Part VII

Part VII VSAT – Very Small Aperture Terminal Introduction VSAT/WLL Implementation of VSAT Access Control Protocols Delay Considerations 1.

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3. 4. 5.

Introduction • VSAT = Very Small Aperture Terminal • Early Earth Stations in commercial systems were very large and expensive (about 30 m). • Need to make system more affordable to end user. • Increased transmit power from satellite. • Higher frequencies • Smaller Earth Station antenna size required.

Large Antenna Systems 

Large antennas are usually implemented using a symmetrical configuration,

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The feed can either be in front of the antenna (a front-fed design) or behind the antenna, as in Cassegrain or Gregorian designs. These different approaches may be axially symmetric or offset. A common break point in the design of antennas is at a main reflector diameter of about 100 wavelengths.

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For ease of construction, with the feed on the boresight axis.

If the diameter is larger than this, the additional cost of a Cassegrain or Gregorian design is more Also the increased gain (up to 1 dB) that can be achieved by shaping the reflectors.

VSAT Earth stations with antenna aperture diameters less than 100 wavelengths were called very small aperture terminals (VSAT).  As the size reduced, the term VSAT was coined and then USAT (ultra small aperture terminal). 

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VSAT antennas are also much larger than the ultimate USAT

Large Antenna Systems Cassegrain and Gregorian antennas require a sub reflector with a minimum diameter of ~ 10 wavelengths.  If the main reflector is less than 100 wavelengths in diameter, the sub reflector becomes an appreciable fraction of the main reflector diameter and causes significant blockage and scattering problems. 

Figure shows a typical VSAT antenna on the roof of a commercial building

VSAT 

The standard VSAT antennas are not as small as the Ku-band direct-to-home (DTH) antennas used for direct broadcast satellite television reception, 

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Typically 0.5-0.8 m in diameter.

DBS-TV satellite use very powerful transponders, typically 160-240 W compared to 20 to 50 W of Ku-band satellites used for VSAT service. A handheld satellite telephone as used in Iridium, Globalstar, New ICO, and other mobile satellite service (MSS) systems, have an omnidirectional antenna.

VSAT 

The size of the VSAT antenna is a key factor in making the service both economically attractive to the user and environmentally acceptable to the community, 

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A careful balance has to be drawn between     

The underlying concept behind most VSAT systems is to bring telecommunications service directly to the end user without any intermediate distribution hierarchy.  Traffic from individual users was bundled together into larger groups and 

Carried over trunk transmission lines via terrestrial microwave systems, satellite systems, or optical fiber cables, before being divided up (demultiplexed) into smaller traffic streams and redistributed to the users at the far end.

VSAT 

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In many regions of the world, the potential users are either widely distributed or the existing telecommunications infrastructure lacks the capacity to expand quickly to meet the demand for new users. This situation applies to most developing countries and, 

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In many cases, network implementations have been adopted.

Geostationary satellites allied to microwave cellular technologies have been used to bypass completely the traditional expansion of analog telephony. 

One such solution is wireless local loop (WLL) coupled with VSAT distribution architectures.

Satellite transponder loading, Transmitted power (both up and down), VSAT antenna off-axis emission (for considerations), Clear sky performance, and Especially for Ku-band frequencies and above 

VSAT 

It places server restrictions on the end-to-end system design.

interference

Availability during impaired propagation conditions (i.e., during rain).

VSAT This is still the most economical transmission architecture for point-to-point communications when the services are being brought into areas with relatively high concentrations of users.  Such conditions do not always apply, 

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VSAT networks take advantage of the wide area broadcast capabilities of GEO satellites.

VSAT/WLL

VSAT/WLL The geostationary satellite is used to link a large number of VSATs with the main switching center in a large city.  Each VSAT acts as the link to the local switching center in the village or rural community, with the final mile of the telephony link being carried over a wireless local loop 

VSAT/WLL 

The VSAT/WLL concept usually has an optimum range of user densities where the economics are most favorable.

VSAT/WLL 

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Approximate economic break points in the implementation choices for serving new regions with different population densities. Physical distances, major transportation routes, and geographic barriers, as well as the individual country’s demographics and political influences, can alter the break points Can be affected by   

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local topography, availability of optical fibers in the country’s telecommunications network, significant transportation routes such as a major rail systems,

Allows a lower cost optical fiber to be laid alongside the railroad tracks or right-of-way.

VSAT/WLL 

VSAT networks allow multimedia traffic to be brought directly to the end user, but generally handle only small traffic streams 

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Sometimes as the equivalent of one voice circuit.

