X ELATEX

Wireless data networksLink Layer

Martin Heusse

Link layer — 2

Aloha (1972)

DATA

ACK

Centralhost

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

0.2

0.4

0.6

0.8

1

0 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 afterthe 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 becomessimply: P[N(D) = 1] = µD exp(−µD)Max for µD = 1

• Used for initial access in GSM…

Link layer — 5

CSMA for Wireless802.11 DCF—basic

Data + ACK

Data

DIFS

Data

Data + ACK

Host #

2

1

3 …

…

…

Elapsed backoff

Residual backoff

collision

Medium busy

exponential backoffCW ← 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 decodethe CTS…

A

B

DataData

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

BSSAP

STA

Link layer — 9

Structure of a 802.11 frame (+ ACK)

MPDU

PLCPpreamble

PLCPheader

DIFS SIFS

50 µs

ACK14 bytes

MAC header30 bytes

Data CRC

4 bytes

PLCPheader

tpr

PLCPpreamble

10 µstpr

ttr

tack

backoff

tcont

LLC IP

Link layer — 10

802.11 Data frame

!"""#$%&'(()'*!%"+",,'+$-'.$/'$-0'#12',#"/!3!/$&!4-, ,56'789:((;988<

/=>?@ABC5'D'988<'!""":'$EE'@ABC5F'@GFG@HG6: !!

"#$%&'()*(+,(&,-+./-0&120*(&()*20&.-&340-,5&6,5&7.2/.8+9&7()*(+,(&:-+./-0&;&!<=&48.&>-'8.8-+&+&-?&.@(

40-,5&6,5&A8.B2>C&8?&'(.&.-&DC&8+E8,2.('&.@2.&2+&"#$%&F8.@&"#$%&'()*(+,(&,-+./-0&120*(&()*20&.-&340-,5

6,5&7.2/.8+9&7()*(+,(&:-+./-0&;&+<&@2'&+-.&A((+&/(,(81(E=&G-/&*+*'(E&?/29B(+.&+*BA(/'&-?&2+&"7$%C&.@(

,-//('>-+E8+9&A8.'&8+&.@(&A8.B2>&2/(&'(.&.-&D=

!"#"#$%&'&$()&*+,

H@(&?/2B(&?-/B2.&?-/&2&E2.2&?/2B(&8'&E(>(+E(+.&-+&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&2+E&8'&2'&E(?8+(E&8+

G89*/( !KL!=

$2.2&?/2B('&F8.@&2&120*(&-?&L&8+&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&2/(&,-00(,.81(0J&/(?(//(E&.-&2'&"#$

%&'&()*&+,-=&M2,@&-?&.@('(&E2.2&'*A.J>('&,-+.28+'&I-7&8+&.@(8/&+2B('C&2+E&.@8'&?/2B(&?-/B2.&8'&E8'.8+9*8'@(E

AJ&.@(&>/('(+,(&-?&2&I-7&:-+./-0&?8(0E&8+&.@(&"6:&@(2E(/=&$2.2&?/2B('&F8.@&2&120*(&-?&D&8+&.@(&I-7&'*A?8(0E

-?&.@(&7*A.J>(&?8(0E&E-&+-.&@21(&.@(&I-7&:-+./-0&?8(0E=

6&I-7&7H6&20F2J'&*'('&I-7&E2.2& ?/2B('& ?-/& E2.2& ./2+'B8''8-+'& .-&-.@(/&I-7&7H6'=&6&I-7&7H6&*'('

?/2B('&F8.@&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&'(.&.-&D&?-/&E2.2&./2+'B8''8-+'&.-&+-+KI-7&7H6'=&6&+-+K

I-7&7H6&20F2J'&*'('&?/2B('&F8.@&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&'(.&.-&D&?-/&E2.2&./2+'B8''8-+'&.-

-.@(/&7H6'=&600&7H6'&*'(& ?/2B('&F8.@& .@(&I-7&'*A?8(0E&-?& .@(&7*A.J>(& ?8(0E&'(.& .-&D& ?-/&A/-2E,2'.&E2.2

?/2B('&*+0(''&2& ./2+'B8..8+9&7H6&5+-F'& .@2.&200&7H6'& 8+&2&477&@21(&I-7&,2>2A808.JC& 8+&F@8,@&,2'(& .@(

./2+'B8..8+9&7H6'&*'(&I-7&E2.2&?/2B('=&600&7H6'&*'(&?/2B('&F8.@&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&'(.

.-&D&?-/&B*0.8,2'.&E2.2&?/2B('&*+0(''&8.&8'&5+-F+&.-&.@(&./2+'B8..(/&.@2.&200&7H6'&8+&.@(&477&.@2.&2/(&B(BA(/'

-?&.@(&B*0.8,2'.&9/-*>&@21(&I-7&,2>2A808.JC&8+&F@8,@&,2'(&7H6'&*'(&I-7&E2.2&?/2B('=

H@(&,-+.(+.&-?&.@(&2EE/(''&?8(0E'&-?&E2.2&?/2B('&2/(&E(>(+E(+.&*>-+&.@(&120*('&-?&.@(&H-&$7&2+E&G/-B&$7

?8(0E'&8+&.@(&G/2B(&:-+./-0&?8(0E&2+E&2/(&E(?8+(E&8+&H2A0( !K!=&N@(/(&.@(&,-+.(+.&-?&2&?8(0E&8'&'@-F+&2'&+-.

