802.11 Wireless LANs and Ad-hoc networks

IFI Master - Ubinet G. Urvoy-Keller

2

Sources of document● Matthew Guast,

– 802.11 Wireless Networks, the definitive guide

– O'Reilly 2005.

● Matti Siekkinen, Aalto University

– 110.5111 Computer Networks II Wireless Local Area networks

● Dino Lopez, Univ. of Nice

– Wireless Networks 1

3

Outline● 802 and 802.11 standard family

● 802.11 primer

● MAC operations

● Framing

● Association

● Physical Layer

● Time vs. Access Fairness

● MANET

4

The big picture: Wireless networks and standards

WPAN

WLAN

WMAN

WWAN3GPP/3GPP2: GSM, CDMA2000, UMTS, LTE

IEEE 802.20

(iBurst)

IEEE 802.15

(Bluetooth, ZigBee, ...)

IEEE 802.11

(Wi-Fi)

IEEE 802.16

(WiMax)

5

802 standard family

● 802.3: CSMA/CD (~ Ethernet)● 802.5: Token Ring● 802.11 flavors differ in physical and not MAC/LLC layer● 802.1 D: bridging/spanning tree and 802.1 Q: VLAN● 802.11b (11 Mb/s) hit the market in 2011, 802.11a (54 Mb/s) around 2005.

6

IEEE 802.11 Wireless LAN:Some history

• 1997: IEEE approved 802.11, which specified the characteristics of devices with a signal rate of 1 and 2 Mb/s– Standard specifies the MAC and the physical layers for transmissions in the 2.4

GHz band

• 1999: IEEE ratified IEEE 802.11b, which works at additional signal rates of 5.5 and 11 Mb/s

• 1999: IEEE approved the specifications of 802.11a, which uses the 5 Ghz band. The signal rates are 6, 9, 12, 18, 24, 36, 48 and 54 Mb/s.

• 2003: IEEE approved 802.11g as a further evolution of the 802.11 standard. – Same performance as 802.11a but works in the 2.4 GHz band– Compatible with 802.11b devices.

• ...

7

802.11: Overview of the many amendments

PHY (.11, a, b, g, j, n, p, y, ac, ad, af, ah)• Change data rate options• Change spectrum

MAC• Security (i, w)• Measurement and Management (k, v)• Flow control and QoS (e, aa, ae)• Time required to establish connection (r, ai) • Spectral Efficiency• Regulatory behavior (d, h)• Radio node connection topology (s, z)• Connection with other networks (u)

8

What are we talking about● Distribution system : the network among access points

– Generally, a single VLAN● Access points : bridging between wired and wireless.

– Current trend : thin AP and a central controller

9

Infrastructure vs. ad-hoc

● Basic Service Set (BSS) : set of communicating stations● Two flavors :

– Infrastructure (WLAN) - all transfers go through AP.

● AP store frames if stations go in power-saving mode

– Independent (Ad-hoc) – stations must route !

10

Extended Service Set

● Multiple APs offering the same SSID (Service Set Identifier)

● Looks like a single layer 2 connections → stations may communicate with each other

● Technologies to interconnect APs : VLANs or tunneling

● A station associated with a single. Need a protocol for inter-AP communication

11

Virtual APs● A single AP can feature 32 to 64 different SSID

● Each SSID is associated to a different VLAN

12

MAC Operations

13

Just to be sure...● MAC: Medium Access Control

– When to transmit

– Manage collisions, retransmissions,etc

● Implemented in physical card with a small hook in the OS

14

Challenges for the MAC: Radio Link Quality

● 802.11 operates in unlicensed band

● Has to cope with noise and interference

● A single transmission at a time (half duplex)

● Relies on positive ACK

● Atomic operation: must complete before turning to the next transmission

● Unicast are all acknowledged. Broadcast frames are not!

15

Challenges for the MAC: hidden nodes

● Stations can hear AP but may not hear each other

16

Challenges for the MAC: hidden nodes

● Stations may ask permission to send (RTS: Request to Send) and the AP answer (CTS: Clear To Send) reaching all other stations

RTS (1)

CTS (2)

17

RTS/CTS● Not used in practice

● Significantly prolong data transmission time

● Plenty of bandwidth in most cases, can afford a few collisions.