The traffic stream is also usually intermittent in nature: 

User accesses the satellite in a demand assigned multiple access (DAMA) mode whenever a message is to be sent and receives a short reply in due course.

VSAT/WLL This is typical in a point of sale (POS) VSAT system that is used to transmit credit card information at a petrol pump or general store.  Information about the sale and the customer’s credit is sent to a central computer facility, and an authorization or denial is received in response.  The interaction between the VSAT and the main hub earth station in the POS transaction is completely automatic and transparent to the user. 

VSAT/WLL  Most

VSAT networks do not generate enough traffic to justify a dedicated satellite.  Many do not even have enough traffic at any given instant to fill one satellite transponder.  Most VSAT networks are designed around the use of leased transponders, in the case of a large network, or  A fractional transponder lease for a medium to small network.

Implementation of VSAT Networks 

One Way Implementation

There are three basic implementations of any telecommunications service: One-way Implementation  Spilt-two-way Implementation (sometimes referred to as split-IP, when referring to Internet traffic, since the outbound and inbound channels are routed over different systems).  Two-way Implementation 

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two-way implementation is further divided into two basic network architectures: 

Star and Mesh Networks

One Way Implementation  

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This is the mode of a satellite used in the broadcast satellite service (BSS). The introduction of digital technology allows the provider and user much greater flexibility in the operation of a broadcast network. By means of software in the user terminals, different parts of the downlink can be accessed by different subscribers according to the programs ordered from the supplier (and paid for by the user). 

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One Way Implementation Schematic of a broadcast satellite service coverage region in which smaller, narrowcasting groups exist within the broader coverage area.  The master control station sends encoded signals within the broadcast stream that enables certain users to have access to particular channel groupings according to the subscriber’s choice. 

This form of channel selection is called narrowcasting.

There can be many narrowcasting groups within a larger broadcasting area.

Split-Two-Way (Spilt IP) Implementation 

This implementation is used when there is no normal return channel. 

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For example, with Ku-band broadcast satellite service (BSS) systems that carry Internet traffic.

The relatively high capacity downlink stream is not complemented by an uplink capability from the user terminal. If the BSS downlink is used as the download channel from an Internet service provider, 

Split-Two-Way (Spilt IP) Implementation

The only option the user has for a return link is via another telecommunications channel, such as a standard telephone line.

The Internet protocol (IP) is therefore split between a satellite downlink (outbound) channel and a terrestrial telephone (inbound, or return) channel; 

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Hence the term split IP for this implementation.

The advantage of this approach is that the VSAT terminal does not require a transmit capability, which significantly reduces its cost and complexity. The disadvantage is that the telephone line connection must usually be through a modem, with a bit rate generally restricted to 56 kbps or less.

Two –Way Implementation – Architecture I

Two –Way Implementation In this case, a return link is designed into the service so that two-way communications can be set up over the same satellite, from the hub to the user and from the user back to the hub.  The VSAT/WLL implementation is a two-way service between the hub (in this case the satellite gateway) and any VSAT terminal.  The architecture selected is the key to the economics of two-way connections: 

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In this network architecture, all of the traffic is routed via the master control station, or hub. If a VSAT wishes to communicate with another VSAT, they have to go via the hub, thus necessitating a “double hop” link via the satellite. Since all of the traffic radiates at one time or another from the hub. This architecture is referred to as a Star network.

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It can be either Mesh or Star.

Two –Way Implementation – Architecture II

Master Control Station

In this network architecture, each of the VSATs has the ability to communicate directly with any of the other VSATs. Since the traffic can go to or from any VSAT, this architecture is referred to as a Mesh network. It will still be necessary to have network control and the duties of the hub can either be handled by one of the VSATs or the master control station functions can be shared among the VSATs.

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Topology of a Star VSAT network Viewed from the satellite’s perspective  VSAT communications links are routed via the satellite to the hub in all cases. 

Topology of a Mesh VSAT network Viewed from the satellite’s perspective.  All of the VSATs communicate directly to each other via the satellite without passing through a larger master control station (hub). 

Two –Way Implementation 

Initially, the most common VSAT architectures were Star networks 

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Since very low receive G/T ratio of the VSATs, coupled with their limited transmit EIRP, was compensated for by using a large hub with high G/T and EIRP.

The cost of the hub was therefore quite high. This led to the concept of a shared hub, where several networks operate through one main hub. 

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The International Standards Organization has specified the open systems interconnection (ISO/OSI) that mandates a seven-layer model for a data communication system as shown in Figure.

A high-speed terrestrial data link is required between the host computers of the networks and the hub, which increases the cost of the network. Rather than have one large hub for all of the VSAT networks sharing the same satellite, the overall network evolved to allow each sub network to have its own hub. In this way the host computer of each VSAT network can be colocated with its own hub. 

