[javajosh]

The serial nature of the physical connection cannot be overstated. Bits flow to your NIC one at a time.

A 1Gb/s NIC is detecting a billion wiggles in voltage per second. Structure is imposed on these wiggles in stages: first, the A/D conversion happens, making the voltage wiggles 1's and 0's, then ethernet framing, then IP packet parsing, then TCP packet/ordering, then the application handles IO (and can, and often does, define even more structure, such as HTTP). You might look up the OSI network layering model (or the OSI 7-layer burrito, as I call it).

My understanding is that MAC filtering happens after ethernet framing, and before putting into the ring via DMA, and packets failing that test do not generate interrupts. Your NIC hardware is choosing to ignore packets not addressed to it because, generally, it's pretty useless to listen in on other people's packets. Especially these days when your most likely to capture HTTPS encrypted data.

[dom0]

> A 1Gb/s NIC is detecting a billion wiggles in voltage per second.

Well technically :) ... 1000BASE-T - regular gigabit ethernet - uses a five-level modulation and four pairs in parallel (which reduces high frequency components, making gigabit over old cables possible).

[javajosh]

No, thanks for the correction! Cool! It's actually something I'd like to learn more about. I get the 5-level modulation, but how are the 4-pairs demultiplexed? The timing constraints must be exquisite.

[dom0]

A DSP in the network card does quite a bit of work to make it work: Since the pairs are used at the same time, near-end and far-end crosstalk need to be cancelled out of the signals, it also removes the currently sent signal and it's reflection (echo cancellation), since each pair can be used in both directions at the same time (!). It also adjusts delay differences between the pairs.

Gigabit Ethernet is really cool and involved tech :)

[eriknstr]

I see you in comments here on HN every so often and I enjoy reading what you have to say. I've been wondering -- and sorry for going totally off-topic here -- whether or not you picked your username inspired by the Xen hypervisor initial domain dom0.

https://wiki.xen.org/wiki/Dom0

[dom0]

Thanks and yes, I did.

[javajosh]

Excellent. You wouldn't happen to have links to someone with an oscilloscope showing what that cross-talk looks like, would you? Would be useful to visualize even at sub-GHz speeds. And also, where is that echo coming from, and how do you avoid generating microwaves if you're sending a GHz signal down a wire? (Or, perhaps you don't avoid it and that's the nature of the cross-talk?)

[dom0]

Since the same pair is used for transmission and reception at the same time, the echo is just removing what you are sending right now from what you are receiving. Similar to how you can hear / listen to someone while talking at the same time (air = ethernet pair).

> how do you avoid generating microwaves if you're sending a GHz signal down a wire?

The main spectral energy of GbE is around 125 MHz & it's harmonics (250 MHz, ...), since that's the symbol rate on the wire, but a cable is still an excellent antenna at these frequencies.

Emission is mainly avoided by using differential transmission over a twisted pair; the small loop area between the conductors minimizes emissions, and also improves rejection of outside electromagnetic noise (EMI) — an antenna always works both ways; a well-shielded mechanism will emit less and will also be less susceptible. Cables are supposed to have an outer shield (for Gigabit anyway), though it works without.

Meanwhile Ethernet avoids creating ground loops by isolating the cable on both ends with small pulse transformers (=high pass filter for the signal). The shields of the cables are also only grounded through small capacitors (=high pass filter for shield currents).

> (Or, perhaps you don't avoid it and that's the nature of the cross-talk?)

Almost! Crosstalk is mainly generate by more "intimate" coupling. Emissions means electromagnetic waves (=long range), while crosstalk in cabling and connectors comes from inductive (magnetic) and capacitive (electric field) coupling. This happens because the conductors and contacts are all very close to each other.

---

I found this video (10BASE-T): https://www.youtube.com/watch?v=i8CmibhvZ0c

[policedemil]

Thank you! This is starting to make a lot more sense! I guess what's still confusing to me has to do with the seriality (continuity?) of the process vs the OSI model, which seems like there are more discrete stages.

Does each stage of the model need to complete for the whole packet before moving on to the next stage? For example, does A/D conversion take place until all of the packet information is converted, then the whole binary blob is enframed as a header and a packet... then we filter for MAC address and move on up the stack in discrete and consecutive stages? Or are the voltages read off the line and once there is enough information to construct the header, compare it, then choose to continue reading the rest from the line or stop the A/D conversion because it is just a waste of energy? The latter makes a lot more sense to me.

EDIT: words

[mschuster91]

> Does each stage of the model need to complete for the whole packet before moving on to the next stage?

Usually, not. Massive core switches could not work if they had to wait for every frame being fully in the buffer before beginning to transmit it to the correct out port. All a core switch needs to look at is the destination MAC address.

Simple math explains why: MTU (max packet size) is usually 1500 bytes (due to most packets originating in Ethernet systems). The dstmac in the Ethernet frame is bytes 9 through 16, which means it would be an absolute waste of time to wait with forward transmission until the remaining 1484 bytes are in the buffer.

Let's calculate this with your ordinary 100 MBit/s home connection (to keep the numbers in reasonable magnitudes). 100 MBit/s means: 0.00000001 s/bit (or 0.01 us/bit, or 0.08 us/byte). With retransmit start after the first 16 bytes, the delay introduced by the equipment is 1.28 us (and needs, basically, 9 bytes of buffering capacity from start of packet to end of packet). Waiting for the full 1500 bytes would introduce 120 us (or 0.12ms) of latency, as well as require 1500 bytes of buffer during the transmission time.

[policedemil]

Excellently put! That's exactly what I was looking for! Thank you!

[mschuster91]

If you want to read further, this is a part of network delay (https://en.wikipedia.org/wiki/Network_delay). A highly interesting field.

By the way, one thing I have forgotten: an instant-forward has much less latency (obviously), but it cannot retract packets that were corrupted during receiving at the ingress port - simply because the checksum can only be calculated when the whole packet is in the buffer.

So basically you choose between safety (corrupted packets do not travel as long, because they don't even reach the final station) and latency (e.g. 10 hops a 0.12ms = 1.20ms delay on 100MBit/s), and also for the cost in buffer memory.

[dom0]

> For example, does A/D conversion take place until all of the packet information is converted, then the whole binary blob is enframed as a header and a packet

The "A/D stage" only gives off a stream of bytes, so something similar to a state machine will be used to separate it into packets that are then further processed.

That doesn't mean you have to wait for a whole packet to come through, though. E.g. a switch (or router) could start to forward the packet as soon as it sees the destination address.