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INTRODUCTION
There are a number of ways to implement a node. All nodes fulfill the
same basic functions: provide a number of interfaces, provide
intelegent routing between interfaces and offer additional services.
+-----------------+ | | | | | +-----+ +------+ | | | | BSS | ---/---
to Cx Nodes or Client Nodes | | R | +------+ | | O | | | | U |
+------+ | | T | | IBSS | +++++++++ to peer Bx Node | | I | +------+ |
| N | | | | G | +------+ | | | | IBSS | +++++++++ to peer Bx Node | |
| +------+ | +-----+ | | | | SERVICES | | +-------------+ | | | DHCP |
| | | DNS | | | | HTTP | | | | FTP | | | +-------------+ | | |
+-----------------+
Implementations fall into one of two main types; Single box and
multiple box.
The Single box approach covers dedicated built-for-purpose devices
such as the WRAP computers, MIni-ITX based, Laptop based and includes
the re-used old PC.
The multiple box approach tends to be based on using multiple
domestic AP's and may have the routing function provided by either a
seperate box or by one of the AP's ( as is the case when using the
Linksys WRT54G ).
LINUX PC BASED NODE
This is relatively attractive as an old PC can be re-used and due to
the strong networking support in the Linux kernel can easily be
configured to perform the required routing. In this case routing is
managed in the box and the interfaces are PCI cards installed in the
same box.
The downside of a PC based node is the difficulty of finding a
suitable location for the PC - ideally you want to keep the cable runs
between the wireless cards and the antennas as short as possible but
the PC is quite large and dificult to place up a mast. Though it can
be done as in the case of NodeGES [1].
LAPTOP BASED NODE
In a similar fashon, an old laptop PC can be used as a node. In this
case the radios are most likely Cardbus (32 bit PCMCIA) with external
antennas. NodeBAE [2] is an example of this implementation.
DEDICATED APPLIANCE
A dedicated appliance such as a WRAP or net4801 is a small form
factor dedicated computer that supports multiple interfaces (either
Mini-PCI or Cardbus). These appliances can run stripped down versions
of Linux ( or other OS) and can boot from compact flash allowing a no
moving parts node to be created.
AP BASED NODE
An AP usually only has a single radio so in itself cannot fulfill all
the functions of a node (despite many AP's being able to provide
routing and other services). To build a higher function node more than
one AP is required. As well as multiple AP's there needs to be a
device providing routing between the AP's and any wired segments in
the node. The use of a hackable AP like the WRT54G allows the routing
function to be combined with or of the radio interfaces reducing the
number of boxes required.
MULTI-BAND AP
A multi band AP could be used to implement a Cx Node if the other end
of the link( to the BX Node ) supports 802.11a. AN example of this
would be the DLink DLW-7100AP [3]
IMPLEMENTATION PRICING COMPARISON
< Not done yet, though there may not be all that much difference
between implemetations when all components are considered. >
OTHER CONSIDERATIONS
AP BRIDGING VS. ROUTING
One problem with AP's in general is that they are normally a bridged
device. This means that they bridge between the wired and wireless
interfaces and because of this extend the network segment they are
connected to rather than provide a route to it.
An example probably makes this clearer.
Node A is a Linux PC (as in the above section). It is connected to an
internal ethernet segment 192.168.10.0 via eth0.
It has three wireless interfaces:
* eth1 10.10.1.64/28 running in IBBS AP mode into a waveguide antenna
* eth2 192.168.11.1 running IBBS mode into a grid antenna connected
to node B
* eth3 192.168.12.20 from DHCP running in BSS client mode connected
to node C
Node A has IP forwarding configured and runs an OSPF daemon. It had
static links configured for the links to node B and node C.
Node D has an AP running in client mode and establishes a link to the
BSS service on the eth1 interface of node A. Node D gets an IP address
from node A via DHCP and huh? the IP address of the wired interface of
node D is also the same as the wireless one. What's happened here?
Unlike a PC interface there is no way to get into the AP and
configure it to have different IP addresses on the two interfaces. The
AP is configured at the factory to bridge the two interfaces rather
than route between them.
What we were hoping to get:
Node A Node D 192.168.1.0 +-----+ BSS +-----+ 192.168.2.0
--------------| | ---/--- >---| |------------ +-----+ +-----+
What we got:
192.168.1.0 +-----+ +-----+ 192.168.1.0 --------------| | ---/---
>---| |------------ +-----+ +-----+
Node D is bridging the two wired lan segments.
Some AP's are able to be configured to run in multiple modes:
* As an AP
* In IBBS mode
* As an AP (BSS) client
* As part of a WDS cloud
When using multiple AP's in combination with a router controled
switch you can use the vlan capabilities of the switch to take
advantage of the bridged nature of an AP. In this way despite the AP
bridging the switch can be configured to perform routing from the port
connected to the AP.
PUBLIC SEGMENT
I am using the notion of a public segment between multiple devices
that forms part of the network. The reason for this was to remove the
dependancy on the (precious) 10.10.0.0/28 subnet for the internal
parts of the node and to get the most flexability on the internal
implementation.
Each of the three uplinks can run as either an IBBS peer to the other
end of the link or as a BSS client(in which case they take one of the
subnet addresses from the AP at the other end. It really depends on
the nature of the link - dedicated radios at each end would use IBBS
mode and could use any subnet to communicate.
P2P dedicated P2P dedicates Client of a P2MP server link link P2MP
server interface
192.168.51.0 192.168.50.0 10.10.x.x/28 10.10.1.64/28 | | | | |2 |2
|(DHCP) |65 +-------+ +-------+ +-------+ +-------+ | IBBS | | IBBS |
| BSS | | BSS | +-------+ +-------+ +-------+ +-------+ |4 |3 |2 |1 |
| | |
|--------+--------------+--------------+--------------+---------|
192.168.100.0
USING VLANS ON YOUR WRT54G
If you have a WRT54G you can use cheap, off-the-shelf AP's (like the
Minitar) as a link interface. Typiclly the AP will be either a client
of the other end of the link (in BSS mode) or part of a P2P link in
IBBS mode. The AP will bridge between the wired and wireless and
"extend" the address space of the remote end of the link. This is not
really a good thing but most simple AP's do not have a routed mode. In
this case the AP can be addressed as a dedicated VLAN in the WRT54G
taking advantage of the routing capability of the WRT.
Links:
------
[1] http://melbournewireless.org.au/?NodeGES
[2] http://melbournewireless.org.au/?NodeBAE
[3]
http://www.dlink.com/products/resource.asp?pid=304&rid=1016&sec=0
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