6 Implementation Considerations

2Mbits/s translates into 256kBytes/s which is significantly less than PC/AT bus and hard disk bandwidth (a few MBytes/s). It is also less than 10Mbits/s Ethernet FTP bandwidth (700-900 kBytes/s). Nevertheless we do not recommend any extra processing besides decoding the frame structure at this stage.

Running data acquisition to hard disk, FTP to dedicated Ethernet, and hard disk to DAT tape all simultaneously should be possible since:

1. Nowadays ``generic'' PCs use PCI buses instead of (or in addition to) the conventional PC/AT bus and its bandwidth is in the order of tens of megabytes per second. The three simultaneous transfers (acquire, FTP, DAT) would require only 3*256=768kBytes/s, so this should leave ample room on the system bus.
2. We can easily use two separate SCSI controller cards, one for hard disks and another for DAT tape. This ensures that there will be no contention on the SCSI bus and that DAT operations will not hamper writing to the hard disk.
3. Linux disk buffer cache effectively prevents fetching newly-acquired data files from disk, since they will already be in cache main memory. Thus the hard disk bandwidth will be mostly needed for only writing new data in. If DAT tapes are written continuously in the background (and not only during LOS), this will require reading oldest blocks from disk at the same time while writing new data.
4. Using a 100Mbits/s-capable network card such as 3C595 will allow increase FTP throughput up to 3--5MBytes/s without changing any software, provided that GSE computers can handle 100BaseT twisted pair 100Mbits/s Ethernet.

We propose using a simple shift register based design which will use the standard PC DMA controller to create a ring buffer in main memory of 128kBytes. This size would suffice for recording 0.5 seconds of 2Mbits/s data without software intervention and this alone may be sufficient to allow for software-only buffer collect. The card can be programmed for different frame lengths but it will ``hunt'' for the sync pattern semi-permanently programmed in firmware.