Introduction



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Introduction

A key requirement in interconnecting networks is to minimize the impact on both networks. A proposal to interconnect networks should not require changes to the two networks. Two machines using incompatible protocols are to communicate. The basic question is how to interconnect two separate networks. Two possible alternative are identified.

The first alternative is not to interconnect two separate networks but to provide a direct user to user CCSDS AOS interface. In this case space craft operations may be connect to the earth stations through a leased line. In this case data is transported back to JPL by treating CCSDS AOS packets merely as a bit stream. This is currently the case for non-manned missions using the CCSDS version for unmanned missions. However, unmanned missions use only a fraction of the bandwidth necessary for manned missions. The cost of the leased lines that connect space craft operations with the earth stations is a major economic factor in considering other alternatives.

In the second alternative ATM is considered in order to interoperate with other scientific users not centrally located at space craft operations. ATM also yields bandwidth efficiency in the use of the leased circuits that connect the earth stations with space craft operations.

In CCSDS AOS and ATM both the sending and receiving network supports virtual circuit service packets. Since the ATM network resolves congestion problems by discarding packets the communications processors at the earth stations and space craft operations need to be prepared to do their own end to end error checking. This requires buffering at both space craft operations and the earth stations. ATM packets are very small. When we wish to send a large packet through a small packet size network fragmentation of the packets is necessary. Packet loss in the ATM network due to congestion may then be a problem.

Address translation is also an issue between the two protocols. In ATM virtual path identifiers can be used to set up a group routing of the individual ATM virtual circuits to the centralized location (JPL) where the individual virtual circuits are routed.

ATM and CCSDS AOS are communications protocols that both provide integrated digital services and thus there are many similarities in the protocols [7] [8]. However, while the physical layer in ATM is primarily optical cable with a low probability of bit error the physical medium of CCSDS AOS, the space channel, has a higher probability of bit errors. The physical layer of CCSDS AOS is made into a low bit error rate through the virtual channel data unit (CVCDUs). Both protocols are capable of providing integrated digital services they operate over vastely different physical channels. Both protocols adhere to the layering principles of the OSI model as is shown in Figure 6, which compares the layers of CCSDS AOS and ATM.

  
Figure 6: CCSDS AOS and ATM Protocol Layers



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Next: Necessary Service Mappings Up: Mapping CCSDS AOS Previous: Mapping CCSDS AOS




Thu Jun 8 12:28:11 BST 1995