Ethernet: Serial or Parallel? Depends on the speed?

[a1111]

Hello,

Can someone pls help me with the following?

These sources say that Ethernet is a serial port:

https://learningnetwork.cisco.com/s/question/0D53i00000Kt6h4CAB/basic-question-is-ethernet-a-parallel-communication

https://www.pusr.com/blog/Ethernet-Parallel-or-Serial#:~:text=Ethernet%20operates%20in%20a%20serial,a%20single%20pair%20of%20wires.

I think it actually depends on the speeds.

Ethernet and FastEthernet ports are serial ports, because bits that belong to the same payload are NOT sent at the same moment. When the sender puts bits onto the wire, they are sent one after the other, rather than at the same moment. That's because only one wire pair is used for sending, and another for receiving.

But, when it comes to GigabitEthernet speeds, consecutive bits that belong to the same payload  ARE sent at the same moment. That's because two wire pairs can be used to send bits, rather than just one (as with Ethernet and FastEthernet).

The above is true for copper. With fiber, if DWDM is used, then I think it's parallel communication. Without DWDM, serial.

With Wifi, if the transmitting access point uses MIMO or MISO, it's parallel. But if it uses SISO or SIMO, it's serial.

So the two key properties of parallel communication seems to be that at least two bits can be sent at the same time, and those bits CAN belong to the same payload.

Here are some sources that describe the difference between parallel and serial communication:

https://www.geeksforgeeks.org/difference-between-serial-and-parallel-transmission/

https://www.sciencedirect.com/topics/computer-science/parallel-communication

https://newhavendisplay.com/blog/serial-vs-parallel-communication/

What do you think?

[Joseph W. Doherty]

Laugh, ah technology continues to get more complicated.

Basically, serial vs. parallel boils down to whether you just using one transmission medium or multiple medium for transmission of data.

Consider, what would you consider it, if rather than sending on bit per time cycle, you sent multiple bits, concurrently, per one time cycle but using just one medium?  For example, parallel like throughput on a single wire.

Can this be done?  Yes it can.  Say, rather than sending a signal of zero or 5 volts, to represent binary zero or 1, we set the voltage to zero, 1 2/3, 3 1/3 or 5 volts, we now can send 2 bits per time cycle.  So, is this serial or parallel transmission?

Or consider, even in classical parallel, where you send 8 bits, concurrently, on 8 wires, but you want to send 16 so you send the first 8 and then the second 8.  BTW, the original 8086 processor was designed for a 16 bit bus, parallel wires, but Intel provided the 8088 which would split its16 bit bus transmissions into two halves so it could work with prior 8 bit bus designs.

Anyway, as Ethernet is designed to send "frames", unless the whole frame is sent in parallel, it's a serial transmission, but particular Ethernet standards do use some parallel transmission techniques to speed up the overall serial transmission.

BTW, my first Microcomputer (late '70s) used a genuine Centronics printer which actually used a parallel cable that sent one character's 8 bits across 8 physical wires, concurrently.  Same Microcomputer also had a 9.6 Kbps RS232 serial port, which I used with a 300 bps acoustic modem.  That Microcomputer also used a bus (parallel) ribbon cable to interconnect the CPU to an expansion box (which provided the aforementioned serial and parallel ports, and also provided a port, also using a ribbon (parallel) cable to connect to external 5.25" diskette 90 KB drives.  Back then, the cables were just serial or parallel, although how data was physically encoded on the diskettes, wasn't just serial/parallel bits.

So, it all boils down to if all you data can be sent in one time interval, its parallel, but it multiple time intervals are required, its serial.

The reason, I mentioned the 8068 vs. an 8088, both transmitted 16 bits words to/from the CPU.  With a 16 bit bus, it was a parallel, one single cycle transmission, but on an 8 bit bus, it was a two cycle transmission.

[Ramblin Tech]

Hi @a1111 

I believe the question as to whether Ethernet is serial or parallel first raises the question: "What is 'Ethernet'?" By that I mean, what do 10Mbps, 100Mbps, 1Gbps, 10Gbps, 100Gbps, and 400Gbps "Ethernets" all have in common such that we call all of them "Ethernet", despite the 40Kx difference in speed between the slowest and fastest? They all have, more or less, the same MAC sublayer which "performs the functions necessary to provide frame-based, connectionless-mode￿ﾠ(datagram style) data transfer" [IEEE 802-2014 5.2.3]. That is, generically, "Ethernet" is an L2 frame format that is specified "in terms of a serial transmission model for the service provided by the supporting PHY" [also IEEE 802-2014 5.2.3]. Peer devices conceptually transmit/receive Ethernet frames with each other serially at the MAC sublayer.

If all "Ethernets" have a frame format in common, then what sets them apart? It is the PHY. The MAC sublayer sends/receives frames via the PHY over a media-independent interface between the two layers which hides the actual encoding and transmission of frame bits via PHY symbols over the media. Those PHY symbols may very be transmitted serially, or they may be transmitted in parallel over parallel lanes as in 100GBASE-LR4. Note that with 100GBASE-LR4 having four parallel lanes and 100GBASE-LR1 (aka, 100GBASE-LR) having a single lane, it is not speed alone that determines whether the symbols are sent strictly serially or in parallel; it is really a technology evolution progression over time (multi-lanes are faster to market, but typically more expensive; single lanes are slower to market, but can be less expensive).

In short, Ethernet at the MAC sublayer is specified in terms of a serial transmission model for standards purposes, whereas the PHY layer can operate over one (serial) or more (parallel) lanes.