The first section aims at providing the introductions and the different aspects of VSAT, which are explained briefly. This section can also be considered as a reader guide to the following section which aims at providing more detail in the most important issues.
1.1 VSAT Networks ;
VSAT stands for ‘'very small aperture terminal'' .VSAT equipment consists of two main units, one is placed indoor for user communication device and the second one is placed outdoor for a line of sight to the satellite. The indoor unit is a small box contains of transmitter and receiver boards and interface to user's equipment. The outdoor unit consists of antenna, electronics and mount for signal transmission and reception.
VSAT is becoming more important for long distance data communications and low density voice. VSAT networks were in rapid growth in 1990s in the United States. Most of the businesses adopted VSAT networks for the communication and transmission of data as an alternative to data systems and terrestrial telephone then available. Growth of VSAT in next decade is expected to see operating in Ka band as a new 30/20 GHz GEO satellites become available. The available VSAT technology offers many advantages and benefits over conventional networks, including lower operating cost, support for multi services, ease of installation and maintenance, the ability to bring location where the cost of leased line is very high into the telecommunication loops, and the integration of VSAT into a single network to provide a cost effective expansion from smaller to larger systems.
1.1.1 VSAT Features:
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VSAT networks uses very small dish antennas therefore, in order to employ the 6/4 -GHz frequency band it has to utilize the spread spectrum also called CDMA code division multiple access) technique to reduce the signal power spectral density. This is due to the interference produced for using the same band as terrestrial microwave transmission does. Alternatively the 14/12 GHz frequency band may be utilized (if available) which provide satellite capacity with much more efficiency although as we know, with the rain as old enemy. Star VSAT network at 2-Mbps are now available using Single Channel Per Carrier (SCPC) access which is provided by most of the vendors however, occasionally , TDMA (Time division multiple access) technique is also used.
VSAT are connected by radio frequency link via satellite. Those links are radio frequency link with a so-called uplink from the station to the satellite and so-called downlink from station to station, sometime called hop consists of an uplink and downlink.
A radio frequency link is modulated carrier conveying information. Basically the satellite receives the uplink carriers from the transmitting earth station within the field of view of its receiving antenna.
Within the field of view Transponder amplifies those carriers, translates their frequency to a lower band in order to avoid possible output/input interference, and transmits the amplified carrier to the station located of its transmitting antenna.
1.2 VSAT Network Architectures
For any telecommunication services there are three basic implementations services: one-way, split-two-way (referred to as split-IP sometimes, when referring to internet traffic) and two-way implementation. Further division of two-way implementation are star and mesh network architectures.
This mode of satellite is used in the BSS (broadcast satellite service). This digital technology allows the user and provider much flexibility in the operations of broadcasting. By using different software in the user terminals, different subscribers can access different parts of the downlink according to programs offered by suppliers. This channel selection form is called narrowcasting
Split-Two-Way (Split IP) Implementation
This implementation is used when there is no normal return channel as, e.g. The KU-BSS (broadcast satellite service) systems that carry internet traffic. From the user end terminal uplink capability is not complemented by high capacity downlink stream relatively. If the BSS downlink is used as the download channel from an ISP (Internet service provider), the only option left for users return link is via another telecommunications channel, such as telephone line. The Internet protocol is therefore split between terrestrial telephone (inbound or return) channel and satellite downlink (outbound) channel. VSAT terminal does not require a transmit capability due to this approach, which significantly reduces its complexity and cast. The disadvantage of this approach is that the terrestrial telephone connection must usually be through a modem, with a bit rate generally less than or 56kbps.
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A return link is designed in this implementation method so that two way communications can be set up over the same satellite, from the user to hub and from the hub back to the user.
The architecture selected is the key to the economics of two way connections; it can be either star or mesh.
1.3 VSAT Network Options
Basically there are two network options.
In Star network architecture, all traffic is routed via the main hub station. If a VSAT want to communicate with another VSAT, they have to go through the hub station. This makes double hop link via the satellite. This architecture is referred to as Star network as shown in diagram.
