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Lesson 3 - The Physical Network

C. Wireless communications

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Wireless communications

• LAN, WAN and mobile computing
• Spread spectrum radio
• Infrared, laser
• Microwave, satellite

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Why wireless

Wireless data communications has its own set of challenges and therefore an organization needs to take a close look before making a decision. The following circumstances suggest a wireless solution.

• Mobile workers who need access to a LAN. Picture an outside salesperson who comes into the office occasionally. There may not be a permanent connection for him to plug into.
• Workers who are mobile in their work environments. This time, picture a doctor making her rounds in a hospital. As she goes from patient to patient, she needs to call up their medical history.
• Locations that are difficult to cable. Bylaws, for example, may prevent holes from being drilled in historic buildings. Another good example is your home. As families acquire multiple personal computers, it is only natural that they should wish them to communicate with each other and with the Internet. Most families, however, do not have the expertise to wire a LAN. Wireless LAN kits are now available that are targeted specifically for home use.

Wireless network categories

Wireless networks are generally categorized into three types.

• LANs — a local area network in which wireless technology substitutes for cable.
• Extended LAN or WAN — when a LAN needs to be extended beyond the allowed distance for cable, wireless technology can be used. Two LANs can also be connected through a wireless bridge.
• Mobile computing — cellular technology allows mobile workers to access their network. This might include a delivery person logging in a parcel pickup or an on-the-road salesperson accessing information from their car on a laptop computer through the cell phone system.

Hybrid networks

Even wireless networks are rarely all wireless. Usually there is a cable based portion which connects the desktop machines. Because there is both wired as well as wireless sections, these are actually hybrid networks. As the signal moves between the two sections, it goes through an access point.

Access point
This is a transceiver (transmitter/receiver) which acts as the conduit from the wireless section to the wired section of the LAN. Typically, this is a small unit with an antenna, attached directly to the cable. Under a different arrangement, a computer can act as the access point if both a wired and a wireless network adapter are installed and the appropriate software forwards the packets.

Wireless LANs

Wireless LANs can use the following technologies: infrared, laser, narrow-band radio and spread-spectrum radio.

Infrared transmission
This technology uses infrared light to transmit data. You are already familiar with this technology because your remote control uses it to communicate with your TV set. Printers and laptop computers are sometimes equipped with infrared ports so that you can print from your laptop without requiring a cable. This can be very handy for mobile workers. As far as networking is concerned, you should note these four variations.

• Line of sight — two nodes can communicate directly as long as there is no obstruction between them.
• Scatter infrared — this mode would be useful if the communication nodes are not in line of sight. The infrared beams are bounced off ceilings and walls until they reach the destination. This method has a 100 ft. restriction and is considered slow.
• Reflective infrared — all nodes aim their signals at a common point from which it is reflected to the other nodes.
• Broadband optical telepoint — using a broadband service, high capacity transmission can be achieved, even suitable for multimedia.

In general, infrared transmission can achieve high output, up to 10Mbps, however, distance is limited, usually to 100 feet. Although it is not affected by EMI, ambient light can degrade its performance.

Laser

Laser is similar to infrared but must be used for line of sight transmission. Any object g etting in the path of the light will break communications.

Narrow-Band radio
Narrow-band radio uses a single frequency to transmit data. This technology is sometimes called single-frequency radio to distinguish it from spread-spectrum radio. To implement narrow-band radio, you normally contract to a service provider who takes care of licensing. Coverage is greater than with infrared, typically up to 5000 square meters. However, the high frequency used preclude it from penetrating steel or masonry walls.

Spread-spectrum radio
This last technology is actually the most popular. It was developed by the military for use in battlefield conditions and appears to be resistant to both EMI and interception.

Transmission uses a range of frequencies, hopping between them at regular intervals. The receiver follows along with the change of frequencies as needed because it knows the pattern that the transmitter is using. Interception of the signal is difficult unless the hopping pattern is known. Further security can be implemented by encrypting the data.

Originally, this was considered a slow technique because it was implemented at 250Kbps. Modern commercial implementations have upped this considerably and are usually found at 11Mbps.

One problem with all wireless techniques has been their lack of standardization and open standards. In other words, products from one company would not work with products from another. The IEEE has finally taken a serious interest in spread-spectrum technology and it has become standardized by the 802.11 subcommittee. With interoperability between different vendor’s equipment, spread-spectrum LANs have become very popular.

Wireless bridging

Imagine the following scenario. Your company has expanded and you have taken over the building across the street. Networks in both building need to communicate with each other. The city won’t allow you to dig up the street in order to lay down a cable between the buildings. What do you do? You could lease a dedicated line from a communications service provider, such as the telephone company, and pay monthly charges. Or you could employ a wireless bridge.

A wireless bridge employs spread-spectrum radio technology and communicates between the access points in the two buildings. As long as the two buildings are in line of sight and within about 3 miles, they can communicate. Special directional antennas may need to be deployed. Special long range wireless bridges can communicate at up to 25 miles.

Mobile Computing

Mobile computing uses the wireless services of public carriers. Technologies involve packet radio communications, cellular networks and satellite stations.

Packet radio communications

The technology of this system is similar to LANs in that the data is broken up into packets in order to transmit it. All packets require basic information such as the source and destination address plus error correction information.

Packet radio uses satellites to forward data. It is sent from a mobile transmitter with an antenna to the satellite and received only by the unit it is addressed to.

Packet radio communications is available from commercial suppliers. One example of an industry which makes heavy use of this service is long haul trucking. It allows the operators of these trucks to keep in touch with home base even when they are a continent away.

Cellular networks

Besides transmitting voice, cellular networks can transmit data using a technology known as Cellular Digital Packet Data (CDPD). The digital refers to the type of data being transmitted, not to the network itself, which can be either analog or digital. The system is very fast and usually transmits the data in the lulls between voice communication.

If you are a laptop user and have a standard modem for communications, you may wonder if you already have the correct components for cellular communications. The answer is no! You require a special cell modem which is not the same as a standard voice modem.

Satellite communications

Microwave transmission is used to communicate via satellite. It is also available for ground based communications.

Microwave is excellent for line of sight transmission. This makes it ideal for satellite to ground links, between two buildings or communication towers and across large flat expanses of land or water. Besides the two transceivers (radio devices that both transmit and receive), directional antennas are required. Microwave transmission is tightly focused and the antennas need to be aimed squarely at each other for optimum reception.

Exercise 3-4: Wireless Network Exercise

Fill in the blanks.
1. The _______________ acts as a bridge between the wired and wireless portion of a network.
2. In ________________ radio communications, the data is broken up into self-contained units with source and destination addresses as well as error correcting information.
3. Connecting two sites across the street could be done with _______________ technology.
4. In reflective infrared transmission, all the nodes aim their signals at _____________.
5. Narrow band radio only uses a ____________ frequency.
6. ______________ radio uses different frequencies during transmission.
7. A salesman who updates information on his laptop while in his customer’s parking lot is probably using a __________ network.
8. Microwave transmission is always ___________, which is why their antennas must be carefully positioned.

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