Chapter 7 Low-Speed Wireless Local Area Networks

Transcription

1 Wireless# Guide to Wireless Communications 7-1 Chapter 7 Low-Speed Wireless Local Area Networks At a Glance Instructor s Manual Table of Contents Overview Objectives s Quick Quizzes Class Discussion Topics Additional Projects Additional Resources Key Terms Technical Notes for Hands-On Projects

2 Wireless# Guide to Wireless Communications 7-2 Lecture Notes Overview Chapter 7 describes low-speed wireless local area networks. Students will learn how WLANs are used. Next, students will see the components and modes of a WLAN. Chapter 7 also describes how an RF WLAN works. Students will review the differences between IR, IEEE , and IEEE b WLANs. Finally, Chapter 7 outlines the user mobility features offered by IEEE networks. Objectives Describe how WLANs are used List the components and modes of a WLAN Describe how an RF WLAN works Explain the differences between IR, IEEE , and IEEE b WLANs Outline the user mobility features offered by IEEE networks s WLAN Applications 1. Describe the various applications and problems solved by wireless networks. 2. Define a wireless residential gateway as a device that combines a router, Ethernet switch, and wireless access point. It also allows Internet and printer sharing. WLAN Components 1. This section describes the hardware components needed to create a WLAN. Wireless Network Interface Card 1. Explain that a network interface card (NIC) allows a computer to be connected to a network. A wireless NIC connects a computer to a network without cables. 2. Describe the various options for wireless NICs available for desktop and notebook computers. Define a Mini PCI as a small card that is functionally equivalent to a standard PCI expansion card. Mini PCI cards are normally used with notebook computers. Use Figure 7-1 to show a Mini PCI wireless NIC.

3 Wireless# Guide to Wireless Communications Describe the various options for wireless NICs available for smaller devices, including optional sled and compact flash (CF) or secure digital (SD) cards. Use Figure 7-2 to illustrate your explanation. 4. Mention that Intel has developed a new chipset called Centrino that integrates all of the functions of a wireless NIC directly on the motherboard. For more information about the Centrino technology, visit: Access Points 1. Explain that an access point (AP) provides wireless LAN devices with a point of access into a wired network. Describe the following parts of an access points: a. Radio transceiver b. Antenna c. RJ-45 wired network port 2. Use Figure 7-3 to describe the following two functions of an access point: a. Acts as the wireless communications base station b. Acts as a bridge between the wireless and wired networks Learn more about access points at: 3. Mention that the range of an AP is approximately 375 feet (115 meters) in an unobstructed environment. However, the data rate will drop as the signal strength, quality, or both begin to fade. The AP will automatically select the highest possible data rate for transmission depending on the strength and quality of the signal. This process is called dynamic rate selection. 4. Describe the benefits of using power over Ethernet (PoE). PoE defines how DC power is delivered to the AP through the unused wires in a standard UTP Ethernet cable. Read more about power over Ethernet (PoE) at: WLAN Modes 1. In an RF WLAN, data can be sent in one of two connection modes: ad hoc mode and infrastructure mode. This section describes both methods.

4 Wireless# Guide to Wireless Communications 7-4 Ad Hoc Mode 1. Explain that in the ad hoc mode, wireless clients communicate directly among themselves without using an AP. Use Figure 7-4 to illustrate your explanation. The drawback is that wireless clients can only communicate among themselves. 2. Mention that the ad hoc mode is also known as peer-to-peer mode, although its formal name is Independent Basic Service Set (IBSS) mode. Infrastructure Mode 1. Explain that infrastructure mode, also known as the Basic Service Set (BSS), consists of wireless clients and an AP. Extended Service Set (ESS) is simply two or more BSS wireless networks installed within the same area, providing users with uninterrupted mobile access to the network. Use Figure 7-5 to illustrate your explanation. All wireless clients and APs must be part of the same network for users to be able to roam freely. 2. Describe the problems of managing a large network. Define subnets as network units that contain fewer computers. Explain the impact of using subnets in an ESS wireless network. Wireless LAN Standards and Operation 1. This section describes the first IEEE standards for wireless LANs. IEEE Standards 1. Explain that the IEEE standard defines a local area network that provides cablefree data access for clients that are either mobile or in a fixed location at a rate of either 1 or 2 Mbps, using either diffused infrared or RF transmission. 2. Mention that the standard specifies that the features of a WLAN be transparent to the upper layers of the TCP/IP protocol stack or the OSI protocol model. Use Figure 7-6 to show the WLAN features in the PHY and MAC layers. For more information about the IEEE standard, visit: IEEE Infrared WLAN Standard 1. Explain that the IEEE standards outline the specifications for infrared as well as RF WLANs. The infrared specification is based on diffused transmissions. 2. Use Figure 7-7 to describe the functions of the following two sublayers of the PHY layer: a. Physical Medium Dependent (PMD) b. Physical Layer Convergence Procedure (PLCP)

