Wireless# Guide to Wireless Communications Chapter 7 Low-Speed Wireless Local Area 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 802.11, and IEEE 802.11b WLANs Outline the user mobility features offered by IEEE 802.11 networks Wireless# Guide to Wireless Communications 2 1
WLAN Applications Wireless networks are increasing in popularity Installing cabling is inconvenient and very expensive Wireless networks solve this problem With a wireless network Multiple users can share a single Internet connection Wireless residential gateway Device that combines a router, Ethernet switch, and wireless access point Also allows Internet and printer sharing Wireless# Guide to Wireless Communications 3 WLAN Components Minimal hardware needed for a WLAN Computer ISP (???) Wireless network interface card Access point (AP) Wireless# Guide to Wireless Communications 4 2
Wireless Network Interface Card Network interface card (NIC) Allows a computer to be connected to a network Wireless NIC Connects a computer to a network without cables Most often a separate card Mini PCI Small card that is functionally equivalent to a standard PCI expansion card Used with notebook computers Wireless# Guide to Wireless Communications 5 Wireless Network Interface Card (continued) Wireless# Guide to Wireless Communications 6 3
Wireless Network Interface Card (continued) Smaller devices have two options for wireless NICs Optional sled Includes a Type II PC Card slot and battery Compact flash (CF) card or secure digital (SD) card Smaller and consume less power Transmit and receive with lower power levels Intel Centrino chipset Integrates all of the functions of a wireless NIC Mounted directly on the motherboard Wireless# Guide to Wireless Communications 7 Wireless Network Interface Card (continued) Wireless# Guide to Wireless Communications 8 4
Access Points Access point (AP) Provides wireless LAN devices with a point of access into a wired network AP parts Radio transceiver Antenna RJ-45 wired network interface port AP functions Wireless communications base station Bridge between the wireless and wired networks Wireless# Guide to Wireless Communications 9 Access Points (continued) Wireless# Guide to Wireless Communications 10 5
Access Points (continued) The range of an AP is approximately 115 meters Dynamic rate selection AP will automatically select the highest possible data rate for transmission Depending on the strength and quality of the signal An AP can generally support over 100 users Power over Ethernet (PoE) DC power is delivered to the AP through the unused wires in a standard UTP Ethernet cable Wireless# Guide to Wireless Communications 11 WLAN Modes Ad hoc mode Infrastructure mode Wireless# Guide to Wireless Communications 12 6
Ad Hoc Mode Also known as peer-to-peer mode Formal name: Independent Basic Service Set (IBSS) mode Wireless clients communicate directly among themselves without using an AP Quick and easy setup of a wireless network Drawback is that wireless clients can only communicate among themselves Wireless# Guide to Wireless Communications 13 Ad Hoc Mode (continued) Wireless# Guide to Wireless Communications 14 7
Infrastructure Mode Also known as the Basic Service Set (BSS) Consists of wireless clients and an AP Extended Service Set (ESS) Two or more BSS wireless networks installed in same area Provides users with uninterrupted mobile access to the network All wireless clients and APs must be part of the same network For users to be able to roam freely Wireless# Guide to Wireless Communications 15 Infrastructure Mode (continued) Wireless# Guide to Wireless Communications 16 8
Infrastructure Mode (continued) Can be difficult to manage one large network Performance and security may also be adversely affected Subnets Network units that contain fewer computers In an ESS divided into subnets A mobile user might not be able to freely roam between APs Wireless# Guide to Wireless Communications 17 Wireless LAN Standards and Operation Most WLANs are based on these same initial IEEE 802.11 standards Wireless# Guide to Wireless Communications 18 9
IEEE 802.11 Standards 802.11 standard Defines a local area network that provides cable-free 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 Specifies that the features of a WLAN be transparent to the upper layers of the TCP/IP protocol stack Or the OSI protocol model Wireless# Guide to Wireless Communications 19 IEEE 802.11 Standards (continued) Wireless# Guide to Wireless Communications 20 10
IEEE 802.11 Infrared WLAN Standard IEEE 802.11 standards outline the specifications for infrared as well as RF WLANs Infrared specification is based on diffused transmissions PHY layer sublayers Physical Medium Dependent (PMD) Physical Layer Convergence Procedure (PLCP) Wireless# Guide to Wireless Communications 21 IEEE 802.11 Infrared WLAN Standard (continued) Wireless# Guide to Wireless Communications 22 11
