Network Interface components

[phray]

While researching the MSI Neo2 Platinum, I noticed something I don't quite understand about network interfaces. I don't know how to flat out ask the question, so lemme give you some background.

The most recent High End System Buyer's Guide says:
"the board [the MSI Neo2 Platinum] also has the advantage of the on-chip nVidia Gigabit LAN, which moves high-speed LAN off the PCI bus."

However, a review at PCPerspectives says:
"MSI has included two Gigabit LAN connections, but neither of them courtesy of the on-board NVIDIA MAC. They are instead powered by the Marvell and Realtek chipsets. "

So I checked w/ MSI. And found this:
"IEEE 802.3 NVIDIA MAC for 1000BASE-T
<snip>
- 1 LAN supports 10/100/1000 Fast Ethernet by Marvell 88E1111 PHY
- 1 LAN supports 10/100/1000 Fast Ethernet by Realtek 8110S (1000Mbps)"

Now, that tells me that PCPerspective was wrong in saying that they're not powered by the nVidia MAC. (Of course I knew it couldn't be right if it differed from what Anandtech had said!)

But it leads me to the question, what is this PHY thingy and what does it do? I know that MAC means "Media Access Control" and that every machine has a unique MAC address. (and yes I know they can be spoofed, etc...) But what does the nVidia MAC actually DO? Does this motherboard have the nVidia hardware firewall that I've read so much about? And more importantly, does it offload the some of the strain from the CPU?

If i were to guess, I'd say the MAC is the brain and does all the 'heavy lifting', while the PHY is just the RJ45 connector. But thats just my guess. Can anyone explain this?

 

[Peter]

The MAC is the engine, and it needs a PHYsical interface chip. The latter has no brain, just does the conversion from the packet frame data to analog ethernet line signals (simply spoken), while the MAC is the engine that takes data out of system RAM, forms them into proper ethernet packets and sends them out, listens to traffic incoming from the PHY and pulls stuff targeting its MAC address into system RAM. (And it does lots of misc housekeeping tasks as well.)

Today's chipsets contain a MAC usually, but need a PHY companion somewhere on the mainboard. This has several reasons - placement of analog components, not having to make a mixed digital/analog chip, etc. etc.

Most standalone PCI ethernet chips however ARE mixed signal digital/analog chips that unify the MAC and PHY into one piece of silicon. The Realtek 8110S is no exception.

 

[FrankSchwab]

In brief terms, the MAC is the digital process that deals with talking on the wire (checking to see if it can transmit, checking for collisions, retransmitting in case of collision, etc).

The PHY is the analog component that actually puts the signal on the wire.

A 10/100 signal comes off the wire as a balanced pair of signals, IIRC in the 0 - 0.7 V range. They go through a transformer/filter that takes care of noise, spikes, common-mode voltages, etc, and into the PHY as low-level analog signals. The PHY does all of the work of converting the signals to digital, detecting the beginning of a packet, detecting collisions, grouping bits into bytes (actually, normally nibbles), and sending them to the MAC.

The MAC checks the destination address on the front of the packet to see if it cares about the packet, checks the received data to verify it's correct via the CRC appended to the packet, and does something with it (DMA's it into processor memory, holds onto it until the processor requests it, etc).

On the way out, the processor sends a packet to the MAC, which generally buffers the packet, calculates and appends the CRC to the end of the packet, checks the physical line to see if it can transmit (not necessary in full-duplex mode), when clear it starts clocking data out to the PHY 4 bits at a time. In the case of a collision, the MAC waits a little bit, then attempts to retransmit the packet.

The PHY starts receiving nibbles from the MAC, and starts shifting them out onto the wire one by one as a balanced pair of low level signals. If it detects a collision, it sends a signal back to the MAC.

Way more information than you were probably interested in.


/frank

 

[phray]

Excellent, thats pretty much what I had guessed. So in this case, it does use the nVidia MAC AND it has the additional Realtek solution as well. The Realtek has the PHY built in, but the nVidia doesn't so they had to get the PHY from Marvell.

I dig it. Thanks for the explination!

 

[Mark R]

Just a follow up question?

Where are the transformers and other magnetics? I don't see any on modern motherboards. Are they now integrated into the sockets?

 

[Lynx516]

I dont know any motherboard that has had a ransformer on. There is no need for magnetics (apart from the occasional inductor) anywhere but the PSU.

 

[PottedMeat]

Originally posted by: Mark R
Just a follow up question?

Where are the transformers and other magnetics? I don't see any on modern motherboards. Are they now integrated into the sockets?

yeah its happening alot now, i noticed it when i was trying to get some transformers for a CS8900A

Halo Electronics FastJack with Integrated Magnetics:

http://www.haloelectronics.com/pdf/fastjack-10base-t.pdf