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HowToLinkToBackbone

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

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 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 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
|================|     |-<      >-|     |===================|
                 +-----+          +-----+


Version 2 (current) modified Tue, 03 Jul 2007 23:12:03 +1000 by dna0xff
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