CWNA Guide to Wireless LANs, Second Edition Chapter Five IEEE Media Access Control and Network Layer Standards

2 Objectives List and define the three types of WLAN configurations List and define the three types of WLAN configurations Tell the function of the MAC frame formats Tell the function of the MAC frame formats Explain the MAC procedures for joining, transmitting, and remaining connected to a WLAN Explain the MAC procedures for joining, transmitting, and remaining connected to a WLAN Describe the functions of mobile IP Describe the functions of mobile IP

3 IEEE Wireless LAN Configurations: Basic Service Set Basic Service Set (BSS): Group of wireless devices served by single AP Basic Service Set (BSS): Group of wireless devices served by single AP infrastructure modeinfrastructure mode BSS must be assigned unique identifier BSS must be assigned unique identifier Service Set Identifier (SSID)Service Set Identifier (SSID) Serves as “network name” for BSS Serves as “network name” for BSS Basic Service Area (BSA): Geographical area of a BSS Basic Service Area (BSA): Geographical area of a BSS Max BSA for a WLAN depends on many factorsMax BSA for a WLAN depends on many factors Dynamic rate shifting: As mobile devices move away from AP, transmission speed decreases Dynamic rate shifting: As mobile devices move away from AP, transmission speed decreases

4 IEEE Wireless LAN Configurations: Basic Service Set Figure 5-1: Basic Service Set (BSS)

5 IEEE Wireless LAN Configurations: Extended Service Set Extended Service Set (ESS): Comprised of two or more BSS networks connected via a common distribution system Extended Service Set (ESS): Comprised of two or more BSS networks connected via a common distribution system APs can be positioned so that cells overlap to facilitate roaming APs can be positioned so that cells overlap to facilitate roaming Wireless devices choose AP based on signal strengthWireless devices choose AP based on signal strength HandoffHandoff

6 IEEE Wireless LAN Configurations: Extended Service Set Figure 5-2: Extended Service Set (ESS)

7 IEEE Wireless LAN Configurations: Independent Basic Service Set Independent Basic Service Set (IBSS): Wireless network that does not use an AP Independent Basic Service Set (IBSS): Wireless network that does not use an AP Wireless devices communicate between themselvesWireless devices communicate between themselves Peer-to-peer or ad hoc modePeer-to-peer or ad hoc mode BSS more flexible than IBSS in being able to connect to other wired or wireless networks BSS more flexible than IBSS in being able to connect to other wired or wireless networks IBSS useful for quickly and easily setting up wireless network IBSS useful for quickly and easily setting up wireless network When no connection to Internet or external network neededWhen no connection to Internet or external network needed

8 IEEE Wireless LAN Configurations: Independent Basic Service Set Figure 5-3: Independent Basic Service Set (IBSS)

9 IEEE Media Access Control (MAC) Layer Standards Media Access Control (MAC) layer performs several vital functions in a WLAN Media Access Control (MAC) layer performs several vital functions in a WLAN Discovering WLAN signalDiscovering WLAN signal Joining WLANJoining WLAN Transmitting on WLANTransmitting on WLAN Remaining connected to WLANRemaining connected to WLAN Mechanics of how functions performed center around frames sent and received in WLANs Mechanics of how functions performed center around frames sent and received in WLANs

10 MAC Frame Formats Packet: Smaller segments of a digital data transmission Packet: Smaller segments of a digital data transmission Strictly speaking, other terms used to describe these smaller segmentsStrictly speaking, other terms used to describe these smaller segments Frames: Packet at MAC layer Frames: Packet at MAC layer Or Data Link layer in OSI modelOr Data Link layer in OSI model IEEE MAC frames different from Ethernet frames in format and functionIEEE MAC frames different from Ethernet frames in format and function Used by wireless NICs and APs for communications and managing/controlling wireless networkUsed by wireless NICs and APs for communications and managing/controlling wireless network

11 MAC Frame Formats Frame control field identifies: Frame control field identifies: Specific protocol versionSpecific protocol version Frame typeFrame type Indicators that show WLAN configurationIndicators that show WLAN configuration All frames contain All frames contain MAC address of the source and destination deviceMAC address of the source and destination device Frame sequence numberFrame sequence number Frame check sequence for error detectionFrame check sequence for error detection

12 MAC Frame Formats Management Frames: Initialize communications between device and AP (infrastructure mode) or between devices (ad hoc mode) Management Frames: Initialize communications between device and AP (infrastructure mode) or between devices (ad hoc mode) Maintain connectionMaintain connection Figure 5-4: Structure of a management frame

