20-12-2009, 11:58 AM
INTRODUCTION
Like in most other areas, space research is also moving into large-scale simulations using powerful computers. In fact, given the high cost, and often the impracticability of conducting live experiments, space research has moved into computer-based simulation long before most other streams. This idea of taking the Internet to the space comes from the need for a low cost, high reliability inter-planetary network. When countries started sending probes into space, each used a unique set of protocols to communicate with earth. This was done using Deep Space Network (DSN). Since communication was done with common ground station, need for a common protocol increased with time. Taking the Internet to space is the offshoot of this need for standardization. The Inter-Planetary Network (IPN), a part of Jet Propulsion Laboratory (JPL), is managing this program. Satellite images of earth have been easily and commercially available for sometime now. These images are archived and distributed in various formats. Satellite images use file formats that can save additional information used for computation. Each satellite used customized systems for communication. For the sake of interpretability with other systems, a set of protocols were designed, specified and implemented. These protocols are currently being tested and they are called Space Communications Protocols Standards (SCPS). These are the protocols used for space communication. Earlier, satellites used customized system for communication using DSN.
" With cooperation among nations and agencies, interpretability becomes important.
" NASA, US Defense department and National Security Agency of the US, jointly designed, specified, implemented and are testing a set of protocols called Space Communication Protocols Standards (SCPS. There were problems in integrating the networks with DSN. The reasons which caused the evolution of standard protocol are: 1. Probes of each country used unique set of protocol. Since the probes communicated with same ground-station, need for a common protocol increased.
2. Cooperation among agencies and countries is increasing. But since each country used different standards, this made the problem worse.
3. Need for a low-cost, high-reliability inter-planetary network.
4. The existing Internet protocols were not sufficient for the Space communication. The error rate and round trip delay were the main factors. WHY NOT TCP/IP? When a common standard was required for space communication, the first plan was to use the existing TCP/IP stack protocol. But it was not practical. A key limitation with TCP in high bit error networks is the lack of error correction capabilities. Since TCP cannot correct bit errors, if even a single bit within a packet is corrupted in transit, the receiver will discard the entire packet. This turns bit errors into packet loss. In addition, TCP can recover from the loss of only one packet per round trip. If the network's round-trip time is 500 milliseconds, then TCP can tolerate only one packet loss per 500 milliseconds. To illustrate the implication of this limitation, consider what happens if this network has a bit error rate of 10-5: TCP can send data at a maximum rate of 200 kbps, no matter how fast the physical network is! Another limitation of TCP is the strength of its data corruption protection. TCP uses a relatively weak checksum scheme to detect bit errors in each packet. The approach fails to detect bit errors relatively often at high bit error rates, allowing corrupted data to be delivered to the application undetected. It has been calculated that Window Based TCP is not suitable for RTT = 40 min 20B/s throughput on 1Mb/s link
