Mesh Routing and Wireless Distribution

Mesh Routing And Wireless Distribution Systems (WDS)

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Wireless Mesh Routing Compared WDS

In a wireless LAN using Mesh Routing (indoor or outdoor) or a Wireless Distribution System, "WDS" (typically mostly outdoor) the end result is the same: an interconnected "cloud" of connectivity over an area (or across an indoor space) is created without having to interconnect all the WiFi access points together with an Ethernet infrastructure. That's the basic end result of a Mesh Router or Wireless Distribution System implementation - eliminate the need for Ethernet interconnectivity between access points. Ethernet connectivity may be prohibitive (or impossible) when outdoor access points are mounted on light poles in a sports stadium, parking lot, across a corporate campus. It may not be practical to run Ethernet cable to a switch from points on the ceiling of an auditorium. Each of these cases are examples of environments where Mesh Routing or a Wireless Distribution System could be beneficial. In its simplest form, a two-node, point-to-point link can be created using either technology (and, that's often how a low-cost point-to-point link is configured.)

To explain and discuss Wireless Mesh Routing and bridging with a Wireless Distribution System (WDS) it's first necessary to lay some groundwork with a discussion of how "routing" differs from "bridging". Based on that, the rest of the comparison between Mesh Routing and WDS Bridging is straightforward. 

How Is "Routing" Different Than "Bridging"

Routing at Layer 3 versus bridging at Layer 2
"Routing" is the term used to define an interconnect device that receives data packets and makes a forwarding decision based on their Layer 3 destination address. "Layer 3" refers to Layer 3 of the OSI Reference Model, the Network Layer. In the case of the TCP/IP protocol family, Layer 3 is the IP (Internet Protocol) portion of each transmitted data packet and the IP address (like 192.168.4.1) is the "Layer 3 address."

"Bridging" is the term used to define an interconnect device that receives data packets and makes a forwarding decision based on their Layer 2 destination address. "Layer 2" refers to Layer 2 of the OSI Reference Model, the MAC Layer. In the case of an Ethernet or 802.11 WiFi protocols, Layer 2 is the Ethernet or 802.11 header in each transmitted data packet and the MAC address (like 9C:2A-79:27:DF:A3) is the "Layer 2 address."

A "Router" is an interconnect box that makes forwarding decisions based on Layer 3 and a "Bridge" is a box that makes forwarding decisions based on Layer 2. Sometimes you hear the terms "Layer 2 Switch" or "Layer 3 Switch." Normally, when you talk about a "switch" you're talking about a Layer 2 device. Bridges/switches were initially (circa 1990) much faster than routers because the decision-making process at Layer 2 could be implemented in silicon without requiring table lookup and maintenance in a devices memory (they way routing required - maintaining a table of IP addresses). By the mid-1990's the market began to see the introduction of devices that could process IP address tables (perform routing functions) in silicon. These routers were referred to as "Layer 3 Switches" because their switching fabric (the silicon that does the processing) was using the same technology as had previously only been used in bridges. "Layer 3 Switch" is, essentially, saying a device acts as a router; at Layer 3.

In the case of Wireless Mesh Routers and wireless Bridges there is also, typically, more manual configuration required for the Layer 2, bridge, Wireless Distribution System. As we'll see in this discussion 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 in a WDS system often depend on more manual configuration of path priorities to establish connectivity.

The wireless link between mesh routers and wireless bridges is, in both cases, exactly the same at Layer 2. That is, in both cases there is 802.11 WiFi connectivity between two devices (routers or bridges) in exactly the same way as two Ethernet Layer 2 switches or two IP Layer 3 routers would be interconnected with a length of Ethernet cable. The WiFi connection serves as the Layer 2 Data Link Layer connection and the two communicating devices don't know, don't care, and have no awareness of the fact that the link is wireless as opposed to wired Ethernet. 

Router and Bridge Links
An interconnected system of mesh routers is called a Wireless Mesh and an interconnected system of wireless bridges is called a Wireless Distribution System or "WDS". Unlike the dynamic nature of wired bridges, wireless bridges demand more configuration attention. 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). 

Selecting Wireless Bridges Versus Mesh Routers
The topology of the desired network must first be considered. If the network is going to be a static set of hierarchical links then bridging might be a more cost-effective alternative. If the links are going to vary, or need redundancy, then mesh routers may be a better choice.

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.

Terminology Used When Referring To Wireless Bridge And Router Implementations
The following terms are defined in the context of wireless mesh routers, WiFi, WDS, and point-to-point/point-to-multipoint outdoor network system design engineering and deployment
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.

802.11ac

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