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Preamble: I don't have that much experience with lower level computer networking yet.

I am currently trying to wrap my head around the workings of the WLAN protocol. While I am starting to get the hang of how the communication between AP and stations works with the different management packets and the protocol for data fragments I am currently still not really seeing how the data is passed to the outside world.

From my understanding if a station S wants to send data to the host 1.2.3.4 (which is neither the AP nor any other station in the network) it will create some IEEE 802.11 data packets (lets assume that data needs to be fragmented into 2 data packets) and encrypt them and send them to the AP. The AP will identify the station, get its current key and decrypt it.

But what next? For example, what will my router send over my fiber cable? And how does it work with fragmentation? As far as I understand there are some rules for fragmentation in WLAN, but what if the connection between my router and its next hop could transfer much more/less? Will it do some buffering or splitting in the router?

I think all of the questions above boil down to the question in my title: What does an access point do when it receives a packet that is addressed for the outside world?

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You need to take a look at the various layers involved. WLAN only covers L1 and L2 of the OSI stack (the physical layer and the data link layer) and doesn't care at all about L3 (the network layer, most commonly IPv4 or IPv6).

Don't think about conversion which is (somewhat) an analog concept. A packet-based network transports payload in packets between nodes using various mechanisms.

The network layer's job is to provide end-to-end connectivity between any two nodes on a global scale. The data link layer provides connectivity between any two nodes in a local network segment. The physical layer finally provides actual data transport between two nodes connect by a common medium - a cable, fiber or a WiFi channel.

An IP packet (L3) sent to 1.2.3.4 from a wireless node is encapsulated in an IEEE 802 frame (L2) which is then sent over the air using an IEEE 802.11 physical layer (L1). In the WAP, the frame is received and forwarded to the router over wire using an Ethernet (IEEE 802.3) physical layer. The router extracts the IP packet and forwards it depending on your WAN protocol (may be Ethernet, xDSL, DOCSIS or anything else).

The WAP is basically a network bridge, forwarding between the wireless nodes and the wired nodes based on their MAC addresses (L2). WiFi encryption is an option with the 802.11 L2, it is added and removed on the WAP (and on the wireless clients, of course).

IPv4 fragmentation happens on the host or on the router. If the WAN MTU is smaller than the LAN MTU, your router needs to fragment the IPv4 packet or you need to reduce the LAN MTU.

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  • That corrected a fundamental misunderstanding I had. I always assumed that the layers more or less stay fixed and not that it "goes up" again to extract the data of the upper layer, this also makes some other things much clearer, thank you!
    – Cromon
    Dec 9 '18 at 11:20

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