4.2.1 ATM
Experimental Wide area ATM switches included Bellcore's Sunshine switch used in Aurora, AT&T's Xunet switch used in Blanca, and a Fujitsu Fetex 150 prototype switch used in Vistanet. In addition, desk area ATM switching was developed as part of MIT's VuNet work in Aurora, and Fore Systems ATM switches were used in experiments at the University of Illinois in the Blanca testbed.
The Sunshine switch had been designed and simulated by Bellcore prior to the start of the testbed project, and was intended as a prototype for carrier wide area ATM switching. The Aurora testbed provided an opportunity for Bellcore to build and experiment with the Sunshine design, with switch prototypes completed and deployed within Aurora during 1993. The design was based on a Batcher-Banyan switching fabric which switched internal 155 Mbps ATM cell streams. To provide the 622 Mbps connections required for the testbed, a set of four STS-3c channels could be defined to be a trunk group and treated by the switch as a single 622 Mbps VC. This was accomplished by input and output port controller modules which interfaced the switching fabric to external OC-12 SONET interfaces. Due to the subsequent discovery by Bellcore of significant skew being introduced on the individual 155 Mbps channels by external SONET equipment, the STS-3c channels sent on the OC-12 SONET link were instead simply treated as individual VCs by the switch in later experiments. This is discussed further in the Transmission section.
The Sunshine switch also provided an important vehicle for exploring ATM Quality-of-Service (QoS) issues through implementation by Bellcore of a second-generation output port controller design later in the project (discussed further in the Network Management section). Prototype versions of the Sunshine switch were deployed by Bellcore at the Upenn, MIT, and Bellcore sites in Aurora.
From a switching perspective, lessons learned from the Sunshine development experience were that using fixed size cells significantly reduces switching fabric cost and the cost is independent of cell size, but if processors are used for output port queuing or other cell-handling functions the small ATM cell size does impose a significant cost penalty. While all-hardware port designs would eliminate this penalty, evolving network-layer QoS and congestion control algorithms may in fact benefit significantly from processor-based designs [2].
Wide area ATM switching in the Blanca testbed was accomplished using Xunet experimental switches developed by AT&T Bell Labs. A version of this switch known as Xunet-II had been developed prior to the start of the testbed project to provide wide area switching of 45 Mbps ATM links. For its use in Blanca, AT&T developed 622 Mbps interfaces and associated internal handling of the higher rate data streams, with this version of the switch called Xunet-III.
The Xunet switch design evolved from AT&T's earlier Datakit data switch design. It was based on a shared bus architecture and used a modular design to attach line interfacing and queuing cards to the bus structure. The switch used a modified version of the standard ATM cell format, a non-SONET proprietary trunk transmission protocol, and a direct proprietary optical link on a user-side line card of the switch to send and receive ATM cells with an ATM-HIPPI adapter.
The Xunet-III version of the switch was deployed by AT&T at Madison, Chicago, and Champaign for experimentation by Blanca researchers. A separate control computer connected to the switch architecture provided call setup, teardown and other control functions, and provided a software platform for network control algorithm investigations carried out by Blanca researchers and discussed in other sections.
The Fetex 150 prototype switch used in Vistanet supported standard ATM cells and 622 Mbps OC-12c user ports as part of a modular, general purpose wide area carrier switch architecture. This switch was notable both for its direct support of synchronous 622 Mbps data streams and for the fact that it was located in a telephone company central office as part of the Vistanet topology. This was the first instance of a central office broadband ATM switch, and provided an opportunity for the participating testbed carriers to gain hands-on experience in an environment involving real user data traffic.
The switch was connected over a 2.5 Gbps OC-48 SONET trunk to GTE's SONET crossconnect switch located across town, and within the central office user area to two Vistanet sites on the UNC campus. Thus Vistanet switching topology differed from the other testbeds on two significant counts: the metropolitan area switching was performed in a telephone company central office, and the switching between the two UNC campus sites was also done by the central office switch.
A prestandard SVC signaling protocol was used in the Fetex 150, allowing VCs to be dynamically established for experiments through the use of special communication channels. Traffic management was not implemented in the switch, however, and only minimal cell buffering was provided. Cell loss was avoided by relying on testbed experimenters to control their traffic through the use of acknowledgments at higher protocol layers.
Local and Desk Area ATM Switching
As part of its investigations into ATM networking, MIT developed what they termed a Desk Area Network (DAN) which interconnected workstations and peripheral I/O devices through a small local network. The DAN architecture was based on the use of small crossbar switches supporting 700 Mbps ports and a non-standard 56-byte ATM cell. The prototypes developed for the testbed activity consisted of 4 or 6-port switches and 500 Mbps links, where the latter used the HP Glink transmission protocol over optical fiber and other media. The switches were designed to simplify the hardware interfaces required to connect devices to the DAN switching fabric, with as much functionality as possible pushed out to host software.
The other local area ATM activity carried out in the testbeds made use of Fore Systems switches at UIUC., where they were used as part of their network control software investigations. Fore Systems in fact came into existence as a direct result of the testbed initiative-its founders were part of the Nectar testbed research group at CMU at the beginning of the project, and were stimulated to form a startup company addressing high speed networking needs as a result of their testbed involvement. The company has gone on to become a major ATM switch provider for both local and wide area environments.