In Mesh network architecture, each of the VSAT s has the ability to communicate with each other directly. The traffic in this architecture can go to or from any VSAT, is referred to as a Mesh network as shown in diagram.
The factor shows which architecture is appropriate than the others one:
- The structure of information flow within the network.
- The requested link quality and capacity.
- The transmission delay.
1.3.1 Structure of Information Flow
VSAT network can support different type of application and each has an optimum network configuration.
Broadcasting: A central site distributes information to many remote sites with no back flow of information. Hence a start shaped one-way network supports the service at the lowest cost.
Corporate Network: Most often companies have a centralized structure with administration and management performed at a central site, and manufacturing or sales performed at site scattered over a geographical area information from the remote sites needs to be gathered at the central site has to be distributed to the remote ones, such as task sharing. Such an information flow can be partially supported by a start one-way VSAT Network.
Interactivity Between Distributed Sites: Other companies or organization with decentralised structure are more likely to compromise many interacting one with a meshed VSAT network using direct single hop connections from VSAT to VSAT is hence mostly desirable.
1.3.2 Link Quality and Capacity
The link considered here is the link from the transmitting to the receiving one. Such a link may comprise several parts. For instance a single hop link would comprise an uplink and downlink; a double hop link would comprise two single hop links, one being inbound and outbound. Fig 1.3 indicates the general trend which relates EIRP to G/T in a VSAR network, considering a given baseband signal quality in term of constant BER. EIRP designates the equivalent isotropic radiated power of the transmitting equipment and G/T is the figure of merit of the receiving equipment.
- BER-Bit Error rate
- EIRP-Emitted isotropic radiated power
- G/T Figure of merit.
1.3.3 Transmission Delay
With a single hop link from VSAT in a humbles network, the propagation delay is about 0.25s. With a double hop from VSAT to VSAT via the hub, the propagation delay is twice as much, i.e. about 0.5s.
Double hop may be a problem for voice communication. However, it is not a severe problem for video or data transmission. Table 1.2 summarizes the above discussion given the EIRP and G/T values for a VSAT, the designer can decide for both a large delay from VSAT to VSAT and a large capacity or a small delay and a lower capacity, by implementing either a start network, or mesh one.
Table1.2 Characteristics of star and mesh network configuration.
1.4 Geostationary Satellite
The Satellite orbiting in the equatorial plane of the earth at an altitude above the earth surface of 22000 miles are known as geostationary satellite. The orbit period at this altitude is equal to that of the rotation of earth. As the satellite works on its circular orbit in the same direction as the earth rotates, the satellite appears from any station on the ground as the fixed relay in the sky, the satellite can be used 24 hours a day as a permanent relay for the uplink radio frequency carrier. Those carriers are down linked to all earth station from the satellite. Present VSAT networks use geostationary satellites
1.4.1 Fixed and Demand Assignment
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There are two basic alternatives
- Fixed assignment of channel
- Demand assignment of channel
Needs are shared between the two following extreme cases.
- High capacity links between a few international transit centres.
- Low capacity links between a large numbers of small low traffic stations, sometimes only necessitating a single channel.
18.104.22.168 Fixed Assignment
Fixed assignment is best applied to high capacity commercial system. This approach is not appropriate for low traffic links such as are encountered in many national system where isolated villages are distributed over vast areas with little infrastructure. So links are often pre assigned on the basis of small number of voice channels per carrier, or even only one channel per carrier(SCPC: Single channel per carrier). Each carrier uses a very narrow band, and the spacing between two carriers can be as small as few tens of KHz, compared with frequencies of 4,6,11 and 14 GHz. This possesses a problem with the stability of the received frequency.
The problem with fixed assignment SCPC is that an unused channel cannot be used to establish any other link. The less each circuit is used, the greater the penalty. Demand assignment offer a solution where channels are pooled, and so one channel can be assigned to a link between two stations just for the duration of the communication.
