A small earth station used for satellite
Low cost business terminals with small antennas (generally less than 2 metres in diameter) are often termed Very Small Aperture Terminals (VSATs). These are usually perceived as being two way data terminals, though strictly speaking many of the systems used for data broadcast are really one-way VSATs. Taking the USA as an example, approximately half of all installed VSATs are only used for one way data links.
ETSI take a different definition for a VSAT as a one or two-way terminal used in a star, mesh or point to point network. Antenna size is restricted to being less than or equal to 3.8 m at Ku band and 7.8 m at C band.
A more general definition is that a network is a VSAT network if it consists of a large high performance hub earth station (with an antenna of up to 9 m in diameter) and a large number of smaller, lower performance terminals. Being completely general, these small terminals can be receive only, transmit only or transmit/receive. Even this definition is not universal. Meshed VSAT networks exist in which all terminals have the same size and performance.
As terminal technology advances, the size of the antenna required to achieve a particular link quality (bit error rate) decreases. A class of terminals smaller than VSATs is now available; these are termed Ultra Small Aperture Terminals (USATs). For most practical purposes, USATs are just VSATs with smaller antennas. It must always be remembered, however, that as antenna size decreases, the antenna beam widens and that a point is rapidly reached when there is no further advantage in decreasing antenna size because of increased interference with other systems. The practical current lower limit on antenna size is 55 cm diameter.
Typical applications for interactive VSAT networks are:
The most common VSAT configuration is the TDM/TDMA star network. These have a high bit rate outbound carrier (TDM) from the hub to the remote earth stations, and one or more low or medium bit rate Time Division Multiple Access (TDMA) inbound carriers.
With its star configuration network architecture, interactive VSAT technology is appropriate for any organisation with centralised management and data processing.
This configuration has been developed to minimise overall lifetime costs for the complete network including satellite transmission costs. The use of a single high performance hub allows the use of low cost remote VSAT terminals and optimises use of satellite capacity. Even so, in most VSAT networks, the cost of the VSAT terminals usually far exceeds the cost of the hub (typically a VSAT terminal is 0.1 to 0.2% of the price of the hub).
In a typical VSAT network, remote user sites have a number of personal computers, dumb terminals and printers connected to the VSAT terminal which connects them to a centralised host computer either at the organisation's head office or data processing centre. Data sent to the VSAT terminal from the DTEs is buffered and transmitted to the hub in packets.
Interactive VSAT Network
The principle characteristics of an interactive VSAT network are:
VSAT networks are designed to be flexible and to evolve with user needs. VSAT terminals are controlled by microprocessors and can generally be reprogrammed remotely using downloaded software from the hub. If additional interfaces or capacity are required this can usually be provided by adding or replacing cards in the VSAT terminal.
Three different transmission schemes are used for interactive hubbed VSAT networks:
Of these TDM/TDMA is by far the dominant technique with only CDMA being used to a small extent. Demand assigned SCPC has been virtually abandoned as a transmission scheme for the present.
It is also common for VSAT systems to support one-way TV transmission from the host to the remote stations.
Two-way, 2 Mb/s transmissions can also be supported by some VSAT systems.
To make VSAT networks more affordable it is possible to share the hub between several users, thereby spreading the cost. In this case the hub is usually owned by a service provider who retains overall control of the network and who manages the hub itself.
Each user, however, is allocated his own time slots or carriers and can so operate his own private network using the shared hub facility without any loss of privacy. The operation and management of these subnetworks is performed by the users themselves completely independently of the service supplier.
VSAT Shared Hub Network Configuration
In this configuration, each user has his own "mini-hub" which is much smaller and simpler, and hence cheaper, than a conventional hub. An approximate price for a mini-hub is 250 kEuro. The antenna diameter is typically only 2.4 m. Each user organisation has complete control over his own communications. Overall management of the complete network is provided by the service supplier who has a "super hub" which provides network supervision and diagnostic support.
VSAT Mini-Hub Network Configuration
Current interactive VSAT networks generally have distributed, rather than a centralised, network management. Multiple points of control and intelligent operator interfaces are common features. The network manager not only has the ability to perform diagnostics on the network, but can also reconfigure the network from his own console. Where multiple consoles are available, the network can be configured, monitored and operated either locally or remotely. In addition, many VSAT network management systems have interfaces available for working with other vendor's network management systems such as IBM's Netview and DEC's EMA.
