There are a few important aspects of data transfer. Security (pros vs cons), power, and what most people care about, transfer of data speed. I will only focus on the only reasonable connections for the iPod. Bluetooth, USB and Firewire 400 - IEEE 1394. (I will include USB 1.1 where I can, since you can use USB 1.1, though i wouldn't recommend it.)
For starters, all Input/Output (I/O) ports have limitations. Despite facts given by manufacturers for optimal potential, they can be tested in real world scenarios and an average can be found. These averages are what I will be using for comparison since the difference on the speeds is small between the top and low end of computers made in the last 3-4 years.
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Security
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This really only applies to WiFi transmissions (Bluetooth and wireless network). Besides the flaw in iPod OS in allowing access to the files themselves to take them on and off the iPod, there really is no worry of someone hacking your iPod if theyre not connected to the wire on the other end.
Source =
Tom's Hardware
Hackers have found many flaws with Bluetooth devices. As these devices gain in popularity, the public needs to be made aware of vulnerability issues with the various Bluetooth devices such as phones, PDAs and wireless headsets.
Three of the most interesting attacks were Bluesnarfing, Bluetracking and Bluebugging. Bluesnarfing is attacking the Bluetooth device, usually a phone, to rip out information. Hackers can obtain phonebooks, calendars and stored SMS messages.
Bluetracking is tracking a person's movement by tracking their Bluetooth device. All Bluetooth devices have a unique address, similar to a MAC address on computer network cards. By using special sensors or antennas you can see where a particular Bluetooth device pops up and record a person's movement.
(^cool story on Hacker's device)
Even if DRM security increased, access to your files would still exist. Then it's only a matter of another Scandinavian hacking the new DRM, or being able to stream media with a WAP application and something like wiretap to record.
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Power
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Unless you can harness the power of the sun, there is really no way to send power successfully through the air without large possibilities of shocking yourself. So that limits us to USB and Firewire.
Max Power Capabilities:
USB 1.1 = 100 mA/3.3V
USB 2.0 = 500 mA/5V
Firewire (400) = 1.25A/12V
Under 5m of cable, USB has a max capability of 127 devices, and firewire can support 63 devices. Really not important, Voltage potential will remain constant, if anything less resistance (from less devices) should result in more power per device. But it would be hard to tell, firewire is 6-pin and USB only 4-pin. so there could be more resistance per device, but I diverge.
USB 2.0 will charge the battery, but it takes about an hour longer compared to firewire. A USB car charger (12v) will charge faster than the computer USB bus itself.
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Speed
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Now to the heart of it. The real question, how much time will it take to put a song on there? Filling an iPod depends on capacity and available memory. So I will compare speeds for one song (about 3Mbytes).
USB:
Source =
Adaptec
USB limitations.... it has a maximum bandwidth limit of 12 mbytes per second (1.5Mbytes/s for USB 1.1). Furthermore, the same inherent simple design that allows for it to be low-cost also makes it "host centric" -- meaning that the host CPU must initiate every transaction occurring between every device. The practical ramifications of this are that once you've added more than a handful of devices on the USB bus, bus latencies increase, efficiency goes down and real sustained throughput is only a fraction of the 12 Mbytes/second upper bound.
FireWire:
Unlike USB, Firewire supports direct memory access (DMA) -- meaning that it does not require host intervention with every transaction. So when connecting one or many devices Firewire speeds still average about 21Mbytes per second
Bluetooth:
Source :
CSR
Bluetooth EDR (the newest Bluetooth standard) offers a maximum data transfer rate of 3 Megabits per second (Mbps) - compared to the current 1Mbps for standard rate Bluetooth.
meaning Bluetooth EDR has a max of 3/8 of a Mbyte (8 bits per byte)
SO we have:
one song (4Mbytes) / Firewire (@21Mbytes/s) = 0.19sec
one song (4Mbytes) / USB 2.0 (@ 12Mbytes/s) = 0.33sec
one song (4Mbytes) / USB 1.1 (@ 1.5Mbytes) = 2.7sec
one song (4Mbytes) / BluetoothEDR (@ 0.38Mbytes) = 10.7sec
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Conclusion
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Bluetooth just doesn't fit the bill for a device like the iPod. It's perfect for phones, for short distance, small data transfer between personal communication devices. It would be good for sharing contacts and calendars between iPods, and maybe even games. But its just too limited for the epitome of mp3 players.
For USB in Windows by default the song must first be copied to Page/Swap file from the disk, then from the Page/Swap file to the iPod. Where as in Unix (ie. OS X), the file is loaded in memory and the address directly sent to the iPod to access from memory, just like Firewire, but controlled by the computer, not device.
I think its fairly obvious that Firewire is the way to go on a storage device, like the iPod. Lots of data and power needed for music on the go. While USB 2.0 isn't much slower, I would recommend using the Firewire wall plug in to charge.