Use alternative sine wave phase to encode bits
Simple to implement
Efficient use of Bandwidth
MSK – Minimal Shift Keying
Special form of FSK
Spectrally efficient, better noise performance at receiver
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Forward Error Correction (FEC)
FEC provides the ability for transmitted data to be
‘self-correcting’ without the need for re-transmission (As in
ARQ)
Thus, we can transmit with LESS POWER - The price is Overhead and
Bandwidth !
FEC ½ means that for every bit sent, an additional bit of overhead
is sent; ¾ means for every 3 bits, one bit of overhead, and so
on…
BER
10E-1
10E-2
10E-3
10E-4
10E-5
10E-6
3
4
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6
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Eb/N0
Convolutional Codes and Block Codes
Convolutional Coding (Viterbi Decoding)
Based on minimum hamming distance “code words” feed through a shift
register
Reed Solomon Code (RS) is a form of Block Code that breaks the data
stream up into fixed size blocks and adds redundancy symbols
On the other side of the link, the data is decoded using linear
algebraic algorithms . This type of code adds considerable
overhead
Concatenated Viterbi – refers to an error correction technique
which uses Viterbi in conjunction with Reed Solomon coding. Adds
approximately 2dB to the link budget
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Bit error rate is Directly Proportional to the Eb/N0
Threshold
Typical BER in some VSAT Systems can be <1.00E -08 (Less than
one error in every 100,000,000 bits) for an Eb/No of only
4.8dB
“Robust” in the digital worlds describes a system that can be
(near) error-free in a noisy signal path
Place Picture of C/N Here
Place MSK Signal Here
Eb/No is classically defined as the ratio of Energy per Bit (Eb) to
the Spectral Noise Density (No). If this definition leaves you with
a empty, glassy-eyed feeling, you're not alone. The definition does
not give you any insight into how to measure Eb/No or what it's
used for.
Eb/No is the measure of signal to noise ratio for a digital
communication system. It is measured at the input to the receiver
and is used as the basic measure of how strong the signal is.
Different forms of modulation -- BPSK, QPSK, QAM, etc. -- have
different curves of theoretical bit error rates versus Eb/No as
shown in Figure 1. These curves show the communications engineer
the best performance that can be achieved across a digital link
with a given amount of RF power.
In this respect, it is the fundamental prediction tool for
determining a digital link's performance. Another, more easily
measured predictor of performance is the carrier-to-noise or C/N
ratio.
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Our strategy for determining the transmit power is to:
Determine Eb/No for our desired BER;
Convert Eb/No to C/N at the receiver using the bit rate; and
Add the path loss and fading margins.
First we must decide what is the maximum BER that we can tolerate.
For our example, we choose 10-6 figuring that we can retransmit the
few packets that will have errors at this BER.
Looking at at BER to Eb/No look-up-table, we find that for DQPSK
modulation, a BER of 10-6 requires an Eb/No of 11.1 dB.
Now we convert Eb/No to the carrier to noise ratio (C/N) using the
equation:
Where: fb is the bit rate, and
Bw is the receiver noise bandwidth.
So for our example, C/N = 11.1 dB + 10log(2x106 / 1x106) = 11.1 dB
+ 3dB = 14.1dB.
Since we now have the carrier-to-noise ratio, we can determine the
necessary received carrier power after we calculate the receiver
noise power.
Noise power is computed using Boltzmann's equation:
N = kTB
T is the effective temperature in Kelvin, and
B is the receiver bandwidth.
Therefore, N1 = (1.380650x10-23 J/K) * (290K) *(1MHz) = 4x10-15W =
4x10-12mW = -114dBm
Our receiver has some inherent noise in the amplification and
processing of the signal. This is referred to as the receiver noise
figure. For this example, our receiver has a 7 dB noise figure, so
the receiver noise level will be:
N = -107 dBm.
We can now find the carrier power as C = C/N * N, or in dB C = C/N
+ N.
C = 14.1 dB + -107dBm = -92.9 dBm
This is how much power the receiver must have at its input. To
determine the transmitter power, we must account for the path loss
and any fading margin that we are building in to the system.
The path loss in dB for an open air site is:
PL = 22 dB + 20log(d/λ)
d is the distance between the transmitter and receiver; and
λ is the wavelength of the RF carrier (= c/frequency)
C
N
Bit Error Rate (BER) & Eb/N0
This assumes antennas with no gain are being used. For our
example,
PL = 22 dB + 20log(100/.122) = 22 + 20*2.91 =
22 + 58.27 = 80.27 dB
Finally, adding our 30 dB fading margin will give the required
transmitter power:
P = -92.9 + 80.27 + 30 = 17.37 dBm = 55 mW
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The process of correctly sizing uplink and downlink paths
for:
Satellite
Hub
Remotes
Download antenna size and receiver noise figure
Path Loss at 12 GHz over 36,000 km can exceed –205 dB !
