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802.11 specifies two principle modes of operation, IBSS [1] and BSS
[2]. The reason this is important is because it can change the way
that the layer two connectivity appears to hosts.
To begin, have a look at this explanation [3].
If the host has an interface running in IBSS [4] mode, then it will
be able to contact a subset of other hosts within the IBSS domain. For
those hosts that are reachable, the network will look as if it is a
broadcast domain (ie. like ethernet, not like ATM Asynchronous
Transfer Mode [5]. For those hosts that aren't in range, it simply
will not be able to address. However for each individual host, the set
of reachable and unreachable hosts will differ. The implications of
this on IP networking in IBSS mode are explained below.
BSS
If the host has an interface in BSS [6] mode though, the point
control function ensures that each host in the BSS domain can hear all
other hosts within the same domain. This way everyone has the same set
of reachable hosts.
So if a host is operating in BSS mode, the layer two looks the same
as any other broadcast layer two medium, e.g. the most common, 802.3
ethernet. You can assign a subnet to the interface, and can be certain
that if an IP within that subnet is active on another interface, that
it will be addressable at layer two by every other host on the same
subnet. In other words, if host A needs to talk to host B, A can use
ARP (address resolution protocol) to discover B's hardware address,
and then proceed to talk directly.
-------------------------
Is this correct? Router A1 is running in BSS [7] mode (therefore it's
an access point) and has three Clients (A,B,C) connected with IP
addresses allocated from the 10.10.0.0/28 subnet.
We just turned our point on and it worked. Everyone's happy, traffic
routes through all interfaces (automagically) - life is good, Ya?
CLIENT B 10.10.0.3/28 eth0 | | | | CLIENT A | CLIENT C 10.10.0.2/28
10.10.0.4/28 eth0 (- ----------------------- | ---------------------
-) eth0 wlan 0 10.10.0.1/28 ROUTER A1
-------------------------
IBSS
In contrast, in IBSS mode, if you allocate a subnet to the interface,
you cannot be certain that you will be able to address other hosts on
the same subnet at layer two. You may need to rely on another node/s
to forward your frames/packets in order for them to reach the
destination. However if you decide that this is no good, and try to
assign a subnetwork for the hosts that are reachable at layer two,
then in general, the hosts in the IBSS domain will be unable to agree
on who is in the set and who is out. This implies that you need
several sets, possibly one per host - so you have a large number of IP
addresses assigned to the one interface, sub-interfaces if you like.
Additionally, every time another host is added to the IBSS domain,
these networks change, so configurations must be changed as well.
-------------------------
And likewise is this correct? Points A,B,C are all running in IBSS
[8] (old AdHocMode [9])
Point A can see B AND C
Points B and C can ONLY see Point A
We subnet and give A-B 10.10.0.0/30 and A-C 10.10.0.4/30
A-B and A-C have to be told about each other, so routes are setup so
that A-B and B-C can talk.
We turned our point on but it didn't work straight out of the box. We
did a bit of extra work though, had to find the config files and RTFM,
and it's now working - life is also good, but a bit exhausting - Ya?
POINT A POINT B 10.10.0.1/30 10.10.0.2/30 eth0 wlan0 (-
------------------------------------- -) eth0:0 10.10.0.5/30 POINT C
-) 10.10.0.6/30 wlan0
-------------------------
Links:
------
[1] http://melbournewireless.org.au/?IBSS
[2] http://melbournewireless.org.au/?BSS
[3] http://seattlewireless.net/index.cgi/PointToPoint
[4] http://melbournewireless.org.au/?IBSS
[5] http://www.iec.org/online/tutorials/atm_fund/
[6] http://melbournewireless.org.au/?BSS
[7] http://melbournewireless.org.au/?BSS
[8] http://melbournewireless.org.au/?IBSS
[9] http://melbournewireless.org.au/?AdHocMode
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