IEEE 802.11 Infrared WLAN Standard (continued) Diffused infrared PHY layer convergence procedure standards Reformats the data received from the MAC layer Into a frame that the PMD sublayer can transmit Frame s size is not measured in bits but in time slots Start frame delimiter for infrared is always 1001 Data rate value determines transmission speed Direct current level adjustment Contains a pattern of infrared pulses Allows receiving device to determine the signal level Wireless# Guide to Wireless Communications 23 IEEE 802.11 Infrared WLAN Standard (continued) Wireless# Guide to Wireless Communications 24 12
IEEE 802.11 Infrared WLAN Standard (continued) Wireless# Guide to Wireless Communications 25 IEEE 802.11 Infrared WLAN Standard (continued) Diffused infrared physical medium dependent standards PMD translates the binary 1s and 0s of the frame into light pulses that are used for transmission PMD transmits data using a series of light impulses 16-pulse position modulation (16-PPM) Translates four data bits into 16 light impulses 4-pulse position modulation (4-PPM) Used for transmissions at 2 Mbps Each time slot is 250 ns (nanoseconds) Wireless# Guide to Wireless Communications 26 13
IEEE 802.11 Infrared WLAN Standard (continued) Wireless# Guide to Wireless Communications 27 IEEE 802.11 Infrared WLAN Standard (continued) Wireless# Guide to Wireless Communications 28 14
IEEE 802.11b Standard Added two higher speeds, 5.5 Mbps and 11 Mbps Specified RF and direct sequence spread spectrum (DSSS) as the only transmission technology Also known as Wi-Fi Physical layer Divided into two parts Physical Medium Dependent (PMD) Physical Layer Convergence Procedure (PLCP) Wireless# Guide to Wireless Communications 29 IEEE 802.11b Standard (continued) Physical layer convergence procedure standards Based on direct sequence spread spectrum (DSSS) Must reformat the data received from the MAC layer into a frame that the PMD sublayer can transmit PLCP frame is made up of three parts: the preamble, the header, and the data Frame preamble and header are always transmitted at 1 Mbps Allows communication between slower and faster devices Wireless# Guide to Wireless Communications 30 15
IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 31 IEEE 802.11b Standard (continued) Physical medium dependent standards Translate the binary 1s and 0s of the frame into radio signals that can be used for transmission 802.11b standard uses the Industrial, Scientific, and Medical (ISM) band Specifies 14 available frequencies, beginning at 2.412 GHz and incrementing by.005 GHz 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 Wireless# Guide to Wireless Communications 32 16
IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 33 IEEE 802.11b Standard (continued) Physical medium dependent standards (continued) 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) is used A table containing 64 8-bit code words Media access control layer 802.11b Data Link layer consists of two sublayers Logical Link Control (LLC) Media Access Control (MAC) Wireless# Guide to Wireless Communications 34 17
IEEE 802.11b Standard (continued) Media access control layer (continued) Changes for 802.11b WLANs are confined to the MAC layer On a WLAN, all devices share the same RF spectrum Distributed coordination function Channel access methods can prevent collisions Carrier sense multiple access with collision avoidance (CSMA/CA) Based on CSMA/CD Wireless# Guide to Wireless Communications 35 IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 36 18
IEEE 802.11b Standard (continued) Distributed coordination function (continued) CSMA/CD is designed to handle collisions CSMA/CA attempts to avoid collisions altogether With contention-based channel access methods Most collisions occur after a device completes its transmission CSMA/CA makes all devices wait a random amount of time CSMA/CA also reduces collisions by using explicit packet acknowledgment (ACK) Wireless# Guide to Wireless Communications 37 IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 38 19
IEEE 802.11b Standard (continued) Distributed coordination function (continued) Additional mechanisms to reduce collisions Request to Send/Clear to Send (RTS/CTS) protocol Fragmentation Polling Another type of channel access method Each computer is asked if it wants to transmit Polling effectively eliminates collisions Point coordination function (PCF) AP serves as the polling device Wireless# Guide to Wireless Communications 39 IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 40 20
IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 41 IEEE 802.11b Standard (continued) Association and reassociation Allow a client to join a WLAN and stay connected Association begins with the client scanning the airwaves Types of scanning Passive scanning Client listening to each available channel for a set period of time Client listens for a beacon frame transmitted from all available APs Frame includes the AP s SSID Wireless# Guide to Wireless Communications 42 21
