VSAT

VSAT Very Small Aperture Terminals.

 1.0                   Introduction

 VSAT is the acronym for 'Very Small Aperture Terminal' and is one of the intermediate steps of the general trend in earth station size reduction that has been observed in satellite communications since the launch of the first communication satellites in the mid 1960s. VSATs are at the lower end of the product line. At the higher end are large stations that support large capacity satellite links. These are used mainly for international switching networks to support trunk style telephony services between continents. These links are typically in the range of 100Mb/s and are owned and operated by national telecom operators. At the lower end are the VSATs. These are small stations with antenna diameters from 2.4 meters down to 45 centimetres, hence 'small aperture' which is in reference to the area of the antenna. These stations cannot support satellite links with large capacities however they are cheap and are easy to install. Typically their capacity is in the range of a few tens of kb/s.  VSATs are available at ones premises and this avoids the need to use public network links to access the earth station. The user can directly plug the VSAT equipment into their own communication terminals, such as telephone, video or personal computer. As such VSATs are a means to bypass public network operators by directly accessing satellite capacity. They are tools for establishing private and public networks communications solutions that can support the internet, data, LAN and voice/fax.

Generally VSATs operate in the Ku band and C band frequencies. As a rule of thumb C- band (which suffers less from rain attenuation but requires larger antennas) is used in Asia, Africa and Latin America. Ku-band (which can use smaller antennas but suffers from rain fade in monsoon like downpour) is used in Europe and North America. Interactive antenna sizes range from 75 centimetres to 1.8 meters for Ku-band and C-band from 1.8 meters to 2.4 meters. One way systems can use antennas as small as 45 centimetres. Ref: [1] Page 1-6. 'VSAT Networks' - [7] Page 2 'What do you mean by Vsat'

1.1                   Network Configurations / Options / Design

                      

1.1.1                Star VSAT Networks

The most common VSAT configuration os the TDM/TDMA star network. These have a high bit rate outbound carrier (Time Division Multiplexed) from the hub to the remote earth stations, and one or more low or medium bit rate (Time Division Multiple Access) inbound carriers. In a typical VSAT network, remote users have a number of personal computers or dumb terminals that are connected to the VSAT terminal that in turn is connected to a centralised host computer either at individual sites or at a data processing centre. Data sent to the VSAT terminal from the data terminal equipment (DTEs) is buffered and transmitted to the hub in packets.

1.1.1.2                      Hub Station

Ref: [8] Page 4. 'The White Papers - Interactive VSATs'

The hub station is a relatively large high performance earth station with an antenna diameter of anything between 6 to 9 meters. The hub consists of a control centre, which manages the network as well as microwave equipment, including the outdoor antenna for the transmission and reception of signals.

Hub stations are expensive and typically cost upwards of 1 million (euro). Hub stations can be shared across several networks. The hub station consists of several main subsystems:

·       A switch (generally a packet switch) which controls routing between the host ports and the modulator ports.

·       One or more modulators that modulate the outbound carriers with the TDM stream.

·       A bank of demodulators which receive the inbound carriers and extract the data packets.

·       A RFT (radio frequency terminal).

·       A NCC (network control centre) which controls and monitors the operation of the hub.

·       The primary power substation.

1.1.1.3             Remote Terminals

To minimise costs VSAT networks are designed to have a single expensive hub and a large number of much smaller remote terminals. A dish antenna is generally 0.55 to 2.4 meters in diameter. These antennas are usually offset parabolic dishes although offset dual reflectors have become more common. The outdoor unit contains the microwave electronics for the terminal. The outdoor unit is usually all solid state with GaAs FETs used in the Low Noise Receiver and the High Power amplifier. An indoor unit which provides the modulation, demodulation, multiplexing, demultiplexing and synchronisation with the rest of the network and supports the user interfaces.

