Satellite Communication Lecture 7 Two-way Interactive Communication for Fixed and Mobile Users.

Satellite CommunicationSatellite Communication

Lecture 7Lecture 7

Two-way Interactive Communication for Fixed and Mobile Users

OverviewOverview Two-way Interactive Communication for Fixed and Mobile Users

• Introduction to VSAT Networks for Interactive Applications

•Principle of Protocol Layering

• Protocols supported by VSAT Networks

• Satellite Point to Point Connectivity

• Satellite Point to Multipoint Connectivity

• VSAT Star Networks Applications Architecture

• Personal Computer Integration with the VSAT

• Operation of Multiple Access Protocols in VSATs

• Mobile Satellite Services

IntroductionIntroduction VSAT networks are composed of low-cost Earth

stations for use in a wide variety of telecommunications applications.

Unlike the point-to-multipoint systems VSATs are two-way communications installations designed to achieve interactivity over the satellite

Interconnection with various terrestrial networks is also a feature.

Internet has taken over the role of the common structure for integrating data communications for the majority of applications in information technology (IT).

This has rationalized the field to the point that a single protocol and interface standard provide almost all of what an organization needs.

Introduction (Contd.)Introduction (Contd.) The same approach works equally well for

individuals and the small office/home office (SOHO) environment.

Satellite communications technology has adapted to this new world as well.

Oddly, it was not until the early 1980s that satellite systems found a direct place in this expanding field.

The overriding principle of the VSAT is that it is a small bidirectional Earth station that delivers integrated data, voice, and video services within a package that is often cost justified when compared to terrestrial alternatives.

Introduction (Contd.)Introduction (Contd.) Today, terrestrial copper, fiber lines, data routing

and switching in conjunction with VSATs provide a fast and effective mix to advance the competitive strategy of many medium to large businesses.

VSAT networks also address the needs of small businesses and individuals.

The three classic architectures for IT networks are host-based processing (utilizing centralized large-scale computers like mainframes), peer-to-peer networks (usually employing minicomputers or large servers that are deployed at different locations to serve local requirements), and client/server networks (which tie together personal computers, servers, and peripherals using LANs and WANs).

Introduction (Contd.)Introduction (Contd.) The three classic architectures for IT networks are:

• Host-based processing (utilizing centralized large-scale computers like mainframes),

• Peer-to-peer networks (usually employing minicomputers or large servers that are deployed at different locations to serve local requirements), and

• Client/Server networks (which tie together personal computers, servers, and peripherals using LANs and WANs).

VSAT networks now address the needs of small businesses and individuals in all these areas.

Principle of Protocol LayeringPrinciple of Protocol Layering Modern data communications theory and practice Modern data communications theory and practice

is literally built upon the concept of protocol is literally built upon the concept of protocol layering, where the most basic transmission layering, where the most basic transmission requirement is at the bottom and more complex requirement is at the bottom and more complex and sophisticated features are added one on top of and sophisticated features are added one on top of each other.each other.

While this concept is abstract, it is important to While this concept is abstract, it is important to understanding how the data in a network is understanding how the data in a network is assembled, processed, and reliably transferred assembled, processed, and reliably transferred between sender and receiver.between sender and receiver.

Principle of Protocol LayeringPrinciple of Protocol Layering It has evolved over decades of telecommunications It has evolved over decades of telecommunications

development, beginning with the most simple development, beginning with the most simple voice radiotelephone network, through networks voice radiotelephone network, through networks that support national air defense, applied in that support national air defense, applied in business for large-scale data processing, and business for large-scale data processing, and evolved into the pervasive structure of the evolved into the pervasive structure of the Internet.Internet.

The layering concept is embodied in the Open The layering concept is embodied in the Open Systems Interconnection (OSI) model shown in Systems Interconnection (OSI) model shown in Figure 8.1 and contained in relevant standards of Figure 8.1 and contained in relevant standards of the International Standards Organization (ISO) and the International Standards Organization (ISO) and the ITU-Telecommunication Sector (ITU-T). the ITU-Telecommunication Sector (ITU-T).

