On Jul 23, 2004, at 01:35 am, Jeff Walther wrote:
If the cable is good, then the problem could be your transceiver, although I've never seen one fail.
I have a DaynaPort twisted pair transceiver that only works intermittently.
I'm assuming the hub is on before you power up the PM. If the hub is off, for some reason the Mac will decide no ENet was available at boot time. This is what leads to the three boot circus when connecting two (older) Macs by cross over cable. But you'll get the same effect if the hub isn't powered up before the Mac powers up.
I might be doin' the grandmother 'n' sucking eggs thing here, but I'll explain why:
Ethernet (802.x protocol) is a 2-way system that relies on a certain of feedback information in order to function. It is in effect a 'smart' system. A Router, Switch or Hub will monitor all it's ports for attached devices constantly. When you plug in a device it senses that and immediately interrogates it to discover it's hardware MAC (note this is nothing to do with Apple ;-) ) address. This address is what all ethernet devices communicate by, regardless of any overlying protocols such as TCP/IP, IPX/SPX or whatever. The addresses used for those are intrinsically linked to the MAC address. On discovery of the MAC address the switching device will then know a device exists there and to send packets there. In the case of a Hub is just blindly sends all packets to all devices in a hope one will respond. A Switch, Bridge or Router however has a memory with which it sets up a table that allows it to send data only to the machine it is destined for. The computers part in this is that it responds to interrogations and at the same time makes a note of the switching devices MAC address (they have them as well as computers - it's usually on a label on the device - everything connected to ethernet is required to have a unique MAC address) and fire packets at it when it needs to. You will notice that if you leave an ethernet link idle it will flash every so often. This is the switching device checking that line for the presence of another device.
Not quite. Switches don't poll. When it "hears" a MAC address on a port it remembers it as being there. If it doesn't know where to send a packet it sends it to all ports. The problem with a polling approach is with large networks. Every switch would have to learn every MAC address over the entire network. This would cause a lot of traffic periodically and will fill up the MAC table. Normally these are large so it's not likely it would overflow but it would slow down MAC lookup unnecessarily. It's far simpler to wait till a device responds and remember the address as needed.
A switch may or may not have a MAC address but it isn't used when doing ordinary transfers from one computer to another (or other equipment). The Switches MAC address is only used in communicating directly to the Switch if it has the intelligence to.
If your hub is not on when you boot the Mac it won't find it, because it obviously won't respond to interrogation from the Mac. Mac OS thusly assumes you are not connected to a network. It is possible to make a Mac recognise an ethernet link after boot, I have found, by opening the 'Appletalk' panel and switching to Ethernet manually. Mac OS defaults to the Printer Port (for Localtalk) if no ethernet link is found.
The Mac is sensing the link condition to determine if the connection is valid. In 10BaseT this is the presence of a signal on the received line. It doesn't require any other communication. I've seen the Mac treat a line as okay when only the receive line was working (a bad cable had the transmit line open).
--
Clark Martin
Redwood City, CA, USA
Macintosh / Internet Consulting
"I'm a designated driver on the Information Super Highway"
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