                                   

1.1.1.4             Bit rates

Remote user sites have several low bit data terminal equipment (DTEs) operating at 1.2 to 9.6 kb/s. These are connected through the VSAT network to a centralised host processor. The DTEs are connected to the host through a X.25 Packet Assembler/Dissembler or through a conventional or statistical multiplexer which concentrates the traffic. The typical amount of data that is transferred in each transaction is relatively small, typically between 300 and 10^5 bits. Each VSAT operates with a low duty cycle.

1.1.1.5             Signal Types

Outbound data from the hub is transmitted at relatively high data rates (56 to 1024 kb/s) using TDM. The bit stream consists of a synchronisation word followed by a series of messages in time slots directed towards individual VSAT terminals. Broadcast messages to all remote VSAT terminals are also admissible. Outbounds are transmitted continuously (100% duty cycle) as a TDM stream and are usually grouped at the top or bottom of the leased bandwidth.

The inbound carrier is often initiated using ALOHA and information rates are between 2.4 and 16kb/s. Inbound TDMA or SCPC carriers are used for file transfer have data rates arouund 56 to 256kb/s. Error bit rates are kept low using carriers that are BPSK or QPSK modulated. Remote terminals transmit in TDMA bursts in either a pre-assigned inbound channel or in any inbound channel slot depending on the manufacturer.

Ref: [8] Page 12. 'The White Papers - Interactive VSATs'

1.1.1.6             TDM/TDMA Connection Set Up

When the network is established or when additional remote terminals are added to the network, remote terminal characteristics and addresses are entered into the network database, which is used as a routing table by the operational system. This database establishes permanent virtual circuits between ports at the user interface of the hub and the ports at the user interfaces of the remote terminals. It is this arrangement that allows the normal transaction traffic carried by the network to be switched without an individual call set up procedure. Ref [8] Page 1-21 "White papers, Interactive VSATS" 'Interactive Vsat'.

1.1.2                           Meshed  VSAT Networks

Meshed VSAT networks provide a way to set up a switched point to point data network that can have the capability for high data rates of up to 2Mb/s. Links are set up directly between remote terminals usually on a call by call basis. These networks are usually configured to operate without a large central earth station and carry a mix of data traffic and telephony traffic or only data traffic. These networks generally will have a network control station, which controls the allocation of resources across the network. This control centre is only involved in the signalling for the call setup/teardown and in monitoring the operation of the network.

Typical applications for the meshed networks are:

·       LAN - LAN interconnections

·       Connections between PABX's

·       Video-conferencing

·       Remote printing

·       Back up for terrestrial circuits

1.1.2.1             Mesh Technology

A meshed VSAT network generally consists of two types of terminals. A single control station, which is only involved in the call set up and tear down processes and a number of user terminals each with an antenna of diameter 2 to 3.7 metres. Connections are generally on a call by call basis or as a required basis.

Users on remote sites tend to be relatively heavy users with bursty data requirements at high data rates. The network is a switched network with circuits provided as required. Data transferred in each transaction can be very large and connections between remote terminals are made directly without the need for a double hop through the hub as is required in a star setup.

Three main technologies are used in modern meshed VSAT systems:

1.1.2.2.1          DAMA SCPC Meshed VSAT Networks

Here SCPC (Single channel per Carrier) channel pairs are automatically set up on a call by call basis for the duration of each call. Typically a DAMA ( Demand assignment multiple access ) SCPC meshed VSAT system consists of the following terminals

Remote sites

·       A DAMA controller

·       A control channel modem which communicates with the NCS in the control system terminal over the control and request channels

·       A modem bank

·       A out door unit ( (ODU), antenna, feed assembly, Low noise block, solid state power amp)

Network control system site.

