Introduction to Computer Networks CS640 Wireless Networking

Ming Liu [email protected]

Introduction to Computer Networks

CS640 https://pages.cs.wisc.edu/~mgliu/CS640/F21/

Wireless Networking

1

Today

Last lecture • Error detection

• Ethernet

2

Today • Wireless basics • WiFi medium access control • WiFi distribution system

Announcements • Lab1 due on 09/30/2021 11:59PM

Wireless Link v.s. Wired Link

Error rates are higher • Environment is inherently noisier

Power limitations • Energy scarce nodes

Limitations on transmit power • Interference; how much power a device may emit at any given frequency

Wireless signals attenuate fast • Rate of attenuation increases with frequency

3

Wireless Link v.s. Wired Link (cont’d)

Cannot do collision detection • Collision happens at receiver

Usually shorter in range

Slower compared to wired links

Dealing with mobile nodes

Limitations on security (eavesdropping)

4

Overview of Wireless Technologies

Which portion of the spectrum • Licensed: satellite, TV, cellular • Unlicensed: Bluetooth, WiFi

Unlicensed device • Maximum power limitation

• Define the range of transmitter -> Signal becomes weak if outside the range

• A metric: signal-to-noise (SNR)

• The higher the better

5

Two Kinds of Hardware Devices

#1: base station #2: mobile endpoint

6

Two Kinds of Hardware Devices

#1: base station #2: mobile endpoint

6

Various WiFi Technologies

Based on IEEE 802.11 standards • 802.11a, 802.11b, 802.11g, 802.11n, 802.11be, …

Difference • #1: the underlying physical properties of how wireless signals are encoded

• #2: what hardware is employed in encoding/decoding

• #3: what portion of the spectrum they operate in

• 802.11 b, g, n —> 2.4GHz • 802.11 a —> 5GHz • 802.11 n —> multiple antennas • Others —> single antenna

7

WiFi Encoding

Direct sequence spread spectrum —> 802.11b • Each bit XOR’ed with an N bit chipping sequence

• Receiver knows the sequence and XOR’s again to retrieve the bit • N > 1 ensures redundancy and protection

OFDM —> 802.11a/g • Orthogonal frequency division multiplexing

• Spread bits across multiple frequency slices • Different slices have different attenuate rates, which can carry over time

• Different slices result in different bit error rates over times • Speeding redundant bit across slices -> increase chances of receiving bits correctly

8

User’s View

Manifest as differences in maximum supported rate • 802.11b —> 11 Mbps • 802.11 a, g —> 54 Mbps • 802.11 n —> 600 Mbps • 802.11 be —> Extreme High Throughput (EHT), WiFi 7 —> 40Gbps —> ongoing

Often support lower rates • e.g., 1Mbps, 2Mbps, 6Mbps, 11Mbps, 27Mbps, 54Mbps, …

• Lower rates are picked based on signal quality • Better the signal, the higher the rate picked —> rate adaptation

9

Medium Access Control

Similar in some respect to Ethernet — CSMA

WiFi uses CSMA/CA — collision avoidance • #1: Wireless nodes cannot transmit and receive at the same time (on the same

frequency) • #2: Senders cannot detect collisions —> collisions happen at the receiver and there is no

way for the collided signal to travel back to the sender

10

Collision avoidance is crucial

Hidden terminal •Both A and C want to send to B. Because they are both out of range of each, they both

sense the medium to be idle and transmit. Unfortunately, the transmissions collide at the

receiver

A B C

11

Collision avoidance is crucial (cont’d)

Exposed terminal • B wants to send to A, where C wants to send to D. But when C tries to send data, it senses the medium to be busy and wait • In realty though, since D is out of B’s range, it could transmit the data as there is no

signal colliding

A B C D

12

Collision Avoidance

Tackle collisions in the first case, and improves efficiency in the second case

Key ideas • RTS/CTS control packet: request to send and clear to send

• ACK (acknowledgement) packet

13

How does CA work?