● Stations are not sending continuously

18

MAC Access Modes● DCF (Distributed Coordination Function): CSMA/CA mode of

operation

– Sender uses random backoff after each frame transmission

– Listen before talk

● PCF (Point Coordination Function): contention free

– Master/slave approach

– Unused in practice!

19

MAC Access Modes● EDCA (Enhanced Distribution Channel Access)

– Evolution of DCF with different traffic priorities (voice, video, best effort, background application)

– A FIFO stack for each priority

20

Carrier Sensing● 2 types:

– Physical carrier sensing

– Virtual carrier sensing

● Physical carrier sensing performed by the physical layer before transmission

– Important: cards are not full duplex. Not possible to hear (e.g., for collision) during transmission

21

Carrier Sensing● Virtual carrier sensing provided by the Network Allocation

Vector (NAV)

● 802.11 frames carry duration field, which is used to reserve medium

● Example with RTS/CTS

22

Inter-frame spacing● SIFS (Short Interframe Space) for high priority traffic, RTS/CTS and ACKs

● DIFS (DCFS Interframe Space)

– A machine that senses the medium idle can send immediately … after its back-off

● PIFS: for PCF – don't care

23

Exercise● A station arrives during :

– RTS

– In between RTS and CTS

– During frame transmission

● Q: what is going to happen neglecting NAV?

● Q: what does the NAV further enable?

24

Frame loss recovery in DCF

● Rule 1: a frame that is not acknowledged is resent

● Rule 2: if a frame is lost, the station uses a backoff approach to determine next transmission time

● When is it triggered ? After frame transmission + DIFS and no ACK

25

Back-off window● Contention window are 2^n-1: 31,63,127, etc...

● Its units is slot time, which decreases with the maximum rate achievable for the standard

– 802.11b: 20 microseconds

– 802.11: 9 (or 20) microseconds

26

Back-off window

27

Framing

28

frame

controlduration

address

1

address

2

address

4

address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seq

control

Address 2(transmitter): MAC address of wireless host or AP transmitting this frame

Address 1(receiver): MAC address of wireless host or AP to process this frame

Address 3: MAC address of router interface to which AP is attached

Address 4: used only in ad hoc mode

802.11 frame and addressing

29

802.11 frame and addressing

Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1(rcv) address 2 (tx) address 3

802.11 frame

R1 MAC addr H1 MAC addr

dest. address source address

802.3 frame

30

802.11 frame and addressing

• Q: Why are three addresses needed?– Can’t sender just specify router’s MAC address as receiver?

• A: Must address frame to AP so that it processes it– Otherwise, AP does not know it is supposed to process it

31

Association, power saving

32

Association

• Host must associate with an AP– Scan channels listening for beacon frames containing AP’s

name (SSID) and MAC address– Select AP to associate with– Optionally perform authentication– Typically run DHCP to get IP address in AP’s subnet

33

Passive/active scanning

AP 2AP 1

H1

BBS 2BBS 1

122

3 4

Active Scanning:

(1) Probe Request frame broadcast from H1

(2) Probes response frame from APs

(3) Association Request frame from H1 to selected AP

(4) Association Response frame from AP to H1

AP 2AP 1

H1

BBS 2BBS 1

1

23

1

Passive Scanning: (1) Beacon frames from APs

(2) Association Request frame from H1 to selected AP

(3) Association Response frame from AP to H1

Q: Which one is better?

