Cable Modem setup help

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Cable Modem setup help

Postby arlojanis » Thu Jun 02, 2011 2:47 pm

I have a cable modem with USB and cat 5 ports. Which connection is faster and gives the best results?
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Re: Cable Modem setup help

Postby tvsjr » Thu Jun 02, 2011 6:01 pm

Ethernet. The USB port is probably a USB-Ethernet adapter in the modem, anyway.
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Re: Cable Modem setup help

Postby Wowbagger » Fri Jun 03, 2011 3:24 am

Shortest answer: it won't matter much, but Ethernet.

Longer answer:
First of all, your actual throughput on a cable modem is likely in the neighborhood of 20Mbit/sec. The modem likely has 100Base-T Ethernet and USB 2.0 High speed (480 MBit/sec). Both are faster than the DOCSIS link, so both are unlikely to be a limiting factor. Now, you might think USB would be better, but you would be wrong.

USB is a HORRIBLE protocol. It was designed for one thing only: to be cheap to implement. First of all, unlike Ethernet, which is full duplex (you can send and receive at the same time), USB is simplex - you are either sending XOR receiving. So there goes OVER half of your throughput right there (send + receive + turn-around time). Second, USB is like a directed net: NOTHING happens unless the master (your computer) commands it. Everything in USB happens in a "frame", which happens once a millisecond. Devices like your modem, which may or may not have something to send, have to be polled by your computer. It looks something like this:
Start of frame:
Computer: OK, modem - do you have anything to say?
Modem: Yes.
Computer: OK, modem - send what you have.
Modem: OK, here are the 5 IP packets I received in the past millisecond.
Computer: OK, here's the 10 packet I want you to send
Computer: [twiddles thumbs for the rest of the millisecond]

So unlike Ethernet, where the modem can send a packet as soon as it has it, USB builds in a one millisecond latency. If a new packet comes in after the modem has been told to send it's data, it will have to wait until the next frame. Moreover, your computer's CPU has to be involved in that polling, setting up the USB request blocks (URBs) in the buffer chain for the EHCI USB interface. Granted, for a modern CPU that's not a great burden, but it still diverts the CPU from doing other things. Ethernet will almost ALWAYS have the ability to get the packets from the wire to memory and vise versa all by itself, freeing the CPU. Also, in USB you have to break everything down into URBs, which are only a few tens of bytes. A normal IP packet on DOCSIS can be 1500 bytes, so that is MANY URBs (plus the overhead of wrapping IP into USB).

That's also why Firewire400 can kick USB's ass, take it's lunch money, give it a swirly, and leave it sobbing in the bathroom when it comes to moving freight off a hard drive, even though Firewire has a nominally slower transfer rate - it is a MUCH better protocol, it is full duplex, it allows the hardware to do more of the work, etc.

USB is cheap and simple. But it isn't the best.
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