CWNA Guide to Wireless LANs, Second Edition

Chapter FiveIEEE 802.11 Media Access Control and

Network Layer Standards

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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

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IEEE Wireless LAN Configurations: Basic Service Set

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• 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: Extended Service Set

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• 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: Independent Basic Service Set

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• 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 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

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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

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MAC Frame Formats -Management

• Management Frames: Initialize communications between device and AP (infrastructure mode) or between devices (ad hoc mode)– Maintain connection

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Figure 5-4: Structure of a management frame

MAC Frame Formats - Types

• Types of management frames:1. Authentication frame2. Association request frame3. Association response frame4. Beacon frame5. Deauthentication frame6. Disassociation frame7. Probe request frame8. Probe response frame9. Reassociation request frame10. Reassociation response frame

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MAC Frame Formats - Control

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

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Figure 5-5: Control frame

MAC Frame Formats - Data

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

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Figure 5-6: Data frame

Discovering the WLAN: Beaconing

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• 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 Fields

• Beacon frame body contains following fields:1. Beacon interval2. Timestamp3. Service Set Identifier (SSID)4. Supported rates5. Parameter sets6. 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

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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

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Discovering the WLAN: Active Scanning

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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

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Open System Authentication

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Shared Key Authentication

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Joining the WLAN: 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

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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

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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

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Carrier Sense Multiple Access with Collision Detection (CSMA/CD)Used on Ethernet LANs

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• 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

Hidden node problem

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Laptop A ,Laptop B and Laptop C can not see the transmission of each other (more later on this topic)

Transmitting on the WLAN: Distributed Coordination Function

and CSMA/CA• 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

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CSMA/CA

• 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

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CSMA/CA and ACK

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Request to Send/Clear to Send

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• 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

Interframe Spacing

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• 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

CSMA/CA with two stations transmitting

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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

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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

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DIFS and DCF frames

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• 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)

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

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Quality of Service and 802.11e

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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

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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

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Power Management

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• A WLAN laptop 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

Power Management

• 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

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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

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Mobile IP

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• 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

Mobile IP-Computer relocated

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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

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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

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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

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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

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