Could USB ever take over PCI Post: 302866215

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Special Forums Hardware Could USB ever take over PCI Post 302866215 by lupin..the..3rd on Monday 21st of October 2013 01:44:50 PM

10-21-2013

Registered User

Quote:

Originally Posted by Lost in Cyberia

Internally speaking, for your motherboard bus. Would it be possible for a usb standard to be used in place of the PCI or PCIe bus that is now nearly standard on all PC's? If not, why wouldn't this work?

It would not work for several reasons.

Firstly USB does not support DMA transfer modes. USB operates only in PIO mode, which is crufty and old and slow. PATA, SATA, Firewire, SCSI all operate in DMA mode, which is the biggest factor for why they are so much faster interfaces for attaching hard drives than USB.

Secondly USB has very high protocol overhead and poor performance with bulk data transfers. For example, USB 2.0 has a raw bitrate of 480 Mbits/s. That is 60 MB/s. Yet anyone who's used a USB hard drive knows that even on the newest fastest computers, real world USB 2.0 transfer rates are barely over 30 MB/s. I.e. USB can barely achieve 50% efficiency. Firewire, SCSI, SATA/PATA, on the other hand, achieve 90%+ of their raw bitrate in real world performance.

Another point worth mentioning is the abysmal bus power provided by USB. Only 500 mW. It's the worst of all external peripheral busses. Firewire has been providing 3x the bus power for many years now as compared with USB. And since you're evaluating it as a replacement for PCI-E, which carries 75w, you can see that you'd need about 150 USB ports to provide the bus power of a single PCI-E slot.

Lastly USB performs very poorly at isochronous transfers. Isochronous transfers are critical for audio and video work, where precise timing is key. This is why all Pro Audio/Video gear has been on Firewire for many years, instead of USB, because even though the USB standard includes isochronous support, it's a hack, and doesn't work very well in the real world.

USB was originally designed as a replacement for the RS232 serial port, PS/2 keyboard and mouse port, and the joystick port. All very low speed serial devices. And USB works quite well for these devices. For high bitrate bulk data transfers, low latency, etc. USB is a real turd.

As for one replacing the other, if anything, its the other way round. Look at Thunderbolt. It's essentially an external PCI-E, and as you might expect, it performs leagues ahead of USB for bandwidth intensive devices like hard disks or VGA displays.

Last edited by lupin..the..3rd; 10-21-2013 at 04:15 PM..

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LEARN ABOUT REDHAT

lsusb

lsusb(8)							Linux USB Utilities							  lsusb(8)

NAME

       lsusb - list all USB devices

SYNOPSIS

       lsusb [options]

DESCRIPTION

       lsusb is a utility for displaying information about all USB buses in the system and all devices connected to them.

       To make use of all the features of this program, you need to have Linux kernel 2.3.15 or newer which supports the /proc/bus/usb interface.

OPTIONS

       -v     Tells lsusb to be verbose and display detailed information about all devices.

       -vv    Tells lsusb to be very verbose and display even more information (actually everything the PCI device is able to tell).

       -s [[<bus>]:][<devnum>]]
	      Show only devices in specified bus and devnum.

       -d [<vendor>]:[<product>]
	      Show only devices with specified vendor and product ID. Both ID's are given in hexadecimal and may be omitted.

       -p <procpath>
	      Use another path instead of /proc/bus/usb.

       -D <device>
	      Do not scan the /proc/bus/usb directory, instead display only information if the device whose device file is given.

       -t     Tells lsusb to dump the physical USB device hierarchy as a tree.

FILES

       /usr/share/hwdata/usb.ids
	      A list of all known USB ID's (vendors, products, classes, subclasses and protocols).

       /proc/bus/usb
	      An  interface  to USB devices provided by the post-2.3.15 Linux kernels. Contains per-bus subdirectories with per-device files and a
	      devices file containing a list of all USB devices.

SEE ALSO

       lspci(8)

AUTHOR

       Thomas Sailer, <sailer@ife.ee.ethz.ch>.

usbutils-0.2							 14 September 1999							  lsusb(8)