Why do some companies list their IEEE 1394 drives throughput at 400Mbps or 50MBps?
The maximum theoretical throughput of the FireWire IEEE 1394 serial standard is 400 Megabits (Mbps) or 50 Megabytes (MBps) per second. However, it is misleading when vendors state their devices, particularly hard drives can achieve those speeds. First, IEEE 1394 is like any other serial bus in that there is an approximate 8 to 10% overhead in transmission. Furthermore, all current IEEE 1394 hard drives use ATA (EIDE) mechanisms. A bridge chip translates IEEE 1394 to IDE commands and there is more overhead associated in this translation. Newer bridge chip technologies like the OXFW911 are much faster than the previous generations, but they still have some overhead. Additionally, even if IEEE 1394 and the IDE bridge chips could provide 50MBps speeds, drive mechanisms within the various EIDE specifications (ie. Ultra ATA 100) don’t sustain data rates saturating the bus. Only in burst mode do these drives approach their interface maximums, their sustained rates are considerably slower.
