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
5
6
7
8
9
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
006
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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25
21
21
14
14
14
f
31
25
17
17
17
17
17
17
17
17
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Optimal for Constant Bit Rate (CBR) applications, such as voice
Guarantees fixed response time
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25
t
21
21
14
14
14
f
31
25
15
32
15
32
15
32
17
17
17
17
17
17
17
17
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
t
f
17
17
17
17
17
17
17
17
15
32
32
15
32
15
DA
PDA
76.8
76.8
153.6
36
11
25
21
21
14
14
14
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)