OSI and TCP/IP (DARPA) ModelOSI and TCP/IP (DARPA) Model

Principle of Protocol LayeringPrinciple of Protocol Layering Layer 1, physical: provides the mechanism for Layer 1, physical: provides the mechanism for

transmitting raw bits over the communication transmitting raw bits over the communication medium (e.g., fiber, wireless, and satellite).medium (e.g., fiber, wireless, and satellite).

It specifies the functional, electrical, and It specifies the functional, electrical, and procedural characteristics such as signal timing, procedural characteristics such as signal timing, voltage levels, connector type, and use of pins.voltage levels, connector type, and use of pins.

The familiar RS-232 connector definition is a good The familiar RS-232 connector definition is a good example of the physical layer. example of the physical layer.

A way to look at this is that the physical layer A way to look at this is that the physical layer takes the raw bit stream at the sending end and takes the raw bit stream at the sending end and introduces it to the network. introduces it to the network.

All together, most of the investment in a satellite All together, most of the investment in a satellite network is at the physical layer.network is at the physical layer.

Principle of Protocol LayeringPrinciple of Protocol Layering Layer 2, data link: provides for the transfer of data between Layer 2, data link: provides for the transfer of data between

adjacent nodes or connection points either by a dedicated adjacent nodes or connection points either by a dedicated point-to-point line (e.g., a T1 private line or a satellite duplex point-to-point line (e.g., a T1 private line or a satellite duplex link) or a medium capable of shared bandwidth (e.g., an link) or a medium capable of shared bandwidth (e.g., an Ethernet cable or satellite TDMA channel). Ethernet cable or satellite TDMA channel).

The link layer can offer a one-to-one connection (the most The link layer can offer a one-to-one connection (the most common approach) or one-to-many delivery (associated with common approach) or one-to-many delivery (associated with broadcast or multicast).broadcast or multicast).

Layer 3, network: responsible for routing information from Layer 3, network: responsible for routing information from end to end within the network, which would consist of end to end within the network, which would consist of multiple data link paths.multiple data link paths.

This may involve decisions about the most effective route This may involve decisions about the most effective route through the point-to-point links that comprise the network. through the point-to-point links that comprise the network.

A VSAT network may serve as one of these links and hence A VSAT network may serve as one of these links and hence would have to interface properly with the network layer.would have to interface properly with the network layer.

Principle of Protocol LayeringPrinciple of Protocol Layering Layer 4, transport: provides another level of assurance that the Layer 4, transport: provides another level of assurance that the

information will properly traverse the network, fromend user to information will properly traverse the network, fromend user to end user. end user.

Two services are commonly available: connectionless, which Two services are commonly available: connectionless, which transfers packets of data, one at a time; and connection transfers packets of data, one at a time; and connection oriented, where a virtual circuit is first established before oriented, where a virtual circuit is first established before sending multiple packets that make up the entire conversation. sending multiple packets that make up the entire conversation.

The familiar TCP layer of TCP/IP provides a connection-oriented The familiar TCP layer of TCP/IP provides a connection-oriented service to computer applications.service to computer applications.

Layer 5, session: somewhat more complicated than layers 3 and Layer 5, session: somewhat more complicated than layers 3 and 4 but provided to instill yet greater degrees of reliability and 4 but provided to instill yet greater degrees of reliability and convenience of interface to applications.convenience of interface to applications.

It manages the data exchange between computer systems in an It manages the data exchange between computer systems in an orderly fashion to provide full-duplex or half-duplex orderly fashion to provide full-duplex or half-duplex conversations. conversations.

One important service is that of reestablishing the connection in One important service is that of reestablishing the connection in the event that the transport layer is interrupted for some reason.the event that the transport layer is interrupted for some reason.

Principle of Protocol LayeringPrinciple of Protocol Layering Layer 6, presentation: provides syntactic and semantic Layer 6, presentation: provides syntactic and semantic

services to the application layer above. services to the application layer above. The presentation layer is inserted to resolve the complexities The presentation layer is inserted to resolve the complexities

between transport/network layers and the more simplistic between transport/network layers and the more simplistic needs of the actual application that employs the network in needs of the actual application that employs the network in the first place.the first place.

