Comment by dragontamer | Hacker News Reader

Comment by dragontamer | Hacker News Reader

I feel like walkie talkies are something all network engineers need to play with. Furthermore, remember that real world telephone and radio networks were in fact... Networks. (Called 'nets' in the radio world)

Ethernet, and 802.11, are both based on the concept of a singular channel shared by many individuals. When Alice talks, she says 'I'm Alice calling Bob, can you hear me Over'.

Alice is the source address. Bob is the destination address. Over is the stop code/stop bit signifying the end of frame. The payload is 'can you hear me'.

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This is everything up to layer 3. But if you wanted to send a radio/telegram across the country, you need one more layer, IP4 or the ARRL protocol (American Radio Relay League meets on the air at predestinated times to pass messages to each other on Ham radio frequencies. It's a human network)

I'm Alice, calling the Eastern Operators. I'd like you to relay a message to Bob for me. The message is 'Can you Hear me', Over.

This is a radio frame like all others, but an additional layer has been added. The radio operator has its own protocol built on top of the Frames.

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Dave may overhear all of this chatter, but Dave knows to stay off the channel (otherwise he will cause interference). Only if Dave is addressed specifically would Dave respond (Ex: This is Alice, calling Dave. How are you doing? Over)

We can see that the operator is itself just another person on the network. The operator may have a message from the rest of the world.

This is the Eastern Operator calling Alice. I'm relaying a message from Bob. The message is 'I got your message Alice!'. Over.

This is how a router may take a message from the greater internet and pass it to your local network.

> Ethernet, and 802.11

full duplex ethernet circuits (eg: a basic cheap 10Gbps 1310/LR carried over two $30 transceivers and a six foot fiber patch cable between two pieces of equipment) and a half-duplex TDD/CSMA regime like 802.11b/g/n/ac/ax are not the same thing.

yes they both enable ethernet connectivity between devices on fabrics of things that can see and arp each other.

the ethernet 10GbE might be shared by many individuals. but it's full speed both directions. one is half duplex, the other is not. one has contention on the shared media airtime, the other does not.

they are both possibly shared in the sense that many peoples' traffic may be carried across the 10GbE full duplex circuit, the the real world implementation is very different.

the real world implementations of FDD point to point (FCC part 101) or similar radio systems are closer to wired ethernet in concept, though with less capacity.

however I do agree with your general point that more people should familiarize themselves with the limitations and usage methodologies for half duplex IP data mediums and the best real world implementations.

You're right for sure.

I probably should have clarified that when I said "Ethernet", I meant classic 10 Mbps Ethernet that today's kids probably don't know about. The Ethernet hub acted like a bus: anything one connection said was broadcast to _ALL_ other connections.

Of course, today we have far faster point-to-point switches. But it helps to remember that the Ethernet-protocol was built in the days of old: half-duplex shared communication broadcast to everyone on your hub.

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Today's wired 1Gbps or 10Gbps ethernet are way way faster than half-duplex channels that 802.11 / Wireless technologies are forced to use (after all: the physics behind 802.11 and walkie talkies remains fundamentally the same)

There probably isn't a need for CSMA/CD systems in Ethernet anymore (since today's networks are all dedicated wires). But still, the CSMA/CD system exists for a reason, a legacy of the old "shared ether network communications" that gives rise to the name "Ethernet". (One Ether that everyone on the network can hear simultaneously, like a walkie talkie)

Raises hand. I remember discovering promiscuous mode, and a stern talking to from our network engineers. It was obvious I didn't know what the hell I was doing, so I got a great tutorial about the differences between hubs and switches.

Ahah. As opposed to token ring.

I'd argue that the legacy of CSMA/CD still alive in ethernet is what makes it harder to implement efficient networks, as the switched network kinda tries to emulate the shared bus behaviour.

I admit that I'm not actually a network engineer at this point. I took a networking course back in college and what I've discussed so far is largely just the basics and how I've internalized my lessons from back then.

Looking at CSMA/CD more specifically, as well as "full-duplex Ethernet" (which are 100Mbit, 1Gbit, and 10Gbit modern Ethernet), it seems like CSMA/CD is no longer used in any full-duplex Ethernet Protocol.

I'm not 100% sure about that, since its outside the scope of my old studies. But it makes sense to me. It seems like "full duplex Ethernet" can only work with point-to-point communications. If you have point-to-point connections between exactly two stations, there's no need for carrier-sense (CS), multiple-access (MA), or collision-detection (CD) protocols at all.

CS: Wait for the current person on the walkie-talkie to say "over" before you yourself start talking.

MA: Walkie talkies: many people can access the same channel.

