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HowToBuildANode

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.

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 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 External linkDLink DLW-7100AP

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:

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:

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.


Version 3 (current) modified Fri, 09 Oct 2009 17:07:46 +1100 by DeannaWest
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