Telecom Deja Vu

Yes, we've seen it before-at least the architecture. The Wireless Mesh Network looks a lot like the Public Switched Telephone Network (PSTN).

Voice and data are converging and can be sent over wireless options. Some wireless is “fixed” and some is “untethered.” The WMN, which is an often-mentioned topology, is similar to the PSTN, which has existed for many, many years. Check out “wireless mesh” with an Internet search and check out the hardware you might want to make available to customers, just as you would hardware that makes up the PSTN.

We are looking at the similarities between these two types of networks because the WMN is here, it's a growing technology and it's time to get familiar with it. One way to do that is to relate it to the PSTN.

The PSTN (public switched telephone network)

The PSTN is the joining of the public circuit-switched telephone networks. It was a network of fixed-line analog telephone systems that is now almost entirely digital except for the final link from the central office to the user, unless DSL or ISDN is implemented.

The PSTN is governed by standards created by the ITU-T [the International Telecommunications Union (ITU), Telecommunications Standardization Sector-one of the three sectors of the union; the ITU is associated with the United Nations]. ITU-T exists to ensure efficient and high-quality standards in all telecommunications fields.

The PSTN is also a Signal Switching 7 (SS7)-based switched network that makes switched-access services available. It is possible to access your ISP (Internet service provider) from your business or home, through a central office to the PSTN, then over ISDN to your ISP. Route management and packet switches/routers play a big part. The software used and the redundancy features help provide service to users at a level of quality they can accept.

This evolution of the PSTN shows how it has adapted to today's technology. It no longer only makes calls possible from one coast to the other (the earlier function), but it now has ways of linking people with their Internet provider.

Figure 1 is a diagram of how the PSTN works. A telephone call from A to B meets up with congestion on the direct path. The exchange selects Route A, but that is also congested because Exchange 1 is overloaded. Selection of Route B to Exchange 2 results in a successful set-up. The next time a call meets congestion on the direct route between A and B, the system remembers the path through Exchange 2 was open the last time and selects that route first. This is how the system avoids making an unsuccessful attempt to switch the call through Exchange 1 if it is still congested. More advanced functions are referred to as dynamic routing. That is when each network node records information about the load in other nodes and can take that situation into account when selecting a route.

The wireless mesh network (WMN)

The new WMN isn't here to replace the PSTN. It is a basic structure that resembles the PSTN. It's also being implemented on a smaller scale-private customers, municipalities, etc. And it's not a “fixed” wireless network such as a WiFi network, because that relies on additional wiring and also requires “line of sight” for reception.

The word “mesh” is what distinguishes this wireless network from other topologies because of the number of interconnections among nodes throughout the network. (See Figure 2.)

The “mesh” topology includes many interconnections that makes duplicating this topology with wires impractical or impossible. The interconnections are among network nodes. The mesh node can contain software; a PC, Ethernet card and wireless card; an optional second wireless card; and various types of antennas.

The nodes can be uplink-connected to the Internet by a wired connection; downlink-connected to the mesh and able to serve clients, both wireless and wired; and repeaters-a node disabled from serving clients but which is connected to the mesh.

This connectivity among neighboring nodes may appear complex, but it can make WMNs very simple to implement and operate. To make this work effectively, the mesh has to be self-configuring-where new nodes automatically become full members of the mesh topology shortly after booting up. Thus, MACs (moves, adds and changes) of nodes and their attached Ethernet devices (clients, servers, access points, surveillance cameras, gateways, routers, etc.) are simple and quick.

Another feature is referred to as self-running end to end, which means it allows traffic to move dynamically along optimal paths. It's similar to how the PSTN handles traffic between its nodes by using traffic-engineering techniques.

Improvements to the PSTN

Today, the PSTN provides support to the Internet with long-distance capabilities. The collection of circuit-switching telephone networks is also almost entirely digital, except for the final link from the central office (local telephone office) to the user.

The PSTN also uses signaling, routers and redundancy; it includes route management; and it is a distributed system that is highly reliable. One reason for that reliability is its Internet protocol (IP)-based architecture. It is a vast improvement to meet today's digital and analog communication needs.

Here are a few advantages and challenges that you or your customer should bear in mind when considering a WMN.

Advantages to the WMN configuration

Advantage No. 1: The wireless mesh is fully wireless. Unlike a WiFi setup, there are no wires involved so you are able to build out networks quickly-without the expense and/or labor-intensive installation of copper wiring.

Advantage No. 2: You can self-reconfigure the mesh network. The “self-reconfiguration” aspect lends itself well to allowing traffic to move dynamically along the best paths. Similar again to the PSTN-when the paths change, so do the route tables that send traffic on the shortest, fastest and least-congested route.

These self-configuring (including new node members), self-tuning (moving dynamically), self-healing (with redundancy), and self-monitoring features all parallel features of the PSTN.

Advantage No. 3: One important difference today is that the mesh network topology is on a small scale-more customer-owned compared to a nationwide network. Even though the PSTN is basically a mesh-type network, it is the largest and could implement an essentially wired network for all these “mesh-type” connections.

Advantage No. 4: Your costs and your workload are reduced. Eliminating wires reduces implementation costs and reduces the labor expense associated with new installations, moves and changes, and on-going operation.

Challenges for wireless mesh networks

Challenges No. 1 and No. 2: Accommodating Ethernet connectivity so that a wireless mesh can be integrated with existing public or private networks and interoperability-The WMN has to fit in with existing network standards, such as Ethernet. It has to be compatible and interoperable with all Ethernet switching and routing protocols because that will allow different vendors' products to work with Layers 2 and 3 of the OSI Model-thus ensuring interoperability.

Challenge No. 3: Security-The WMN must exclude unauthorized user traffic from accessing its mesh traffic. Solutions can be digital signatures and/or encryption.

Challenge No. 4: End-to-End Quality of Service-A good quality of service (QoS) needs to be available to the user for end-to-end transmissions. Interference cannot be allowed just because transmitting devices are close to each other. The QoS relates to the amount of bandwidth available; extending or growing the mesh can increase the bandwidth, but that could also increase latency because of the added hops certain traffic must experience. IEEE is working on 802.11e for this QoS.

Challenge No. 5: Trusting the network to “self manage”-Since one strong feature is the WMN's self-managing capabilities, network managers or administrators must become familiar enough with this technology to have confidence in its self-management capabilities. When well-designed, it will be traffic decisions that consider options and handle security controls.

In Summary

Think of a WMN where coverage is needed within a large building or across a campus or a metropolitan area. Or maybe for a building that doesn't have the pathways and spaces to handle wiring, or where there are “line of sight” obstructions, or finally, where something must be set up quickly. Some physical areas where a WMN has been deployed are at educational facilities, healthcare institutions, or for wireless access for Internet service providers, metropolitan networks, transportation/shipping, entertainment or hospitality facilities, manufacturing or warehousing, military and/or Homeland Security locations, and retail locations. There are benefits to a WMN configuration; you can expect to see more implementation. EC

MICHELSON, president of Jackson, Calif.-based Business Communication Services and publisher of the BCS Reports, is an expert in TIA/EIA performance standards. Contact her at or

About the Author

Marilyn Michelson

Freelance Writer
Marilyn Michelson, president of Jackson, Calif.-based Business Communication Services and publisher of the BCS Reports, is an expert in TIA/EIA performance standards.

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.