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Overview of Wireless Mesh and Bridging - An On-Line Tutorial
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Connect802 is a nationwide wireless network equipment reseller providing system design consulting, equipment configuration, and installation services.


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An Overview of Wireless Mesh Routing
and Wireless Bridging Architecture

Connect802 can help with the selection and system design for many manufacturer's Wi-Fi Wireless Mesh Router and Bridging equipment. We understand Mesh Router technology, point-to-point and point-to-multipoint wireless bridging and we know how to build a system that will meet your requirements.

If you have questions, please don't hesitate to call us today! We'll be happy to provide you with any technical explanation that you need to help you assure a successful wireless networking deployment.

Wireless mesh routers and wireless bridges both provide point-to-point and point-to-multipoint connectivity. Mesh routers automatically discover the best path between end-points. Wireless bridges depend on more manual configuration to establish connectivity.
 

The Basic Communication Functionality of a Wireless Bridge and Wireless Mesh Router Is Identical

Here's a definition. The term "wireless bridge" and the term "wireless mesh router" can either be used with the definition because at this level of consideration both technologies do the same thing:

A [ wireless bridge / wireless mesh router ] creates a Layer 2 connection to one or more other [ wireless bridges / wireless mesh routers ] to allow data to be passed between them. Links are transparent at the Data Link layer which means that devices on either side of the [ wireless bridge / wireless mesh router ] link have no awareness of the presence of the link. A client computer communicates to a server exactly as if it were hard-wired with CAT5 Ethernet cable. The [ wireless bridge / wireless mesh router ] link is transparent to any IP configuration in the network.

  • An interconnected system consisting of mesh routers is called a Wireless Mesh.
  • An interconnected system consisting of wireless bridges is called a Wireless Distribution System or "WDS"
 

How Do These Two Technologies Differ?

In general, a WDS bridge link assumes a relatively static configuration without significant redundancy. A bridge link is created by configuring the end-points with the correct MAC address of each one's adjacent peer. A bridge device does not automatically discover new end-points and, if an end-point fails, the bridge is unable to automatically locate a new, previously unconfigured, alternative path.

A mesh router link potentially provides greater redundancy because it assumes a certain degree of dynamic change in the environment. This could be as simple as creating a redundant path to use in the event of node failure or as elaborate as a mesh router on a passenger train maintaining a dynamic connection to a series of mesh routers installed at intervals along the railroad track. The mesh router automatically discovers new end-points and dynamically determines the best path between Point A and Point B.

 

Bridges and Spanning Tree Algorithm

If bridge links were set up to create a circular set of connections a serious problem would occur. Broadcast traffic would be retransmitted from bridge to bridge, around in a circle, without stopping. This "bridging loop" would almost immediately bring the network to a complete standstill. There are two ways to avoid a bridging loop: 1) the network is manually configured without loops or 2) the bridges communicate to each other and detect the presence of loops. They then block the connections that would cause loops. The software mechanism and accompanying set of inter-bridge communications is called the Spanning Tree Algorithm ("STA"). It is an algorithm that considers "path cost" and computes the best path between two points while setting all other paths into "blocking mode" (so they don't forward any traffic).

 

Hierarchical Network Topology Considerations

To the right is a network diagram showing a hierarchical topology. These could be building rooftops on a corporate campus, streetlight poles in a community, or rooms in a building. What must be imagined from a wireless perspective is that the nodes are installed in locations such that there are no other possible ways for the nodes to link to each other.

Assume that the nodes are all wireless bridges. The diagram would then be referred to as a Wireless Distribution System. The RED lines denote links that have been blocked by Spanning Tree Algorithm.

Using wireless bridges it was the responsibility of the person performing the configuration to identify and configure the end-point addresses for all links that could be used. The configuration included the red line links and STA knew to block them. If one of the other links were to go down then STA could activate the red link to compensate.

If these had been mesh routers then there would have been no up-front link configuration. The mesh routers would have discovered the possible paths through the mesh network and handled them automatically.