13 MAC Frame Formats Types of management frames: Types of management frames: Authentication frameAuthentication frame Association request frameAssociation request frame Association response frameAssociation response frame Beacon frameBeacon frame Deauthentication frameDeauthentication frame Disassociation frameDisassociation frame Probe request frameProbe request frame Probe response frameProbe response frame Reassociation request frameReassociation request frame Reassociation response frameReassociation response frame

14 MAC Frame Formats Control frames: Provide assistance in delivering frames that contain data Control frames: Provide assistance in delivering frames that contain data Figure 5-5: Control frame

15 MAC Frame Formats Data frame: Carries information to be transmitted to destination device Data frame: Carries information to be transmitted to destination device Figure 5-6: Data frame

16 Discovering the WLAN: Beaconing At regular intervals, AP (infrastructure network) or wireless device (ad hoc network) sends beacon frame At regular intervals, AP (infrastructure network) or wireless device (ad hoc network) sends beacon frame Announce presenceAnnounce presence Provide info for other devices to join networkProvide info for other devices to join network Beacon frame format follows standard structure of a management frame Beacon frame format follows standard structure of a management frame Destination address always set to all onesDestination address always set to all ones

17 Discovering the WLAN: Beaconing Figure 5-7: Beaconing

18 Discovering the WLAN: Beaconing Beacon frame body contains following fields: Beacon frame body contains following fields: Beacon intervalBeacon interval TimestampTimestamp Service Set Identifier (SSID)Service Set Identifier (SSID) Supported ratesSupported rates Parameter setsParameter sets Capability informationCapability information In ad hoc networks, each wireless device assumes responsibility for beaconing In ad hoc networks, each wireless device assumes responsibility for beaconing In infrastructure networks beacon interval normally 100 ms, but can be modified In infrastructure networks beacon interval normally 100 ms, but can be modified

19 Discovering the WLAN: Scanning Receiving wireless device must be looking for beacon frames Receiving wireless device must be looking for beacon frames Passive scanning: Wireless device simply listens for beacon frame Passive scanning: Wireless device simply listens for beacon frame Typically, on each available channel for set periodTypically, on each available channel for set period Active scanning: Wireless device first sends out a management probe request frame on each available channel Active scanning: Wireless device first sends out a management probe request frame on each available channel Then waits for probe response frame from all available APsThen waits for probe response frame from all available APs

20 Discovering the WLAN: Scanning Figure 5-8: Active scanning

21 Joining the WLAN: Authentication Unlike standard wired LANS, authentication performed before user connected to network Unlike standard wired LANS, authentication performed before user connected to network Authentication of the wireless device, not the userAuthentication of the wireless device, not the user IEEE authentication: Process in which AP accepts or rejects a wireless device IEEE authentication: Process in which AP accepts or rejects a wireless device Open system authentication: Most basic, and default, authentication method Open system authentication: Most basic, and default, authentication method Shared key authentication: Optional authentication method Shared key authentication: Optional authentication method Utilizes challenge textUtilizes challenge text

22 Joining the WLAN: Authentication Figure 5-9: Open system authentication

23 Joining the WLAN: Authentication (continued) Figure 5-10: Shared key authentication

24 Joining the WLAN: Authentication Open system and Shared key authentication techniques are weak Open system and Shared key authentication techniques are weak Open System: Only need SSID to connectOpen System: Only need SSID to connect Shared Key: Key installed manually on devicesShared Key: Key installed manually on devices Can be discovered by examining the devices Can be discovered by examining the devices Digital certificates: Digital documents that associate an individual with key value Digital certificates: Digital documents that associate an individual with key value Digitally “signed” by trusted third partyDigitally “signed” by trusted third party Cannot change any part of digital certificate without being detectedCannot change any part of digital certificate without being detected

25 Joining the WLAN: Association Association: Accepting a wireless device into a wireless network Association: Accepting a wireless device into a wireless network Final step to join WLANFinal step to join WLAN After authentication, AP responds with association response frame After authentication, AP responds with association response frame Contains acceptance or rejection noticeContains acceptance or rejection notice If AP accepts wireless device, reserves memory space in AP and establishes association ID If AP accepts wireless device, reserves memory space in AP and establishes association ID Association response frame includes association ID and supported data rates Association response frame includes association ID and supported data rates

26 Transmitting on the WLAN: Distributed Coordination Function (DCF) MAC layer responsible for controlling access to wireless medium MAC layer responsible for controlling access to wireless medium Channel access methods: Rules for cooperation among wireless devices Channel access methods: Rules for cooperation among wireless devices Contention: Computers compete to use mediumContention: Computers compete to use medium If two devices send frames simultaneously, collision results and frames become unintelligible If two devices send frames simultaneously, collision results and frames become unintelligible Must take steps to avoid collisions Must take steps to avoid collisions