IEEE 802.11b Standard (continued) Association and reassociation (continued) Types of scanning (continued) Active scanning Client sends a probe frame to each channel Client then waits for the probe response frame Associate request frame Includes the client s capabilities and supported rates Associate response frame Sent by the AP Contains a status code and client ID number for that client Wireless# Guide to Wireless Communications 43 IEEE 802.11b Standard (continued) Association and reassociation (continued) Reassociation Client may drop the connection with one AP and reestablish the connection with another Necessary when mobile clients roam beyond the coverage area of one AP Client sends a reassociation request frame to new AP AP sends back a reassociation response frame New AP sends a disassociation frame to old AP Wireless# Guide to Wireless Communications 44 22
IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 45 IEEE 802.11b Standard (continued) Power management Most clients in a WLAN are portable devices To conserve battery power, they can go into sleep mode 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 Wireless# Guide to Wireless Communications 46 23
IEEE 802.11b Standard (continued) Power management (continued) When a mobile 802.11b client goes into sleep mode, the AP is informed of the change 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) Traffic indication map (TIM) List of clients with buffered frames waiting at the AP Wireless# Guide to Wireless Communications 47 IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 48 24
IEEE 802.11b Standard (continued) MAC frame formats Three types of MAC frame formats Management frames Used to set up the initial communications Control frames Provide assistance in delivering the frames that contain the data Data frames Carry the information to be transmitted to the destination client Wireless# Guide to Wireless Communications 49 IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 50 25
IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 51 IEEE 802.11b Standard (continued) Wireless# Guide to Wireless Communications 52 26
Distributed Coordination Function Rules 802.11 standard defines a number of interframe spaces (IFS) To handle the contention for the medium Interframe space types Short interframe space (SIFS) Distributed coordination function interframe space (DIFS) Basic rules of communication in an 802.11 network Device that wants to transmit begins listening for an RF signal Wireless# Guide to Wireless Communications 53 Distributed Coordination Function Rules (continued) Wireless# Guide to Wireless Communications 54 27
Distributed Coordination Function Rules (continued) Basic rules of communication in an 802.11 network (continued) Size of a frame includes both the length of time necessary to send the data plus an SIFS After receiving an ACK, the transmitting client begins to wait a random backoff interval If the transmitting device does not receive an ACK within the SIFS It is allowed to maintain control of the medium If the frame was acknowledged correctly Transmitting device listens to the medium while waiting its backoff interval Wireless# Guide to Wireless Communications 55 Distributed Coordination Function Rules (continued) Wireless# Guide to Wireless Communications 56 28
Distributed Coordination Function Rules (continued) In the case of traffic in the medium Client A is using DSSS with a backoff interval of 3 While Client B has a backoff interval of only 2 Client A transmits its first frame Client B detects traffic and waits Both devices begin carrier sensing during second DIFS At the end, clients A and B begin their backoff interval Client B will finish its backoff interval before Client A Then begins transmitting its first packet Next time, client A will wait only for 1 interval Wireless# Guide to Wireless Communications 57 Distributed Coordination Function Rules (continued) Wireless# Guide to Wireless Communications 58 29
Summary The wireless technology that attracts the most attention today Radio frequency (RF) wireless local area networks (WLANs) Wireless NIC performs same functions as wired NIC Access points parts Antenna Radio transceiver RJ-45 wired network interface Wireless# Guide to Wireless Communications 59 Summary (continued) Data can be sent and received in an RFWLAN either in ad hoc or infrastructure mode IEEE 802.11 standard defines a local area network that provides cable-free data access 802.11 standard uses an access method known as the distributed coordination function (DCF) Specifies that carrier sense multiple access with collision avoidance (CSMA/CA) be used 802.11 standard provides for an optional polling function known as point coordination function (PCF) Wireless# Guide to Wireless Communications 60 30
Summary (continued) Mobile WLAN devices often depend on batteries as their primary power source Sleep mode conserves battery power 802.11 standard specifies three different types of MAC frame formats Management frames Control frames Data frames Wireless# Guide to Wireless Communications 61 31