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The difficulty with this approach for large countries is that the host computers for the small VSAT networks are rarely close to the hub.

Access Control Protocols 

Two –Way Implementation

Thus eliminating the cost of the interconnection between the hub earth station and the computer controlling the service offered through the VSAT network.

Whether the hub is shared/dedicated or the VSAT is connected to a single user/local area network (LAN) with multiple users sharing access through an Ethernet connection on the other. 

An access control protocol is needed.

Protocol architecture of a Star VSAT. VSAT networks are normally maintained as independent, private networks, with the packetization handled at the user interface units of the VSAT terminals. The satellite access protocol (with a larger time-out window) is handled in the routing and switching, and network management functions. Protocol conversion is handled by the gateway equipment.

Access Control Protocols 

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A satellite communications link occupies primarily the physical layer, which is the place where the bits are carried between the terminals. A VSAT network must have terminal controllers at each end of the link and these occupy the network and link layers. The network control center typically controls the system and is responsible for the remaining layers. It is very useful as a conceptual model which identifies functions that must be performed somewhere in every data communication network.

Access Control Protocols 

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Most data communication networks use some form of packet transmission, in which blocks of data are tagged with an address, error control parity bits, and other useful information before transmission. The receiving end of a link checks arriving packets for errors. 

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Then sends an acknowledgement signal (ACK) that the packet was received correctly, or a not acknowledge signal (NAK) that tells the transmit end to resend a particular packet because the packet had an error.

Some systems do not send acknowledgements, only NAK signals to request a retransmission of a packet with an error, since this speeds up data transmission. This is the error control method used in the Internet protocol TCP/IP. 

Generically, such systems are known as automatic repeat request (ARQ) systems.

Access Control Protocols 

The ISO-OSI stack was initially developed for terrestrial communications systems. 

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For this reason, the protocols that implement the functions of each layer were designed for use in terrestrial circuits with low delay and low bit error rate (BER), with very high performance levels.

Many of the early protocols had a connection time-out of a few milliseconds. 

Access Control Protocols

If no reply was received from the recipient in this interval, transmissions ceased. Similarly, an errored signal received from the source or an intervening node would trigger an automatic error recovery sequence. 

Frame relay and ATM (asynchronous transfer mode) systems flag the error but continue the flow of information (continuous transmission ARQ). 

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More errors that occur in the link,  

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For example, in ARQ approach, on detecting an error in a packet, immediately requests a retransmission and halts further transmissions until the corrected packet is received.

In both cases, the errored transmission must be corrected and suitable buffers at the receiver end (or intermediate node) used to restore the packets in their original order. Need many retransmission of packets, Reduces the effective data throughput rate of the link becomes.

The potential for delay and propagation induced errors are therefore critical design elements in digital VSAT connections

Delay Considerations 

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A typical slant range to a GEO satellite is 39,000 km. The one-way delay over such a GEO link (earth station to satellite to earth station) is 2 × (range/velocity) = 260 ms. The one-way delay in a typical 4000-km transcontinental link via fiber-optic cable is a little over 13 ms. Neither example includes processing delay 

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Delay Considerations The time out element of a protocol is often referred to as the window of the connection.  As long as the window is “open,” communications can continue without interruption. 

(e.g., source coding and/or compression, channel coding, baseband processing in the switching elements, frame length)

Can add several tens or hundreds of milliseconds.

Delay Considerations Clearly, satellite systems have to operate satisfactorily, and seamlessly, with existing terrestrial networks.  There are two ways to make terrestrial protocols work with a satellite link. 

First, the protocols can be changed so that the timeout window is well in excess of 260 ms;  Second, the satellite element of the packet network can be configured to exist as a separate sub network within the global packet network.

Delay Considerations 

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In practice, both solutions are adopted

That is, the protocol will transmit only 7 unacknowledged frames before it stops transmissions; 

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A typical data link layer protocol used in a low delay, terrestrial link employs modulo-8 operation.

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This leads to the low throughput particularly for GEO satellite links.

High level data link control (HDLC) protocol used in layer 2 for satellite systems employs a modulo-128 operation. That is, 127 frames may be sent without receiving any acknowledgements before it stops the transmissions.

Moving from modulo - 8 to modulo - 128 operation significantly increases the “window” size permitted for the link layer control. 

This concept is called as protocol emulation.

References 

Timothy Pratt, and Charles Bostian, “Satellite Communications”, Wiley Publications

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Dennis Roddy, “Satellite Communications”, Tata Mcgraw Hill Publications