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

CWNA Guide to Wireless LANs, Second Edition2 Objectives List and define the three types of WLAN configurations Tell the function of the MAC frame formats Explain the MAC procedures for joining, transmitting, and remaining connected to a WLAN Describe the functions of mobile IP

CWNA Guide to Wireless LANs, Second Edition3 IEEE Wireless LAN Configurations: Basic Service Set Basic Service Set (BSS): Group of wireless devices served by single AP –infrastructure mode BSS must be assigned unique identifier –Service Set Identifier (SSID) Serves as “network name” for BSS Basic Service Area (BSA): Geographical area of a BSS –Max BSA for a WLAN depends on many factors Dynamic rate shifting: As mobile devices move away from AP, transmission speed decreases

CWNA Guide to Wireless LANs, Second Edition4 IEEE Wireless LAN Configurations: Basic Service Set (continued) Figure 5-1: Basic Service Set (BSS)

CWNA Guide to Wireless LANs, Second Edition5 IEEE Wireless LAN Configurations: Extended Service Set 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 –Wireless devices choose AP based on signal strength –Handoff

CWNA Guide to Wireless LANs, Second Edition6 IEEE Wireless LAN Configurations: Extended Service Set (continued) Figure 5-2: Extended Service Set (ESS)

CWNA Guide to Wireless LANs, Second Edition7 IEEE Wireless LAN Configurations: Independent Basic Service Set Independent Basic Service Set (IBSS): Wireless network that does not use an AP –Wireless devices communicate between themselves –Peer-to-peer or ad hoc mode 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 –When no connection to Internet or external network needed

CWNA Guide to Wireless LANs, Second Edition8 IEEE Wireless LAN Configurations: Independent Basic Service Set (continued) Figure 5-3: Independent Basic Service Set (IBSS)

CWNA Guide to Wireless LANs, Second Edition9 IEEE Media Access Control (MAC) Layer Standards Media Access Control (MAC) layer performs several vital functions in a WLAN –Discovering WLAN signal –Joining WLAN –Transmitting on WLAN –Remaining connected to WLAN Mechanics of how functions performed center around frames sent and received in WLANs

CWNA Guide to Wireless LANs, Second Edition10 MAC Frame Formats Packet: Smaller segments of a digital data transmission –Strictly speaking, other terms used to describe these smaller segments Frames: Packet at MAC layer –Or Data Link layer in OSI model –IEEE MAC frames different from Ethernet frames in format and function –Used by wireless NICs and APs for communications and managing/controlling wireless network

CWNA Guide to Wireless LANs, Second Edition11 MAC Frame Formats (continued) Frame control field identifies: –Specific protocol version –Frame type –Indicators that show WLAN configuration All frames contain –MAC address of the source and destination device –Frame sequence number –Frame check sequence for error detection

CWNA Guide to Wireless LANs, Second Edition12 MAC Frame Formats (continued) Management Frames: Initialize communications between device and AP (infrastructure mode) or between devices (ad hoc mode) –Maintain connection Figure 5-4: Structure of a management frame

CWNA Guide to Wireless LANs, Second Edition13 MAC Frame Formats (continued) Types of management frames: –Authentication frame –Association request frame –Association response frame –Beacon frame –Deauthentication frame –Disassociation frame –Probe request frame –Probe response frame –Reassociation request frame –Reassociation response frame

CWNA Guide to Wireless LANs, Second Edition14 MAC Frame Formats (continued) Control frames: Provide assistance in delivering frames that contain data Figure 5-5: Control frame

CWNA Guide to Wireless LANs, Second Edition15 MAC Frame Formats (continued) Data frame: Carries information to be transmitted to destination device Figure 5-6: Data frame

CWNA Guide to Wireless LANs, Second Edition16 Discovering the WLAN: Beaconing At regular intervals, AP (infrastructure network) or wireless device (ad hoc network) sends beacon frame –Announce presence –Provide info for other devices to join network Beacon frame format follows standard structure of a management frame –Destination address always set to all ones

CWNA Guide to Wireless LANs, Second Edition17 Discovering the WLAN: Beaconing (continued) Figure 5-7: Beaconing

CWNA Guide to Wireless LANs, Second Edition18 Discovering the WLAN: Beaconing (continued) Beacon frame body contains following fields: –Beacon interval –Timestamp –Service Set Identifier (SSID) –Supported rates –Parameter sets –Capability information In ad hoc networks, each wireless device assumes responsibility for beaconing In infrastructure networks beacon interval normally 100 ms, but can be modified

CWNA Guide to Wireless LANs, Second Edition19 Discovering the WLAN: Scanning Receiving wireless device must be looking for beacon frames Passive scanning: Wireless device simply listens for beacon frame –Typically, on each available channel for set period 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 APs

