On Wednesday 22 June 2005 20:36, David P. Reed wrote: > Though GigE sounds like a good idea to pursue, has anyone thought about > using 2 or more USB 2 interfaces as an alternative? I don't know what > the typical controller interfaces can handle, but I see three on my > AMD64-based laptop, plus a firewire interface - that's potentially about > 1.6 Gb/sec combined, but I only have a 100 Mb/s Ethernet interface.
Typically all the USB ports on a PC are tied to a single host controller channel, although there are exceptions. The entire bus is limited in bandwidth, and adding another channel into a single controller isn't going to help the throughput. The biggest problem with GigE is the potential for interference in RF spectra that we might want to receive. As to throughput, you are still limited in IO bandwidth at the PC level, particularly in the HD interface. While the interface itself might be substantially faster than the necessary throughput (ATA133 supposedly can transfer 133MB/s, SATA 150 can do 150MB/s) in reality the HD platter transfer rate is typically much lower: my ATA100 drive in my laptop, for instance: [EMAIL PROTECTED] ~]# hdparm -t /dev/hda /dev/hda: Timing buffered disk reads: 70 MB in 3.05 seconds = 22.95 MB/sec [EMAIL PROTECTED] ~]# hdparm -i /dev/hda This means that there is no way I can store data at the current USB rates. My faster server could: [EMAIL PROTECTED] ~]# hdparm -t /dev/hda /dev/hda: Timing buffered disk reads: 172 MB in 3.02 seconds = 57.04 MB/sec [EMAIL PROTECTED] ~]# But even then it would be hard pressed to keep up for long. The server has a fast ATA133 Maxtor 250GB drive as /dev/hda, and is one of the faster ones around. EVEN THEN I could not keep up with full-bore GigE. Next, to fully take advantage of GigE, you have to have multiple PCI buses in your system, if you have 32 bit PCI especially. 32 bit PCI at 33MHz rate is capable of 133MB/s max; GigE at full rate is very close to this. Many if not most GigE cards have a 64 bit PCI-X interface; 66MHz or better 133MHz PCI-X at 64 bits is capable of up to ten times the 32 bit PCI rate, and is typically only available in servers. Next, as the USRP USB page states, finding a GigE chip that can be handled with the fab process being used is a challenge, and finding one with open docs (from the hardware design point of view) is even harder. Prototyping BGA package based designs is exceeding difficult unless you have very special equipment or produce high enough volume to justify the expense of the necessary equipment (like motherboard manufacturers can afford to have, for instance). GigE would likely more than double the cost. > The other thought is that if you are considering putting the peripheral > remotely close to an outdoor antenna, perhaps an optical fiber solution > would be better - why risk frying your CPU or your body? At PARI, we have gone the gamut from thinking about a full fledged embedded PC in the feedbox to bringing the RF down fiber. RF down fiber has its own challenges; fiber transmitters and receivers that can handle the low signal levels in a linear fashion are very expensive. But having all the frequency hash of the PC and USRP at the feedbox (close to the focal plane!) presents its own challenge, not the least of which is lightning protection. The fiber itself is cheap; troll eBay looking for single mode fiber pigtails in the 225 foot length range; they can be had very inexpensively: I have seen 225 foot 8 fiber pigtails with one end terminated in the $25 neighborhood. The equipment to drive this fiber is where you will run into cost. We at PARI have 12 strands of good quality singlemode fiber run to each of the two 26m antennas' feedboxes. I get around 1dB of loss for the shorter of the two runs, and around 2dB for the longer. With transmit lasers approaching 30mW, I may very well have to put fiber attenuators in the line to prevent the receiver from being blinded. See, lasers can't just be turned down; they won't 'lase' if they're not fed enough power. But then receivers have their own distortion curves to worry about, and too much light is just as bad as not enough light. In many ways bringing RF down is easier, and in many other ways putting the PC and USRP in the feedbox is easier, and like all engineering decisions, there are significant tradeoffs. Fiber transceivers for ethernet, even in the singlemode variety, are much less expensive and much easier to deal with than RF transmitters/receivers. But the RF transmitter doesn't need as much shielding to keep its hash out of the front-end. -- Lamar Owen Director of Information Technology Pisgah Astronomical Research Institute 1 PARI Drive Rosman, NC 28772 (828)862-5554 www.pari.edu _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
