How network devices are sending the actual packets?

[NetworkingGeek1]

Hello community. I have some general question regarding on how to network devices are sending the packets (frames, segments, etc.). I'll try to clarify what I mean: let's say switch received 2 frames with size of each packet being 1518 bytes (1500 + 14 Ethernet header + 4 FCS). These two packets are to be sent out from the same egress interface. How switch will send these packets out of the same egress interface? The minimum physical data is bit, so will switch first send all bits of one packet or it will send like this: 1 bit from one packet, another bit from another packet and so on? I think switch (or any other network device) should first send all bits of the same packet and then start to send all bits of another packet and so on. But I want to make sure, that I'm correct in my assumptions. Thank you.

[MHM Cisco World]

Sorry. 

Thanks 

MHM

[NetworkingGeek1]

Another question would be regarding twisted pair. If bits are sent in parallel over the twisted pair, are these bits of the same frame?  Meaning, frame's bits are just splitted between wires? Or each frame is being sent over the dedicated wire?

[Joseph W. Doherty]

Transmission, and reception, is frame by frame.

Twisted pair, depending on modulation technique might send parts of a frame in parallel and/or send multiple bits during one signal transition time.

[NetworkingGeek1]

Hello @Joseph W. Doherty  Thanks for the reply. Can you please clarify this part?

"Twisted pair, depending on modulation technique might send parts of a frame in parallel and/or send multiple bits during one signal transition time."

Below question is only relevant for 1000BASE-T, which uses 4 twisted pairs for transmitting and receiving the data:

If bits are sent in parallel over the twisted pairs, are these bits of the same frame? Meaning, frame's bits are just splitted between twisted pairs? Or each frame is being sent over the dedicated wire? There is some picture from https://learningnetwork.cisco.com/s/question/0D53i00000Kt6h4CAB/basic-question-is-ethernet-a-parallel-communication which should kind of answer this questions, but the picture is confusing:

In one hand it's written that frames are divided at the transmission side and re-assembled at the receiving side, so, to me it means that single frame is divided between 4 pairs and bits of that frame are sent in parallel over those 4 pairs. But in another hand, on the same picture, every pair is associated with different Frame number, for example: green pair associated with 1st Frame, brown pair with 2nd Frame, etc. To me it means that every single frame is sent over one twisted pair. Could you please clarify?

[Joseph W. Doherty]

In your posting, I didn't find that illustration in its original context, but that noted, its reference to sending "frames", I believe, is incorrect.

What 1000Baset-T does is send a frame's bytes, serially, at 125 MBps or 1,000 Mbps.  (Each byte is split into four two bit pairs, and each two bit pair is sent, in parallel, on each of the 4 wire pairs.  I.e. 2 bits, times 4 wire pairs = 8 bits [per baud], or 1 byte @ 125 million per second [baud, per pair, is 125 million/4 per second].)

More info on how this is accomplished:

https://www.rfwireless-world.com/Terminology/Advantages-and-Disadvantages-of-4D-PAM5-line-coding.html

http://units.folder101.com/cisco/sem1/Notes/ch7-technologies/encoding.htm

And if you want to learn more about Ethernet, you might look at:

https://www.practicalnetworking.net/stand-alone/ethernet-wiring/

If you're now getting totally befuddled, that's often to be expected unless you're an electrical engineer designing L1 hardware.  For practical network engineering, we usually only concern ourselves whether a particular cable meets its Ethernet Cat standard, and/or interface stats are showing CRC errors and such.  Or, we just concern ourselves with, we have "gig" interfaces and suitable media.

; )

[NetworkingGeek1]

@Joseph W. Dohertythanks for the reply. Yes, I'm confused a little bit.

I already read: https://www.practicalnetworking.net/stand-alone/ethernet-wiring/ and this is a really great article.

Regarding this part:

"What 1000Baset-T does is send a frame's bytes, serially, at 125 MBps or 1,000 Mbps. (Each byte is split into four two bit pairs, and each two bit pair is sent, in parallel, on each of the 4 wire pairs. I.e. 2 bits, times 4 wire pairs = 8 bits [per baud], or 1 byte @ 125 million per second [baud, per pair, is 125 million/4 per second].)"

Does it mean that two bit send in parallel over a single wire pair (two physical cable) ? I thought that a twisted pair is a single communication channel, meaning these two physical wires act like one, so only single bit can be sent over those two wires at the same time, so, it's not like every wire of the pair can transmit different bits, both wires can transmit only the same bits.

[Joseph W. Doherty]

Two bits can be sent at the same time.

A single bit can only be sent if there's just two values, e.g. on or off.

Two bits can be sent if we have four values, you can send.. For example rather than blinking a white light on or off, what if the light could be off, white, red or blue?  We can then send two bits.

[NetworkingGeek1]

Hi @Joseph W. Doherty  Thanks for the reply. But those two bits are sent serially, I mean one after the other, right? Why am I asking, because if I understand correctly, one twisted pair (two physical cables) is the one communication channel. So, information is not sent in parallel inside one twisted pair, it's sent in serial way, because this two physical cables inside twisted pair is the one communication channel.

And just to make sure: transmission, and reception, is frame by frame. Meaning network device doesn't mix bits of different frames, it's send all bits of one frame, then all bits of another frame and etc. Correct?

[Joseph W. Doherty · Accepted solution]

"But those two bits are sent serially, I mean one after the other, right?"

No, they are not sent serially.  During "one bit time" a single signal is sent with four possible values, i.e. 00, 01, 10 or 11.  I.e. one signal transition, but again, not two values but four values.  This allows two bits to be set concurrently.

"And just to make sure: transmission, and reception, is frame by frame. Meaning network device doesn't mix bits of different frames, it's send all bits of one frame, then all bits of another frame and etc. Correct?"

Yes, correct.

[NetworkingGeek1]

@Joseph W. Doherty  Thank you so much for the clarification. The one thing, I still don't understand is how it's network device which receives frames is able to reassemble frame in a correct order. I mean, if frame is divided and sent in parallel over 4 twisted pairs, then other end should somehow reassemble it.

Also, as far as I understand, in 1 Gbps all 4 twisted pairs are used for both transmitting and receiving. So, there is no different twisted pair for transmitting and receiving anymore. But how is it possible that there is no collisions?

[Joseph W. Doherty]

Each frame's bytes are transmitted in sequence. Each byte is split into four two bit pairs.

I believe tx, in both directions, is done by using different frequencies.

[NetworkingGeek1]

@Joseph W. Doherty  this is from your previous post "What 1000Baset-T does is send a frame's bytes, serially, at 125 MBps or 1,000 Mbps.  (Each byte is split into four two bit pairs, and each two bit pair is sent, in parallel, on each of the 4 wire pairs.  I.e. 2 bits, times 4 wire pairs = 8 bits [per baud], or 1 byte @ 125 million per second [baud, per pair, is 125 million/4 per second].)" From here I understood that each Byte is split into four bit pairs (4*2 = 8 bit = 1 Byte) and those bits are sent in parallel over 4 twisted pairs, the question is how receiving side can reassembly those bits in one Byte in the correct order?

[Joseph W. Doherty]

Simply by both ends agreeing pins 1 and 2 will always be the first two bits of a transmitted byte, pins 3 and 4 will be next two bits of the same byte, etc.

I.e., pins have specific purposes.