27 Transmitting on the WLAN: Distributed Coordination Function Carrier Sense Multiple Access with Collision Detection (CSMA/CD): Before networked device sends a frame, listens to see if another device currently transmitting Carrier Sense Multiple Access with Collision Detection (CSMA/CD): Before networked device sends a frame, listens to see if another device currently transmitting If traffic exists, wait; otherwise sendIf traffic exists, wait; otherwise send Devices continue listening while sending frameDevices continue listening while sending frame If collision occurs, stops and broadcasts a “jam” signal If collision occurs, stops and broadcasts a “jam” signal CSMA/CD cannot be used on wireless networks: CSMA/CD cannot be used on wireless networks: Difficult to detect collisionsDifficult to detect collisions Hidden node problemHidden node problem

28 Transmitting on the WLAN: Distributed Coordination Function Figure 5-11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

29 Transmitting on the WLAN: Distributed Coordination Function Figure 5-11 (continued): Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

30 Transmitting on the WLAN: Distributed Coordination Function Figure 5-12: Hidden node problem

31 Transmitting on the WLAN: Distributed Coordination Function Distributed Coordination Function (DCF): Specifies modified version of CSMA/CD Distributed Coordination Function (DCF): Specifies modified version of CSMA/CD Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) Attempts to avoid collisions altogetherAttempts to avoid collisions altogether Time when most collisions occur is immediately after a station completes transmissionTime when most collisions occur is immediately after a station completes transmission All stations must wait random amount of time after medium clearAll stations must wait random amount of time after medium clear Slot time Slot time

32 Transmitting on the WLAN: Distributed Coordination Function CSMA/CA also reduces collisions via explicit frame acknowledgment CSMA/CA also reduces collisions via explicit frame acknowledgment Acknowledgment frame (ACK): Sent by receiving device to sending device to confirm data frame arrived intactAcknowledgment frame (ACK): Sent by receiving device to sending device to confirm data frame arrived intact If ACK not returned, transmission error assumedIf ACK not returned, transmission error assumed CSMA/CA does not eliminate collisions CSMA/CA does not eliminate collisions Does not solve hidden node problemDoes not solve hidden node problem

33 Transmitting on the WLAN: Distributed Coordination Function Figure 5-13: CSMA/CA and ACK

34 Transmitting on the WLAN: Distributed Coordination Function Request to Send/Clear to Send (RTS/CTS) protocol: Option used to solve hidden node problem Request to Send/Clear to Send (RTS/CTS) protocol: Option used to solve hidden node problem Significant overhead upon the WLAN with transmission of RTS and CTS framesSignificant overhead upon the WLAN with transmission of RTS and CTS frames Especially with short data packets Especially with short data packets RTS threshold: Only packets that longer than RTS threshold transmitted using RTS/CTSRTS threshold: Only packets that longer than RTS threshold transmitted using RTS/CTS

35 Transmitting on the WLAN: Distributed Coordination Function Figure 5-14: Request to Send/Clear to Send (RTS/CTS)

36 Transmitting on the WLAN: Interframe Spacing Interframe spaces (IFS): Intervals between transmissions of data frames Interframe spaces (IFS): Intervals between transmissions of data frames Short IFS (SIFS): For immediate response actions such as ACKShort IFS (SIFS): For immediate response actions such as ACK Point Coordination Function IFS (PIFS): Time used by a device to access medium after it has been asked and then given approval to transmitPoint Coordination Function IFS (PIFS): Time used by a device to access medium after it has been asked and then given approval to transmit Distributed Coordination Function IFS (DIFS): Standard interval between transmission of data framesDistributed Coordination Function IFS (DIFS): Standard interval between transmission of data frames

37 Transmitting on the WLAN: Interframe Spacing (continued) Figure 5-15: CSMA/CA with one station transmitting

38 Transmitting on the WLAN: Interframe Spacing Figure 5-16: CSMA/CA with two stations transmitting

39 Transmitting on the WLAN: Fragmentation Fragmentation: Divide data to be transmitted from one large frame into several smaller ones Fragmentation: Divide data to be transmitted from one large frame into several smaller ones Reduces probability of collisionsReduces probability of collisions Reduces amount of time medium is in useReduces amount of time medium is in use If data frame length exceeds specific value, MAC layer fragments it If data frame length exceeds specific value, MAC layer fragments it Receiving station reassembles fragmentsReceiving station reassembles fragments Alternative to RTS/CTS Alternative to RTS/CTS High overheadHigh overhead ACKs and additional SIFS time gaps ACKs and additional SIFS time gaps

