4.1.2 Other Transmission Technologies
Wide Area All-optical Transmission
While four of the testbeds advanced the understanding and use of SONET transmission equipment, the Blanca testbed explored the use of all-optical wide area transmission through the use of optical fiber amplifiers over a total distance of nearly 300 miles (Figure 4-4). Eighteen amplifiers were installed on a Madison-Chicago-Champaign path, with optical termination equipment connected to Xunet switches at each of the three sites. The all-optical links provided protocol-transparent transmission, allowing the proprietary transmission format used by the Xunet switches to be replaced at a later date by SONET formats without changes to the link installations.
Figure 4-4. All-Optical Transmission
The link provided a 622 Mbps channel for use by the testbed in both directions between Madison and Champaign, and in addition was shared with other AT&T operational services using other capacity available on the optical link. This was the first deployment of optical fiber amplifiers as repeaters on a service basis by a carrier. The testbed-based link deployment allowed data to be collected by AT&T on link dispersion, signal-to-noise ratio, and bit error rates.
Most of the testbeds used HIPPI for their local distribution technology. This was primarily due to the fact that, at the beginning of the project in 1990, HIPPI was the only standards track gigabit-range technology on the scene. HIPPI originated in the late 1980s at LANL as a solution to local high speed interconnection of supercomputers, providing an 800 Mbps full duplex point-to-point connection over parallel copper wire. By 1990 it was in the process of becoming an ANSI standard and was also beginning to be supported by industry, most notably Cray Research (whose supercomputers at the time dominated the computation centers participating in the testbeds) and Network Systems Corporation (NSC). The latter company had won a contract to develop a HIPPI-based device for workstation interconnection for one of the testbeds, and was also developing a local area HIPPI switch.
HIPPI thus provided a practical solution to local interconnection for the testbeds, but was significantly limited by the relatively short 25 meter distance allowed by its parallel copper wire technology and by the size, complexity and cost of the associated host interface. Two 100-wire cables were required for full duplex data transmission, resulting in a large host connector footprint and relatively inflexible and bulky cables. Nevertheless, because of its growing support for supercomputers and a commercially available switch, it became the dominant interconnection technology within supercomputer centers in the early 1990s, and by extension to the workstations used in conjunction with the supercomputers within the testbeds. The Aurora testbed was the exception, since its research was focused on workstations and ATM in both local and wide area environments, and also was not dependent on the constraints imposed by supercomputers.
Local Area Fiber Technologies
HIPPI's 25-meter distance constraint and the needs of the testbeds inspired the development of an optical fiber-based HIPPI extender by industry. The first extender product became available from Broadband Communication Products (BCP) through their involvement with testbed researchers in the early years of the project.
The HIPPI extender allowed a full duplex HIPPI connection to be extended across a pair of single-mode optical fibers over distances of up to 10 km. In addition to relaxing the constraints on interconnecting HIPPI devices within a campus environment, these extenders were able to support cross-town metropolitan area connections for some of the testbeds as a workaround while waiting on the development of SONET-related equipment. While the direct extension of HIPPI flow control signaling across longer distances can result in significant throughput degradation, the extender's usefulness nevertheless resulted in the definition of an optical fiber serial HIPPI standard.
Two other fiber-based technologies used for local interconnection in the testbeds were HP's Glink and IBM's Orbit, both of which operated at a rate of 1 Gbps. Glink was used to provide point-to-point links between hosts and local switches in local Aurora configurations. Orbit is a buffered ring LAN technology developed by IBM, and was used in the Planet/Orbit networking portion of Aurora.
An alternative to HIPPI, Fibre Channel (FC), was also being developed for ANSI standardization at the time the project began, and was targeted for use as the local area networking technology in the Blanca testbed. FC provided the same 800 Mbps user rate as HIPPI, but with much smaller host connector profiles and longer operating distances through its direct use of optical fiber instead of HIPPI's parallel copper wires. The realization of a well-defined FC standard and useable products was significantly delayed, however, due at least in part to FC's large functional scope and complexity. When by spring 1992 FC products were still not available for 800 Mbps operation, Blanca researchers decided to use HIPPI and so FC was not used within the testbeds.