A: Passive probing usually takes longer time prefer active probing (handovers and energy consumption)

34

Power saving

• PSM = Power Saving Mode• Allows Rx/Tx circuitry to be temporarily shut down• Coordinated with the AP

– Node-to-AP: “I am going to sleep until next beacon frame”– AP knows not to transmit frames to this node, buffers them– Node wakes up before next beacon frame– Beacon frame: contains list of mobiles with AP-to-mobile frames waiting

to be sent– Node will stay awake and request frames buffered at AP if any;

otherwise sleep again until next beacon frame

• Sleep mode consumes ten times less power than idle mode

35

PHY

Wireless Networks: The Frequency bands

• In 1985, the FCC designated 3 frequency band for the Industry, Scientific and Medical purposes. Used without license

• The ISM bands (adopted later in several countries) are 902-928MHz, 2.400-2.4835GHz and 5.725-5.850GHz

• In Europe, the frequency band between 890MHz and 915MHz is reserved for mobile communication (GSM). Only the bands at 2.4GHz and 5GHz can be used without license

37

802.11 in practice: PHY

• 802.11b– 2.4 GHz band– up to 11 Mbps– Uses DSSS– legacy

• 802.11a – 5 GHz band– up to 54 Mbps– Uses OFDM

• 802.11g – OFDM for 2.4 GHz band– up to 54 Mbps– Still most widespread(?)

• 802.11n:– 2.4/5 GHz range– OFDM with multiple antennas (MIMO)– 2 channel widths (20/40MHz)– up to 600 Mbps– Widely supported now by commercial

products

• 802.11ac:– 5GHz band– Even wider channels (80 and 160

MHz)– MIMO using max. 8 spatial streams– Multi-user MIMO– Up to 7Gb/s aggregate throughput

OFDM is dominantDifferent frequency bands

MIMO (multiple antenna) has entered practical use

38

DSSS● Direct-sequence systems spread

● Spread signal in a controlled way

39

OFDM● OFDM divides an available channel into

several subchannels

● encodes a portion of the signal across each subchannel in parallel

● Signals are orthogonal

40

PHY: Channels and interference

• 2.4GHz-2.485GHz spectrum divided into 14 channels• Interference possible

– Channel can be same as that chosen by neighboring AP– Channels overlap– Only three non-overlapping channels

• Admin chooses channels for APs– No self configuration

2.412

2.417

2.422 2.432 2.442 2.452 2.462 2.472 2.484

2.427 2.437 2.447 2.457 2.467

23

45

17

89

10

6 1112

1314

41

PHY: 2.4 vs. 5 GHz bands

• 802.11n can make a choice between the two– Which is better?

• 2.4GHz band enables longer range• 2.4GHz band more crowded

– Interference from microwave ovens, cordless phones, garage door openers…

– Bluetooth, Zigbee

• 5GHz band is wider– More non-overlapping channels

42

Why 54 is not 54 ?

43

A typical cycle● Delayed ack in TCP → 2 data for one ack

44

A typical cycle

45

For the 802.11 b case

● Even with no contention, we reach 6.5 and not 11.

46

Time fairness against Access Fairness

47

Rate Anomaly

• Rate anomaly is well-known in WiFi 802.11 networks– Low-rate stations degrade throughput of

high-rate stations

• Why does rate anomaly exist?– Stations reduce data rates when signal

strength is poor– Low-rate stations’ packets consume more

airtime– Per-packet fairness in basic 802.11

Higher-rate stations receive less airtimethroughput degrades

54Mbps

AP

A

54Mbps

B

Thro

ughp

ut (M

bps)

10

20

30

0

A, B near AP A far from AP

B

A

BA

18Mbps

48

The “TCP unfairness problem”

49

Reference Scenario

● TCP connections to wireless stations

● Downloads and upload

● Sporadic UDP traffic with real-time requirements (VoIP)

● Hypothesis: wireless LAN is the bottleneck

● Reasonable for enterprise/campus networks

● Possible in home networking for in-home flows but a priori not at ADSL/wifi boundary

wired

wireless

TCPconnections

50

TCP Unfairness Problem

● Upload/download asymmetry:

● Long lived flows

● Stations performing uploads obtain higher TCP throughput

51

802.11 DCF characteristics

● Access Point (AP) behaves like a station

● Equal channel access opportunity for all contending entities (AP and N stations)

● Statistical share of 1/(N+1)

52

N TCP Uploads

2N data segments at stations, N ACKs at AP

AP share needs to be N/(N + 2N)= 1/3

DCF ® share of 1/(1+N)= 1/4

● Losses of ACKs segments, but ACKs are cumulative

wired N=3

ACK segmentsDATA segments

53

N TCP Downloads

2N data segments at AP, N ACKs at stations

AP share needs to be 2N/(N + 2N)=2/3

DCF ® share of 1/(1+N)=1/4

● Losses, limited by congestion control

wired N=3

DATA segments ACK segment

54

Mixed upload-download

AP share needs to be something between 1/3 and 2/3

With DCF, the share is ¼ for N=3!

