Let's say for example we have the following hypothetical topology:
In each network, can each user communicate with each other?
For example, can User 1 ping User 2?
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can each user communicate with each other?
Ultimately, yes, as that's the point of any internet connection. The question is thus where do these two rivers meet. Routing (layer-3) or switching (layer-2) can happen at the DSLAM/OLT, or it can happen further into the network, or even in a 3rd party's network. (In resold situations, users could be isolated into different logic networks. I.e. with standard ethernet switching, if I'm on port 1 in vlan 10, and you're on port 2 in vlan 20, we cannot talk to each other directly, we may actually be many layer-3 hops apart.)
If the ISP is not providing end users with public addresses [CGN - Carrier Grade NAT], then who can see who depends on how the specific network is setup.
Do DSLAM and OLT function like a switch?
On the physical (L1) and data link (L2) layers ("like a switch"), each CPE can only communicate with the DSLAM or OLT (COE). If those were to forward data between CPEs they could communicate directly, but that is highly unlikely. Forwarding L2 between CPEs would open the door to various attacks including MAC/IP spoofing, MAC or ARP flooding, so it's usually filtered.
Some AMs route already, so L3 communication (see below) might even work directly.
can User 1 ping User 2?
Of course, the users can usually communicate across the access server (BRAS) off the diagram to the far right. So
ping, working on the network layer (L3), should be working between users.