Wireless Mesh Networks

Gilbert Held

• Éditeur: Auerbach Publications
• Année: 2005
• ISBN:
  9781000612172
• Édition: 1
• Sujet: Informatique
• Sous-sujet: Technologies de l'information

Wireless mesh networking is a new technology that has the potential to revolutionize how we access the Internet and communicate with co-workers and friends. Wireless Mesh Networks examines the concept and explores its advantages over existing technologies. This book explores existing and future applications, and examines how some of the networking

Chapter 1

Introduction to Wireless Mesh Networking

The purpose of an introductory chapter is to provide readers with basic information concerning the subject of a book. Because this book is about wireless mesh networking, as you might surmise the goal of this introductory chapter is to become familiar with this topic, terms associated with this topic, and even a few associated abbreviations.

In this chapter we begin with an explanation of mesh networking and wireless mesh networks. This explanation includes a brief examination of different types of networking, with this author discussing networking structures commonly referred to as networking topology and the manner by which such structures evolved. As we review different types of networking topologies, we note some of the advantages and disadvantages associated with each structure, which will provide a foundation for examining the advantages and disadvantages associated with wireless mesh networking. Because modern mesh networks are built upon over-the-air transmission and primarily use existing wireless LAN networking components, it will come as no surprise that we also focus our attention upon this area in this chapter.

Once we complete our initial examination of networking topology, we turn our attention to the different types of mesh networks, their advantages, and disadvantages.

In concluding our introduction to the topic of mesh networks and wireless mesh networking, we examine some of the existing and evolving applications that have the potential to make wireless mesh networking into an ubiquitous technology. That said, let’s grab a Coke, Diet Pepsi, or another drink and our favorite munchies and explore the wonderful world of wireless mesh networking.

1.1 Mesh Networking Defined

To understand mesh networking, we first need to obtain an appreciation for what a mesh topology represents. If we have n nodes in a network, where the term “node” refers to a communications device that can transport data from one of its interfaces to another, then the ability of each node to communicate with every other node in the network represents a mesh network topology. We can view the structure of a mesh network by simplifying the number of nodes in the network from a value of n, which is what mathematicians like to work with, to an easy-to-visualize number, such as three, four, or five.

Nodes and Links

Figure 1.1 illustrates three-, four-, and five-node mesh network structures, in which each node has a communications connection to all other nodes in the network. The connection between each node is referred to as a link.

If we examine the number of links associated with each network shown in Figure 1.1, it’s obvious that the number of links increases as the number of nodes increases. Although only three links are required to interconnect three nodes, six are required to interconnect four nodes, and ten are required to interconnect five nodes. If you take the time to draw six nodes and interconnect each, you would then note the need for fourteen links. What this means is that a classical mesh network structure in which each node is interconnected to every other node in the network becomes impractical as the number of nodes in the network increases. After all, when networks were first constructed, the links interconnecting nodes were dedicated or leased telephone lines. This meant that a separate physical interface was required by a node to connect to each link. That interface primarily performed parallel to serial and serial to parallel conversion, because data flows bit by bit on a serial link. Because each interface requires buffer memory and a node is a computing device with a finite amount of processing power, adding interfaces increases the amount of processing the node needs to perform until one interface too many is added that saturates the processing capability of the node. Thus, from a classical perspective, a mesh network in which every node can directly communicate with every other node has physical constraints that limit the number of nodes that can be interconnected.

Figure 1.1 In a true mesh network structure, each node has a connection to every other node in the network.

Control Issues

Recognizing the previously mentioned constraints associated with network nodes resulted in the development of more cost-effective partial mesh network structures. A good example of a partial mesh network would be the public packet networks constructed by Tymnet and Sprint during the 1970s and 1980s. Such networks consisted of hundreds to thousands of nodes, however, instead of each node being directly interconnected to every other node, they simply had two or more links to other nodes to provide an alternate routing and traffic balancing capability. Because nodes are not directly connected to one another, traffic would typically flow through one or more intermediary nodes to its destination, requiring the development of routing protocols that are based upon the transfer of control messages between nodes. Similarly, alternate routing and traffic balancing operations also required coordination, with control messages transmitted between nodes and a centralized network operations center required for controlling the flow of messages between nodes, which in turn controlled the use of alternate links for backup and traffic balancing operations. This concept of the use of a partial mesh holds true for the “mother of all interconnected networks” better known as the Internet.