A Hierarchical Network
 

Selecting Wireless Bridges Versus Mesh Routers

The topology of the desired network must first be considered. The following characteristics of the desired network would support the use of wireless bridges:

  • The number of nodes is small enough that the required manual configuration for each node can be performed in a reasonable amount of time.
  • The topology of the network is basically hierarchical.
  • There is little or no need for redundancy. A few Spanning Tree links will be sufficient.
  • The links, once established, won't be intermittently disrupted (by passing vehicles, opening and closing of warehouse dock doors, etc).
  • There is little or no need to consider increasing the number of nodes in the network over time to support increased network size.

The following characteristics of the desired network would support the use of wireless mesh routers:

  • Manual configuration for all the nodes would be too labor-intensive or otherwise unrealistic
  • The topology of the network introduces multiple links from node to node and these multiple links are considered an important part of providing redundancy or load balancing.
  • Redundancy is an important consideration. The nodes need to be able to independently discover alternative paths rather than depending on manual pre-configuration of links to be blocked and activated by Spanning Tree Algorithm.
  • The links may be disrupted (by passing vehicles or other factors).
  • The network will be moved from location to location and the relationship (topology) can not be determined in advance. For example, a portable public-safety network deployed at a disaster scene or a trade show or fair ground network set up and taken down from venue to venue).
  • The network is going to scale to many more nodes than the initial deployment.

In general, wireless bridges are much more simplistic than their mesh counterparts. A wireless bridge is typically significantly less expensive than its mesh counterpart. On the other hand, mesh routers often offer a number of sophisticated features (auto channel and power selection, firewall, captive portal web screens, etc). that would make them a reasonable choice for even a simple point-to-point or point-to-multipoint network. For example, a noisy environment could benefit from a mesh router's ability to switch to the best channel.

Definitions

Client Association : A user's notebook computer or other device connects to an Access Point and an Ethernet-like wireless link is formed. The idea of an "Ethernet-like" link refers to the fact that once the client machine has connected ("associated") to an Access Point then the transfer of data occurs just as if the client device were attached to an Ethernet cable (i.e.: DHCP, IP, TCP, UDP, and behaviors consistent with a normal Ethernet connection).

Access Point: A central radio to which client devices associate to form wireless links. Multiple Access Points can communicate to each other through a matrix of connectivity called the Distribution System. The Distribution System is necessary so that an aggregate of Access Points can operate as if they were one, single Local Area Network (which is to say, a single IP subnet and broadcast domain).

Distribution System: The interconnection between access points allowing clients to roam within a particular area and remain constantly connected to the network with the same IP address. Typically the distribution system is a wired Ethernet network where the access points are wired back to a central Ethernet switch. The distribution system can, however, consist of RF connections between Access Points. The entire Distribution System acts as a single Layer 2 broadcast domain and a single IP subnet. A client device can roam anywhere in the Distribution System and be handed off from one Access Point to another automatically (as per the rules of 802.11), retaining the same IP address and remaining part of the same network as they roam.

Wired Distribution System: The Access Points are wired with Ethernet cable to one or more Ethernet switches. The Distribution System for communication between Access Points is based on wired Ethernet. Note that both a wired, and wireless Distribution System is also the portal through which clients access the Internet or other back-end file servers or resources. A broadband modem or other Internet connection attaches to the Distribution System through a router or HotSpot access controller gateway. So, whether the Distribution System is wired, or wireless, there will always be some type of Portal connection OUT of the Distribution System to the rest of the world.

Wireless Distribution System: When Access Points are manually configured so that they're interconnected using radio links (as opposed to Ethernet cables) a Wireless Distribution System (WDS)is formed. In essence, the WDS is a matrix of Access Point to Access Point links. Each of the links is manually configured by the network administrator so each Access Point knows the address of its peers in the connectivity matrix. The Distribution System between Access Points is based on wireless communication links.

Mesh Router System: When Access Points are designed to automatically discover their peers and automatically create a matrix of Access Point to Access Point communication links, a Mesh Router system is formed. With regard to the functionality of the mesh, it serves the same purpose as WDS: to connect Access Points to each other. The fundamental difference between WDS and Mesh Routing is the automatic discovery and configuration mechanism in a Mesh Router which also makes the Mesh Router fault tolerant. If a Mesh Router link were to fail, the mesh would attempt to find an alternative path around the failed section. If a WDS link fails there is no fail-over mechanism provided.