2>>08,2A0(&3OP6<C&.@(&?8(0E&8'&-B8..(E=&O-.(&.@2.&6EE/(''&L&20F2J'&@-0E'&.@(&/(,(81(/&2EE/(''&-?&.@(&8+.(+E(E

/(,(81(/&3-/C&8+&.@(&,2'(&-?&B*0.8,2'.&?/2B('C&/(,(81(/'<C&2+E&.@2.&6EE/(''&Q&20F2J'&@-0E'&.@(&2EE/(''&-?&.@(

7H6&.@2.&8'&./2+'B8..8+9&.@(&?/2B(=

-&./+$!0!1233)+,,$(4+/3$567'+7',

!"#$% &'"(#$% )**'+,,#- )**'+,,#. )**'+,,#/ )**'+,,#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

849:)+$!0;!1%&'&$()&*+

(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'#12',#"/!3!/$&!4-, ,56'789:((;988<

/=>?@ABC5'D'988<'!""":'$EE'@ABC5F'@GFG@HG6: !!

"#$%&'()*(+,(&,-+./-0&120*(&()*20&.-&340-,5&6,5&7.2/.8+9&7()*(+,(&:-+./-0&;&!<=&48.&>-'8.8-+&+&-?&.@(

40-,5&6,5&A8.B2>C&8?&'(.&.-&DC&8+E8,2.('&.@2.&2+&"#$%&F8.@&"#$%&'()*(+,(&,-+./-0&120*(&()*20&.-&340-,5

6,5&7.2/.8+9&7()*(+,(&:-+./-0&;&+<&@2'&+-.&A((+&/(,(81(E=&G-/&*+*'(E&?/29B(+.&+*BA(/'&-?&2+&"7$%C&.@(

,-//('>-+E8+9&A8.'&8+&.@(&A8.B2>&2/(&'(.&.-&D=

!"#"#$%&'&$()&*+,

H@(&?/2B(&?-/B2.&?-/&2&E2.2&?/2B(&8'&E(>(+E(+.&-+&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&2+E&8'&2'&E(?8+(E&8+

G89*/( !KL!=

$2.2&?/2B('&F8.@&2&120*(&-?&L&8+&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&2/(&,-00(,.81(0J&/(?(//(E&.-&2'&"#$

%&'&()*&+,-=&M2,@&-?&.@('(&E2.2&'*A.J>('&,-+.28+'&I-7&8+&.@(8/&+2B('C&2+E&.@8'&?/2B(&?-/B2.&8'&E8'.8+9*8'@(E

AJ&.@(&>/('(+,(&-?&2&I-7&:-+./-0&?8(0E&8+&.@(&"6:&@(2E(/=&$2.2&?/2B('&F8.@&2&120*(&-?&D&8+&.@(&I-7&'*A?8(0E

-?&.@(&7*A.J>(&?8(0E&E-&+-.&@21(&.@(&I-7&:-+./-0&?8(0E=

6&I-7&7H6&20F2J'&*'('&I-7&E2.2& ?/2B('& ?-/& E2.2& ./2+'B8''8-+'& .-&-.@(/&I-7&7H6'=&6&I-7&7H6&*'('

?/2B('&F8.@&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&'(.&.-&D&?-/&E2.2&./2+'B8''8-+'&.-&+-+KI-7&7H6'=&6&+-+K

I-7&7H6&20F2J'&*'('&?/2B('&F8.@&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&'(.&.-&D&?-/&E2.2&./2+'B8''8-+'&.-

-.@(/&7H6'=&600&7H6'&*'(& ?/2B('&F8.@& .@(&I-7&'*A?8(0E&-?& .@(&7*A.J>(& ?8(0E&'(.& .-&D& ?-/&A/-2E,2'.&E2.2

?/2B('&*+0(''&2& ./2+'B8..8+9&7H6&5+-F'& .@2.&200&7H6'& 8+&2&477&@21(&I-7&,2>2A808.JC& 8+&F@8,@&,2'(& .@(

./2+'B8..8+9&7H6'&*'(&I-7&E2.2&?/2B('=&600&7H6'&*'(&?/2B('&F8.@&.@(&I-7&'*A?8(0E&-?&.@(&7*A.J>(&?8(0E&'(.

.-&D&?-/&B*0.8,2'.&E2.2&?/2B('&*+0(''&8.&8'&5+-F+&.-&.@(&./2+'B8..(/&.@2.&200&7H6'&8+&.@(&477&.@2.&2/(&B(BA(/'

-?&.@(&B*0.8,2'.&9/-*>&@21(&I-7&,2>2A808.JC&8+&F@8,@&,2'(&7H6'&*'(&I-7&E2.2&?/2B('=

H@(&,-+.(+.&-?&.@(&2EE/(''&?8(0E'&-?&E2.2&?/2B('&2/(&E(>(+E(+.&*>-+&.@(&120*('&-?&.@(&H-&$7&2+E&G/-B&$7

?8(0E'&8+&.@(&G/2B(&:-+./-0&?8(0E&2+E&2/(&E(?8+(E&8+&H2A0( !K!=&N@(/(&.@(&,-+.(+.&-?&2&?8(0E&8'&'@-F+&2'&+-.

