The HIPPI-SONET gateway developed by LANL for the Casa testbed addressed the issues associated with interconnecting islands of HIPPI-based local area networks with wide area SONET links, without use of a wide area ATM switching (Figure 4-1D). Major aspects of this gateway's operation have already been discussed in earlier sections on switching and striping in the testbeds. The striping of HIPPI data across multiple 155 Mbps SONET channels played a central role in the gateway design, allowing incremental amounts of bandwidth to be allocated to a HIPPI connection rather than just the full 1.2 Gbps available on the SONET trunks. While the gateways did not perform switching directly, they were designed to maximize the use of local HIPPI switches for wide area switching among the testbed sites, as discussed in the Switching section above.
A key difference from the Vistanet NTA approach was local termination of HIPPI connections by the Casa gateway, with a special gateway-gateway flow control mechanism used to prevent overrunning gateway buffers, and reliance on TCP for end-to-end recovery when a packet was discarded by a gateway due to destination host blocking. Another important difference was the use of forward error correction to correct single-bit errors, reducing the frequency with which hosts needed to retransmit packets to achieve reliable end-to-end operation.
The gateways were installed at Casa sites and used both to support Casa applications and for network studies. Transfer rates of 790 Mbps were measured using HIPPI testers at endpoints, and throughputs of 550 Mbps and 580 Mbps were measured when using TCP/IP and UDP/IP respectively on Cray computers at network endpoints.