CD: If two people accidentally start talking at exactly the same time, they need to independently try to restart-talking. A random-number generator with exponential backoff is common for this sort of thing IIRC, though I'm not keen on the exact details.

The problem is that the switched network has to emulate behaviours that were free on CSMA/CD system. So we get MAC learning, Spanning-Tree Protocol, and ultimately if you want performant large scale ethernet switching (and not just terminate ethernet at point2point) you end up with complex software directly managing forwarding tables (all sorts of SDNs) or essentially turn ethernet into routed network (using IS-IS routing, two standards exists for implementing it, one of them - TRILL - is reportedly backbone of Cisco Nexus Fabric system).

There's a reason why another name of Layer 2 ethernet network is "broadcast area".

I learned that during my studies two years ago. Also the leaky abstration in the OSI model was a topic. We had a good prof it seems :)

The book “CODE” uses exactly this pattern. It starts with two people talking to each other via cans connected with string, and works its way up to complex networks and processor architecture.

That sounds like an enjoyable read. Could you share the ISBN or link? Thanks

https://www.amazon.com/Code-Language-Computer-Hardware-Softw...

Thanks, just picked this up, look forward to reading this.

Thanks. Seems there is a second edition due October 26, 2022.

https://www.amazon.co.uk/Code-Language-Computer-Hardware-Sof...

I feel tempted to buy the current edition, but also to wait for the second.

Oh that's brilliant, I'm looking forward to that!

There is still so much of the tech stack that hasn't changed since this book was published in 2000 - TCP, IP, DNS, binary, logic gates, etc. But there's also much that has changed that could be written about - virtualisation, containers, wifi, fibre, cloud infrastructure, USB, GPUs.

I've gone ahead and bought the first edition, and then when the other is out, I'll add that to my collection too. Looking forward to reading it. No matter how much I learn, I find going over the basics again, reading introductions again, really helps to keep everything active up there. Let's face it, we learn so much but we also seem to lose so much as well.

I recently picked up CODE after seeing it recommended on HN a few months ago and it's really filled in the gaps in my knowledge of how computing technology evolved.

I absolutely love this. Makes the core concepts easy to grasp and to build upon.

When I write software I think of objects and messages in a similar way.

Well, there is already a term for that: "Bottom up". I used to think learning abstract design "Top down" is a better way to get good at design, but the "Bottom Up" way, by walking me through the implementation and design, sticks with me in the long run.

Many of us are amateur radio operators, which is basically walkie talkies. And furthermore, when we're all having a group discussing, it very much follows your last point: https://en.wikipedia.org/wiki/Amateur_radio_net

To nitpick a bit, both Ethernet and Wi-Fi frames start with the destination address, so it's more like "Bob? This is Alice, can you read me, over".

That way Dave knows to ignore the message if it doesn't start with his name (or "Everyone?" for broadcasts).

> Dave knows to stay off the channel (otherwise he will cause interference)

What would be the Bob-Alice-Dave equivalent to replacing the network hub with a switch?

If you wanted to add a learning switch to the example, you'd also need to add the concept of multiple channels. Once a switch learns which channel the recipient is on, it would only send to that channel instead of broadcasting to everyone. It still starts to break down unless you torture the analogy a bit more.

Asking the FCC for dedicated channels and preventing everyone else in the world from using those channels.

So I guess cellphones instead of one-at-a-time walkie talkies, since everyone on the cellphone network has a unique connection.

There is no true radio analogue here in the sense of how OP was explaining it.

At least in a wireless context, CDMA and TDMA are flawed examples because, CDMA uses fancy math to split communications over multiple channels and TDMA cuts the channels up into timeslots. You don't get full bandwidth allotment and utilization.

To make a wireless switch, you'd need to have PTP optical/microwave/radio links which have highly directional antennas. This would give you full-bandwidth per user.

Even in a wired context, frame-based protocols inherently utilize TDMA (over multiple channels), just not on timescales you normally think about.

If I'm not mistaken, the radio example uses both frequency division and time division. You're on a particular channel (typically frequency, although AM exists too), and only one person can talk at a given instant, or else you get a collision which results in probably garbled output. (There's also space division at play, since radios are not infinitely powerful.)

If the relay hop is also done over radio, then it's most likely achieved by hopping to dedicated frequencies for backbone comms, ideally with a separate antenna per channel so we can listen on all of them simultaneously. (You could constantly sweep the frequency space, but it's simpler to just have multiple antennas / radios.)

A (modern) real-world example is that I can use my phone as a wifi hotspot. It relays packets to/from its upstream link (4G or even wifi again). (It also presumably behaves like a NAT, but that's an implementation detail.)

That’s a great summary! Thanks for taking the time to write it up.

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