Retro networking / WAN communities

[Paul Koning]

> Begin forwarded message:
> 
> From: Grant Taylor via cctalk <cctalk at classiccmp.org>
> Subject: Re: Retro networking / WAN communities
> Date: April 12, 2022 at 2:08:22 AM EDT
> To: Wayne S <wayne.sudol at hotmail.com>, "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
> Reply-To: Grant Taylor <cctalk at gtaylor.tnetconsulting.net>, "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
> 
> On 4/11/22 11:38 PM, Wayne S wrote:
>> In the beginning there was thick ethernet limited to 100 m.
> 
> Um....
> 
> I *REALLY* thought the 5 & 2 in 10Base5 and 10Base2 was the number of hundreds of meters that the cable segment could exist on it's own.
> 
> My understanding is that the 100 meter limit came about with 10Base-T.

Yes, that is correct.  10Base5 is 500 meter segment limit, 10Base2 is 200 meters max.  There are some other small differences: 10Base5 wants you to put the transceiver attachments on the marks on the cable (to avoid having impedance bumps aligned with the wave length); 10Base2 omits that requirement.

See the 802.3 spec for all the gory details.

>> People wanted computers that were on different floors  connected together between floors and buildings.  That exceeded the 100 meter spec so the repeater was born to connect two 100 m thick ethernet seqments.
> 
> I feel like even only 100 meters / 300(+) feet gives quite a bit of flexibility to connect multiple floors.  Especially if you consider the AUI drop cable.
> 
> Aside:  I'm not sure how long an AUI drop cable could be.  I'm anticipating between single and low double digit feet.

The spec says 50 meters.

And given the 500 meter segment limit, 10Base5 would handle quite a large building.   Repeaters serve several purposes.  One is to allow a larger station count than is permitted on a single segment.  Another is to allow multiple segments either for still greater distances or for convenience.  For example, it would make a lot of sense to run a segment per floor, a backbone segment up the elevator shaft, and repeaters to connect floor to backbone, even if in principle you can zig-zag a single segment across several floors within the distance limits.

>> A repeater was basically a signal booster between two ethernet segments. As you added segments interference and collisions became a problem as traffic on one segment was passed to all the other connected segments.
> 
> Yep, the 3, 4, 5, rule.  Up to five segments of cable with four repeaters and stations on no more than three of the segments.
> 
>> Hence the bridge was born. It had some intelligence And didn’t pass packets intended for computers on its own segment to the other segments thereby reducing congestion and collisions.
> 
> Didn't repeaters operate in the analog domain?  Meaning that they would also repeat ~> amplify any noise / distortion too?

No, they are digital devices (except that collision sense of course is an analog function, but that lives in the transceiver).  They do clock recovery and regeneration.  So you'd get some added jitter but not noise or distortion.

> Conversely bridges operated in the digital domain.  Meaning that they received an Ethernet frame and then re-generated and transmitted a new pristine Ethernet frame?

Yes, except that bridges were encouraged to repeat the CRC rather than recalculate it, if possible.  You can't do that on mixed LANs (like Ethernet to FDDI) but for the Ethernet to Ethernet case you can, and it's a very good thing to do so.

>> Then the router was born to connect multiple segments together at one point. And it had intelligence to determine what segment a packet should go to and route it there. It also prevented packets from going onto segments that didn’t have the packet’s intended target thereby reducing congestion.

Yes, except that historically speaking this is not accurate; routers predate Ethernet by 10 years or so.

>> Hubs were born approximately the same time to get over the ethernet tap distance because by this time there were more computers in the single area that needed to be connected together to the Ethernet.
> 
> Hum....
> 
> I can see problems with having enough actual taps on a network segment to connect all the machines in a given area with AUI drop cable length issues.
> 
> But I know that multi-port taps were a thing.  I've read about them and seen pictures of them for sale.  I think I've read about a singular tap that had eight AUI ports on it.  I've seen pictures of four AUI ports on a single tap.

Yes, DEC came out with that very early on, the DELNI.

> ...
>> The switch came about. It was a smart hub that had intelligence. It could filter out packets that were not intended for other computers connected to it thereby reducing congestion.
> 
> I feel like the switch and the bridge are doing the same thing from a learning / forwarding / blocking perspective.

Learning and spanning tree were part of bridges from the start (the DECbridge 100).  That's precisely what made bridges offer scaling benefits vs. repeaters.  And DEC worked very hard to figure out how to do that at wire speed, in the early days of Ethernet that was an extremely hard thing to do.

	paul