Many VSAT systems can be configured to support virtual subnetworks within a VSAT network. These can be set up to give closed groups of users their own private networks.
This facility allows groups of users to have complete control over their own subnetwork and to be able to manage it independently of the main network.
Virtual subnetworks are exploited by many VSAT service vendors in "shared hub" networks. Within a single organisation, however, virtual subnetworks can be used, for example, for each division in the organisation, so that communications costs can be accurately charged.
All the established interactive hubbed VSAT systems use TDM/TDMA access as the primary access technique (TDM on the outbounds and TDMA on the inbounds).
The outbound data stream from the hub is transmitted at a relatively high data rate (typically 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 generally permitted.
Outbounds are transmitted continuously (i.e. duty cycle 100%) as a TDM stream. The number of outbounds per network is determined by the traffic statistics, packet length as well as the outbound data rate.
The outbounds for a network are generally grouped together at either the top or the bottom of the leased bandwidth.
The inbound carrier is often accessed using ALOHA or Slotted ALOHA. If a higher capacity is required, a separate channel can be dedicated to ALOHA or Slotted ALOHA access requests and a demand assigned TDMA access scheme established.
Inbound slotted ALOHA carriers information rates are usually between 2.4 and 16 kb/s. Inbound TDMA or SCPC carriers used for file transfer usually have information data rates between 56 kb/s and 256 kb/s. All carriers are BPSK or QPSK modulated and have rate 1/2 or 2/3 Forward Error Correction (FEC). This ensures that bit error rates are low (typically 10-6 or 10-7 which is comparable to ISDN).
Remote terminals transmit in TDMA bursts in either a pre-assigned inbound channel slot or in any inbound channel slot depending on the manufacturer.
Several different inbound TDMA access systems are used depending on traffic characteristics and the manufacturer.
In a shared hub network, individual customers are often, but not always, allocated one or more dedicated outbounds and several inbounds.
If the traffic mix is a combination of short interactive messages and long file transfers it is often worthwhile to use a technique called Adaptive ALOHA/TDMA. VSATs which have large blocks of data to transmit request dedicated TDMA time slots and use TDMA. The other VSAT terminals in the network use slotted ALOHA and avoid the assigned time slots. Alternatively, dedicated SCPC carriers can be temporarily assigned for file transfer.
Typical Interactive Hubbed VSAT Network Spectrum
Typical Interactive Hubbed VSAT Frame and Packet Format
Each TDM outbound carries a continuously transmitted bitstream which is divided into frames.
The start of a frame is denoted by a framing packet contain a unique word (UW) and a control word (CNTRL) which, together, provide framing, timing and control information.
The rest of the frame is filled by (generally) fixed length data packets which each contain:
Outbound data packets typically contain between 50 and 250 bytes in transactional networks.
Each TDMA inbound contains frames which are synchronised to the outbound frames. Each inbound frame is divided into slots. Individual IDUs transmit in these slots in a manner depending on the access modes available to the particular system and how the network has been set up.
Each inbound packet consists of:
Inbound data packets typically contain between 50 and 250 bytes in transactional networks.
The main inbound transmission modes used are:
Aloha, in which an IDU can transmit data packets at any time in a particular inbound frequency slot. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of 10 to 15%.
Slotted Aloha, in which an IDU can transmit data packets in any slot (or any of a predetermined number of slots) in a particular inbound frequency slot. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of 25 to 30%.
Fixed Assignment, in which specific time slots in an inbound frequency slot are permanently, or for the duration of a particular transmission, assigned to a particular IDU. This is often used for batch transmission and for telephony. Transmissions in any particular frequency slot are intermittent but can have a peak traffic duty cycle of 100% if that particular inbound is carrying telephony traffic or several batch file transfers from different IDUs.
Dynamic Assignment, in which time slots in an inbound frequency slot are dynamically assigned to a particular IDU in line with ongoing traffic demands. Transmissions in any particular frequency slot are intermittent with a peak traffic duty cycle of from 25 to 30% to approaching 100%, depending on the traffic mix.
Most interactive hubbed VSATs now have protocol stacks which map, at least notionally, onto the OSI stack.
Network layer spoofing is provided by many VSATs to minimise the impact of the data layer protocol and, particularly, the satellite transmission delay, on the throughput of the satellite link.