A (sender) -> B (receiver) • Step 1: A sends RTS to B

• Step 2: If B is willing to accept, then it responds with a CTS; If B has already accepted

someone else’s RTS, then it will not send the CTS

• Step 3: A sends data and waits • Step 4: B receives data and sends ACKs back • Step 5: If ACK is not received by A after time T, A tries again with Step 1

14

How does CA handle the hidden terminal?

15


Because B will send CTS to either A or C

The RTS and correspondingly the CTS indicates how long the medium will be busy. So the hidden terminal will stop sending data

15




What happens when two RTS collide at the receiver?

15




What happens when two RTs collide at the receiver? - Senders wait for a random amount of time, and retry using binary exponential back off (similar to Ethernet)

15

How does CA handle the exposed terminal?

C will hear the RTS from B but not CTS from A to B

C knows it is an exposed terminal and so it will send RTS to D

16

WiFi Distribution System

802.11 networks use a mesh topology

17

Access point (AP)

Multiple APs provide coverage

APs are connected to each other via a switched network, which is then connected to the Internet

The distribution system runs at the link layer

18

A

An Example

B

C D

GF

E

Distribution system

AP-1AP-2

AP-3

19

How to select an AP?

Active probing • The node sends a Probe frame

• All APs within reach reply with a Probe Response frame

• The node selects one of the access points and sends that AP an Association Request frame

• The API replies with an Association Response frame

20

AP hand-over

A B

C D

GF

E

Distribution system

AP-1AP-2

AP-3

C

21

How to select an AP?

Active probing • The node sends a Probe frame

• All APs within reach reply with a Probe Response frame

• The node selects one of the access points and sends that AP an Association Request frame

• The API replies with an Association Response frame

Passive probing • AP broadcasts beacons • Nodes send association request in response

22

802.11 Frame Format

Ctrl2B

Describe the frame control information, like frame type (RTS/CTS), protocol version, ToDS/FromDS, etc.

23

802.11 Frame Format

Ctrl2B

Carries the value of the Network Allocation Vector (NAV). Access to the medium is restricted for the time specified by the NAV.

Duration2B

23

802.11 Frame Format

Ctrl2B

Duration2B

Addr1 Addr26B 6B

SeqCtrlAddr3 Addr46B 2B 6B

Sequence number and fragment number

23

802.11 Frame Format

Ctrl2B

Duration2B

Addr1 Addr26B 6B


Addr1 Addr2 Addr3 Addr4

ToDS = 0, FromDS = 0 Target node Sender node N/A N/A

ToDS = 1, FromDS = 0 Target node N/A Intermediate

target N/A

ToDS = 0, FromDS = 1 N/A Intermediate

sender N/A Sender node

ToDS = 1, FromDS = 1 Target node Intermediate

senderIntermediate

target Sender node

23

802.11 Frame Format

Ctrl2B

Duration2B

Addr1 Addr26B 6B


Payload CRC0-2312B 4B

Preamble8B

Dest Source Type Data CRCPad6B 6B 2B 4B

Ethernet frame

23

Other Wireless Network

Bluetooth (802.15.1) • Very short range communication (<10m)

• Low power transmission

• Operate @ 2.45GHz • Bandwidth 1-3 Mbps

Piconet • A master device + up to 7 slave devices • Apply the frequency-hopping technique

• A slave device can be parked, which can be reactived by the master

24

Other Wireless Network (cont’d)

Cellular network • licensed spectrum

• Europe: 900MHz and 1800MHz; USA: 850-MHz and 1900MHz • A base station support 1+ cells • 3G standards are based on CDMA

• No hard limit on how many users can share a piece of spectrum

[1] 5G Mobile Networks: A Systems Approach, https://5g.systemsapproach.org/index.html [2] Ambient Backscatter: Wireless Communication Out of Thin Air, Sigcomm’13 [3] Enabling Deep-Tissue Networking for Miniature Medical Devices, Sigcomm’18

25

https://5g.systemsapproach.org/index.html

Summary

Today • Wireless basics • WiFi medium access control • WiFi distribution system

Next lecture • L2 switching

26

Documents

Introduction to Computer Networks CS640 Wireless Networking