Some specialized services like encryption and data structure Some specialized services like encryption and data structure definition are considered to be part of the presentation layer.definition are considered to be part of the presentation layer.

Interaction of the presentation layer with elements of a Interaction of the presentation layer with elements of a satellite network may cause incompatibility, requiring satellite network may cause incompatibility, requiring additional processing to be performed in Earth station additional processing to be performed in Earth station equipment or user terminals.equipment or user terminals.

Principle of Protocol LayeringPrinciple of Protocol Layering Layer 7, application: includes the actual data communication Layer 7, application: includes the actual data communication

applications that are common in open systems, such as file applications that are common in open systems, such as file transfer, virtual terminal, e-mail, and remote database transfer, virtual terminal, e-mail, and remote database access. access.

We refer to these as applications because they include not We refer to these as applications because they include not only the protocol elements that support specific types of only the protocol elements that support specific types of information but also features and facilities that ultimately information but also features and facilities that ultimately interact with the end user.interact with the end user.

Most non-expert users will not use the application layer Most non-expert users will not use the application layer directly, instead relying on specialized software within the directly, instead relying on specialized software within the computer to improve the interface and functionality.computer to improve the interface and functionality.

For example, most subscribers to on-line information services For example, most subscribers to on-line information services use the e-mail package supplied by the provider.use the e-mail package supplied by the provider.

This package, in turn, will engage layer 7 e-mail services to This package, in turn, will engage layer 7 e-mail services to do the actual function of sending and receiving message do the actual function of sending and receiving message traffic.traffic.

Principle of Protocol LayeringPrinciple of Protocol Layering The VSAT network is ideal for centralized computer networksThe VSAT network is ideal for centralized computer networks

—that is, those that employ a host computer. —that is, those that employ a host computer. The majority of such installations are assembled from The majority of such installations are assembled from

standard computer components supplied in the United States standard computer components supplied in the United States by Dell, HP, IBM, and Sun Microsystems; by Dell, HP, IBM, and Sun Microsystems;

Major European and Japanese suppliers like Bull, Siemens, Major European and Japanese suppliers like Bull, Siemens, Fujitsu, and Hitachi are in this market as well.Fujitsu, and Hitachi are in this market as well.

Following the layering concept, each computing or terminal Following the layering concept, each computing or terminal device in the network has a unique address that identifies device in the network has a unique address that identifies that device at the specified layer. that device at the specified layer.

Some examples of addressing schemes are given in Table Some examples of addressing schemes are given in Table 8.18.1

Principle of Protocol LayeringPrinciple of Protocol Layering

Protocols supported by VSAT NetworksProtocols supported by VSAT Networks A summary of the protocols in general use and their support A summary of the protocols in general use and their support

over typical VSAT networks is provided in Table 8.2.over typical VSAT networks is provided in Table 8.2. When first introduced in the 1980s, VSATs played heavily on When first introduced in the 1980s, VSATs played heavily on

the traditional IBM proprietary protocol, Systems Network the traditional IBM proprietary protocol, Systems Network Architecture (SNA), which followed the same centralized Architecture (SNA), which followed the same centralized approach as the VSAT star network.approach as the VSAT star network.

While still in existence in some legacy environments, it has While still in existence in some legacy environments, it has been replaced with the more open Internet Protocol suite been replaced with the more open Internet Protocol suite (TCP/IP). (TCP/IP).

TCP/IP has its shortcomings, which are being addressed by TCP/IP has its shortcomings, which are being addressed by standards bodies and major vendors like Cisco.standards bodies and major vendors like Cisco.

Employing TCP/IP in a private network is very straightforward Employing TCP/IP in a private network is very straightforward and is well within the means of any organization or and is well within the means of any organization or individual.individual.

Protocols supported by VSAT NetworksProtocols supported by VSAT Networks

Protocols supported by VSAT NetworksProtocols supported by VSAT Networks However, the complexity comes when an organization wishes However, the complexity comes when an organization wishes

to interconnect with the global Internet and with other to interconnect with the global Internet and with other organizations. organizations.

