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Wireless LAN (WLAN): infrastructure mode

Access Point

F1

F1

F1

F1F1

F1

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

WLAN: Infrastructure Network

−→ shared uplink & downlink channel F1

−→ single baseband channel

• basic service set (BSS)

• base station: access point (AP)

• mobile stations must communicate through AP

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WLAN: ad hoc mode

F1

F1

F1

F1F1

F1

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

MobileMobile

F1

F1

F1

F1

F1

WLAN: Ad Hoc Network

−→ homogeneous: no base station

−→ everyone is the same

−→ share forwarding responsibility

• independent basic service set (IBSS)

• mobile stations communicate peer-to-peer

→ also called peer-to-peer mode

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WLAN: internetworking

Access Point

F1

F1

F1

F1F1

F1

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

Access Point

F1

F1

F1

F1F1

F1

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

Access Point

F1

F1

F1

F1F1

F1

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

Mobile

WLAN: Extended Service Set

Distribution System

−→ internetworking between BSS’s through APs

−→ mobility and handoff

• extended service set (ESS)

• APs are connected by distribution system (DS)

→ typically: Ethernet switch

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• How do APs and Ethernet switches know where to

forward frames?

→ spanning tree

→ IEEE 802.1 (Perlman’s algorithm)

→ learning bridge: source address discovery

→ if unclear: broadcast

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Additional headache: mobility

−→ how to perform handoff

−→ mobility management at MAC vs. IP

Mobility between BSSes in an ESS

• association

→ registration process

→ AP sends out periodic beacon

→ mobile station (MS) associates with one AP

• disassociation

→ upon permanent departure: notification

• reassociation

→ movement of MS from one AP to another

→ inform new AP of old AP

→ forwarding of buffered frames

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IEEE 802.11b/g WLAN spectrum 2.4–2.4835 GHz:

−→ 11 channels (U.S.)

−→ 2.412 GHz, 2.417 GHz, . . ., 2.462 GHz

−→ unlicensed ISM (Industrial, Scientific, Medical) band

−→ global: 2.4–2.4835 GHz

−→ up to 14 channels

IEEE 802.11a: 5.15–5.35 GHz and 5.725–5.825 GHz

−→ UNNI (unlicensed National Information Infrastructure)

−→ non-global

IEEE 802.11n: both 2.4 and 5 GHz

−→ 2.4 GHz: backward compatible

−→ also uses multiple antennae

−→ called MIMO (multiple input multiple output)

−→ e.g., Apple’s 802.11n has 3 antennae

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Non-interference specification for 802.11b:

• each channel has 22 MHz bandwidth

• require 25 MHz channel separation

−→ thus, only 3 concurrent channels possible

−→ e.g., channels 1, 6 and 11

−→ 3-coloring. . .

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Examples:

Purdue Univ.: IEEE 802.11b (11 Mbps) WLAN network

−→ PAL (Purdue Air Link)

−→ partial mobility: MAC roaming (within ESS)

−→ no mobile IP

−→ football scores at Ross-Ade through PDAs

Dartmouth College: IEEE 802.11b WLAN (500+ APs)

−→ full VoIP

−→ free long distance

Seattle, SF, San Diego, Boston, etc.: WiFi communities

−→ “free” Internet access

−→ city: lamp-post; called mesh networks

−→ private: roof-top

−→ cable & DSL companies don’t like it

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IEEE 802.11 MAC

−→ CSMA/CA with exponential backoff

−→ almost like CSMA/CD

−→ drop CD

−→ explicit positive ACK frame

−→ added optional feature: CA (collision avoidance)

Two modes for MAC operation:

• Distributed coordination function (DCF)

→ multiple access (default mode)

• Point coordination function (PCF)

→ polling-based priority

. . . neither PCF nor CA used in practice

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Timeline without collision:

DATA BOBO DIFSDIFS

SIFS

2−way handshake

. . .

. . .

