[Cerowrt-devel] treating 2.4ghz as -legacy?
cra at WPI.EDU
Wed Dec 18 09:17:57 EST 2013
On Tue, Dec 17, 2013 at 08:33:03PM -0500, Theodore Ts'o wrote:
> I don't mind using multiple routers, if at some point CeroWRT were to
> gain the advanced feature of talking to other routers and forcing a
> disassociation when the signal strength talking to a particular client
> gets significantly weaker than compared to the signal strength from
> another AP. Is there any special hardware support needed to do this
> kind of AP-to-AP handoff, or is it just really complicated and no one
> has bothered to do it in an open source implementation?
It is always the Wi-Fi client that decides when/where to roam to in a
multi-AP environment based on whatever criteria it chooses--usually
the signal strength. The APs don't generally influence the clients'
roaming decisions, except for some advanced enterprise systems which
can sometimes use "tricks" like playing with transmitted
signal-strength or Beacon timing or forced disassociation to "steer"
clients to a certain band or AP, usually for capacity or policy
reasons. These are imperfect solutions because the AP can't know what
the client's signal strength actually is--it can only infer it from
the AP's received signal strength of a particular client which is
imprecise due to the different antenna and radio characteristics of
each end. This gets more complicated with MIMO on 802.11n, since
different paths could be used in each direction of transmission.
Simply setting the same ESSIDs on all APs and making sure they are all
connected to the same wired layer 2 network (so IP connectivity wont't
be broken after roaming) is all you should need to do to have a basic,
working multi-AP environment. Clients usually have a configurable
"roaming aggressiveness" setting that determines how sensitive they
are to changes in the signal srength and how "sticky" they are to one
AP before deciding to roam to another one.
In the context of CeroWRT specifically, a multi-AP environment
presents problems because CeroWRT explicity does NOT come configured
to do layer 2 switching between networks. In order to support proper
Wi-Fi roaming, though, it would have to provide some solution for the
client to keep working with the same IP address after roaming to
another AP. This could be as simple as configuring the same layer 2
network between APs, or using a tunneling technology such as Mobile IP
or GRE as some advanced enterprise systems do.
Here is a good paper on how Wi-Fi roaming works;
Here is a relavent quote:
"Wi-Fi network connection establishment and roaming is decentralized,
being controlled almost entirely by the client. The 802.11 standard
explicitly places control of wireless connection establishment in the
hands of clients by defining various logical services and breaking
implementation out between clients and access points."
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