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© 2008 The McGraw-Hill Companies 1 Understanding Wireless Networking
© 2008 The McGraw-Hill Companies 2 Walkie-Talkie Network If you want to understand wireless networking at its simplest level, think about a pair of walkie-talkies. These are small radios that can transmit and receive radio signals. Recall, when you talk into a Walkie-Talkie, your voice is picked up by a microphone, encoded onto a radio frequency and transmitted with the antenna.
© 2008 The McGraw-Hill Companies 3 Walkie-Talkie Network (Cont’d) Another walkie-talkie can receive the transmission with its antenna, decode your voice from the radio signal and drive a speaker. Simple walkie-talkies like this transmit at a signal strength of about 0.25 watts, and they can transmit about 500 to 1,000 feet.
© 2008 The McGraw-Hill Companies 4 Walkie-Talkie Network (Cont’d) In order to do this, we require Each computer is equipped with a walkie-talkie. We would give each computer a way to set whether it wants to transmit or receive. And we would give the computer a way to turn its binary 1s and 0s into two different beeps that the walkie-talkie could transmit and receive and convert back and forth between beeps and 1s/0s.
© 2008 The McGraw-Hill Companies 5 Walkie-Talkie Network (Cont’d) This would actually work. The only problem would be that the data rate would be very slow. A walkie-talkie is designed to handle the human voice (and it's a pretty scratchy rendition at that), so you would not be able to send very much data this way. Maybe 1,000 bits per second. Another problem: the walkie-talkies could not be used to connect to the internet.
© 2008 The McGraw-Hill Companies 6 WiFi’s Radio Technology The radios used in WiFi are not so different from the radios used in walkie-talkies. They have the ability to transmit and receive. They have the ability to convert 1s and 0s into radio waves and then back into 1s and 0s. There are major differences, of course.
© 2008 The McGraw-Hill Companies 7 WiFi’s Radio Technology (Cont’d) WiFi radios that work with the 802.11b and 802.11g standards transmit at 2.4 GHz, while those that comply with the 802.11a standard transmit at 5 GHz. Normal walkie-talkies normally operate at 49 MHz. The higher frequency allows higher data rates. WiFi radios use much more efficient coding techniques (process of converting 0’s and 1’s into efficient radio signals) that also contribute to the much higher data rates.
© 2008 The McGraw-Hill Companies 8 WiFi’s Radio Technology (Cont’d) The radios used for WiFi have the ability to change frequencies. For example, 802.11b cards can transmit directly on any of three bands, or they can split the available radio bandwidth into dozens of channels and frequency hop rapidly between them. The advantage of frequency hopping is that it is much more immune to interference and can allow dozens of WiFi cards to talk simultaneously without interfering with each other.
© 2008 The McGraw-Hill Companies 9 WiFi Range Regardless of which setup you use, once you turn your Wireless Access Point on, you will have a WiFi hotspot in your house. In a typical home, this hotspot will provide coverage for about 100 feet (30.5 meters) in all directions, although walls and floors do cut down on the range. Even so, you should get good coverage throughout a typical home. For a large home, you can buy inexpensive signal boosters to increase the range of the Hotspot.
© 2008 The McGraw-Hill Companies 10 Another Way to Amplify WiFi Signals A WiFi repeater is installed to extend coverage. Wireless Access Point
© 2008 The McGraw-Hill Companies 11 How are Multiple Transmitters Supported? Recall the method for supporting multiple transmitter is called the multiple access method. In 802.11 systems, only one user is allowed to communicate with a receiver at a time (cannot use another frequency channel support a second or third additional user). The way the one user is selected depends on the carrier sense multiple access with collision avoidance (CSMA/CA) random access method.
© 2008 The McGraw-Hill Companies 12CSMA To help illustrate the operation of CSMA, we will use an analogy of a dinner table conversation. Let’s represent our wireless medium as a dinner table, and let several people engaged in polite conversation at the table represent the wireless nodes.
© 2008 The McGraw-Hill Companies 13 CSMA (Cont’d) The term multiple access covers what we already discussed above: When one wireless device transmits, all other devices using the wireless medium hear the transmission, just as when one person at the table talks, everyone present is able to hear him or her. Now let's imagine that you are at the table and you have something you would like to say. At the moment, however, I am talking.
© 2008 The McGraw-Hill Companies 14 CSMA (Cont’d) Since this is a polite conversation, rather than immediately speak up and interrupt, you would wait until I finished talking before making your statement. This is the same concept described in the CSMA protocol as carrier sense. Before a station transmits, it "listens" to the medium to determine if another station is transmitting. If the medium is quiet, the station recognizes that this is an appropriate time to transmit.
© 2008 The McGraw-Hill Companies 15CSMA/CA Carrier-sense multiple access gives us a good start in regulating our conversation, but there is one scenario we still need to address. Let’s go back to our dinner table analogy and imagine that there is a momentary lull in the conversation. You and I both have something we would like to add, and we both "sense the carrier" based on the silence, so we begin speaking at approximately the same time. In 802.11 terminology, a collision occurs when we both spoke at once.
