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USB and FireWire
By admin | May 18, 2007
FireWire
USB was originally seen as a complement to FireWire (IEEE 1394), which was designed as a high-speed serial bus which could efficiently interconnect peripherals such as hard disks, audio interfaces, and video equipment. USB originally operated at a far lower data rate and used much simpler hardware, and was suitable for small peripherals such as keyboards and mice.
However, Intel was not interested in paying the near-dollar license fee to add an IEEE 1394 subsystem to their board. The fee was reduced to a flat 25 cents, but Intel prefered to push for its own USB 2.0 standard. As a result, they were rarely provided as standard equipment on computers other than Apple Macintosh computers (Apple owns rights to the FireWire standard), and peripheral manufacturers offered many more USB devices. Moreover, USB 2.0 Hi-Speed reached a performance level sufficient for consumer equipment while retaining compatibility with older devices. An example of how the popularity of USB displaced FireWire in a commercial device is the Apple iPod. It was originally released with a FireWire connector, which was eventually modified to allow for both USB and FireWire connections when the product was released for Windows. The iPod now relies solely on USB for data and only allows a FireWire connection to charge the battery.
Today, USB Hi-Speed is rapidly replacing FireWire in consumer products. FireWire retains its popularity in many professional settings, where it is used for audio and video transfer, and data storage.
Technical differences
The most significant technical differences between FireWire and USB include the following:
- USB networks use a tiered-star topology, while FireWire networks use a repeater-based topology.
- USB uses a “speak-when-spoken-to” protocol; peripherals cannot communicate with the host unless the host specifically requests communication. A FireWire device can communicate with any other node at any time, subject to network conditions.
- A USB network relies on a single host at the top of the tree to control the network. In a FireWire network, any capable node can control the network.
These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video.
USB 2.0 Hi-Speed vs FireWire
The signaling rate of USB 2.0 Hi-Speed mode is 480 megabits per second, while the signaling rate of FireWire 400 (IEEE 1394a) is 393.216 Mbit/s [6]. USB requires more host processing power than FireWire due to the need for the host to provide the arbitration and scheduling of transactions. USB transfer rates are theoretically higher than FireWire due to the need for FireWire devices to arbitrate for bus access. A single FireWire device may achieve a transfer rate for FireWire 400 as high as 41 MB/s, while for USB 2.0 the rate can theoretically be 55 MB/s (for a single device). In a multi-device environment FireWire rapidly loses ground to USB: FireWire’s mixed speed networks and long connection chains dramatically affect its performance.
The peer-to-peer nature of FireWire requires devices to arbitrate, which means a FireWire bus must wait until a given signal has propagated to all devices on the bus. The more devices on the bus, the lower is its peak performance. Conversely, for USB the maximum timing model is fixed and is limited only by the host-device branch (not the entire network). Furthermore, the host-centric nature of USB allows the host to allocate more bandwidth to high priority devices instead of forcing them to compete for bandwidth as in Firewire.
Despite all this and despite USB’s theoretically higher speed, in real life benchmarks the actual speed of FireWire hard drives nearly always beats USB 2 hard drives by a significant margin (for example[7]). In addition to this some operating systems take a conservative approach to scheduling transactions and limit the number of transfers per frame, reducing the maximum transfers from, say, the theoretical 13 per frame to 10 or 9.
However, on Bare Feats, the Mac only USB 2.0 vs. FireWire speed comparison, the poster notes the measured PC speed of USB 2.0 instead of Mac “The Windows PC implementation of USB 2.0 puts the Mac to shame. Today we tested the same USB 2.0 drive/enclosure on a Windows PC (3GHz Pentium 4) with built-in USB 2.0 on the motherboard, similar to Apple’s approach. We measured 33MB/s READ and 27MB/s WRITE.”
In 2003, FireWire was updated with the IEEE 1394b specification. This provides a new mode called S800, which operates at 786.432 Mbit/s. S800 requires a new physical layer, but S800 nodes can be connected to existing FireWire 1394a ports, just as USB Hi-Speed nodes will operate with older full-speed hosts. However, unlike USB Hi-Speed systems, which can change the speeds on each branch, a 1394a device on a 1394b system requires all devices to fall to 1394a speeds. IEEE 1394b also provides rates up to approximately 3.2 Gbit/s; however, the higher rates use special physical layers which are incompatible with 1394a devices.
Source: http://en.wikipedia.org/wiki/Usb
Topics: Computers & Software |