5 Wireless# Guide to Wireless Communications Explain that the diffused infrared PHY layer convergence procedure reformats the data received from the MAC layer into a frame that the PMD sublayer can transmit. Use Figure 7-8 to illustrate your explanation. 4. Mention that the data rate value determines the speed of the transmission. Use Table 7-1 to show the infrared data rate and direct current level adjustment values. The direct current level adjustment contains a pattern of infrared pulses that allows receiving devices to determine the signal level. 5. Explain that PMD translates the binary 1s and 0s of the frame into light pulses that are used for transmission. PMD transmits the data using a series of light impulses. 6. Use Table 7-2 and Table 7-3 to describe how 16-pulse position modulation (16-PPM) and 4-pulse position modulation (4-PPM) work. Quick Quiz 1 1. A(n) is a small card that is functionally equivalent to a standard PCI expansion card. Answer: Mini PCI 2. The AP will automatically select the highest possible data rate for transmission, depending on the strength and quality of the signal. This process is called. Answer: dynamic rate selection 3. The standard defines a local area network that provides cablefree data access for clients that are either mobile or in a fixed location at a rate of either 1 or 2 Mbps, using either diffused infrared or RF transmission. Answer: Even though it contains data, a frame s size is not measured in bits but in. Answer: time slots IEEE b Standard 1. Explain that the b standard added two higher speeds, 5.5 Mbps and 11 Mbps, to the original 1 or 2 Mbps standard, and specified RF and direct sequence spread spectrum (DSSS) as the only transmission technology. 2. Mention that the b standard is also known as Wi-Fi.

6 Wireless# Guide to Wireless Communications 7-6 Read more about the IEEE b standard at: 3. Use Figure 7-7 to describe the two sublayers that compose the IEEE PHY layer. 4. Explain that physical layer convergence procedure (PLCP) standards are based on direct sequence spread spectrum (DSSS). PLCP must reformat the data received from the MAC layer into a frame that the PMD sublayer can transmit. Use Figure 7-9 to illustrate your explanation. 5. Describe the structure of the PLCP frame, as explained in this section. Mention that the frame preamble and header are always transmitted at 1 Mbps to allow for communication between slower and faster devices. 6. Explain that the physical medium dependent standards translate the binary 1s and 0s of the frame into radio signals that can be used for transmission b standard uses the Industrial, Scientific, and Medical (ISM) band that specifies 14 available frequencies, beginning at GHz and incrementing by.005 GHz. Use Table 7-4 to illustrate your explanation. 7. Mention that PMD can transmit the data at 11, 5.5, 2, or 1 Mbps. For transmissions at 1 Mbps, two-level differential binary phase shift key (DBPSK) is used. Transmissions at 2, 5.5, and 11 Mbps use differential quadrature phase shift keying. To transmit at rates above 2 Mbps, Complementary Code Keying (CCK), a table containing 64 8-bit code words, is used. 8. Describe the two sublayers that compose the b Data Link Layer: a. Logical Link Control (LLC) b. Media Access Control (MAC) 9. Mention that changes for b WLANs are confined to the MAC layer. 10. Explain that in the b standard, channel access methods are used to prevent collisions. Use Figure 7-10 to illustrate your explanation. One of these methods is carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CA is based on CSMA/CD used in Ethernet. CSMA/CD is designed to handle collisions when they occur. CSMA/CA attempts to avoid collisions altogether. 11. Describe how CSMA/CA works and the differences with CSMA/CD. Use Figure 7-11 to better describe CSMA/CA. Learn more about CSMA/CA at: Describe the following additional mechanisms to reduce collisions in b: a. Request to Send/Clear to Send (RTS/CTS) protocol

