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IP Transport over Satellite
|The Satellite Advantage
As IP traffic volume increases, so does competitive pressure among Internet service offerings. As a result, service providers continually seek ways to reduce costs, improve functionality, and ensure quality of service (QoS) for Internet- based applications. These issues are easily addressed with satellite links. Customers quickly realized the value of transporting Internet-related traffic via satellite. As a result, the market for satellite-based Internet link capacity quadrupled in 1998 (per Internet Via Satellite 99 Report, http://www.spotbeam.com/mansum99.htm).Distance Insensitive Service.
Internet traffic routed via terrestrial networks is often subject to bandwidth limitations and latency due to congested routers and Internet backbones. IP datagrams queue at congested routers, where they contend for router and backbone capacity. They are then routed based on order in which they arrived in the queue or on other criteria, such as the Type of Service (ToS) required. As distance increases between the source and destination address, so does the number of routing points, connection latency, and the likelihood of congestion.
Alternatively, a geo-synchronous earth orbit (GEO) satellite parked at 22,500 miles altitude can provide simultaneous direct access to large geographic areas— as large as one-third of the earth’s surface, depending on the type of beam used. Satellite exploits beam coverage to deliver IP data in a single hop (with minimal end-to-end routing) to one or more locations within the beam
Terrestrial network topology does not lend itself easily to point-to-multipoint
(i.e., multicast) distribution of IP traffic across the Internet. In terrestrial networks, multicast traffic requires a simultaneous delivery of the same information to a number of hosts along a delivery tree (to individual destinations of a class D address) and substantial replication of datagrams. In addition, routers must periodically track each host’s membership activity in order to avoid unnecessary datagram replication and delivery to inactive hosts that taxes network capacity.
In contrast, IP data can be multicast simultaneously
to any number of points within the coverage area with a single transmission,
rather than as multiple point- to-point transmissions on terrestrial networks.
By delivering IP multicast traffic via satellite, complex routing algorithms
and management of delivery trees are virtually eliminated. (Figure 1).
Satellites, on the other hand, adjust easily to asymmetrical traffic. Their bandwidth is equally available to all the traffic within a beam, independent of which direction traffic flows.
Quality of Service
The lowest, or Physical Layer, is the medium that transports electrical or optical signals— a cable, a wireless link, or an optical fiber link. The next layer is the Link Layer, which is commonly a LAN, WAN, or serial data line and uses its own protocols depending on the specific medium used. The next higher layer is the Network Layer. IP operates in this layer and transports datagrams across the lower layers to deliver them to the destination. However, IP does not provide a guaranteed delivery of datagrams. For this reason it is called connectionless. TCP/IP was designed to work transparently across virtually any type of Physical or Link Layer media.
For a reliable point-to-point connection, the Internet requires a protocol such as the connection-oriented Transmission Control Protocol (TCP). TCP operates in the fourth layer, the Transport Layer. TCP achieves a reliable connection by sending and tracking all datagrams, re-transmitting lost datagrams, and passing datagrams in the proper order at the destination to the Application. TCP compensates for latency differences between the various paths over which the individual IP datagrams may have traversed the network. To verify proper delivery of all datagrams, TCP sends receipt acknowledgements from the destination back to the source. TCP also has the means to prevent and recover from network congestion. However, all these make a TCP connection susceptible to the roundtrip time (RTT) of the network.
Network Congestion Control
A connection’s maximum throughput is limited by
the supported data rate of a link. However, actual throughput is further
limited by TCP’s Congestion Window. TCP increments the value of
the window by the size of each datagram sent. The window value is decremented
each time the originating address receives a confirmation that a datagram
has been delivered. The Slow-Start and
Terrestrial networks normally work with a high signal-to-noise
ratio (SNR) and virtual absence of bit errors. Thus, TCP was not equipped
with any special algorithm for datagram corruption due to bit errors