This chapter is from the book
This chapter is from the book
This chapter is from the book
The Wireless LANs
A wireless LAN (WLAN) is a networking method that delivers all benefits of a local area network (LAN) to the enterprise, with one very important advantage—no wires. No wires means that you now have the flexibility to immediately deploy workgroups wherever and whenever needed. Wireless LANs allow different workstations to communicate and to access a network using radio propagation as a transmission medium. The WLAN can then be connected to an existing wired LAN as an extension, or it can act as a standalone network. The advantage here is that WLAN combines data connectivity with user mobility and gives the user a movable LAN. Wireless LANs are especially suited for indoor locations such as hospitals, universities, and office buildings.
Configuration of a WLAN
The keystone to a wireless LAN is the cell. The cell is the area where all wireless communication takes place. In general, a cell covers a more or less circular area. Within each cell, there are radio traffic-management units also known as access points (repeaters). The access point, in turn, interconnects cells of a wireless LAN and also connects to a wired Ethernet LAN through some sort of cable connection, as shown in Figure 9 27.
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Figure 9 Configuration of a wireless LAN.
The number of wireless stations per cell is dependent on the amount of data traffic (and the type of data traffic). Each cell can carry anywhere from 50 to 200 stations, depending on how busy the cell is. To allow continuous communication between cells, individual cells overlap. Cells can also be used in a standalone environment to accommodate traffic needs for a small to medium-sized LAN between workstations or workgroups. A standalone cell would require no cabling. Another option is wired bridging. In a wired bridging configuration, each access point is wired to the backbone of a wired Ethernet LAN (see Figure 9). Once connected to a wired LAN, network-management functions of the wired and the wireless LANs can be controlled. Wireless bridging is also an option that allows cells to be connected to remote wireless LANs. In this situation, networking can stretch for miles if it were linked successively and effectively from access point to access point. Finally, by connecting several access points to external directional antennas instead of their built-in omnidirectional antennas, access points can provide multicells. This is useful for areas of heavy network traffic, since, with this configuration, they are able to automatically choose the best access point to communicate with. Roaming can also be provided for portable stations. Roaming is seamless, and it allows a work session to be maintained when moving from a cell to a cell (there is a momentary break in data flow).
Pros and Cons
Now let's look at some of the following pros and cons of wireless LANs:
- Range/coverage
- Throughput
- Integrity and reliability
- Interoperability
- Simplicity
- Security
- Cost
- Scalability
- Power consumption
Range/Coverage
Most wireless LANs use radio frequencies (RF) to function (normally in the range of 2.4GHz). RF is used because of its ability to propagate through objects. In wireless LAN, objects blocking the path of communication between access points limits the range that a wireless LAN can cover. Typically, the radius of coverage is anywhere from 100 feet to more than 300 feet. Coverage can be extended via roaming, which was previously defined.
Throughput
Airwave congestion contributes to data rates for a wireless LAN. Typical rates range from 1Mbps to 10Mbps. Just like in wired Ethernet LANs, wireless LANs slow down as traffic intensifies. In traditional Ethernet LANs, users experience a minimal difference in performance when going from wired to wireless LANs.
Integrity and Reliability
Radio interference can cause degradation in throughput. Such interference is rare in the workplace, and existing robust designs of WLAN prove that such problems are nothing compared to similar problems in existence with cellular phone connections. After all, wireless data technology has been used by the military for more than 50 years.
Interoperability
Wireless and wired infrastructures are interoperable yet dependent on technology choice and vendor implementation. Currently, vendors make only their products to be interchangeable (adapters access points, etc.). The IEEE 802.11 ensures compliant products that are able to interoperate between vendors.
NOTE
802.11 is the standard for wireless local area networks (WLANs) developed by the Institute of Electrical and Electronics Engineers (IEEE). It can be compared to the 802.3 standard for Ethernet wired LANs. The goal of this standard is to tailor a model of operation in order to resolve compatibility issues between manufacturers of WLAN equipment manufacturers. Thus far, the IEEE 802.11 standards committee is revising a version of a Media Access Control–Physical Level (MAC-PHY) level.
Simplicity
Wireless LANs, due to their nature, are transparent to a user's networking operating systems (OS). This allows excellent compatibility to the existing OS and minimizes having to use any type of new OS. Also, since only the access points of wireless LANs require cabling, moving, adding, and setting up is much easier. Finally, the portable nature of wireless LANs allows networking managers to set up systems at remote locations.
Security
The military has been using wireless technology for a long time; hence, security has been a strong design criterion when designing anything that is wireless. Components are built so that it is extremely difficult for eavesdroppers to listen in on wireless LAN traffic. Complex encryption makes unauthorized access to network traffic virtually improvable, if not impossible.