7 Wireless# Guide to Wireless Communications 7-7 b. Fragmentation Use Figure 7-12 to explain how RTS/CTS works. 13. Define polling as another type of channel access method. With polling, each computer is sequentially polled, or asked if it wants to transmit. Use Figure 7-13 to show how polling works. Polling effectively eliminates collisions. 14. Mention that the b standard provides for an optional polling function known as point coordination function (PCF). With PCF, the AP serves as the polling device, or point coordinator, and queries each client in an orderly fashion to determine if the client needs to transmit. 15. Explain that association and reassociation allow a client to join a WLAN and stay connected. Association begins with the client scanning the airwaves. 16. Describe the following two types of scanning: a. Passive scanning b. Active scanning 17. Explain how the associate request and associate response frames are used to conclude the association process. 18. Use Figure 7-14 to describe the reassociation process, as explained in this section. 19. Explain that since most clients in a WLAN are portable devices, they can go into sleep mode to conserve battery power. When a client is part of a WLAN, it must remain fully powered. Power management allows the mobile client s NIC to be off as much as possible but still not miss out on data transmissions. The key to power management is synchronization. 20. Explain that when a mobile b client goes into sleep mode, the AP is informed of the change. The AP temporarily stores the synchronized frames (this function is called buffering). At predetermined times, the AP sends out a beacon frame to all clients containing the traffic indication map (TIM). The traffic indication map (TIM) is a list of the clients that have buffered frames waiting at the AP. Use Figure 7-15 to illustrate your explanation. 21. Use Figure 7-16 through Figure 7-18 to describe the following three types of MAC frame formats: a. Management frames b. Control frames c. Data frames Distributed Coordination Function Rules 1. Explain that to work with the distributed coordination function, the standard defines a number of interframe spaces (IFS) to handle the contention for the medium. 2. Use Table 7-5 to describe the following two interframe space types:

8 Wireless# Guide to Wireless Communications 7-8 a. Short interframe space (SIFS) b. Distributed coordination function interframe space (DIFS) 3. Explain the basic rules of communication in an network, as described in this section. Use Figure 7-19 and Figure 7-20 to illustrate your explanation. Quick Quiz 2 1. One type of channel access method is. With this method, each computer is sequentially, or asked if it wants to transmit. Answer: polling, polled b standard allows the mobile client s NIC to be off as much as possible to conserve battery life but still not miss out on data transmissions. Answer: power management 3. In the b standard, frames are used to set up the initial communications between a client and the AP. Answer: management 4. The is a standard time interval during which all clients must wait between transmissions of data frames. Answer: Distributed coordination function interframe space (DIFS) Distributed coordination function interframe space DIFS Class Discussion Topics 1. What is a wireless network interface card (WNIC)? 2. What are the three types of MAC frame formats? Additional Projects 1. Ask your students to use the Internet to research for access points that support power over Ethernet (PoE). Is this technology exclusively reserved to expensive equipment? 2. Many commercial APs have the SSID broadcast option enabled by default, so clients can easily detect the presence of an AP. Ask your students to write a one-page report explaining the security implications of having the SSID broadcast option enabled.

9 Wireless# Guide to Wireless Communications 7-9 Additional Resources 1. Centrino: 2. Power over Ethernet: 3. IEEE : 4. CSMA/CA & CSMA/CD: 5. CSMA: Key Terms 4-pulse position modulation (4-PPM) A modulation technique that translates two data bits into 4 light impulses. 16-pulse position modulation (16-PPM) A modulation technique that translates four data bits into 16 light impulses standard An IEEE standard released in 1990 that defines wireless local area networks at a rate of either 1 Mbps or 2 Mbps. All WLAN features are contained in the PHY and MAC layers b standard A 1999 addition to the IEEE standard for WLANs that added two higher speeds, 5.5 Mbps and 11 Mbps. Also known as Wi-Fi. active scanning The process of sending frames to gather information. ad hoc mode A WLAN mode in which wireless clients communicate directly among themselves without using an AP. associate request frame A frame sent by a client to an AP that contains the client s capabilities and supported rates. associate response frame A frame returned to a client from the AP that contains a status code and client ID number. Basic Service Set (BSS) A WLAN mode that consists of wireless clients and one AP. buffering The process that the AP uses to temporarily store frames for clients that are in sleep mode. channel access methods The different ways of sharing resources in a network environment. collision The scrambling of data that occurs when two computers start sending messages at the same time in a shared frequency. compact flash (CF) card A small expansion card that is used with PDA devices. Complementary Code Keying (CCK) A table containing 64 8-bit code words used for transmitting at speeds above 2 Mbps. This table of codes is used instead of the process of adding a Barker code to the bit to be transmitted.