This is due to the somewhat complex nature of routing This is due to the somewhat complex nature of routing protocols like the Border Gateway Protocol (BGP) and a new protocols like the Border Gateway Protocol (BGP) and a new scheme called Multi Protocol Label Switching (MPLS).scheme called Multi Protocol Label Switching (MPLS).

Frame Relay has been popular in WANs for more than a Frame Relay has been popular in WANs for more than a decade, thanks to its ease of interface at the router and decade, thanks to its ease of interface at the router and availability in (and between) major countries.availability in (and between) major countries.

It is capable of near-real-time transfer and can support voice It is capable of near-real-time transfer and can support voice services. With access speeds generally available at 2 Mbps or services. With access speeds generally available at 2 Mbps or less. less.

Satellite provision of Frame Relay has been limited to point-Satellite provision of Frame Relay has been limited to point-to-point circuits as the protocol is not directly supported in to-point circuits as the protocol is not directly supported in VSATs currently on the market. VSATs currently on the market.

The best approach would be to use TCP/IP in lieu of Frame The best approach would be to use TCP/IP in lieu of Frame Relay when VSAT links are interfaced at the router. Relay when VSAT links are interfaced at the router.

Satellite Point to Point ConnectivitySatellite Point to Point Connectivity

The first satellite networks to be implemented were The first satellite networks to be implemented were employed for point-to-point connectivity to complement employed for point-to-point connectivity to complement the cross-country microwave and undersea cable links the cross-country microwave and undersea cable links of the time. of the time.

This topology remains an effective means of This topology remains an effective means of transferring information with minimum delay between transferring information with minimum delay between pairs of points. pairs of points.

As illustrated in Figure 8.2, node 1 in a point-to-point As illustrated in Figure 8.2, node 1 in a point-to-point service conducts a full-duplex conversation with node 2 service conducts a full-duplex conversation with node 2 (shown with heavy arrows), and node 3 does likewise (shown with heavy arrows), and node 3 does likewise with node 4 (shown with broken arrows). with node 4 (shown with broken arrows).

For applications such as Fixed Telephony Satellite For applications such as Fixed Telephony Satellite Services, Point-to-point connectivity between node 1 Services, Point-to-point connectivity between node 1 and node 3 can be changed on demand.and node 3 can be changed on demand.

Satellite Point to Point ConnectivitySatellite Point to Point Connectivity

Satellite Point to Multipoint ConnectivitySatellite Point to Multipoint Connectivity

The point-to-multipoint connectivity is illustrated in The point-to-multipoint connectivity is illustrated in Figure 8.4. Figure 8.4.

The thick, shaded arrows represent the digital The thick, shaded arrows represent the digital broadcast “outroute” from the hub to the remote nodes broadcast “outroute” from the hub to the remote nodes (other acceptable terms for the hub transmitted signal (other acceptable terms for the hub transmitted signal are “outbound,” “forward,” and “downstream”). are “outbound,” “forward,” and “downstream”).

It contains all hub-originated data to be delivered to the It contains all hub-originated data to be delivered to the VSATs throughout the network. VSATs throughout the network.

This transmission is received by all remotes within the This transmission is received by all remotes within the satellite footprint; however, it would typically contain satellite footprint; however, it would typically contain address information that allows only the desired address information that allows only the desired remotes to select the information destined for them. remotes to select the information destined for them.

The thin lines represent the “inroutes” from the The thin lines represent the “inroutes” from the individual remote nodes (likewise, acceptable terms individual remote nodes (likewise, acceptable terms include “inbound,” “return,” and “upstream”).include “inbound,” “return,” and “upstream”).

Satellite Point to Multipoint ConnectivitySatellite Point to Multipoint Connectivity

VSAT Star NetworksVSAT Star Networks

Organizations employ VSATs primarily as replacements Organizations employ VSATs primarily as replacements for terrestrial data networks using private lines in a for terrestrial data networks using private lines in a variety of applications, including retailing, postal and variety of applications, including retailing, postal and package delivery, automobile sales and service, package delivery, automobile sales and service, banking and finance, travel and lodging, and banking and finance, travel and lodging, and government administration and security. government administration and security.

