IE 419/519 Wireless Networks

IE 419/519Wireless Networks

Lecture Notes #4IEEE 802.11 Wireless LAN Standard

Part #2

2

IEEE 802.11 MAC Layer Key to the 802.11 specification

It “rides” on every PHY layer and controls the transmission of user data into the air

Provides core framing operations Provides interaction with a wired network

backbone Covers three functional areas

Reliable data delivery Medium access control Security

3

MAC – Reliable Data Delivery WLAN using the IEEE 802.11 PHY and MAC

layers is subject to considerable unreliability

Even with error-correction codes, a number of MAC frames may not be received successfully

More efficient to deal with errors at the MAC level than higher layer (e.g., TCP)

4

MAC – Reliable Data Delivery (cont.) 802.11 incorporates positive acknowledgement Frame exchange protocol

Source station transmits data Destination responds with acknowledgment

(ACK) If source does not receive ACK, it retransmits

frame Four frame exchange

Source issues request-to-send (RTS) Destination responds with clear-to-send (CTS) Source transmits data Destination responds with ACK

5

MAC – Reliable Data Delivery (cont.)

Atomic Operation 802.11 allows stations to lock out contention during

atomic operations so that atomic sequences are not interrupted by other stations attempting to use the transmission medium

MAC – Medium Access Control The 802.11 working group considered two

types of proposals for a MAC algorithm Distributed (like Ethernet), or Centralized

The end result is a MAC algorithm called Distributed Foundation Wireless MAC (DFWMAC)

The DFWMAC architecture provides a distributed access control mechanism with an optional centralized control built on top of that

6

MAC – Medium Access Control (cont.) DFWMAC architecture

7

8

802.11 MAC Frame Format

9

MAC Frame – Address FieldsFunction ToD

SFromDS

Address 1

Address 2

Address 3

Address 4

IBSS

Function ToDS

FromDS

Address 1

Address 2

Address 3

Address 4

To AP (infra.)

APClient Server

802.11 DS

10

MAC Frame – Address Fields (cont.)

Function ToDS

FromDS

Address 1

Address 2

Address 3

Address 4

From AP (infra.)WDS (bridge)

APClient Server

802.11 DS

802.11

Server

AP AP

Client

11

MAC – Security In wireless networks, the word

“broadcast” takes on an entirely new meaning

Original 802.11 standard Privacy

Wired Equivalent Privacy (WEP) algorithm RC4 algorithm using a 40-bit key ( 104-bit key

later) Authentication

Shared-key authentication For more information, go to

http://www.isaac.cs.berkeley.edu/isaac/wep-faq.html

12

MAC – Security (cont.)

WEP only addressed protection for the radio link Nothing beyond the AP

Did not include a framework for authentication & authorization

Employed a pre-shared key for encryption Suffered from severe weaknesses Key had to be manually entered/changed on

the APs and all the stations Used CRC for data integrity

13

MAC – Security (cont.)

Types of Attacks Unauthorized association with the AP Man-in-the-middle

Rogue AP

Eavesdropping MAC Spoofing Denial of Service

14

MAC – Security (cont.)

The 802.11i task group developed a set of security mechanisms that eliminates most 802.11 security issues

802.11i addresses several security areas Access Control Authentication Authorization Confidentiality Data Integrity Key management Protection against known attacks

15

MAC – Security (cont.)

Security for WLANs focuses on Access Control (i.e., authentication)

To prevent unauthorized users from communicating with APs

To ensure that legitimate client units associate only with trusted APs (not rogue or unauthorized APs)

Privacy Only intended audience understands transmitted

data Encryption is key

16

MAC – Security (cont.)

Four distinct WLAN security solutions exist Open Access Basic Security Enhanced Security

Requires a Remote Authentication Dial-In User Service (RADIUS) server

Also known as an Authentication, Authorization and Accounting (AAA) server

Remote Access Security Uses a VPN to allow access to corporate network

and access business applications

17

MAC – Security (cont.)

Basic Security SSID

“Sniffing” is a problem Open or Shared-Key Static WEP keys

40 or 128 bits Very time consuming process, especially if they

change Stolen devices are a problem

MAC Authentication Optional APs have access to a list MACs can be forged

18

MAC – Security (cont.)

Basic Security II WPA or WPA2 Pre-Shared Key (PSK) Uses a password or identification code

PassphraseNetwork Type WPA WPA2

Enterprise mode(business, government, education)

Authentication: IEEE 802.1x/EAPEncryption: TKIP/MIC

Authentication: IEEE 802.1x/EAPEncryption: AES-CCMP

Personal mode (SOHO, home/personal)

Authentication: PSKEncryption: TKIP/MIC

Authentication: PSKEncryption: AES-CCMP

19

IEEE 802.11 PHY Layer PHY media defined by original 802.11 standard

Direct-sequence spread spectrum Operating in 2.4 GHz ISM band Data rates of 1 and 2 Mbps 11 channels in the US, 13 in Europe, 1 in

Japan Frequency-hopping spread spectrum

Operating in 2.4 GHz ISM band Data rates of 1 and 2 Mbps 70 channels in the US, 23 in Japan

Infrared 1 and 2 Mbps Wavelength between 850 and 950 nm

20

IEEE 802.11 PHY Layer (cont.)

21

IEEE 802.11 PHY Layer (cont.)

22

IEEE 802.11a Channel structure

Makes use of the U-NII frequency bands Standard specifies a transmit spectrum

mask Purpose is to constrain the spectral properties of

the transmitted signal such that signals in adjacent channels do not interfere with one another

23

IEEE 802.11a (cont.)

Channel structure Available channels

24

IEEE 802.11a (cont.)

Channel structureA

f

f

f

FDM

OFDM

OFDM

25

IEEE 802.11a (cont.)

Coding and Modulation Uses Orthogonal Frequency Division

Multiplexing (OFDM) Also called multicarrier modulation Uses multiple carrier signals at different

frequencies, sending some of the bits in each channel

Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-QAM

26

IEEE 802.11a (cont.)

Coding and Modulation

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IEEE 802.11b Extension of the 802.11 DSSS scheme Provides data rates of 5.5 and 11 Mbps in

the ISM band Uses chipping rate of 11 MHz thus

occupying the same bandwidth as original DSSS scheme

Higher data rate is achieved by using complementary code keying (CCK) as modulation scheme

28

IEEE 802.11b (cont.)

Channel structure1 2 3 4 5 6 87 9 10 11 12 13 14

1 2 3 4 5 6 87 9 10 11 12 13 14

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IEEE 802.11g Extension of 802.11b

Achieves data rates above 20 Mbps up to 54 Mbps

Operates in the 2.45 GHz range Compatible with 802.11b

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Other IEEE 802.11 Standards 802.11f

Multi-vendor AP interoperability (IAPP) 802.11i

Security and authentication mechanisms at the MAC layer

802.11n Range of enhancements to both PHY and

MAC layers to improve throughput Multiple antennas Smart antennas Changes to MAC access protocols

31

References The following references were used to

complement the material presented in this module: Gast, M.S., 802.11 Wireless Networks: The

Definitive Guide, 1st Edition, O’Reilly, 2002 Rivero, J., Porter, J.D., Puthpongsiriporn, T.,

Lemhachheche, R., Layton, W.T., Campus Wireless Environment Deployment Guide, 2005.