1

CWNA Guide to Wireless LANs, Second Edition

Chapter FiveIEEE 802.11 Media Access Control and

Network Layer Standards

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

3

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

4

IEEE Wireless LAN Configurations: Basic Service Set (continued)

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

• APs can be positioned so that cells overlap to facilitate roaming– Wireless devices choose AP based on signal

strength– Handoff

6

IEEE Wireless LAN Configurations: Extended Service Set (continued)

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– 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

8

IEEE Wireless LAN Configurations: Independent Basic Service Set

(continued)

Independent Basic Service Set (IBSS)

9

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

10

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

11

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

12

MAC Frame Formats (continued)

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

Structure of a management frame

13

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

14

MAC Frame Formats (continued)

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

Control frame

15

MAC Frame Formats (continued)

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

Data frame

16

802.11 MAC Addressing

• Address 1 – Receiver address

• Address 2 – Transmitter address

• Address 3 – Ethernet SA, Ethernet DA, or BSSID

• Transmitter: Sends a frame on to the wireless medium, but doesn’t necessarily create the frame.

• Receiver: Receives a frame on the wireless medium, but may not be the destination, i.e. may be the access point.

Distribution System (DS)

A B CD

Access Point 1 Access Point 2

XY

General 802.11 Frame

Host A to Host B

aaa bbb

xxx

111

17

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

18

Discovering the WLAN: Beaconing (continued)

Beaconing

19

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

20

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

21

Discovering the WLAN: Scanning (continued)

Active scanning

22

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

23

Joining the WLAN: Authentication (continued)

Open system authentication

24

Joining the WLAN: Authentication (continued)

Shared key authentication

25

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

26

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

27

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

28

Medium Access – CSMA/CA

• Both CSMA/CD and CSMA/CA are half-duplex architectures • Ethernet uses CSMA/CD – Collision Detection

– Ethernet devices detect a collision as when the data is transmitted

• 802.11 uses CSMA/CA – Collision Avoidance– 802.11 devices only detect a collision when the transmitter has not

received an Acknowledgement (coming).– Stations also use CS/CCA – coming– Stations also use a virtual carrier-sense function, NAV (coming)

CSMA/CD CSMA/CA

ACK

All stations detect the collision

29

Medium Access – CSMA/CA

• The 802.11 standard makes it mandatory that all stations implement the DCF (Distributed Coordination Function), a form of carrier sense multiple access with collision avoidance (CSMA/CA). Coming!

• CSMA is a contention-based protocol making sure that all stations first sense the medium before transmitting (physically and virtually). Coming!

• The main goal of CSMA/CA is to avoid having stations transmit at the same time, which will then result in collisions and eventual retransmissions. Coming!

• However, collisions may still occur and when they do stations may or may not be able to detect them (hidden node problem). Coming!

CSMA/CD CSMA/CA

ACK

All stations detect the collision

30

DCF and PCF

• IEEE mandated access mechanism for 802.11 is DCF (Distributed Coordination Function)– Basis for CSMA/CA– Discussed in detail next

• There is also the PCF (Point Coordination Function)– Point Coordinators (PC), ie.Access Points, provide point

coordination for contention-free services.– Restricted to Infrastructure BSSs– Stations can only transmit when allowed to do so by PC (AP).– PCF is not widely implemented and will not be discussed

31

DCF Operation

• In DCF operation, a station wanting to transmit :– Checks to see if radio link is clear, CS/CCA – Carrier Sense,

Clear Channel Assessment (Later in PHY presentation)– Checks its Network Allocation Vector (NAV) timer to see if

someone else is using the medium.– If medium is available DCF uses a random backoff timer to avoid

collisions and sends the frame.• Transmitting station only knows the 802.11 frame got there if it

receives an ACK.• May also use RTS/CTS to reduce collisions (coming)

32

Duration Field

• Duration/ID field – The number of microseconds (millionth of a second) that the medium is expected to remain busy for transmission currently in progress.– Transmitting device sets the Duration time in microseconds.– Includes time to:

• Transmit this frame to the AP (or to the client if from an AP)• The returning ACK • The time in-between frames, IFS (Interframe Spacing)

• All stations monitor this field!• All stations update their NAV (Network Allocation Vector) timer.

