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Working on a C# Tcp server that is communicting with an embedded IOT device and I was having serious communication problems. Packets getting lost and or delayed. Not receiving ACK...ect

It turns out the Network I was using has 3 access points available. Two are on Channel 1 and one is on Channel 11 all for the same SSID.

I was looking around for information on what happens if an packet goes out on one channel 1 and comes back on the other.

I know that collisions will happen because of this. Is it possible for packet corruption as well.

nate_weldon
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    Any one wireless client can only be associated with a single AP at a time. So unless your client is hopping around (unlikely, look at AP logs to verify), the situation you describe is not happening. Even if clients move between APs, TCP connections are generally not affected. You may see a retransmit or two, but TCP generally handles that well. – EEAA Apr 27 '17 at 23:25
  • So either your server is misbehaving, or the device is misbehaving, or you are operating in a very noisy RF environment. – EEAA Apr 27 '17 at 23:26
  • Additionally, wifi collisions aren't really a thing thanks to [CSMA/CA](https://en.m.wikipedia.org/wiki/Carrier-sense_multiple_access_with_collision_avoidance). – EEAA Apr 27 '17 at 23:30
  • "*I know that collisions will happen because of this.*" What? How do you figure? Can you clarify what your concern is? How can the same packet be received on a different channel from the one on which it was sent? – David Schwartz Apr 27 '17 at 23:40
  • Unless the wireless client is moving, the connection should stay with the same BSSID or access point. The channel overlap may cause some interference. Can you plug the IOT device in wired to test if that changes things? – Cory Knutson May 05 '17 at 21:33

2 Answers2

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There is nothing at all unusual about a single wireless network with a single SSID having numerous access points, some of which share channels and some of which don't. A single client will only be associated with a single access point at a time.

There may be a momentary loss of connectivity when a client roams from one access point to another. Unless the network is badly designed, this should only happen when the client physically moves and, even then, fairly infrequently. The network administrator should be able to check logs for evidence of excessive roaming, but this is not particularly likely.

There's no particular reason to think this kind of setup is causing problems.

David Schwartz
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Packet loss and inexplicable delays are a common issue with setups where there are several APs on the same channel and with the same SSID. Client roaming is not that seamless, especially with heterogeneous setups.

Keep in mind that each client device will choose which APs to connect to based on, for instance, signal strength and/or available modulations. Once connected to a particular AP, that is hardly static. Clients can decide to jump to a different AP anytime, and they do. Just someone standing too close to an AP antenna can make a distant client see a huge loss of signal, making it start a handover process.

When changing from one BSSID to another, the client should sent a disassociation request to the AP it is connected to, and then associate to the new AP. Some do both things at the same time. Some even connect to the new AP without properly disconnecting from the first one. That is typical of embedded WiFi chips used create "IoT" from existing hardware.

If you consider that each of the AP is at the same time a layer 2 bridge between the wireless and the wired network, you know each AP will be actively forwarding frames to and from the client until it disassociates from it. So there are lots of things that can go wrong, specially if the APs are from different vendors. For instance, one AP may be answering ARP packets on the wired network on behalf of a client that already decided to go to a different AP. If that happens just when the upstream wired switch expired the client entry on the ARP table, you may have 10-30 seconds of disconnection between those two hosts.

This creates all sorts of weird and hard to reproduce issues.

That is why each vendor has come to a particular solution for extended coverage with roaming under a single SSID. There are protocols to create vendor-independent solutions, like IAPP. You can see what I mean looking at this (quite old) presentation.

With the addition of WPA2 or higher security, encryption, Quality of Service (QoS) and Wireless Multimedia Extensions (WMM) the complexity of roaming increased and new inter-AP protocols were required: 802.11k, 802.11v and 801.11r.

I am still not allowed to include more than 2 links, but you can easily find references online about them.

Your experience in a homogeneous, well behaving extended network with cooperating APs would be completely different.

Arduous
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Pablo
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