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Higher Layer Protocols (TCP/IP) over Satellites - Tutorial










Transport control protocol/Internet protocol (TCP/IP) is the protocol suite on which the Internet is based. TCP/IP is now very widely deployed. However, it was developed without taking into consideration its performance over very high speed (fiber optic) links or long-delay (satellite) links, with the result that efforts are now underway to remedy some of the shortcomings that are evident on links that have large bandwidth-delay product. At the present time, considerable low bit rate TCP/IP commercial traffic is being carried over GEO satellites. With suitable TCP/IP performance enhancements, data rates in excess of 500 Mbps have been demonstrated over GEO located satellites.


Internet Protocol

IP is a network layer protocol whose function is to permit data traffic to flow seamlessly between different types of transport mechanisms (Ethernet, ATM, Frame Relay, etc). IP resides in the terminal devices and in routers which function as switches in the network, routing datagrams (packets) towards their destination based on an address field contained in the datagram.

The routers in the network are required to translate between different addressing schemes. For example, local area networks operating on the IEEE 802 LAN standard address attached devices with 16 or 48-bit binary addresses. An X.25 public packet-switching network, on the other hand, uses 12-digit decimal addresses. IP provides a global addressing scheme and a directory service. The current version (Ipv4) of IP has address space limitations that threaten to inhibit the growth of the Internet, with the result that a new version (Ipv6) is under development (Stallings 1997).

Fig. 4.21. The protocol stack for a network running on TCP/IP (Stallings 1997).


Routers are also required to handle differences in the size of packets that can be carried on different networks. X.25 networks commonly operate with packets having a maximum size of 1,000 bytes-in contrast to Ethernet, which permits packets of 1,500 bytes. To overcome these differences datagrams may have to be broken into smaller packets (this is known as fragmentation) and reassembled when they reach their destination.

The IP protocol does not guarantee delivery, or that packets will arrive in the proper sequence. (Packets can get out of order since they may follow different paths through the network, thereby encountering different amounts of delay.) Packets can fail to be delivered for several reasons. If the network becomes congested one or more routers may become overloaded and their buffers may begin to overflow. Rather than simply discarding all newly arriving packets, the routers are programmed discard packets in a random fashion to prevent buffer overflow. This is best implemented in a "fair" way so that the data stream having the largest volume suffers the largest number of dropped packets. The links in the network are not error free so that it is possible for a packet's address to become corrupted making the packet undeliverable. Again it must be discarded lest the network become clogged with undeliverable traffic. In sum, IP is engineered to make a best effort to deliver a message but does not guarantee to do so.


Transport Control Protocol (TCP)

It is the function of the TCP protocol residing in the end devices (computers)-see Figure 4.21-to ensure the proper delivery of a complete message. TCP achieves this by assigning each byte of information a unique sequence number. The receiver keeps track of these sequence numbers and sends acknowledgements (ACKS) to indicate that it has received each datagram up to a particular byte number.


Performance of TCP Over Satellite

Satellites are an attractive option for carrying Internet and other IP traffic to the many locations across the globe where terrestrial options are limited or price prohibitive. However, data networking over satellites is faced with overcoming the large latency and high bit error rates typical of satellite communications, as well as the asymmetric bandwidth design of most satellite networks.

Communications over geosynchronous satellites, orbiting at an altitude of 22,300 miles, have round-trip times of approximately 540 ms, an order of magnitude larger than terrestrial networks. The journey through the atmosphere can also introduce bit errors into the data stream. These factors, combined with back channel bandwidth which is typically much smaller than that available on the forward channel, reduce the effectiveness of TCP which is optimized for short hops over low-loss cable or fiber. Satellite conditions adversely interact with a number of elements of the TCP architecture, including its window sizing, congestion avoidance algorithms, and data acknowledgment mechanisms, which combine to severely constrict the data throughput rate that can be achieved over satellite links.


  Window Size:

TCP utilizes a sliding window mechanism to limit the amount of data in flight. When the window becomes full, the sender stops transmitting until it receives new acknowledgments. Over satellite networks, where acknowledgments are slow to return, the TCP window size generally sets a hard limit on the maximum throughput rate. The minimum window size needed to fully utilize an error-free link, known as the "bandwidth-delay product," is 100 KB for a T1 satellite link and 675 KB for a 10 Mbps link. However, many implementations of TCP are limited to a maximum window size of 64 KB and most operating systems use a default window size of only 8 KB, imposing a maximum throughput rate over a satellite link of only 128 Kbps per connection, regardless of the bandwidth available.
Congestion Avoidance:

In order to avoid the possibility of congestive network meltdown, TCP assumes that all data loss is caused by congestion and responds by reducing the transmission rate. However, over satellite links, TCP misinterprets the long round-trip time and bit errors as congestion and responds inappropriately. Similarly, the TCP "Slow Start" algorithm, which over the terrestrial infrastructure prevents new connections from flooding an already congested network, forces an excessively long ramp-up for each new connection over satellite. While these congestion avoidance mechanisms are vital in routed environments, they are ill-suited to single-path satellite links.

Data Acknowledgements:

The simple, heuristic data acknowledgment scheme used by TCP does not adapt well to long latency or highly asymmetric bandwidth conditions. To provide reliable data transmission, the TCP receiver constantly sends acknowledgments for the data received back to the sender. The sender does not assume any data is lost or corrupted until a multiple of the round-trip time has passed without receiving an acknowledgment. This algorithm does not respond well over satellite networks where the round-trip time is long and error rates are high. Further, this constant stream of acknowledgments wastes precious back channel bandwidth and if the back channel is small, the return of the acknowledgments to the sender can become the system bottleneck.


The better satellite IP implementations take the above issues into account in maximising throughput and the end user experience.


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