·       A control modem

·       A network control system to operate the network

·       A modem bank

·       An ODU

Ref: [8] Page 4. 'The White Papers - Meshed VSATs'

1.1.2.2.1.1            Signal Types and Characteristics.

Most systems have one control channel using a high capacity TDM and two or more request channels using lower capacity ALOHA. Traffic is two way as traffic channels are assigned in pairs. Once the channels are established through a call up procedure, these channels are generally transparent and are transmitted continuously for the call duration. The TDM channel carries a continually transmitted bit stream, which is divided into frames. A framing packet containing a unique word and a control word denotes the start of the frame, which together provide framing, timing and control information. On the other side of the equation each TDMA request channel contains frames which are synchronised to the control channel frames and each request channel frame is divided into slots. Transmissions are intermittent in the range of 25 to 30% for slotted ALOHA.

Ref: [8] Page 5. 'The White Papers - Meshed VSATs'

1.1.2.2.2          TDM SCPC Meshed VSAT Networks

With TDM SCPC meshed VSAT networks each remote terminal transmits on its own TDM channel. All other remote terminals receive all other transmissions and process the packets carrying their own address in the headers. All other packets are ignored. As such no network control site is needed.

Remote sites

·       A modulator for the transmitted carrier

·       A bank of demodulators. One for each remote site.

·       An ODU.

Ref: [8] Page 9. 'The White Papers - Meshed VSATs'

1.1.2.2.2.1       Signal Types and Characteristics.

The TDM carrier is modulated by a continuously transmitted bitstream, which is divided into frames. The start of the frame is denoted by a framing packet that contains a unique word and a control word which together provide framing, timing and control information. The rest of the frame contains fixed length data packets. Call setup is not required as permanent virtual circuits are established between each remote site and all other parts of the network at its inception.

Ref: [8] Page 10. 'The White Papers - Meshed VSATs'

1.1.2.2.3          TDMA Meshed VSAT Networks

With TDMA meshed VSAT networks each remote terminal transmits signalling and traffic data in bursts with pre-assigned time slots using TDMA on a single channel. All the network terminals use the same frequency. Transmissions are not continuous because some of the time slots may not be used. (There is a variant of this network configuration that uses multiple frequencies in which several parallel TDMA carriers are used for network traffic.)

Remote Sites

·       A burst modem

·       An ODU

Network control system site.

·       A burst modem

·       An NCS to control the network burst plan

·       An ODU

Ref: [8] Page 12. 'The White Papers - Meshed VSATs'

1.1.2.2.3.1       Signal Types and Characteristics.

The start of a TDMA frame is denoted by a framing packet that contains a unique word and a control word which together provide framing, timing and control information. The NCS generates the framing packet and defines network timing. Signalling from the NCS to remote sites is contained in the control word and signalling from remote sites to the NCS is carried in the header. Ref [8] Page 1-16 "White papers, MeshedbVSATS" 'Meshed Vsat'.

Ref: [8] Page 13. 'The White Papers - Meshed VSATs'

2.0                   Competing Terrestrial Technology

The VSAT industry has always had to compete with terrestrial transmission technology. However it is very difficult to make general comparisons between VSAT services and their terrestrial equivalents. This is due to the fact that terrestrial services are almost always distance dependent while VSAT connections cost the same whether the sites are 1 or 1000 kilometres apart. In the mid to late 80s telecommunication managers where turning to intelligent T1 multiplexers. These multiplexers drove the convergence of voice and data traffic onto the same corporate backbone, which led to the decrease of costs for remote sites. These new networks were cost effective and gave telecom managers additional control through features such as dynamic bandwidth and automatic route configuration. These intelligent multiplexers connected by fibre optics have resulted in highly reliable communication systems, which has made it increasingly difficult to entice users away from terrestrial solutions. Today new VSAT products have incorporated some of the most attractive network management features in T1 multiplexers such as dynamic bandwidth allocation and route configuration. Ref: [1] Page 9. 'VSAT Networks' - [9] Page 1. '"Emerging Technology: Broadband from outer space.'