Perhaps the first major installation was for Wal-Mart, Perhaps the first major installation was for Wal-Mart, the leading U.S. retailer with stores throughout the the leading U.S. retailer with stores throughout the United States and other locations around the world.United States and other locations around the world.

Today, there are more than 250,000 two-way VSATs Today, there are more than 250,000 two-way VSATs installed in the United States and over 600,000 installed in the United States and over 600,000 worldwide. worldwide.

Not included is the consumer VSAT designed to provide Not included is the consumer VSAT designed to provide Internet accessInternet access



VSAT technology should only be used as a supplement VSAT technology should only be used as a supplement to high-quality digital fiber optic and wireless networks to high-quality digital fiber optic and wireless networks of the world. In fact, the best strategy is often to of the world. In fact, the best strategy is often to complement the terrestrial network infrastructure with complement the terrestrial network infrastructure with VSATs so as to achieve an optimum and reliable mix.VSATs so as to achieve an optimum and reliable mix.

For example, a European company needing to connect For example, a European company needing to connect only five domestic locations to a data center would find only five domestic locations to a data center would find that conventional VSATs may not be cost-effective.that conventional VSATs may not be cost-effective.

Likewise, a large industrial organization that needs Likewise, a large industrial organization that needs high-capacity links between major sites is not a high-capacity links between major sites is not a candidate for existing VSATs. candidate for existing VSATs.

This would clearly be a better application for fiber optic This would clearly be a better application for fiber optic links, if that were feasible, or point-to-point satellite links, if that were feasible, or point-to-point satellite links.links.

VSAT Star Networks ApplicationsVSAT Star Networks Applications

Many centralized companies build their IT systems Many centralized companies build their IT systems around the host computer that is located at the around the host computer that is located at the headquarters or outsourced hosting facility. headquarters or outsourced hosting facility.

This is an ideal starting point for VSAT network adoption This is an ideal starting point for VSAT network adoption since it is centralized. since it is centralized.

Table 8.4 provides a listing of popular IT applications Table 8.4 provides a listing of popular IT applications now provided over enterprise VSAT networks.now provided over enterprise VSAT networks.

Examples include:Examples include:• Retail Marketing—Wal-Mart and JD GroupRetail Marketing—Wal-Mart and JD Group• Automotive—Daimler-Chrysler and ToyotaAutomotive—Daimler-Chrysler and Toyota• US Postal ServicesUS Postal Services• Retail Banking—BanamexRetail Banking—Banamex

The architecture of the typical VSAT star network is The architecture of the typical VSAT star network is provided in Figure 8.6provided in Figure 8.6

VSAT Star Networks ApplicationsVSAT Star Networks Applications

The architecture of the typical VSAT star network is The architecture of the typical VSAT star network is provided in Figure 8.6 that depicts how the user provided in Figure 8.6 that depicts how the user connects computers, PCs and other terminals, PBX and connects computers, PCs and other terminals, PBX and telephone systems, and video equipment used in telephone systems, and video equipment used in private broadcasting. private broadcasting.

The hub of the star is shown on the right in the form of The hub of the star is shown on the right in the form of a complete Earth station facility with a relatively large a complete Earth station facility with a relatively large antenna (typically 4.7m at Ku-band and 9m at C-band).antenna (typically 4.7m at Ku-band and 9m at C-band).

The most common implementations of the star network The most common implementations of the star network use TDMon the outroute and TDMA as well as a use TDMon the outroute and TDMA as well as a derivative called ALOHA on the inroute. derivative called ALOHA on the inroute.

VSAT Star Networks ArchitectureVSAT Star Networks Architecture

Personal Computer Integration Personal Computer Integration with the VSATwith the VSAT

The PC is the ideal direct user interface with the VSAT The PC is the ideal direct user interface with the VSAT in applications where on-line information delivery is in applications where on-line information delivery is required. required.

Typical telephone networks have a real throughput of Typical telephone networks have a real throughput of about 40 Kbps; lower rates are common in areas where about 40 Kbps; lower rates are common in areas where line quality is poor.line quality is poor.