40 Transmitting on the WLAN: Point Coordination Function (PCF) Polling: Channel access method in which each device asked in sequence if it wants to transmit Polling: Channel access method in which each device asked in sequence if it wants to transmit Effectively prevents collisionsEffectively prevents collisions Point Coordination Function (PCF): AP serves as polling device or “point coordinator” Point Coordination Function (PCF): AP serves as polling device or “point coordinator” Point coordinator has to wait only through point coordination function IFS (PIFS) time gap Point coordinator has to wait only through point coordination function IFS (PIFS) time gap Shorter than DFIS time gapShorter than DFIS time gap

41 Transmitting on the WLAN: Point Coordination Function (continued) If point coordinator hears no traffic after PIFS time gap, sends out beacon frame If point coordinator hears no traffic after PIFS time gap, sends out beacon frame Field to indicate length of time that PCF (polling) will be used instead of DCF (contention)Field to indicate length of time that PCF (polling) will be used instead of DCF (contention) Receiving stations must stop transmission for that amount of time Receiving stations must stop transmission for that amount of time Point coordinator then sends frame to specific station, granting permission to transmit one framePoint coordinator then sends frame to specific station, granting permission to transmit one frame standard allows WLAN to alternate between PCF (polling) and DCF (contention) standard allows WLAN to alternate between PCF (polling) and DCF (contention)

42 Transmitting on the WLAN: Point Coordination Function Figure 5-18: DIFS and DCF frames

43 Transmitting on the WLAN: Quality of Service (QoS) and e DCF does not work well for real-time, time-dependent traffic DCF does not work well for real-time, time-dependent traffic Quality of Service (QoS): Capability to prioritize different types of frames Quality of Service (QoS): Capability to prioritize different types of frames Wi-Fi Multimedia (WMM): Modeled after wired network QoS prioritization scheme Wi-Fi Multimedia (WMM): Modeled after wired network QoS prioritization scheme e draft: defines superset of features intended to provide QoS over WLANs e draft: defines superset of features intended to provide QoS over WLANs Proposes two new mode of operation for MAC LayerProposes two new mode of operation for MAC Layer

44 Transmitting on the WLAN: Quality of Service and e Table 5-1: Wi-Fi Multimedia (WMM)

45 Transmitting on the WLAN: Quality of Service and e e draft (continued): e draft (continued): Enhanced Distributed Channel Access (EDCA): Contention-based but supports different types of trafficEnhanced Distributed Channel Access (EDCA): Contention-based but supports different types of traffic Four access categories (AC) Four access categories (AC) Provides “relative” QoS but cannot guarantee service Provides “relative” QoS but cannot guarantee service Hybrid Coordination Function Controlled Channel Access (HCCA): New form of PCF based upon pollingHybrid Coordination Function Controlled Channel Access (HCCA): New form of PCF based upon polling Serves as a centralized scheduling mechanism Serves as a centralized scheduling mechanism

46 Remaining Connected to the WLAN: Reassociation Reassociation: Device drops connection with one AP and establish connection with another Reassociation: Device drops connection with one AP and establish connection with another Several reason why reassociation may occur:Several reason why reassociation may occur: Roaming Roaming Weakened signal Weakened signal When device determines link to current AP is poor, begins scanning to find another APWhen device determines link to current AP is poor, begins scanning to find another AP Can use information from previous scans Can use information from previous scans

47 Remaining Connected to the WLAN: Power Management When laptop is part of a WLAN, must remain “awake” in order to receive network transmissions When laptop is part of a WLAN, must remain “awake” in order to receive network transmissions Original IEEE 802 standard assumes stations always ready to receive network messagesOriginal IEEE 802 standard assumes stations always ready to receive network messages Power management: Allows mobile devices to conserve battery life without missing transmissions Power management: Allows mobile devices to conserve battery life without missing transmissions Transparent to all protocolsTransparent to all protocols Differs based on WLAN configurationDiffers based on WLAN configuration AP records which stations awake and sleepingAP records which stations awake and sleeping Buffering: If sleeping, AP temporarily stores framesBuffering: If sleeping, AP temporarily stores frames