When the network is established, or when additional remote terminals are added to the network, remote terminal addresses and characteristics (i.e. card fits and port addresses) are entered into a 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. In those products which permit the dedication of the assignment of capacity on request, or dynamic variable assignment, the database also establishes permanent virtual circuits between the IDU controllers at the remote terminals and the NCC.
This arrangement allows the normal transactional traffic carried by the network to be switched without an individual call set up procedure.
A packet sent by a particular IDU carries addressing information identifying both the source and destination. This allows the hub switch to route the packet to the correct user interface port without additional signalling traffic.
This same procedure is used for intra network signalling to set up assignments for the temporary or permanent assignment of channels to a particular IDU port/hub port pair (for example, telephony or batch data transfers). Call set up information is sent as a transactional data packet as described above, except that the destination address at the hub is the NCC.
The hub station is usually a relatively large, high performance earth station with an antenna diameter of anything between 6 and 9m. The hub consists of a control centre which manages the network as well as microwave equipment, including an outdoor antenna, for the transmission and reception of signals. A substantial amount of interfacing equipment necessary to support the wide range of terrestrial interfaces required at the hub completes the installation. This equipment is usually mounted in several racks.
VSAT Hub Station Block Diagram
Hub stations are expensive and typically cost upwards of 1 MEuro. Hub stations can be shared between several networks, resulting in a sharing of costs. Two principal options for network implementation can be adopted. Firstly, some very large users will wish to purchase their own dedicated VSAT networks including a hub. Other users will choose to buy or lease the user terminals and to lease access to a hub which will be owned by the system operator.
The hub station consists of several main subsystems, except for the antenna these are usually fully redundant with automatic switchover in the event of failure:
The hub is usually very expensive, costing typically between 0.5 million Euro to 2 million Euro, depending on the configuration and manufacturer. This cost excluded the price of the RFT, antenna and civil works.
A few small, simple VSAT systems intended for very low data rate applications such as SCADA (for example the TSAT) have low cost hubs, costing of the order of 25,000 to 50,000 Euro.
In contrast to the hub station, the remote terminals are much simpler. To minimise total system costs, VSAT networks are designed to have a single expensive hub and a large number of much smaller remote terminals.
VSAT Remote Terminal Block Diagram
Remote terminals consist of:
Remote terminals usually support a wide range of common electrical interfaces such as RS-232, RS-422, V.35, as well as voice and TV. Several common protocols are also generally supported including SDLC, 3270 bisync, X.25, asynch and Ethernet. Asynchronous data rates are typically available up to 9.6 kb/s. Synchronous data rates between 1.2 and 32 or 64 kb/s are also generally available.
Remote terminals have now become very reliable, with MTBFs of typically 25000 hours. Link availability is also usually designed to be high, with an end to end availability of better than 99.7% being quite common.
The price of a remote terminal, like that of a hub station, can vary a great deal, but typical prices are in the range 3 to 8 kEuro (for a complete installation consisting of antenna, mount, ODU and IDU).
This is similar to TDM/TDMA networks.
This technique is used in networks which, unlike most interactive VSAT networks, are required to transfer large files.
When the VSAT terminal wants to transmit it requests an SCPC inbound channel over an ALOHA or Slotted ALOHA access request channel. The hub assigns a specific SCPC channel to the VSAT terminal which then has full use of that channel until it stops transmission. The SCPC channel is then allocated to the next terminal requesting a channel.
The outbound channel can also use DA/SCPC or TDMA depending on the traffic statistics.
This is similar to the hub in TDM/TDMA networks.
These are similar to the remote terminals in TDM/TDMA networks.
This is similar to TDM/TDMA networks.
Each VSAT in the network is assigned a unique pseudorandom number (PN) which is used to code and decode its transmissions. Several VSATs can transmit simultaneously on the same frequency and be separated on reception by the hub.
The outbound transmission from the hub is also usually coded in a similar way, except only a single PN code is used allowing reception by all the VSATs in the network.
CDMA is an inefficient method of using satellite capacity, however it has great resistance to external interference and generates substantially lower levels of interference than other methods. CDMA is therefore used primarily where external interference restricts the use of other solutions.
This is similar to the hub in TDM/TDMA networks.
These are similar to the remote terminals in TDM/TDMA networks.
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