General 802.11 Frame (more on this later)

33

NAV Timer

• All stations have a NAV (Network Allocation Vector) timer.• Virtual carrier-sensing function• Protects the sequence of frames from interruption.• Martha sends a frame to George.• Since wireless medium is a “broadcast-based” (not broadcast frame)

shared medium, all stations including Vivian receive the frame.• Vivian updates her NAV timer with the duration value.• Vivian will not attempt to transmit until her NAV is decremented to 0.• Stations will only update their NAV when the duration field value

received is greater than their current NAV.

General 802.11 Frame (more on this later)

34

Interframe Spacing (IFS)

• 802.11 uses four different interframe spaces used to determine medium access (note: microsecond = millionth of a second):– DIFS – DCF Interface Space (50 microseconds in DSSS)

• Minimum amount of medium idle time until contention-based services begin.

– PIFS – PCF Interframe Space (30 microseconds in DSSS)• Used by PCF

– SIFS – Short Interframe Space (10 microseconds in DSSS)• Used for highest priority transmission, ACKs, RTS, CTS

35

Wanting to transmit (1/3)

• Station wanting to transmit.

• Carrier Sensing:– Physical: Physically senses medium is idle (CS/CCA – coming).– Virtual: NAV timer is 0

• Waits DIFS (DCF Interface Space) period of 50 microseconds– Minimum amount of medium idle time until contention-based services begin.– Once DCF is over, stations can contend for access.

• Contention window begins.– Uses random backoff algorithm to determine when it can attempt to access

the medium. (next)

Random backoff slots

36

Wanting to transmit (2/3)

• (Detail of random backoff algorthim has been left out, but this will be sufficient.)• The random backoff algorithm randomly selects a value from 0 to 255

(maximum value varies by vendor and stored in the NIC).• The random value is the number of 802.11 slot times the station must wait

after the DIFS, during the contention window before it may transmit.• Stations pick a random slot and wait for that slot before attempting to access

the medium.• With several stations attempting to transmit, the station that picks the lowest

slot, lowest random number, wins.

Contention Window Begins

37

Example

Scenario:

• Both Vivian and George want to transmit frames.

• Both stations have same NAV values and physically sense when the medium is idle.

• Both are waiting for Martha’s transmission to end and the medium to become available.

• The medium now becomes available.

I’m waiting

I’m waiting

38

Example

• George and Vivian are both wanting to transmit.• Both perform the following:• Both sense that medium is available using Physical and Virtual

Carriers Sensing:– Physical: Physically senses medium is idle (CS/CCA – coming).– Virtual: NAV timer is 0

• Both waits DIFS (DCF Interface Space) period of 10 microseconds• Contention window begins.

– Uses random backoff algorithm to determine when it can attempt to access the medium. (next)

Random backoff slots

39

Example

• Both Vivian and George calculate their random backoff algorithm to randomly selects a value from 0 to 255.

• Vivian has a slot time of 7, George a slot time of 31.

• Vivian wins.

• The destination of her frame is George

Vivian (7), George (31)

40

Example

• Vivian transmits, setting the Duration ID to the time needed to transmit, ACK and IFSs.

• George with a higher slot will see the 802.11 frame from Vivian and wait to transmit.

• Assuming their was not a collision from another station, Martha and George update their NAVs.

General 802.11 Frame (more on this later)

Others update NAV

Martha and George receive “broadcast-based” 802.11 frame.

( ( ( ) ) )

41

Transmitting on the WLAN: Distributed Coordination Function (continued)

Hidden node problem

42

RTS/CTS Solution

• The hidden node stations cannot see the RTS.• The AP replies to Vivian with a CTS, which all nodes, including the

hidden node can see.• Vivian transmits the frame.• The AP returns an ACK to Vivian.• The AP sends the message to George who returns an ACK to the AP.

• Vivian attempts to reserve the medium using an RTS control frame to the AP.

