Wireless Bridges

A wireless bridge provides connectivity between two wired LAN segments, and is used
in point-to-point or point-to-multipoint configurations. A wireless bridge is a half-duplex device capable of layer 2 wireless connectivity only. The above figure shows an example of a wireless bridge, while the following figure illustrates where a wireless bridge is used on a wireless LAN.


Wireless Bridge Modes
Wireless bridges communicate with other wireless bridges in one of four modes:

Root Mode
Non-root Mode
Access Point Mode
Repeater Mode

Root Mode
One bridge in each group of bridges must be set as the root bridge. A root bridge can
only communicate with non-root bridges and other client devices and cannot associate
with another root bridge. The above figure illustrates a root bridge communicating with nonroot bridges.

Non-root Mode
Wireless bridges in non-root mode attach, wirelessly, to wireless bridges that are in root mode. Some manufacturers’ wireless bridges support client connectivity to non-root
mode bridges while in bridging mode. This mode is actually a special mode where the
bridge is acting as both an access point and as a bridge simultaneously. When using the
Spanning Tree Protocol, all non-root bridges must have connectivity to the root bridge.

Access Point Mode
Some manufacturers give the administrator the ability to have clients connect to bridges, which is actually just giving the bridge access point functionality. In many cases, the bridge has an “access point” mode that converts the bridge into an access point.

Repeater Mode
Wireless bridges can also be configured as repeaters, as shown in the above figure. In repeater configuration, a bridge will be positioned between two other bridges for the purpose of extending the length of the wireless bridged segment. While using a wireless bridge in this configuration has the advantage of extending the link, it has the disadvantage of decreased throughput due to having to repeat all frames using the same half duplex radio. Repeater bridges are non-root bridges, and many times the wired port will be disabled while the bridge is in repeater mode.


Common Options
The hardware and software options of a wireless bridge are similar to those of an access
point, and for many of the same purposes:

Fixed or Detachable Antennas
Advanced Filtering Capabilities
Removable (Modular) Radio Cards
Variable Output Power
Varied Types of Wired Connectivity

Access Points

Second only to the basic wireless PC card, the access point, or “AP”, is probably the most common wireless LAN device with which you will work as a wireless LAN administrator. As its name suggests, the access point provides clients with a point of access into a network. An access point is a half-duplex device with intelligence equivalent to that of a sophisticated Ethernet switch.

Access Point Modes
Access points communicate with their wireless clients, with the wired network, and with other access points. There are three modes in which an access point can be configured:

Root Mode
Repeater Mode
Bridge Mode

Root Mode
Root Mode is used when the access point is connected to a wired backbone through its wired (usually Ethernet) interface. Most access points that support modes other than root mode come configured in root mode by default. When an access point is connected to the wired segment through its Ethernet port, it will normally be configured for root mode. When in root mode, access points that are connected to the same wired distribution system can talk to each other over the wired segment. Access points talk to each other to coordinate roaming functionality such as re-association. Wireless clients can communicate with other wireless clients that are located in different cells through their respective access points across the wired segment, as shown in above.

Bridge Mode
In bridge mode, access points act exactly like wireless bridges, which will be discussed later in this chapter. In fact, they become wireless bridges while configured in this manner. Only a small number of access points on the market have bridge functionality, which typically adds significant cost to the equipment. We will explain shortly how wireless bridges function, but you can see from above figure that clients do not associate to bridges, but rather, bridges are used to link two or more wired segments together wirelessly.

Repeater Mode
In repeater mode, access points have the ability to provide a wireless upstream link into the wired network rather than the normal wired link. As you can see in the following figure, one access point serves as the root access point and the other serves as a wireless repeater. The access point in repeater mode connects to clients as an access point and connects to the upstream root access point as a client itself. Using an access point in repeater mode is not suggested unless absolutely necessary because cells around each access point in this scenario must overlap by a minimum of 50%. This configuration drastically reduces the range at which clients can connect to the repeater access point. Additionally, the repeater access point is communicating with the clients as well as the upstream access point over the wireless link, reducing throughput on the wireless segment. Users attached to the repeater access point will likely experience low throughput and high latencies in this scenario. It is typical for the wired Ethernet port to be disabled while in repeater mode.

Applications of Wireless LANs

Access Role
Wireless LANs are deployed in an access layer role, meaning that they are used as an entry point into a wired network. The following figure illustrates mobile clients gaining access to a wired network through a connection device (access point).


Network Extension
Wireless LANs can be easily implemented to provide seamless connectivity to remote areas within a building, as illustrated by the floor plan image in the following. Because little wiring is necessary to install a wireless LAN, the costs of hiring installers and purchasing Ethernet cable might be completely eliminated.


Building-to-Building Connectivity
There are two different types of building-to-building connectivity. The first is called point-to-point (PTP), and the second is called point-to-multipoint (PTMP). Point-to-point links are wireless connections between only two buildings, as illustrated in the following figure. PTP connections almost always use semi-directional or highly-directional antennas at each end of the link.


Last Mile Data Delivery
"Last mile" refers to the communication infrastructure—wired or wireless—that exists between the central office of the telecommunications company (telco) or cable company and the end user. Currently the telcos and cable companies own their last mile infrastructure, but with the broadening interest in wireless technology, WISPs are now creating their own wireless last mile delivery service, as illustrated in the following figure.

Mobility
As an access layer solution, wireless LANs cannot replace wired LANs in terms of data rates (100BT at 100Mbps versus IEEE 802.11a at 54Mbps). What wireless LANs do offer is an increase in mobility (as can be seen in the following figure) as the trade off for speed and quality of service.

Radio Frequency Behaviors

Gain
Gain, illustrated in the following figure, is the term used to describe an increase in an RF signal's amplitude. Gain is usually an active process; meaning that an external power source, such as an RF amplifier, is used to amplify the signal or a high-gain antenna is used to focus the beamwidth of a signal to increase its signal amplitude.


Loss
Loss describes a decrease in signal strength (the following figure). Many things can cause RF signal loss, both while the signal is still in the cable as a high frequency AC electrical signal and when the signal is propagated as radio waves through the air by the antenna.

Reflection
Reflection, as illustrated in the following figure, occurs when a propagating electromagnetic wave impinges upon an object that has very large dimensions when compared to the wavelength of the propagating wave.



Refraction
Refraction describes the bending of a radio wave as it passes through a medium of different density. As an RF wave passes into a denser medium (like a pool of cold air lying in a valley) the wave will be bent such that its direction changes.

Principles of Antennas

Line of Sight (LOS)
The LOS is an apparently straight line because light waves are subject to changes in direction due to refraction, diffraction, and reflection in the same way as RF frequencies. The following figure illustrates LOS.


Fresnel Zone
A consideration when planning or troubleshooting an RF link is the Fresnel Zone. The Fresnel Zone occupies a series of concentric ellipsoid-shaped areas around the LOS path, as can be seen in the following figure.


Intentional Radiator
As defined by the Federal Communication Commission (FCC), an intentional radiator is an RF device that is specifically designed to generate and radiate RF signals. In terms of hardware, an intentional radiator will include the RF device and all cabling and connectors up to, but not including, the antenna, as illustrated in Figure below.