This was once adequate for applications such as on-line This was once adequate for applications such as on-line service connection, dial-up terminal access to e-mail, service connection, dial-up terminal access to e-mail, and fax.and fax.

With the growth of the World Wide Web and the With the growth of the World Wide Web and the increasing demand for the transfer of large files for increasing demand for the transfer of large files for graphics, database, and engineering applications, the graphics, database, and engineering applications, the analog telephone network ceases to be adequate. analog telephone network ceases to be adequate.

The marketplace is provided with VSAT networks that The marketplace is provided with VSAT networks that have typical inbound throughputs in the range of 128 have typical inbound throughputs in the range of 128 Kbps to 2Mbps. Kbps to 2Mbps.


An example of using the point-to-multipoint feature and An example of using the point-to-multipoint feature and the PC is shown in Figure 8.9. the PC is shown in Figure 8.9.

The data files or streams are uplinked from a hub Earth The data files or streams are uplinked from a hub Earth station at the right. station at the right.

A public ISP or content delivery network service would A public ISP or content delivery network service would own and operate the hub. own and operate the hub.

Information is delivered to the hub over backhaul Information is delivered to the hub over backhaul circuits from one or more servers or other information circuits from one or more servers or other information sources (e.g., a stock market ticker).sources (e.g., a stock market ticker).

Subscribers purchase and install a receive-only VSAT, Subscribers purchase and install a receive-only VSAT, which need only receive the high-speed forward link which need only receive the high-speed forward link broadcast from the hub. broadcast from the hub.


Operation of Multiple Access Operation of Multiple Access Protocols in VSATsProtocols in VSATs

The inbound channel is shared by multiple The inbound channel is shared by multiple VSATs that transmit their data in bursts.VSATs that transmit their data in bursts.

Two basic multiple access methods are Two basic multiple access methods are used for this purpose:used for this purpose:• TDMATDMA• ALOHAALOHA

Operation of Multiple Access Operation of Multiple Access Protocols - TDMAProtocols - TDMA

An example of a TDMA burst time frame lasting about 45 ms An example of a TDMA burst time frame lasting about 45 ms is provided in Figure 9.4.is provided in Figure 9.4.

As applied to the inbound channel, the transmissions from As applied to the inbound channel, the transmissions from the VSATs are coordinated and highly synchronized so as to the VSATs are coordinated and highly synchronized so as to prevent overlap and a resulting loss of information.prevent overlap and a resulting loss of information.

Each station (numbered 1 through 10) is allotted a fixed Each station (numbered 1 through 10) is allotted a fixed interval of time in which to transmit data. interval of time in which to transmit data.

The frame repeats every 45 ms, producing an average delay The frame repeats every 45 ms, producing an average delay per inbound channel burst due to multiple access of 45/2 = per inbound channel burst due to multiple access of 45/2 = 22.5ms.22.5ms.

Obviously, the shorter the frame, the less the average delay.Obviously, the shorter the frame, the less the average delay.

Operation of Multiple Access Operation of Multiple Access Protocols - TDMAProtocols - TDMA

Operation of Multiple Access Operation of Multiple Access Protocols - ALOHAProtocols - ALOHA

Another approach for separating the inbound channel Another approach for separating the inbound channel transmissions in time is the ALOHA protocol. transmissions in time is the ALOHA protocol.

The scheme is simpler in that the transmissions are The scheme is simpler in that the transmissions are uncoordinated; however, the complexity occurs because uncoordinated; however, the complexity occurs because there are occasional overlaps that result in lost there are occasional overlaps that result in lost communication. communication.

This is overcome by retransmissions from the affected This is overcome by retransmissions from the affected VSATs. VSATs.

For example, a slotted ALOHA channel with three users is For example, a slotted ALOHA channel with three users is shown in Figure 9.5. shown in Figure 9.5.

Slotting refers to requiring that the ALOHA packets fall within Slotting refers to requiring that the ALOHA packets fall within timed periods, indicated by the vertical lines. timed periods, indicated by the vertical lines.

The upper three horizontal lines represent three VSAT The upper three horizontal lines represent three VSAT uplinks; the bottom timeline depicts the downlink showing uplinks; the bottom timeline depicts the downlink showing how the ALOHA packets appear after passing through the how the ALOHA packets appear after passing through the satellite repeater. satellite repeater.