© 2008 The McGraw-Hill Companies 16 CSMA/CA (Cont’d) The collision will result in an undecipherable message to the intended receivers (listeners). What we need is a polite contention method to get access to the medium; this is the collision avoidance part of CSMA/CA. 802.11 has come up with two ways to deal with this kind of collision. One uses a two-way handshake when initiating a transmission. The other uses a four-way handshake.
© 2008 The McGraw-Hill Companies 17 2 Way Handshake Node with packet to send monitors channel. If channel idle for specified time interval called DIFS, then node transmits. If channel busy, then node continues to monitor until channel idle for DIFS. At this point, terminal backs-off for random time (collision avoidance) and attempts transmitting after waiting this random amount of time.
© 2008 The McGraw-Hill Companies 18 2 Way Handshake If the node does not back-off the random time, then it will definitely collide with another node that has something to send. Reason for random back-off time is that if I choose a random time and you choose a random time, the probability that we choose the same random time is slim. This way we both back-off transmitting and will therefore will probably not interfere with each other when we are ready to transmit.
© 2008 The McGraw-Hill Companies 19 2 Way Handshake (Cont’d) First way of the 2 way handshake was for the transmitter to send its information packet to the destination node, after following the collision avoidance method described above. If the packet reaches the destination without problems, the destination sends a short packet over the wireless medium acknowledging the correct reception. This packet is typically called an ACK packet. ACK is the second way of the 2 way handshake.
© 2008 The McGraw-Hill Companies 20 4 Way Handshake “Listen before you talk” If medium is busy, node backs-off for a random amount of time after waiting DIFS, just as before. But now, instead of packet, sends a short message: Ready to Send (RTS). This message is basically attempting to inform others that “I have something to send.”
© 2008 The McGraw-Hill Companies 21 4 Way Handshake (Cont’d) RTS contains destination address and duration of message. RTS tells everyone else to back-off for the duration. If RTS reaches the destination okay (no one else collides with this message), the destination sends a Clear to Send (CTS) message after waiting a prescribed amount of time, called SIFS.
© 2008 The McGraw-Hill Companies 22 4 Way Handshake (Cont’d) After getting the CTS, the original transmitter sends the information packet to its destination. In these systems, the transmitter cannot detect collisions. The receiver uses the CRC to determine if the packet reached correctly. If it does then, it sends out an ACK packet. If the information packet not ACKed, then the source starts again and tries to retransmit the packet.
© 2008 The McGraw-Hill Companies 23 4 Way Handshake (Cont’d) Access Point Laptop RTS CTS Data ACK
© 2008 The McGraw-Hill Companies 24 RFID
© 2008 The McGraw-Hill Companies 25 Radio-Frequency Identification and Near-Field Communications Another growing wireless technique is radio frequency identification (RFID). RFID uses thin, inexpensive tags or labels containing passive radio circuits that can be queried by a remote wireless interrogation unit. The tags are attached to any item that is to be monitored, tracked, accessed, located, or otherwise identified.
© 2008 The McGraw-Hill Companies 26 Radio-Frequency Identification and Near-Field Communications The tag is a very thin label-like device into which is embedded a simple passive single-chip radio transceiver and antenna. The chip also contains a memory that stores a digital ID code unique to the tagged item. For the item to be identified, it must pass by the interrogation or reader unit, or the reader must physically go to a location near the item.
© 2008 The McGraw-Hill Companies 27 Radio-Frequency Identification and Near-Field Communications The reader unit sends out a radio signal that may travel from a few inches up to no more than a hundred feet or so. The radio signal is strong enough to activate the tag. The tag rectifies and filters the RF signal into direct current that operates the transceiver.
© 2008 The McGraw-Hill Companies 28 Radio-Frequency Identification and Near-Field Communications This activates a low-power transmitter that sends a signal back to the interrogator unit along with its embedded ID code. The reader checks its attached computer, where it notes the presence of the item and may perform other processing tasks associated with the application.
© 2008 The McGraw-Hill Companies 29 Radio-Frequency Identification and Near-Field Communications Basic concept and components of an RFID system.
© 2008 The McGraw-Hill Companies 30 Radio-Frequency Identification and Near-Field Communications The standard is under the auspices of EPCGlobal, the organization that also standardizes the Electronic Product Code (EPC) used on all tagged items. A key benefit of the new standard is that it is designed to read multiple tags faster. Tag read rates as high as 1500 tags per second are possible. The tags can operate reliably in an environment with multiple readers transmitting and receiving simultaneously.
© 2008 The McGraw-Hill Companies 31 Radio-Frequency Identification and Near-Field Communications Near-Field Communications One of the newest forms of wireless is a version of RFID called near-field communications (NFC). It is an ultrashort-range wireless whose range is rarely more than a few inches. It is a technology used in smart cards and cell phones to pay for purchases or gain admittance to some facilities.
© 2008 The McGraw-Hill Companies 32Reference Shalinee Kishore, LUCID Summer Workshop on Wireless Communications, Lehigh University, July 26-August 3, 2004 32
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