Wireless Guide to Wireless Communications Chapter 7 LowSpeed
- Slides: 61
Wireless# Guide to Wireless Communications Chapter 7 Low-Speed Wireless Local Area Networks Wireless# Guide to Wireless Communications
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. 11 b WLANs • Outline the user mobility features offered by IEEE 802. 11 networks Wireless# Guide to Wireless Communications 2
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
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
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
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
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 (Po. E) – 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
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
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
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
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
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
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
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 1 s and 0 s 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
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
IEEE 802. 11 b 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. 11 b 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
IEEE 802. 11 b Standard (continued) Wireless# Guide to Wireless Communications 31
IEEE 802. 11 b Standard (continued) • Physical medium dependent standards – Translate the binary 1 s and 0 s of the frame into radio signals that can be used for transmission – 802. 11 b 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
IEEE 802. 11 b Standard (continued) Wireless# Guide to Wireless Communications 33
IEEE 802. 11 b 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. 11 b Data Link layer consists of two sublayers • Logical Link Control (LLC) • Media Access Control (MAC) Wireless# Guide to Wireless Communications 34
IEEE 802. 11 b Standard (continued) • Media access control layer (continued) – Changes for 802. 11 b 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. 11 b Standard (continued) Wireless# Guide to Wireless Communications 36
IEEE 802. 11 b 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. 11 b Standard (continued) Wireless# Guide to Wireless Communications 38
IEEE 802. 11 b 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. 11 b Standard (continued) Wireless# Guide to Wireless Communications 40
IEEE 802. 11 b Standard (continued) Wireless# Guide to Wireless Communications 41
IEEE 802. 11 b 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
IEEE 802. 11 b 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. 11 b 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
IEEE 802. 11 b Standard (continued) Wireless# Guide to Wireless Communications 45
IEEE 802. 11 b 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
IEEE 802. 11 b Standard (continued) • Power management (continued) – When a mobile 802. 11 b 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. 11 b Standard (continued) Wireless# Guide to Wireless Communications 48
IEEE 802. 11 b 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. 11 b Standard (continued) Wireless# Guide to Wireless Communications 50
IEEE 802. 11 b Standard (continued) Wireless# Guide to Wireless Communications 51
IEEE 802. 11 b Standard (continued) Wireless# Guide to Wireless Communications 52
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
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
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 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
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
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
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