Cost
Infrastructure costs are dependent on the number of access points and the number of wireless LAN adapters. Typically, access points range anywhere from $2,000 to $3,000. Wireless LAN adapters for standard computer platforms range anywhere from $400 to $2,000. Installation and maintenance costs vary depending on the size of the LAN. Installation costs of installing and maintaining a wireless LAN are lower in general when compared to the costs of installing and maintaining a traditional wired LAN.
Scalability
Complexity of each network configuration varies depending on the number of nodes and access points. The ability of wireless LANs to be used in a simple or complex manner is what makes them so influential to current offices, hospitals, and universities.
Power Consumption
Power consumption of a wireless LAN is very low when compared to that of a handheld cellular phone. Wireless LANs must meet very strict standards posed by government and industry regulations, hence making them a safe device to have around you at a workplace. Finally, no detrimental health affects have ever been attributed to wireless LANs.
Technology Options
There is a range of available technologies out there for manufacturers to select from. For each individual technology, there are individual advantages and limitations.
Narrowband Technology
Narrowband technology uses narrow frequency on the radio signal. Communications channels are apportioned to this signal, each with different channel frequencies. This technology works just like a radio station. Each channel in this technology could be similar to a radio station on your FM stereo. Nevertheless, the frequencies used in narrowband technology are much higher (in the gigahertz range).
Spread Spectrum
Spread spectrum is the most commonly used technology among wireless LANs component manufacturers. This technology has been adopted from the military and provides secure and reliable communication. The disadvantage to this is that it consumes a large amount of bandwidth. The advantage is that it produces a louder and more detectable signal. Within the spread spectrum exist two types of spread spectrum radio: One type is frequency hopping, and the other is direct sequence.
Frequency Hopping
Frequency hopping (FHSS) uses frequency diversity to combat interference. Basically, what happens is that the incoming digital stream gets shifted in frequency by a certain amount (determined by a code that spreads the signal power over a wide bandwidth). If the signal is seen by an unintended receiver, it will appear as a short-duration impulse noise.
Direct-Sequence Spread Spectrum Technology
Direct-sequence spread spectrum (DSSS) technology generates a chipping code that encodes each data bit. Effectively, this produces a low-power wideband noise in the frequency domain (thus rejected by narrowband receivers). The greater the number of chips in the chipping code, the less likely it will be that the original data will be lost. This is the most commonly used among spread spectrum technology.
Infrared Technology
Infrared (IR) systems is another option of available technologies for wireless LANs for the enterprise. This technology uses very high frequencies just below visible light in the electromagnetic (EM) spectrum to carry data. The disadvantage here is that IR cannot penetrate opaque objects, hence limiting its line of sight. Ranges of IR are approximately 3 feet, which makes them useless for most WLAN enterprise applications.
IEEE 802.11 Standard
The 802.11 standard, as shown in Figure 10, is the new IEEE standard for wireless LANs27. The goal of this standard is to standardize wireless LAN development in the industrial, scientific, and medicine (ISM) frequency bands allocated by the Federal Communications Commission (FCC) in the mid-1980s. The bands allocated include the frequency ranges 902–928MHz, 2400–2483.5MHz, and 5725–5850MHz. The advantage of these ISM bands is that they do not require a license. As long as the device operating in the ISM bands meets special FCC regulations, no license of operation is necessary. The IEEE 802.11 standard focuses on the Media Access Control (MAC) and the physical (PHY) protocol levels.
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Figure 10 IEEE 802.11 protocol.
Medium Access Control
A Medium Access Control (MAC) layer is built to allow overlapping of multiple networks in the same area and channel space. It has the ability to share media and to be robust for interference. The distributed coordination function is used to provide efficient medium sharing without any overlapping constrictions. Its frame formats are built to support the infrastructure and the ad-hoc network support as well as the wireless distribution system. The MAC layer provides the following services: authentication, deauthentication, privacy, MSDU delivery, association and disassociation, distribution, integration, and reassociation.
Physical Layer
A physical layer (PHY) is built to connect many stations together. Each station may transmit information to any other station in the network. As in other LANs, packets of the users' data are encoded according to the specific physical-layer protocol and transmitted as a serial data stream over a physical medium to other stations on the LAN. Figure 10 shows a proposed configuration. Also, the decision to discard interpackets takes place at the physical layer as the result of an elasticity buffer overflow or underflow. As previously explained, within the physical layer, the frequency-hopping spread spectrum radio, direct sequence spread spectrum radio, and infrared PHY are all found. Station management (see Figure 10) is used as a mediator between the MAC layer and the physical layer.
IEEE 802.11 Future Development
Finally, a new specification known as the Internet-Access Point Protocol (IAPP) is now in existence. This specification goes beyond the work that has been done by the IEEE 802.11 at the MAC and PHY (physical-layer specification) layers. This new standard works at higher OSI (Open Systems Interconnection) layers to establish the way access points communicate across cells in the wired backbone. This new standard is backed by Aironet, Lucent Technologies, and Digital Ocean, Inc24.