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Network Protocol Layer

The physical-level protocol used by this network is the standard Synchronous Optical Network (SONET) optical data transmission protocol. This is the level at which the ACTS satellite operates, i.e., it knows nothing of protocols above the raw SONET data stream.

Figure 5: The protocol stack for our ATM-based TCP/IP network.  
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In order to support standard higher-level (IP) networking protocols, we installed an Asynchronous Transfer Mode (ATM) network on top of the SONET layer. ATM is a packet-switched protocol, similar to frame relay, which is capable of bandwidths exceeding OC-48 (2 Gbit/sec) [2,10]. Data is transferred in 53-byte ``cells'', each containing a 5 byte header and 48 bytes of payload (see Figure 5). The transfer of cells is performed by hardware switches, which have been installed throughout the network in California and Hawaii. The ground network in California includes three ATM switches running at OC-3 (155 Mbit/sec) speeds: one at each end-point (Caltech and JPL), and an intermediate switch belonging to the CalREN (California Research and Education Network) project of Pacific Bell . The ground network in Hawaii includes several ATM switches: one at each end-point (Honolulu, the Keck Telescope dome, and the Keck Headquarters in Waimea; see Figure 3b.), and a number of intermediate switches belonging to GTE Hawaiian Telephone. Because of the initial use of microwave antennae in Hawaii, this portion of the network, and therefore the end-to-end network, was limited to DS-3 (45 Mbit/sec) speeds. The ATM switches provide a point-to-point network connecting the computer in the remote observing room at Caltech with the instrument control computer at the Keck Telescope in Hawaii. We have established Permanent Virtual Circuits (PVCs) in each of the switches, which direct the cells between the end-points of the network. Several vendors have supplied the ATM switches for this network, including FORE Systems , Newbridge Networks , and SynOptics . Fore ATM Network Interface Cards (NICs) are used to interface the Sun SPARCstation 20/51 workstations at each end of the network. This mixed vendor environment has been a stringent test of the compatibility among vendors in the relatively new ATM environment. Although we have encountered several problems associated with interoperability issues, none have been extremely serious, and the ATM vendors and telephone companies have been extremely helpful in attempting to diagnose and solve ATM-level problems. We have also witnessed the increasing popularity of ATM even in the mere 2-year lifetime of this project, and expect to see more widespread use of this protocol at the WAN and enterprise network levels. (For more information on ATM, see the Cell Relay web site .)

next up previous
Next: User Protocol Layer Up: NETWORK ARCHITECTURE Previous: Physical Layer
Patrick Shopbell