Losses that impact more downloads than uploads

wired

55

Exercise● Assume N TCP downloads

● Q: what is the maximum amount of in-flight IP packets, assuming

– MSS packets

– No window scaling

● Q: what if one sets the buffer size to this value?

56

MANET - Routing

57

Routing● Routing protocol is the basis for multihop communication

– Ad hoc mode of 802.11 does not natively support routing protocol

● The historic problem of the community

● Around 40 different routing protocols has been proposed

– http://en.wikipedia.org/wiki/Ad_hoc_protocol_list

● MANET WG at IETF

● Mobility: links are not permanent

58

Objectives of routing protocols

● Minimize control traffic

– Minimize bandwidth utilization

– Minimize energy (optimize battery usage)

● Minimize processing

– Again : optimize battery usage

● Refreshing of routes

● Avoid routing loops

59

Routing● Routing in mobile ad hoc networks can be

– Proactive protocols (traditional routing protocols are proactive). Routes are built even in the absence of traffic

● Ex. DSDV (Destination Sequenced Distance Vector)

– Reactive protocols. Routes are built only when it is required

● Ex. AODV (Ad-hoc On Demand Distance Vector) [RFC3561]

– Hybrids protocols.

– Hierarchical protocols, Geographical protocols....

60

AODV● A node knows all its neighbors

– Hello packets periodically sent

● A Route Request (RREQ) message is broadcasted when a node does not know how to reach a destination broadcasts

● The RREQ message contains several key bits of information: the source, the destination, the broadcast id, hop count, lifespan, source sequence number, destination sequence number.

– The source address and the broadcast id serves as a unique ID.

Charles E. Perkins and Elizabeth M. Royer. "Ad hoc On-Demand Distance Vector Routing." Proceedings of the 2nd IEEE Workshop on Mobile Computing Systems and Applications, New Orleans, LA, February 1999, pp. 90-100.

Where isNode 3?

61

AODV● Route Reply (RREP) contains a “lifetime” which indicates the

time for which nodes consider a route to be valid

● RREQ contains also a “Sequence number”. Every time a node sends out any type of message it increase its own Sequence number. Each node records the Sequence number of all the other nodes it talks to. A higher Sequence number means a fresher route.

● In case of Link failure, an Route Error (RERR) is sent back to the source. All routes to the Destination Address are erased

62

DSDV1

● Destination Sequenced Distance Vector (DSDV) protocol

● Route advertisement

– Each mobile node advertises its own route tables to its current neighbors

– Routing tables are updated periodically to reflect the network dynamics and maintain a consistent routing table

● Route advertisement will contain the new sequence number of the transmitter node and the following information for each new route

– The destination’s address

– The cost (number of hops) to reach the destination

– The sequence number of the information received,originally stamped by the destination.

1. Perkins, Charles E. & Bhagwat, Pravin: Highly dynamic Destination-Sequenced Distance-Vector routing (DSDV) for mobile computers In: SIGCOMM Comput. Commun. Rev. , Vol. 24 , Nr. 4 New York, NY, USA: ACM (1994) , S. 234-244 .

63

DSDV

64

Responding to Topology changes in DSDV

● Two types of packets defined for route updates

– full dump packets

● Carry all available routing information

● Will require multiple network protocol data units (NPDUs)

● Occasionally transmitted

– Incremental packets

● Carry only information changed since last full dump

● Should use only one NPDU

● Frequently transmitted

65

AODV vs DSDV1

1. Performance Comparison of AODV, DSDV and I-DSDV Routing Protocols in Mobile Ad Hoc Networks, European Journal of Scientific Research, ISSN 1450- 216X Vol. 31 No. 4 (2009), pp. 566-576, © EuroJournals Publishing, Inc. 2009, http://www.eurojournals.com/ejsr.htm