CWNA Guide to Wireless LAN's Second Edition - Chapter 5

CWNA Guide to Wireless LANs, Second Edition

Chapter FiveIEEE 802.11 Media Access Control and

Network Layer Standards

CWNA Guide to Wireless LANs, Second Edition 2

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


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


IEEE Wireless LAN Configurations: Basic Service Set (continued)

Figure 5-1: Basic Service Set (BSS)


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


IEEE Wireless LAN Configurations: Extended Service Set (continued)

Figure 5-2: Extended Service Set (ESS)


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


IEEE Wireless LAN Configurations: Independent Basic Service Set

(continued)

Figure 5-3: Independent Basic Service Set (IBSS)


IEEE 802.11 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


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 802.3 Ethernet

frames in format and function– Used by wireless NICs and APs for communications

and managing/controlling wireless network


MAC Frame Formats (continued)

• Frame control field identifies:– Specific 802.11 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



• 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



• 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



• Control frames: Provide assistance in delivering frames that contain data

Figure 5-5: Control frame



• Data frame: Carries information to be transmitted to destination device

Figure 5-6: Data frame


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


Discovering the WLAN: Beaconing (continued)

Figure 5-7: Beaconing


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


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


Discovering the WLAN: Scanning (continued)

Figure 5-8: Active scanning


Joining the WLAN: Authentication

• Unlike standard wired LANS, authentication performed before user connected to network– Authentication of the wireless device, not the user

• IEEE 802.11 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


Joining the WLAN: Authentication (continued)

Figure 5-9: Open system authentication



Figure 5-10: Shared key authentication



• 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


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


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


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



Figure 5-11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)



Figure 5-11 (continued): Carrier Sense Multiple Access with Collision Detection (CSMA/CD)



Figure 5-12: Hidden node problem



• 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



• 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



Figure 5-13: CSMA/CA and ACK



• 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



Figure 5-14: Request to Send/Clear to Send (RTS/CTS)


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


Transmitting on the WLAN: Interframe Spacing (continued)

Figure 5-15: CSMA/CA with one station transmitting


Transmitting on the WLAN: Interframe Spacing (continued)

Figure 5-16: CSMA/CA with two stations transmitting


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


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


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

• 802.11 standard allows WLAN to alternate between PCF (polling) and DCF (contention)


Transmitting on the WLAN: Point Coordination Function (continued)

Figure 5-18: DIFS and DCF frames


Transmitting on the WLAN: Quality of Service (QoS) and 802.11e

• 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

• 802.11e draft: defines superset of features intended to provide QoS over WLANs– Proposes two new mode of operation for 802.11

MAC Layer


Transmitting on the WLAN: Quality of Service and 802.11e (continued)

Table 5-1: Wi-Fi Multimedia (WMM)


Transmitting on the WLAN: Quality of Service and 802.11e (continued)

• 802.11e 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


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


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


Remaining Connected to the WLAN: Power Management (continued)

Figure 5-19: Power management in infrastructure mode


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


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


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


WLAN Network Layer Standards: Mobile IP (continued)

Figure 5-20: Mobile IP components



Figure 5-21: Computer relocated in Mobile IP



Figure 5-22: Encapsulated Mobile IP frame


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


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


Summary (continued)

• The IEEE 802.11 standard specifies two procedures for transmitting on the WLAN, distributed coordination function (DCF) and an optional point coordination function (PCF)

• The 802.11 standard provides for an optional polling function known as Point Coordination Function (PCF)

• The 802.11e draft defines a superset of features that is intended to provide QoS over WLANs


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

Documents

CWNA Guide to Wireless LAN's Second Edition - Chapter 5