1.5 C- Band Verses Ku-Band
The Very small aperture terminal networks are available now in both 12/14 and 6/4 GHz bands. The first business VSAT system was introduced by equatorial communication company operating in 6-4 GHz band. Spread spectrum technology used by this system reduces the power flux density of the transmitted signal and thus avoids interference to adjacent 6/4 GHz terrestrial microwave and 6/4 GHz satellite systems. The spread spectrum signal is spread by sequence or pseudo noise code over a bandwidth of satellite that is much wider than the bandwidth of the data. The received signal correlates with receiving equipment with the same PN code with the same PN code to extract the data which is transmitted. The single hope architecture is used by equatorial VSAT network. The severe interference limitations at 6/4 GHz band even with spread spectrum technology can cause any spread spectrum VSAT network to use the satellite capacity inefficiently, and the link data rate is consequently low. The common effectiveness for a 6/4 GHz VSAT network is approximate to be less than 0.03 b/s Hz and the average data rate is less than 9.6kb/s. The ineffectiveness of spread spectrum VSAT networks at the 6/4 GHz band has provoked adverse criticism. Unquestionably spread spectrum systems use transponder bandwidth inefficiently. But without the use of spread spectrum signals there could be no 6/4 GHz. The VSAT price with small dish antenna as small as 0.6 m in diameter at a probable price of less than 1300 pounds per terminal. It is likely that the market place will decide how the satellite bandwidth is going to be used.
The situation is diverse for VSAT networks operating in the 14/12 GHz band which are free of terrestrial microwave interference. In the US, the FCC (Federal communication union) has calm licensing requirements for relatively larger VSAT networks (approximately 500 terminals) operating in 14/12 GHz band. In this band, VSAT networks can be planned to take advantage of the higher power flu density allowed and the lack of earthly interference. They can offer data rate of 56 kb/s or more using dish antennas with diameters of 1 to 2 m. For the same antenna diameter, the antenna gains in the 14/12 GHz are approximately 7.4db for transmit and 9.5 db for receive higher than those in 6/4 GHz band. Alternatively the attenuation caused by heavy rain will be much higher at 14/12 GHz band than at 6/4 GHz band, and therefore the VSAT networks operating at the high frequency band undergo more and larger outages.
Table 1.3 of Advantages and Drawbacks of C and KU band.
It allows the use of smaller dishes.
It causes fading during rain. Attenuation from 6 db to 10 db
Signal less susceptible to rain fading. Rain attenuation in the range of 0.4 to 1 db.
Needs slightly larger dishes as compared to Ku band.
High transponder power
Not available everywhere in the world
Low transponder power
Narrow beam coverage
Wider and even global beam coverage
Less terrestrial interference
Higher likely hood of terrestrial interference
1.6 VSAT Network Benefits, Application and Types:
1. Benefits of VSAT Networks.
From the application perspective, VSAT networks offer the many benefits as follows
- Wide range of voice, video and data applications.
- Robust and proven technology with high user satisfaction
- Quicker network deployment
- Direct and rapid access to telecommunications
- Rapid response to market needs, because of ease of expansion
- Ease of maintenance and reliability
It is not amazing that VSAT networks are being installed to resolve many telecommunication challenges. VSAT networks remain more effective and reasonable than terrestrial solutions. At present, the largest VSAT network consists of over 12,000 sites and is administered by Space net and MCI for the US Postal Service (USPS).
VSAT network have many applications.
Services supported by VSAT Networks (One Way and Two Way).
One Way VSAT Networks
- Stock market and other news broadcasting
- Training or continuing education at distance
- Distribution of financial trends and analyses
- Introduce new products at geographically dispersed locations
- Update market related data, news and catalogue prices
- Distribute video or TV programs
- Distribute music in store and public areas
- Relay advertising to electronic signs in retail stores
Two Way VSAT Networks
- Interactive computer transactions
- Low rate video conferencing
- Data base inquiries
- Bank transactions, ATM machines
- Distributed remote process control and telemetry
- Reservation systems
- Voice communications
- Emergency services
- Electronic funds transfer at point of sale
- Medical data transfer
- Sales monitoring and stock control
- Satellite news gathering (SNG)
1.7 Types Of Traffic
Depending on the service the traffic flow between the hub and the VSATs may have different characteristics and requirements.