3.0                   Users and Applications

Different VSATS : Generic pictures

Because VSAT costs are distance insensitive, it has made VSATs the ideal choice for corporate users with numerous sites.  VSAT transmissions have tended to be 'transaction' based in support of credit authorisation terminals, inventory systems, reservation systems and other general inquiry / response systems. The past ten years has seen the networks shifting to client server architectures and LAN to LAN interconnections across wide area networks (WANs). This has impacted on the way traditional VSAT users are using their systems and are expanding their functionality to include more than transaction based applications.

When the early adopters selected VSAT technology, it was largely based on cost rather than availability. Landlines were often available, but for point to multipoint and multipoint to multipoint connections VSAT was cheaper. However landlines or any real infrastructure in developing countries is not always available. The choice becomes having a communications service or not. VSATs may be the key to increasing teledensity within these countries.

Since 1987 the internet has grown from 10 thousand to 37 million hosts worldwide. As a consequence there has been a significant increase in the demand for telecommunications services throughout the world. VSATs due to the fact that outbound capacity usually exceeds inbound capacity form an ideal choice for the provision of internet services. These services though will need to provide higher speeds than the typical 9.6 to 64kbps used in transaction oriented applications.

Some typical uses of a VSAT system

Receive only

·       Stock market and news broadcasting

·       Training or education

·       Distribution of financial trends

·       Distribute video or TV

·       Advertising

·       Distribute music

Transmit and Receive

·       Interactive computer transactions

·       Internet

·       Video teleconferencing

·       Bank transactions, ATM

·       Reservation systems

·       Voice communications

·       Emergency services

·       Electronic funds transfers

·       E-mail

·       Medical data transfer

·       Sales monitoring and stock control

The trend has been to fewer and larger players in this industry. The industry has seen steady growth since its inception but that growth has been slower than expected. The competitive landscape has shifted and shifted again over the last 20 years and is now dominated by the larger players such as Spaceway, Astrolink and Euro Skyway. Satellite is now the fastest growing access technology, expected to account for 20.5 percent of all broadband deployments in 2002.

Ref: [4] Page 1-2. 'Quantum Prime Communications' - [9] Page 1-5. '"Emerging Technology: Broadband from outer space.'

4.0                   Regulation and Standards

North America was the original VSAT market due to the fact that there was only one regulatory structure to deal with. Growth in other areas has been slow due to a number of factors. In areas such a Europe and Latin America there are numerous countries each with their own regulatory structure. Cross boarder applications have been hindered by the stumbling block of dealing with multiple and often conflicting regulations. Regional distribution has changed however as deregulation and privitisation of telecommunications has increased throughout the world. Ref [6] Page 6 'VSATs: Industry Changes and Growth from 1988 through 1998'.

5.0                   Future Trends

Satellite networks today are competitive in the local market. This is mainly due to the fact that in 2001 that broadband market suffered some setbacks. Cable, wireless and DSL promised to provide high data rates at low cost but all three have suffered major setbacks. Satellites on the other hand are about to get cheaper and faster as a new broadband generation is about to be launched. These new networks use satellites containing their own switches along with highly focused antennas that target particular neighbourhoods or individual customers. These networks should be able to service anyone from individual customers to larger enterprises. A typical broadband dish transmits at 512Kb/s but receives at 2Mb/s.

The services about to be launched are a variant of the current tried and true fixed satellite technology. The satellites fly in a geostationary orbit, which is tied to the earth's rotation. Customers access them through their VSAT that requires a line of sight. The next generation of VSAT technology will add new technology to overcome 'time delay' and 'bent pipe' architecture where the data has to make two roundtrips into space as the switch is on the ground. The new generation of satellites will have intelligent switching on board the satellite so that two users can communicate directly. The new fixed satellite technologies will also overcome atmospheric fading by increasing the signal strength. They will use targeted beams, which point a transmission directly at those that need it meaning users don't need to filter out data intended for others. This approach also increases network capacity by reusing the same spectrum in different areas.

Ref [9] Page 1-4. '"Emerging Technology: Broadband from outer space.'