Modern Mesh Networking

In a wireless environment, a single radio frequency (RF) transmitter/receiver in one node has the ability to communicate with a virtually unlimited number of other nodes. Thus, the physical constraints associated with wired connectivity becomes less of an issue in a wireless environment. This means it’s both a practical and a relatively simple process for one node to communicate with many other nodes because a single interface in the form of an RF transmitter/receiver can be substituted for the multiple interfaces required in a wired environment. Obviously, other nodes must be within transmission rage for communications to occur.

Wireless Networking Structures

There are two basic types of wireless LAN networking structures, referred to as peer-to-peer and infrastructure. In a peer-to-peer networking structure, each node can directly communicate with every other node, assuming they are in transmission range of one another. In an infrastructure wireless LAN networking environment, all traffic flows through an access point (AP). The access point represents a two-port bridge, with one port connected to a wired network and the second port representing the RF transmitter/receiver. Thus, in an infrastructure wireless network two nodes communicating with each other do so by first transmitting to the access point which then regenerates the data. Because the access point in effect functions as a relay station, when transmission occurs between two wireless nodes the transmission distance between nodes can double in comparison to a peer-to-peer networking environment. However, the access point represents a central control mechanism and, if it fails or if a node is out of range of the access point, communications suffer.

Overcoming Transmission Distance Limitations

A solution to the need for centralized control and transmission range limitations of wireless nodes occurs by enabling each network node to function as a relay. Figure 1.2 provides a comparison of a wireless peer-to-peer network, a wireless LAN infrastructure network, and wireless mesh networking. Note that in a wireless mesh networking environment each node functions as a router and repeater, forwarding data to the next node toward its ultimate destination. In comparison, in a peer-to-peer environment transmission is limited to two nodes communicating with each other whereas in an infrastructure networking environment all transmissions occur through a centralized access point. However, because nodes can be modified to relay information we can group a sequence of peer-to-peer transmissions to obtain a mesh structure operating environment. Based upon the preceding, we can define a wireless mesh network as follows.

Figure 1.2 Comparing wireless LAN topologies.

A wireless mesh network represents a series of peer-to-peer transmissions where each node functions as a router and repeater.

Note from the above definition that there is no requirement for any centralized control and in fact nodes communicate with each other on a peer-to-peer basis.

We can use an analogy to obtain additional information concerning the operation of a mesh network. Assume a room has a group of people, each having a similar communications device, such as a cell phone or notebook computer. If we are located in the room and need to send a message to a person located outside the room we could either move through the room or convey our message to another person, with the expectation that that person would relay our message until a person near the door could open it and deliver the message to the appropriate person located outside the room. If we think of each person in the room as a “client” or “node” then one person can relay our message to another as a peer-to-peer transmission. The person who opens the door to communicate our message to the person behind the door can be thought of as a network gateway. Thus, in a mesh networking environment, messages are passed in the form of electronic signals from client to client or node to node until they reach their destination on the network or a gateway for transmission off the mesh.

Now that we have a general appreciation for the term “wireless mesh networking,” let’s backtrack a bit and discuss the general aspects of how networks evolved. Doing so provides us with a firmer understanding of wireless mesh networking including its relationship to wireless LANs and the advantages and disadvantages associated with the technology.

1.2 Network Evolution

Earlier in this chapter when we examined mesh networking we noted the term “link” was used to refer to the connection between two nodes. Both of those terms, “link” and “node,” are considered by some people as antiquated in today’s modern wireless environment. However, they represent a good starting point for examining the evolution of networking including different types of networks and mechanisms to move or transport data from one network to another.

Network Topologies

There are two generic types of network topologies or structures that evolved over the past half century. Those two generic topologies are referred to as point-to-point and multipoint.

Point-to-Point

The first type of network structure consisted of a link interconnecting two nodes. This network structure is simply referred to as a point-to-point link because it directly interconnects two locations. The top portion of Figure 1.3 illustrates a point-to-point network structure.

Figure 1.3 Generic network topologies.

Multipoint

To conserve the cost of leased lines, vendors developed poll and select software that enabled multiple terminal devices to be connected to a common communications line. Referred to as multidrop or multipoint networking, terminals were either individually located in different geographical areas or were clustered together and connected to a common communications line by a control unit. For either situation pol...