An RF link budget is primarily a series of calculations that
determine the signal loss between a satellite transmitter and a
given earth station or receive antenna. The main consideration in
these calculations is downlink carrier-to-noise density (C/N) which
is represented by equation (1):
C/N = EIRP – PL + G/T + 228.6 (1)
Where:
EIRP = Satellite’s Effective Isotropic Radiated Power expressed in
dBW. The satellite
operator specifies this figure. For the SATNET and DTS C-Band
Service, in the POR,
AOR, the EIRP is 29 dBW, and the SATNET Ku-Band Service’s EIRP is
47.7 dBW.
PL = Path Loss expressed in dB. This is the free space dissipation
of the satellite’s
transmitted power as a function of distance. The PL calculation is
shown in equation (2) below.
G/T = Earth station figure of merit expressed in dB/K. The G/T
calculation is shown in
equation (3) below.
PL = 185.0 + 10LOG[1-(0.295 CosH CosAL)] + 20LOG(Frequency in GHz)
(2)
Where:
H = Earth station latitude
AL = Difference in longitude of the satellite and the earth
station
G/T = Net Antenna Gain – 10LOG(System Noise Temperature) (3)
Where:
Net Antenna Gain = antenna gain – waveguide losses – coupler
mismatch losses
System Noise Temperature = LNB noise temperature + antenna noise
temperature +
VSWR noise contribution and mismatch loss + interface waveguide
noise.
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Bandwidth (and power) = $
Outbound and Inbound BW proportional to:
Number of Users
All VSATs must share the allocated inbound BW
OB
IB
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Transmissions occur on the same frequency from multiple
sources
When a collision occurs, each source waits a random amount of time
before re-transmitting
Time slots are allowed to pass unused
In a loaded network, more collisions will occur, increasing the
random wait time
Frequency 1
Collision Occurs
Transmissions occur on the same frequency from multiple
sources
When a collision occurs, each source waits a random amount of time
before re-transmitting
Time slots are allowed to pass unused
In a loaded network, more collisions will occur, increasing the
random wait time
Frequency 1
Collision Occurs
F1 F2 F3 F4 F5 F6 F7 F8 F9 F10
VSAT 006
VSAT 021
VSAT 053
VSAT 102
Access Schemes
006
053
102
021
006
102
053
021
006
102
021
Retransmission
102
006
Collision
102
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A “private” frequency is allocated to a single VSAT
Collision free, high throughput channel for batch applications and
file transfer
When a DA is required by a VSAT, initiate request is sent in RA
mode, triggered
According to IP-socket or IP address
According X.25 destination address
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f
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Optimal for Constant Bit Rate (CBR) applications, such as
voice
Guarantees fixed response time
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t
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21
14
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f
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32
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32
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RA
Any 2 bit rates can be supported
Each VSAT supports two bit rates with multiple access modes
Lower bit rate for RA and higher bit rate for DA
Each Receiver Cage at the hub can handle two bit rates
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f
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DA
PDA
76.8
76.8
153.6
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RA
The Problem: TCP/IP requires acknowledgment of each and every
packet
The Satellite delay [(36,000/300000)2]2 in addition to all the
routers along the way adds significant latency
Spoofing Concept:
Acknowledge TCP packets locally at the VSAT/Hub – Send ‘Acknowledge
Summary’ periodically
No Spoofing
With Spoofing
Internet Page Acceleration (IPA)
On Terrestrial Based Networks, each HTML object is requested and
acknowledged
IPA requests all the objects on a specific URL
All objects on an HTML Page are sent to the VSATs at once
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VSAT
VHF or UHF
Starband ~44,000 (US ISP); US Postal Service ~33,000
How many VSATs are operation World-Wide ?
>400K installed by Gilat alone
Can a Star VSAT communicate with another VSAT without a Hub ?
Star Topology – No. Mesh Topology VSATs can operate
Point-to-Point.
Can a VSAT work Mobile ?
No, due to the associated delay is some systems and antenna
pointing issues. Mobile systems are under development.
Can a VSAT be used anywhere ?
No. It can not be used at the extreme North and South latitudes due
to coverage of Geostationary satellites. You must have line-of-site
coverage towards the satellite your network is working on.
What are typical upsteam and downsteam speeds that can be achieved
with a VSAT ?
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Can I view DVB-S video stream from the same antenna/LNB ?
Yes. (So long as it is on the same satellite). Elliptical antennas
allow adding two additional LNBs with switchable
polarization.
How many PC’s can I connect to a VSAT ?
Theoretically, as many as you want. The limiting factor is that
they will be sharing the Inbound/Outbound Bandwidths. The other
limiting factor is the total number of TCP/IP sockets and whether
or not the VSAT ISP set up the VSATs to assign an IP address to
connected PC. Up to 4 is recommended.
What applications are NOT suitable for VSATs ?
VSAT traffic has an inherent latency due to the distance. Real-Time
Internet Gamming other time-critical applications will not work as
well as terrestrial lines.
What changes can we expect to see in the future concerning VSAT
technology, markets ?
When Ka-Band Satellite service begins, we can expect to see much
smaller dishes. 8PSK instead of the current QPSK on the Outbound,
Internal Caching on VSAT, plus much more.
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‘Doppler Effect’)
Capability)