On Mon, Jan 19, 2015 at 8:14 AM, Michael Torrie <torr...@gmail.com> wrote:
> I recently set up another access point in my yard to give better
> wireless coverage to my shop. Currently it's on a different channel and
> different SSID than my current one. This works alright and my phone or
> laptop will hop on whichever is closest most of the time. But it got me
> wondering, would I be better off having the second access point have the
> same SSID? Would a device be more likely to switch back and forth
> between APs if the SSID was the same? We did this at BYU with enteprise
> wifi APs, which supposedly did some kind of fancy handoff between APs as
> the client roamed. I realize on a non-enterprise system there's no
> handoff between APs, so connections might drop when the client interface
> reinitializes, but that happens anyway with my existing setup and
> doesn't cause me any heartburn. Do any of you run two APs in your house
> with the same SSID but different channel?
Here's some technical info on WiFi from 802.11-2012:
The basic unit on the network is a STA. These can be fixed (always in
the same location), portable (can move around, but stays in one place
during use), or mobile (moves around while being used).
The basic unit of networking between STAs is the BSS, or Basic Service
Set. The BSA is the physical area in which members of the BSS can
remain in communication. There are multiple kinds of BSSs; any STAs
can create an ad-hoc BSS called an IBSS, or Independent BSS. There
are also Mesh BSSs (MBSS) and Infrastructure BSSs (since IBSS is
taken, this is just a BSS).
To communicate in an Infrastructure BSS, a STA must go through an
"association" process. This involves a special kind of STA called an
access point (AP). It's special in that it has access to a
Distribution System (DS) that uses some other networking technology to
connect multiple BSSs together.
There's a third concept known as an ESS, or Extended Service Set. This
is the union of all infrastructure BSSs connected via a DS with the
same SSID. It's just the union of wireless BSSs; it doesn't include
the DS network itself. In an ESS, the BSSs act as if they were part
of their own IBSS, i.e. they've merged their link-layer network. That
is, all STAs in the ESS have MAC addresses in the same address space!
A MBSS and ESS are similar in some ways, but a MBSS is not an ESS. You
can use a MBSS as the DS for an ESS, though!
Anyway, there's a concept called 'service set transition', in which a
STA moves from one BSS to another BSS. This can be either a BSS
transition or an ESS transition.
So, just because you've configured two APs to use the same BSSID for
their BSS doesn't mean that you've created an ESS. If the APs are set
up as routers, each BSS will be distinct and you'll switch IP
addresses every time. But if they're set up to bridge to the
Ethernet, you should have them all effectively on the same ESS,
assuming their firmware is doing the right thing. There's still some
delay during switchover, and it can have serious impact on your
connections if the STA waits on a lousy connection and TCP connections
get a lot of packet delay/loss before switchover happens, but it ought
to allow TCP connections to stay established.
One point to make is that normally the process of switching from one
AP to another is solely up to the STA. There's a newer bit in 802.11
called 'Fast BSS Transition' or FT that, if supported by the APs,
allows them to set up a 'mobility domain' as a subset of the ESS that
a STA can be reassociated within with a faster process. This is still
initiated by the STA, though, so it has to be supported by all the APs
and the STA as well. These were standardized by 802.11k and 802.11r,
so you can look for those in feature lists. Apparently iOS devices of
recent vintage support them.
There are also some proprietary solutions in the enterprise wifi space
that make "virtual cells"; these are virtual APs composed of multiple
actual APs. They appear to client devices as a single AP, and all
handoff decisions are made by the network controller rather than the
STAs. I have no idea what these cost, but my guess is that it's not
really affordable for home use.
--Levi
/*
PLUG: http://plug.org, #utah on irc.freenode.net
Unsubscribe: http://plug.org/mailman/options/plug
Don't fear the penguin.
*/
