4.3.3 Seamless ATM
A topic of major interest to ATM developers is seamless ATM, the interworking of distinct ATM networks without using an explicit internetworking layer such as IP, and an initial exploration of this topic was done by MIT in the Aurora testbed (Figure 4-1A). They developed devices to interconnect their Desk Area Vunet ATM network with both the Aurora wide area ATM/SONET network and a local area ATM network.
The differences which needed to be resolved for interworking included the underlying physical transmission technologies, cell formats, and VC signaling conventions. VuNet used HP Glink single-mode fiber transmission technology operating at 500 Mbps, in contrast to the use of striped 155 Mbps SONET channels in the wide area ATM network. The VuNet ATM cell format was chosen to be 56 Bytes to simplify cell processing, and a choice was also made not to use the HEC header check defined as part of the standard ATM format, since it was felt that the relatively low error rates experienced in a DAN environment did not warrant the additional complexity associated with HEC processing. VC control signaling was also handled differently than in the Aurora Sunshine switch.
Two different devices were developed by the VuNet group for ATM-ATM interworking. The first, called AVlink, interfaced VuNet directly to a single 155 Mbps channel of the wide area ATM/SONET network. The AVlink device converted the 56-byte VuNet cells to standard 53-byte cells and conversely, computed the HEC field in the cell header, and mapped cells to and from the Glink and SONET physical transmission formats. This was accomplished using a relatively simple design, with a resulting average latency of 4.6 microseconds and transfer rate of 150 Mbps.
A second device called Zebra was also developed to interface VuNet to a Digital Equipment AN2 ATM switch, which effectively represented the presence of a LAN between VuNet and the Aurora wide area ATM network. The latter was connected to a 622 Mbps SONET port on the AN2 switch, which was in turn connected to VuNet through the Zebra board installed on the AN2. Zebra, like AVlink, provided the required cell format mapping and Glink interfacing. More significantly, the use of Zebra with the AN2 provided an opportunity to explore solutions to the general striping problem, since VuNet supported 500 Mbps Glink channels while the wide area path consisted of multiple 155 Mbps SONET channels. This was carried out using source and destination VuNet hosts and an end-to-end path which included a loopback point within the wide area network .