Time

TimeSender

ACK

Receiver

• SIFS (short interframe space): 10 µs

• Slot Time: 20 µs

• DIFS (distributed interframe space): 50 µs

→ DIFS = SIFS + 2 × slot time

• BO: variable back-off (within one CW)

→ CWmin: 31; CWmax: 1023

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Time snapshot with Mira-come-lately:

−→ Sue sends to Arnold

Arnold

DATA

ACK

Time

Time

SIFS

Time

Sue

Mira

Mira wants to send a frame

DIFS BO

DIFS

Slot Time

. . .

BO

WAITCS = BUSY . . .

Contention Window CW

DATA

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Time snapshot with collision (Sue & Mira):

Mira

Sue

Time

SIFS

Time

Time

Arnold

Sue wants to send a frame

Mira wants to send a frame

DATA

SIFS

BACKOFF

BACKOFF

Slot Time

WINNER

NOACK

ACKNO

BO

BODIFS

DIFS

DATA . . .

. . .DATA

COLLISION

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MAC throughput and collision (simulation):

0

1

2

3

4

5

6

3.5 4 4.5 5 5.5 6 6.5

Tho

ughp

ut (

Mb/

s)

Offered Load (Mb/s)

node 2node 5

node 10node 20node 30node 50

node 100

0

10

20

30

40

50

60

70

3.5 4 4.5 5 5.5 6 6.5

Col

lisio

n P

roba

bilit

y (%

)

Offered Load (Mb/s)

node 2node 5

node 10node 20node 30node 50

node 100

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MAC throughput (experiment):

−→ HP iPAQ pocket PC running Linux

0

1

2

3

4

5

6

4 4.5 5 5.5 6 6.5 7 7.5 8

Thr

ough

put (

Mb/

s)

Offered Load (Mb/s)

Node 2Node 5

Node 10Node 12Node 16

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Additional issues with CSMA in wireless media:

Hidden station problem:

MobileMobile Mobile

A B C

Hidden Station Problem

(1) (2)

(3)

(1) A transmits to B

(2) C does not sense A; transmits to B

(3) interference occurs at B: i.e., collision

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But: collision does not always mean junk

−→ i.e., transmission failure

For example:

if A’s frame has stronger signal strength than C’s frame,

B may still be able to successfully decode A’s frame

but not C’s frame

−→ called capture effect

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Biggest problem: starvation problem

−→ related to hidden station problem

−→ A cannot hear C, C cannot hear A

−→ B can hear both A and C

−→ by CS, B gets little chance to speak

−→ hence “sandwiched” B may starve

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Consider 4 APs, each with 10 laptops (i.e., 4 adjacent

BSS):

AP 1 AP 2 AP 3 AP 4

hearing (i.e., CS) range

0

1e+006

2e+006

3e+006

4e+006

5e+006

Througput (bps)

AP0AP1AP2AP3

−→ middle two APs get half the throughput

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400-AP (20 × 20) mesh network:

−→ campus-scale wireless network

−→ could be city block

−→ large residential community

0 1 2 3 4 5 6

0 5 10 15 20 0

5

10

15

20

−→ black squares: regions of near-starvation

−→ note alternating checker pattern

−→ solution?

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Hidden station problem: CA (congestion avoidance)

−→ RTS/CTS reservation handshake

• Before data transmit, perform RTS/CTS handshake

• RTS: request to send

• CTS: clear to send

SIFSSIFSSIFS

CTS

RTS

Sender

Receiver

reservation handshake

20B

14B 14B

29B − 2347B

same as before

. . .

. . .

Time

Time

DIFS DIFSBO

ACK

DATA

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Hidden station problem: RTS/CTS handshake “clears”

hidden area

MobileMobile Mobile

A B CRTS

CTS CTS

RTS/CTS Handshake

"clears the area"

RTS/CTS perform only if data > RTS threshold

−→ why not for small data?

. . . feature available but not used