CWNA Guide to Wireless LANs, Second Edition20 Discovering the WLAN: Scanning (continued) Figure 5-8: Active scanning

CWNA Guide to Wireless LANs, Second Edition21 Joining the WLAN: Authentication Unlike standard wired LANS, authentication performed before user connected to network –Authentication of the wireless device, not the user IEEE authentication: Process in which AP accepts or rejects a wireless device Open system authentication: Most basic, and default, authentication method Shared key authentication: Optional authentication method –Utilizes challenge text

CWNA Guide to Wireless LANs, Second Edition22 Joining the WLAN: Authentication (continued) Figure 5-9: Open system authentication

CWNA Guide to Wireless LANs, Second Edition23 Joining the WLAN: Authentication (continued) Figure 5-10: Shared key authentication

CWNA Guide to Wireless LANs, Second Edition24 Joining the WLAN: Authentication (continued) Open system and Shared key authentication techniques are weak –Open System: Only need SSID to connect –Shared Key: Key installed manually on devices Can be discovered by examining the devices Digital certificates: Digital documents that associate an individual with key value –Digitally “signed” by trusted third party –Cannot change any part of digital certificate without being detected

CWNA Guide to Wireless LANs, Second Edition25 Joining the WLAN: Association Association: Accepting a wireless device into a wireless network –Final step to join WLAN After authentication, AP responds with association response frame –Contains acceptance or rejection notice If AP accepts wireless device, reserves memory space in AP and establishes association ID Association response frame includes association ID and supported data rates

CWNA Guide to Wireless LANs, Second Edition26 Transmitting on the WLAN: Distributed Coordination Function (DCF) MAC layer responsible for controlling access to wireless medium Channel access methods: Rules for cooperation among wireless devices –Contention: Computers compete to use medium If two devices send frames simultaneously, collision results and frames become unintelligible Must take steps to avoid collisions

CWNA Guide to Wireless LANs, Second Edition27 Transmitting on the WLAN: Distributed Coordination Function (continued) 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 send –Devices continue listening while sending frame If collision occurs, stops and broadcasts a “jam” signal CSMA/CD cannot be used on wireless networks: –Difficult to detect collisions –Hidden node problem

CWNA Guide to Wireless LANs, Second Edition28 Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

CWNA Guide to Wireless LANs, Second Edition29 Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-11 (continued): Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

CWNA Guide to Wireless LANs, Second Edition30 Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-12: Hidden node problem

CWNA Guide to Wireless LANs, Second Edition31 Transmitting on the WLAN: Distributed Coordination Function (continued) Distributed Coordination Function (DCF): Specifies modified version of CSMA/CD –Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) –Attempts to avoid collisions altogether –Time when most collisions occur is immediately after a station completes transmission –All stations must wait random amount of time after medium clear Slot time

CWNA Guide to Wireless LANs, Second Edition32 Transmitting on the WLAN: Distributed Coordination Function (continued) CSMA/CA also reduces collisions via explicit frame acknowledgment –Acknowledgment frame (ACK): Sent by receiving device to sending device to confirm data frame arrived intact –If ACK not returned, transmission error assumed CSMA/CA does not eliminate collisions –Does not solve hidden node problem

CWNA Guide to Wireless LANs, Second Edition33 Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-13: CSMA/CA and ACK

CWNA Guide to Wireless LANs, Second Edition34 Transmitting on the WLAN: Distributed Coordination Function (continued) 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 frames Especially with short data packets –RTS threshold: Only packets that longer than RTS threshold transmitted using RTS/CTS

CWNA Guide to Wireless LANs, Second Edition35 Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-14: Request to Send/Clear to Send (RTS/CTS)

CWNA Guide to Wireless LANs, Second Edition36 Transmitting on the WLAN: Interframe Spacing Interframe spaces (IFS): Intervals between transmissions of data frames –Short 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 transmit –Distributed Coordination Function IFS (DIFS): Standard interval between transmission of data frames

CWNA Guide to Wireless LANs, Second Edition37 Transmitting on the WLAN: Interframe Spacing (continued) Figure 5-15: CSMA/CA with one station transmitting

CWNA Guide to Wireless LANs, Second Edition38 Transmitting on the WLAN: Interframe Spacing (continued) Figure 5-16: CSMA/CA with two stations transmitting

CWNA Guide to Wireless LANs, Second Edition39 Transmitting on the WLAN: Fragmentation Fragmentation: Divide data to be transmitted from one large frame into several smaller ones –Reduces probability of collisions –Reduces amount of time medium is in use If data frame length exceeds specific value, MAC layer fragments it –Receiving station reassembles fragments Alternative to RTS/CTS –High overhead ACKs and additional SIFS time gaps

