Wireless data networks Link Layer Martin Heusse X L A TEX E - - PowerPoint PPT Presentation

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Wireless data networks Link Layer

Martin Heusse

Link layer — 2

Aloha (1972)

DATA ACK Central host

i = 1 while (i <= maxAttempts) do send packet wait for acknowledgement or timeout if ack received then leave wait for random time increment i end do

Link layer — 3

Aloha performance with Poisson traffic

0.2 0.4 0.6 0.8 1 0.5 1 1.5 2

P mT

mT exp(-2mT)

Link layer — 4

Slotted Aloha

• Access takes place on discrete time “slots” (D, 2D, 3D ms after

the end of the previous transmission)

• Number of transmission during a slot:

P[N(D) = n] = (µD)n

n!

exp(−µD)

• The probability of transmission without collision becomes

simply: P[N(D) = 1] = µD exp(−µD) Max for µD = 1

• Used for initial access in GSM…

Link layer — 5

CSMA for Wireless 802.11 DCF—basic

Data + ACK Data

DIFS

Data

Data + ACK

Host # 2 1 3 … … … Elapsed backoff Residual backoff collision Medium busy exponential backoff

CW ← 2 x CW

time

Link layer — 6

CSMA/CA (Bharghavan 94: MACAW)

• Collision Avoidance with hidden terminal
• Send a short frame which may collide (RTS)
• The intended receiver reserves the channel around himself

(CTS with explicit Data frame duration) However, this does not work if a station (D) could not decode the CTS…

A B

Data Data

D C

RTS CTS RTS Collision Data Collision NAV

Link layer — 7

xIFS: Interframe spacing

• SIFS
• PIFS
• DIFS
• EIFS

Link layer — 8

802.11 network

DS IBSS BSS AP STA

Link layer — 9

Structure of a 802.11 frame (+ ACK)

MPDU PLCP preamble PLCP header DIFS SIFS 50 µs ACK 14 bytes MAC header 30 bytes Data

C R C

4 bytes PLCP header tpr PLCP preamble 10 µs tpr ttr tack backoff tcont

LLC

IP

Link layer — 10

802.11 Data frame

(QoS control field is generally not present)

• 4 address fields! (what’s a DS—distribution system?)

✓ Address 1: always used for reception decision ✓ Address 2: Address of transmitting station ✓ Address 3: Final recipient (to DS); Original source (from DS); BSSID (IBSS mode) ✓ Address 4: source address if frame is in transit between relays

• Duration: used by other stations to set their NAV

Link layer — 11

Address fields content

!"#$% &'"(#$% )**'+,,#- )**'+,,#. )**'+,,#/ )**'+,,#0 D D R6&S&$6 H6&S&76 477T$ OP6 D L R6&S&$6 H6&S&477T$ 76 OP6 L D R6&S&477T$ H6&S&76 $6 OP6 L L R6 H6 $6 76

Link layer — 12

ACK frame format

RA: address of sender of data frame

Link layer — 13

RTS frame format

• Duration: RTS+SIFS+CTS+SIFS+DATA+SIFS+ACK

Link layer — 14

Management frames

• Beacons:

✓ Timestamp ✓ Beacon interval ✓ Capability (PCF available? Encryption required?) ✓ SSID (up to 32 bytes) ✓ Required rates

• Association requests/responses

Link layer — 15

PLCP

• Phy. Layer Convergence Proto.
• Duration: 802.11b: 192µs @ 1Mb/s

(Short preamble option: 72 bits preamble @ 1Mb/s, PLCP head. @ 2Mb/s → 92µs)

• Signal: data transmission bit rate

Link layer — 16

PLCP OFDM (11a, 11g)

?/;(;X<#6= 6LEL CPIHL> <8(,<#6=,CN3A/: 5C67 E4$: 54;,U$1- >GHIEJ )Y,A$1- KLEG ],A$1- 54'$1N ),A$1 R,A$1- O4'$4A:(,H&3A(',/2,<#6=,CN3A/:- 5>?5,5'(43A:( )Y,CN3A/:- K(-('M(; ),A$1 E4$: R,A$1- ?/;(;X<#6= ,QU5CVB,',W,)XYT ,QKLEG,$-,$8;$941(;,$8,CPIHL>T CGKOP?G )R,A$1- 5>?5,J(4;('

Duration: ≈22µs

Link layer — 17

802.11a PHYs

"#$#!%#$&! '()*$+,+- (./01#$*.2 3./*24!%#$& '5- 3./&/!)*$+! 6&%! +0)7#%%*&% '89:;3- 3./&/!)*$+! 6&%!<="(! +>?).1 '839:;- "#$#!)*$+! 6&%!<="(! +>?).1 '8"9:;- A! B>CD EFG E H6 GH "! B>CD IFH E H6 IA EG! J>CD EFG G "A H6 E6! J>CD IFH G "A 5G GH! EAKJL= EFG H E"G "A IA! EAKJL= IFH H E"G EHH H6! AHKJL= GFI A G66 E"G MH! AHKJL= IFH A G66 GEA

Link layer — 18

One problem in a multirate cell (“Performance anomaly”)

• All stations access the network with same probability
• So in time T, they will all send approx. the same number of

frames

• If one of them transmit at 1Mb/s, everybody gets only a fraction
• f this