Each VSAT remains in an idle state until there is data to be Each VSAT remains in an idle state until there is data to be transmitted.transmitted.

Lets assume that VSAT 1 is the first to need the channel and Lets assume that VSAT 1 is the first to need the channel and so transmits the block of data without waiting. so transmits the block of data without waiting.

VSAT 2 transmits next, independently of what happens at VSAT 2 transmits next, independently of what happens at users 1 and 3. users 1 and 3.

From the downlink timeline, we see that VSAT 1 and VSAT 2 From the downlink timeline, we see that VSAT 1 and VSAT 2 do not overlap and hence get through in the clear.do not overlap and hence get through in the clear.

The next packets from VSATs 1 and 3 have reached the The next packets from VSATs 1 and 3 have reached the satellite at approximately the same time and so have satellite at approximately the same time and so have produced a collision. produced a collision.

In the event of such a time overlap, the signals jameach In the event of such a time overlap, the signals jameach other and the information is lost (indicated by the presence other and the information is lost (indicated by the presence of a dark block in the downlink). of a dark block in the downlink).

Neither packet is received at the hub—a condition that is Neither packet is received at the hub—a condition that is inferred by these VSATs because of non-acknowledgment by inferred by these VSATs because of non-acknowledgment by the hub over the outbound channel. the hub over the outbound channel.


The way that packets are ultimately transferred is through The way that packets are ultimately transferred is through automatic retransmissions, as shown at the ends of the automatic retransmissions, as shown at the ends of the curved arrows in Figure 9.5. curved arrows in Figure 9.5.

The delay between the original and retransmitted packets is The delay between the original and retransmitted packets is selected randomly by each VSAT to reduce the possibility of selected randomly by each VSAT to reduce the possibility of a second collision. a second collision.

The result of this protocol is that the delay is as small as it The result of this protocol is that the delay is as small as it can possibly be for a packet that does not experience a can possibly be for a packet that does not experience a collision. collision.

For one that does, the delay is lengthy since it includes at For one that does, the delay is lengthy since it includes at least two round-trip delays plus the delays of the random least two round-trip delays plus the delays of the random offset as well as from processing within the hub and VSAT.offset as well as from processing within the hub and VSAT.

In an acceptable operating situation, only 1 in 10 ALOHA In an acceptable operating situation, only 1 in 10 ALOHA packets will experience a collision.packets will experience a collision.


Mobile Satellite ServicesMobile Satellite Services Historically, GEO satellites have provided most of the MSS Historically, GEO satellites have provided most of the MSS

capabilities, in terms of land, sea, and air. capabilities, in terms of land, sea, and air. The economy and simplicity of a single satellite along with The economy and simplicity of a single satellite along with

the ability to use fixed antennas on the ground have allowed the ability to use fixed antennas on the ground have allowed GEO to become most suited for these applications. GEO to become most suited for these applications.

In addition to the global capability of Inmarsat, a number of In addition to the global capability of Inmarsat, a number of GEOMSS networks capable of serving handheld satellite GEOMSS networks capable of serving handheld satellite telephones are in service.telephones are in service.

The major benefit of the lower orbits is reduced time delay The major benefit of the lower orbits is reduced time delay for voice services. for voice services.

This factor is very important in terrestrial telephone This factor is very important in terrestrial telephone networks, particularly with high-quality transmission as networks, particularly with high-quality transmission as provided through fiber optic technology.provided through fiber optic technology.

Table 11.3 provides a summary of key attributes of LEO, MEO Table 11.3 provides a summary of key attributes of LEO, MEO and GEO Satellites.and GEO Satellites.

Mobile Satellite ServicesMobile Satellite Services



Figure 11.9: A selection of user terminal equipment for use with Mobile Satellite Services


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Satellite Communication Lecture 7 Two-way Interactive Communication for Fixed and Mobile Users.

Documents

introduction vsat networks

peer networks

vsat networks satellite

clientserver networks

various terrestrial

satellite systems

satellite interconnection

large servers