CWNA Guide to Wireless LANs, Second Edition40 Transmitting on the WLAN: Point Coordination Function (PCF) Polling: Channel access method in which each device asked in sequence if it wants to transmit –Effectively prevents collisions 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 –Shorter than DFIS time gap

CWNA Guide to Wireless LANs, Second Edition41 Transmitting on the WLAN: Point Coordination Function (continued) 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) Receiving stations must stop transmission for that amount of time –Point coordinator then sends frame to specific station, granting permission to transmit one frame standard allows WLAN to alternate between PCF (polling) and DCF (contention)

CWNA Guide to Wireless LANs, Second Edition42 Transmitting on the WLAN: Point Coordination Function (continued) Figure 5-18: DIFS and DCF frames

CWNA Guide to Wireless LANs, Second Edition43 Transmitting on the WLAN: Quality of Service (QoS) and e DCF does not work well for real-time, time- dependent traffic Quality of Service (QoS): Capability to prioritize different types of frames Wi-Fi Multimedia (WMM): Modeled after wired network QoS prioritization scheme e draft: defines superset of features intended to provide QoS over WLANs –Proposes two new mode of operation for MAC Layer

CWNA Guide to Wireless LANs, Second Edition44 Transmitting on the WLAN: Quality of Service and e (continued) Table 5-1: Wi-Fi Multimedia (WMM)

CWNA Guide to Wireless LANs, Second Edition45 Transmitting on the WLAN: Quality of Service and e (continued) e draft (continued): –Enhanced Distributed Channel Access (EDCA): Contention-based but supports different types of traffic Four access categories (AC) Provides “relative” QoS but cannot guarantee service –Hybrid Coordination Function Controlled Channel Access (HCCA): New form of PCF based upon polling Serves as a centralized scheduling mechanism

CWNA Guide to Wireless LANs, Second Edition46 Remaining Connected to the WLAN: Reassociation Reassociation: Device drops connection with one AP and establish connection with another –Several reason why reassociation may occur: Roaming Weakened signal –When device determines link to current AP is poor, begins scanning to find another AP Can use information from previous scans

CWNA Guide to Wireless LANs, Second Edition47 Remaining Connected to the WLAN: Power Management 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 messages Power management: Allows mobile devices to conserve battery life without missing transmissions –Transparent to all protocols –Differs based on WLAN configuration –AP records which stations awake and sleeping –Buffering: If sleeping, AP temporarily stores frames

CWNA Guide to Wireless LANs, Second Edition48 Remaining Connected to the WLAN: Power Management (continued) Figure 5-19: Power management in infrastructure mode

CWNA Guide to Wireless LANs, Second Edition49 Remaining Connected to the WLAN: Power Management (continued) At set times AP send out beacon to all stations –Contains traffic indication map (TIM) –At same time, all sleeping stations switch into active listening mode Power management in ad hoc mode: –Ad hoc traffic indication message (ATIM) window: Time at which all stations must be awake 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 awake

CWNA Guide to Wireless LANs, Second Edition50 WLAN Network Layer Standards: WLAN IP Addressing In standard networking, IP protocol responsible for moving frames between computers –Network layer protocol TCP/IP works on principle that each network host has unique IP address –Used to locate path to specific host –Routers use IP address to forward packets –Prohibits mobile users from switching to another network and using same IP number Users who want to roam need new IP address on every network

CWNA Guide to Wireless LANs, Second Edition51 WLAN Network Layer Standards: Mobile IP Provides mechanism within TCP/IP protocol to support mobile computing –Computers given home address, Static IP number on home network –Home agent: Forwarding mechanism that keeps track of where mobile computer located –When computer moves to foreign network, a foreign agent provides routing services Assigns computer a care-of address Computer registers care-of address with home agent

CWNA Guide to Wireless LANs, Second Edition52 WLAN Network Layer Standards: Mobile IP (continued) Figure 5-20: Mobile IP components

CWNA Guide to Wireless LANs, Second Edition53 WLAN Network Layer Standards: Mobile IP (continued) Figure 5-21: Computer relocated in Mobile IP

CWNA Guide to Wireless LANs, Second Edition54 WLAN Network Layer Standards: Mobile IP (continued) Figure 5-22: Encapsulated Mobile IP frame

CWNA Guide to Wireless LANs, Second Edition55 Summary 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 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

CWNA Guide to Wireless LANs, Second Edition56 Summary (continued) 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) Once a wireless device has discovered the WLAN, it requests to join the network; This is a twofold process known as authentication and association

CWNA Guide to Wireless LANs, Second Edition57 Summary (continued) 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 e draft defines a superset of features that is intended to provide QoS over WLANs

CWNA Guide to Wireless LANs, Second Edition58 Summary (continued) 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