10 Wireless# Guide to Wireless Communications 7-10 control frames MAC frames that assist in delivering the frames that contain data. data frames MAC frames that carry the information to be transmitted to the destination clients. disassociation frame A frame sent by the new AP to the old AP to terminate the old AP s association with the client. distributed coordination function (DCF) The default access method for WLANs. distributed coordination function interframe space (DIFS) The standard interval between the transmission of data frames. dynamic rate selection A function of an AP that allows it to automatically select the highest transmission speed based on the strength and quality of the signal received from a client NIC. Extended Service Set (ESS) A WLAN mode that consists of wireless clients and multiple APs. fragmentation The division of data to be transmitted from one large frame into several smaller frames. Independent Basic Service Set (IBSS) A WLAN mode in which wireless clients communicate directly among themselves without using an AP. infrastructure mode A WLAN mode that consists of wireless clients and one AP. interframe spaces (IFS) Time gaps used for special types of transmissions. management frames MAC frames that are used to set up the initial communications between a client and the AP. Mini PCI A small card that is functionally equivalent to a standard PCI expansion card used for integrating communications peripherals onto a notebook computer. null data frame The response that a client sends back to the AP to indicate that the client has no transmissions to make. packet acknowledgment (ACK) A procedure for reducing collisions by requiring the receiving station to send an explicit packet back to the sending station. passive scanning The process of listening to each available channel for a set period of time. peer-to-peer mode A WLAN mode in which wireless clients communicate directly among themselves without using an AP. point coordination function (PCF) The optional polling function. polling A channel access method in which each computer is asked in sequence whether it wants to transmit. power management An standard that allows the mobile client to be off as much as possible to conserve battery life but still not miss out on data transmissions. power over Ethernet (PoE) A technology that provides power over an Ethernet cable. probe A frame sent by a client when performing active scanning. probe response A frame sent by an AP when responding to a client s active scanning probe. reassociation The process of a client dropping a connection with one AP and reestablishing the connection with another. reassociation request frame A frame sent from a client to a new AP asking whether it can associate with the AP. reassociation response frame A frame sent by an AP to a station indicating that it will accept its reassociation with that AP. Request to Send/Clear to Send (RTS/CTS) An protocol option that allows a station to reserve the network for transmissions.

11 Wireless# Guide to Wireless Communications 7-11 scanning The process that a client uses to examine the airwaves for information that it needs in order to begin the association process. secure digital (SD) card A small expansion card that is used with PDA devices. SD cards are smaller than CF cards. See also compact flash (CF) cards. Service Set Identifier (SSID) A unique identifier assigned to an AP. short interframe space (SIFS) A time gap used for immediate response actions such as ACK. sled An external attachment for a PDA that permits external cards to attach to the device. sleep mode A power-conserving mode used by notebook computers. subnets Smaller units of a network. time slots The measurement units in a PLCP frame. The length of a time slot is predefined by the standard or specification for a particular system. traffic indication map (TIM) A list of the stations that have buffered frames waiting at the AP. Wi-Fi A trademark of the Wi-Fi Alliance, often used to refer to b WLANs that pass the organization s interoperability tests. wireless residential gateway A combination of several technologies that permit home users to have wireless capabilities and also allow Internet and printer sharing and provide better security than connecting a computer directly to the Internet. Technical Notes for Hands-On Projects Project 7-1: This project requires a notebook computer with a WNIC and the latest version of Windows XP. Project 7-2: This project requires a Linksys WRT54G access point. Project 7-3: This project requires a computer running Windows.