• The RTS frame indicates to the AP and all stations within range, that Vivian wants to reserve the medium for a certain duration of time, message, ACK, and SIFS.

43

RTS/CTS Solution

• The RTS/CTS procedure can be enabled/controlled by setting the RTS threshold on the 802.11 client NIC.

• RTS/CTS is also used during frame fragmentation (coming).

• RTS/CTS consumes a fair amount of capacity and overhead, resulting in additional latency.

• Normally used in high capacity environments.

44

Setting the RTS Threshold on a Cisco Client

• Specifies the data packet size beyond which the low-level RF protocol invokes RTS/CTS flow control. A small value causes RTS packets to be sent more often, which consumes more of the available bandwidth and reduces the throughput of other network packets. However, small values help the system recover from interference or collisions, which can occur in environments with obstructions or metallic surfaces that create complex multipath signals.

RTS Threshold

45

Improving WLAN Performance with RTS/CTS by Jim Geier (wi-fiplanet.com)

• If you enable RTS/CTS on a particular station (just the hidden node station), it will refrain from sending a data frame until the station completes a RTS/CTS handshake with another station, such as an access point.

• Keep in mind, though, that an increase in performance using RTS/CTS is the net result of introducing overhead (i.e., RTS/CTS frames) and reducing overhead (i.e., fewer retransmissions). If you don't have any hidden nodes, then the use of RTS/CTS will only increase the amount of overhead, which reduces throughput. A slight hidden node problem may also result in performance degradation if you implement RTS/CTS. In this case, the additional RTS/CTS frames cost more in terms of overhead than what you gain by reducing retransmissions. Thus, be careful when implementing RTS/CTS.

46

Improving WLAN Performance with RTS/CTS by Jim Geier (wi-fiplanet.com)

• One of the best ways to determine if you should activate RTS/CTS is to monitor the wireless LAN for collisions. If you find a large number of collisions and the users are relatively far apart and likely out of range, then try enabling RTS/CTS on the applicable user wireless NICs. You can activate the function by clicking "enable RTS/CTS" somewhere in the user setup screens. You don't need to enable RTS/CTS at the access point in this case. After receiving a RTS frame from a user's radio NIC, the access point will always respond with a CTS frame.

• Of course, keep in mind that user mobility can change the results. A highly mobile user may be hidden for a short period of time, perhaps when you perform the testing, then be closer to other stations most of the time. If collisions are occurring between users within range of each other, the problem may be the result of high network utilization or possibly RF interference.

47

Frame Fragmentation

• Since we have already discussed RTS/CTS, let’s also discuss frame fragmentation.

• Later, we will see that RTS/CTS and fragmentation are typically combined.

• Frame fragmentation is a MAC layer function that is designed to increase the reliability of transmitting frames across a wireless medium.

48

Frame Fragmentation

• In a “hostile wireless medium” (interference, noise) larger frames may have more of a problem reaching the receiver without any errors.

• By decreasing the size of the frame, the probability of interference during transmission can be reduced.

• Breaking up a large frame into smaller frames, allows a larger percentage of frames to arrive undamaged (without errors).

49

Frame Fragmentation

• Frame fragmentation can increase the reliability of frame transmissions but there is additional overhead:– Each frame fragment includes the 802.11 MAC protocol header.

– Each frame fragment requires a corresponding acknowledgement.

• If a frame fragment encounters errors or a collision, only that fragment needs to be retransmitted, not the entire frame.

• The frame control field includes information that this is a fragmented frame.

50

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

51

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

Wi-Fi Multimedia (WMM)

52

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

53

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

54

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

55

Remaining Connected to the WLAN: Power Management (continued)

Power management in infrastructure modeTraffic Indication Map (TIM)

56

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

57

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

58

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

59

WLAN Network Layer Standards: Mobile IP (continued)

Mobile IP components

60

WLAN Network Layer Standards: Mobile IP (continued)

Computer relocated in Mobile IP

61

WLAN Network Layer Standards: Mobile IP (continued)

Encapsulated Mobile IP frame

62

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

63

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

64

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

65

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