Data Transfer or Broadcasting
Typically displays file transfers of 1-100 M bytes of data. This kind of service is not only delay sensitive but requires a high integrity of the data, which are transferred. Example of application is computer download and distribution of data remote sites.
Interactive data is a two-way service corresponding to several transactions per minutes and per terminal of single packets 50 to 250 bytes long on both inbound and outbound links. The required response time is typically a few seconds. Example of applications is bank transactions and electronic funds transfer at point of sale.
Inquiry / response are a two-way service corresponding to several transactions per minute and terminal. Inbound packets (typically 30-100 bytes). The required response time is typically a few seconds Example of applications is airline or hotel reservations and database inquiries.
Supervisory Control and Data Acquisition
Supervisor control and data acquisition (SCADA) is a two way service corresponding to one transaction per second or minute per terminal. Inbound packets (typically 100 bytes) are longer than outbound packets (typically 10 bytes). The required response time ranges from a few seconds to a few minutes. What are most important in the high security level, and the low power consumption the terminal? Example of applications control and monitoring of pipelines, offshore platforms, electric utilities and water resources.
II New Developments
A number of considerable technology enhancements are being projected for new VSAT systems. These are being focused in to two main technical areas that seem to expand quickly in parallel: digital signal processing and microminiaturization. The first enables large amounts of complex ‘'transactions'' to take place in microseconds, while the second enables the product to built into slighter packages.
Conceivably the change that had the biggest bang on satellite developments over the 1990s/2000s was the shift from military spending being the main financial input for space ventures to industrial commercial funding. With that major change has come market focused economics that force the end product to be reliable and low-cost. Behind all of the developments has been the surge in demand for communications capabilities that can bring any stream of bits into any terminal, anywhere: the ultimate multimedia portable unit.
Most advanced satellite is designed to handle the vast increase in the internet like traffic and they take advantage of onboard processing (OBP) technology. On board processing technology was one considered to be overly expensive in both mass and power for commercial satellites but is now the enabling technology of most new systems. The iridium constellation was the first commercial system to employ OBP extensively although, in the civilian area. INTELSAT VI was the first use onboard, satellite switching between beams. In the iridium system, the uplink signals from the handheld units are mixed down to baseband within the satellite payload, the header information stripped off the frames, and the traffic re-assembled and routed to the appropriate output port, whether it be for down-linking to another handheld unit, down-linking to the gateway (hub) earth station, or cross-linking to another satellite. All the new satellites being proposed for internet service at Ka band (30/20) GHz have OBP capabilities. This is not a trivial undertaking. Not only must the OBP design allow for a large variety of traffic types and permit flexible connection between any points in the coverage, it must also incredibly reliable.
Very small aperture terminals (VSATs) have become a part of everyday life around the world. In highly developed regions, they act as links in the retail chain, taking point to of sale data from automated terminals (e.g., a gas pump) from the customer to the credit card authorization centre. In all regions of the world they have become the fastest growing segment of video distribution: the direct-to-home (DTH) receivers in the direct broadcasting service using geostationary satellites. Soon, DTH terminals will be two-way links via satellites in global Information infrastructure, better known as the GII or simply the Internet. The development, and more importantly the universal acceptance, of VSATs required a number of technological breakthroughs, the most important of which were digital compression techniques and very high density integrated circuits. The subjective, such as customer acceptance of outages at Ka band and the ability of onboard processing (OBP) payloads to adapt to changes in traffic mixes over the life time of a particular satellite system. Nevertheless, VSATs will always be a major part of every satellite system, growing in importance as new enterprise seek to provide multimedia streams directly to customer premises.