48 Remaining Connected to the WLAN: Power Management Figure 5-19: Power management in infrastructure mode

49 Remaining Connected to the WLAN: Power Management At set times AP send out beacon to all stations At set times AP send out beacon to all stations Contains traffic indication map (TIM)Contains traffic indication map (TIM) At same time, all sleeping stations switch into active listening modeAt same time, all sleeping stations switch into active listening mode Power management in ad hoc mode: Power management in ad hoc mode: Ad hoc traffic indication message (ATIM) window: Time at which all stations must be awakeAd hoc traffic indication message (ATIM) window: Time at which all stations must be awake Wireless device sends beacon to all other devices Wireless device sends beacon to all other devices Devices that previously attempted to send a frame to a sleeping device will send ATIM frame indicating that receiving device has data to receive and must remain awakeDevices that previously attempted to send a frame to a sleeping device will send ATIM frame indicating that receiving device has data to receive and must remain awake

50 WLAN Network Layer Standards: WLAN IP Addressing In standard networking, IP protocol responsible for moving frames between computers In standard networking, IP protocol responsible for moving frames between computers Network layer protocolNetwork layer protocol TCP/IP works on principle that each network host has unique IP address TCP/IP works on principle that each network host has unique IP address Used to locate path to specific hostUsed to locate path to specific host Routers use IP address to forward packetsRouters use IP address to forward packets Prohibits mobile users from switching to another network and using same IP numberProhibits mobile users from switching to another network and using same IP number Users who want to roam need new IP address on every network Users who want to roam need new IP address on every network

51 WLAN Network Layer Standards: Mobile IP Provides mechanism within TCP/IP protocol to support mobile computing Provides mechanism within TCP/IP protocol to support mobile computing Computers given home address,Computers given home address, Static IP number on home network Static IP number on home network Home agent: Forwarding mechanism that keeps track of where mobile computer locatedHome agent: Forwarding mechanism that keeps track of where mobile computer located When computer moves to foreign network, a foreign agent provides routing servicesWhen computer moves to foreign network, a foreign agent provides routing services Assigns computer a care-of address Assigns computer a care-of address Computer registers care-of address with home agent Computer registers care-of address with home agent

52 WLAN Network Layer Standards: Mobile IP Figure 5-20: Mobile IP components

53 WLAN Network Layer Standards: Mobile IP Figure 5-21: Computer relocated in Mobile IP

54 WLAN Network Layer Standards: Mobile IP Figure 5-22: Encapsulated Mobile IP frame

55 Summary A Basic Service Set (BSS) is defined as a group of wireless devices that is served by a single access point (AP) A Basic Service Set (BSS) is defined as a group of wireless devices that is served by a single access point (AP) An Extended Service Set (ESS) is comprised of two or more BSS networks that are connected through a common distribution system An Extended Service Set (ESS) is comprised of two or more BSS networks that are connected through a common distribution system An Independent Basic Service Set (IBSS) is a wireless network that does not use an access point An Independent Basic Service Set (IBSS) is a wireless network that does not use an access point Frames are used by both wireless NICs and access points for communication and for managing and controlling the wireless network Frames are used by both wireless NICs and access points for communication and for managing and controlling the wireless network

56 Summary The MAC layer provides four major functions in WLANs: discovering the WLAN signal, joining the WLAN, transmitting on the WLAN, and remaining connected to the WLAN The MAC layer provides four major functions in WLANs: discovering the WLAN signal, joining the WLAN, transmitting on the WLAN, and remaining connected to the WLAN Discovery is a twofold process: the AP or other wireless devices must transmit an appropriate frame (beaconing), and the wireless device must be looking for those frames (scanning) Discovery is a twofold process: the AP or other wireless devices must transmit an appropriate frame (beaconing), and the wireless device must be looking for those frames (scanning) Once a wireless device has discovered the WLAN, it requests to join the network; This is a twofold process known as authentication and association Once a wireless device has discovered the WLAN, it requests to join the network; This is a twofold process known as authentication and association

57 Summary The IEEE standard specifies two procedures for transmitting on the WLAN, distributed coordination function (DCF) and an optional point coordination function (PCF) The IEEE standard specifies two procedures for transmitting on the WLAN, distributed coordination function (DCF) and an optional point coordination function (PCF) The standard provides for an optional polling function known as Point Coordination Function (PCF) The standard provides for an optional polling function known as Point Coordination Function (PCF) The e draft defines a superset of features that is intended to provide QoS over WLANs The e draft defines a superset of features that is intended to provide QoS over WLANs

58 Summary Power management allows mobile devices to be off as much as possible to conserve battery life but not miss data transmissions Power management allows mobile devices to be off as much as possible to conserve battery life but not miss data transmissions Mobile IP provides a mechanism within the TCP/IP protocol to support mobile computing Mobile IP provides a mechanism within the TCP/IP protocol to support mobile computing