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INTRODUCTION
It is possible to create a link in different ways, the commonest ways
are supported in most wireless devices (cards and APs). The simplest
links are those that use BSS mode where an AP is the hub for all
transmissions. This is fine for the Dx Node you use in the house and
for connecting Client Nodes to a Cx Node but has some limitations when
trying to use it for backbone links (the major one being that all
traffic is routed through the AP).
In the backbone it is useful to build redundancy through multiple
connections. Within a node this ideally would be through establishing
connections with multiple physical radios (i.e. a dedicated
transciever for each link) but cost and aavilability of other nodes to
connect to may not make this practical. The advantage of having
multiple connections is that as the density of nodes increases it is
possible that multiple nodes can provide alternative routes for
traffic.
ANTENNAS FOR LINKS
There is a limited set of choices here, somewhat dictated by the
protocol used for establishing the link.
omni ------/--- ---/--- omni ( hmmm, Ok, if you really have to... )
omni ------/--- >-------- directional ( Better, hybrid Bx Cx nodes do
this )
directional ----< >-------- directional ( Best!! )
PROTOCOLS FOR LINKS
There are four candidates for protocols that could be used for
establishing links. In reality while any protocol that is supported by
the devices at each end could be used some of tha available protocols
are not really the best for links ( as compared to supporting client
connection ).
* BSS
* WDS
* Bridge
* IBBS
BSS
Using BSS for a backbone link can be done, One end runs as an AP
while the other runs as a BSS client. Each end would also has other
links so this is just a single circuit of the ones that are available.
This is a good choice for establishing a link in the initial instance
and while traffic is light. at the AP end the same radio could support
multiple links while at the client end there are a number of choices
of device that could be used.
Using this protocol the AP end is probably already correctly
configured for routing while the client end will need some further
configuration to work as a router rather than as a stand alone client
of the AP. The client may be an AP in client mode or a PC with a radio
card. Details of how to configure routing in a client device are on
the RoutingHowto [1] page.
Note: BSS is probably the most common protocol for AP-to-client
connection, the protocol being devised for this use. Using BSS for
establishing an inter-node (Bx-to-Bx or Bx-to-Cx) is a different use.
WDS
WDS is a higher level protocol that is used to support a roaming
environment for a Client. Multiple Nodes use the same ESSID and a
mobile client node is passed from Node to node based on signal
strength. WDS is a suitable protocol for the client Node cloud but
inappropriate for establishing the network backbone links.
Well, actually WDS is not so bad for establishing a link if a
dedicated radio is used at one or both ends. A lot of APs support WDS
and it means an AP that cannot be configured in client mode ( like the
AVAYA-AP3 ) can be used as one end of a link. The AP-3 can be set up
to use one radio for the WDS connection and the other as the Node AP.
To the Node at theother end ( also configued to support WDS ) it is
like a special client. The down side is a possible reduction in
available bandwidth if data is going from a wireless client at the
upstream end to the Node at the end of the WDS link. The use of
multiple radio devices ate either end is a good compromise and would
not result in any splitting of the bandwidth.
AP-3s used for WDS link
+---------+ +---------+ | | | | | +-----+ Node 1 AP Node 2 AP
+-----+ | | | A |-< CH 1 CH 11 >-| A | | | +-----+ +-----+ | | | | | |
+-----+ CH 6 WDS link +-----+ | | | B |-< ///////// >-| B | | LAN
Interface| +-----+ +-----+ |LAN interface =============| | |
|============= +---------+ +---------+
In this case CH 6 is used only for the link. Either end could also
use the WDS interface to link to further nodes, so WDS works as a P2MP
link.
BRIDGE
A bridge makes two physicly isolated network segments look like the
same segment. The bridging device at either end forewards traffic
across the bridge. This is a common use of WiFi [2] in a private
network. A bridge could be used to link two nodes but typically each
node would have it's own address range. The bridging device at one end
or another would have to act as a router or would have to feed into a
port on a router to allow each node to control it's own address space.
Bridge spanning two physical segments
192.168.1.0 +-----+ +-----+ 192.168.1.0 |================| |-< >-|
|==================| +-----+ +-----+ Bridge with additional router at
receiving end
192.168.1.0 +-----+ +-----+ 192.168.1.0 +-----+ 192.168.2.0
|================| |-< >-| |=============| |===============| +-----+
+-----+ +-----+
IBBS
IBBS is probably the best protocol to use to establish a link between
two nodes. It is a lower lvel protocol than BSS ( doesn't try to solve
the hidden node problem and routing decisions are promoted to layer 3
rather than being addressed at layer 2 see Layer2Assumptions).
192.168.1.0 +-----+ +-----+ 192.168.2.0 |================| |-< >-|
|===================| +-----+ +-----+
Links:
------
[1] http://melbournewireless.org.au/?RoutingHowto
[2] http://melbournewireless.org.au/?WiFi
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