6.0       Twenty Key Points

·       VSAT is the acronym for 'Very Small Aperture Terminal' and is one of the intermediate steps of the general trend in earth station size reduction that has been observed in satellite communications since the launch of the first communication satellites in the mid 1960s.

·       VSATs are a means to bypass public network operators by directly accessing satellite capacity. They are tools for establishing private and public networks. Communications solutions that can support the internet, data, LAN and voice/fax.

·       Generally VSATs operate in the Ku band and C band frequencies. C- band (which suffers less from rain attenuation but requires larger antennas) is used in Asia, Africa and Latin America. Ku-band (which can use smaller antennas but suffers from rain fade in monsoon like downpour) is used in Europe and North America.

·       The most common VSAT configuration os the TDM/TDMA star network. These have a high bit rate outbound carrier (Time Division Multiplexed) from the hub to the remote earth stations, and one or more low or medium bit rate (Time Division Multiple Access) inbound carriers.

·       The hub station is a relatively large, high performance earth station with an antenna diameter of anything between 6 to 9 meters. The hub consists of a control centre, which manages the network as well as microwave equipment, including the outdoor antenna for the transmission and reception of signals. Outbound data from the hub is transmitted at relatively high data rates (56 to 1024 kb/s) using TDM.

·       VSAT networks are designed to have a single expensive hub and a large number of much smaller remote terminals 'VSATs'. A dish antenna is generally 0.45 to 2.4 meters in diameter. These antennas are usually offset parabolic dishes although offset dual reflectors have become more common. The inbound carrier is often initiated using ALOHA and information rates are between 2.4 and 16kb/s. Inbound TDMA or SCPC carriers are used for file transfer and have data rates of arouund 56 to 256kb/s.

·       A meshed VSAT network generally consists of two types of terminals. A single control station, which is only involved in the call set up and tear down processes and a number of user terminals each with an antenna of diameter 2 to 3.7 metres. Connections are generally on a call by call basis or as a required basis.

·       Three main technologies are used in modern meshed VSAT systems

·       DAMA SCPC Meshed VSAT Networks

Here SCPC (Single channel per Carrier) channel pairs are automatically set up on a call by call basis for the duration of each call.

·       TDM SCPC Meshed VSAT Networks

With TDM SCPC meshed VSAT networks each remote terminal transmits on its own TDM channel. All other remote terminals receive all other transmissions and process the packets carrying their own address in the headers.

·       TDMA Meshed VSAT Networks

With TDMA meshed VSAT networks each remote terminal transmits signalling and traffic data in bursts with pre-assigned time slots using TDMA on a single channel. All the network terminals use the same frequency.

·       The VSAT industry has always had to compete with terrestrial transmission technology. However it is very difficult to make general comparisons between VSAT services and their terrestrial equivalents. Terrestrial services are almost always distance dependent while VSAT connections cost the same whether the sites are 1 or 1000 kilometres apart.

·       VSAT transmissions have tended to be 'transaction' based in support of credit authorisation terminals, inventory systems, reservation systems and other general inquiry / response systems.

·       The past ten years has seen the networks shifting to client server architectures and LAN to LAN interconnections across wide area networks (WANs).

·       VSATs may be the key to increasing teledensity within developing countries.

·       VSATs, due to the fact that outbound capacity usually exceeds inbound capacity, form an ideal choice for the provision of internet services.

·       Spaceway, Astrolink and Euro Skyway are the some of the world's major VSAT players. Satellite is now the fastest growing access technology, expected to account for 20.5 percent of all broadband deployments in 2002.

·       In 2001 that broadband market suffered some setbacks. Cable, wireless and DSL promised to provide high data rates at low cost but all three have suffered major setbacks. Satellites on the other hand are about to get cheaper and faster as a new broadband generation is about to be launched.

·       These networks should be able to service anyone from individual customers to larger enterprises. A typical broadband dish transmits at 512Kb/s but receives at 2Mb/s.