2>>08,2A0(&3OP6<C&.@(&?8(0E&8'&-B8..(E=&O-.(&.@2.&6EE/(''&L&20F2J'&@-0E'&.@(&/(,(81(/&2EE/(''&-?&.@(&8+.(+E(E

/(,(81(/&3-/C&8+&.@(&,2'(&-?&B*0.8,2'.&?/2B('C&/(,(81(/'<C&2+E&.@2.&6EE/(''&Q&20F2J'&@-0E'&.@(&2EE/(''&-?&.@(

7H6&.@2.&8'&./2+'B8..8+9&.@(&?/2B(=

-&./+$!0!1233)+,,$(4+/3$567'+7',

!"#$% &'"(#$% )**'+,,#- )**'+,,#. )**'+,,#/ )**'+,,#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

849:)+$!0;!1%&'&$()&*+

Link layer — 12

ACK frame format!"""#$%&'()*++,)((- ./01.&123&4"56/7/.!512&16"1&2"58/69#:#7"0!;!0&6"<=!6"4"25#

!" 0>?@ABCD$&E&)((-&!"""*&1FF&ABCD$G&AHGHAIH%*

!"#"$"%&'()&*+,-.&*/+-,0

#$%&'()*%&'+(*),&'+(&,$%&-./&'()*%&01&)1&2%'03%2&03&4056(% !789

#$%&:-&'0%;2&+'& ,$%&-./&'()*%&01&<+=0%2&'(+*&,$%&-22(%11&>&'0%;2&+'&,$%& 0**%20),%;?&=(%@0+61&20(%<,%2

2),)A&*)3)5%*%3,A&B;+<C-<C:%D&<+3,(+;A&B;+<C-<C&<+3,(+;A&+(&EF7E+;;&<+3,(+;&'()*%9

4+(&-./&'()*%1&1%3,&G?&3+37H+F&F#-1A&0'&,$%&I+(%&4()5*%3,1&G0,&J)1&1%,&,+&K&03&,$%&4()*%&.+3,(+;&'0%;2&+'

,$%&0**%20),%;?&=(%@0+61&20(%<,%2&2),)&+(&*)3)5%*%3,&'()*%A&,$%&26(),0+3&@);6%&01&1%,&,+&K9&L3&);;&+,$%(&-./

'()*%1A&,$%&26(),0+3&@);6%&01&,$%&@);6%&+G,)03%2&'(+*&,$%&M6(),0+3NLM&'0%;2&+'&,$%&0**%20),%;?&=(%@0+61&2),)A

*)3)5%*%3,A& EF7E+;;A& B;+<C-<C:%DA& +(& B;+<C-<C& '()*%& *0361& ,$%& ,0*%A& 03& *0<(+1%<+321A& (%D60(%2& ,+

,()31*0,&,$%&-./&'()*%&)32&0,1&FL4F&03,%(@);9&L'&,$%&<);<6;),%2&26(),0+3&03<;62%1&)&'()<,0+3);&*0<(+1%<+32A

,$),&@);6%&01&(+632%2&6=&,+&,$%&3%O,&$05$%(&03,%5%(9&

!"#"$"1&2342/55&*+,-.&*/+-,0

#$%&'()*%&'+(*),&'+(&,$%&EF7E+;;&'()*%&01&)1&2%'03%2&03&4056(% !7P9

#$%& BFFLM& 01& ,$%& )22(%11& +'& ,$%& F#-& <+3,)03%2& 03& ,$%& -E9& #$%& #-& '0%;2& 01& ,$%& )22(%11& +'& ,$%& F#-

,()31*0,,035&,$%&'()*%9&#$%&-LM&01&,$%&@);6%&)11053%2&,+&,$%&F#-&,()31*0,,035&,$%&'()*%&G?&,$%&-E&03&,$%

)11+<0),0+3&(%1=+31%&'()*%&,$),&%1,)G;01$%2&,$),&F#-Q1&<6((%3,&)11+<0),0+39

#$%&-LM&@);6%&);J)?1&$)1&0,1&,J+&IFB1&%)<$&1%,&,+&R9

!"#"$"6&(7489:&*+,-.&*/+-,0

#$%&'()*%&'+(*),&'+(&,$%&.47S32&'()*%&01&)1&2%'03%2&03&4056(% !7RK9

7;<=+.&!4>?'()&*+,-.

7;<=+.&!4@?2342/55&*+,-.

7;<=+.&!4$A?(7489:&*+,-.

RA: address of sender of data frame

Link layer — 13

RTS frame format

ISO/IEC 8802-11: 1999(E)MEDIUM ACCESS CONTROL (MAC) AND PHYSICAL (PHY) SPECIFICATIONS ANSI/IEEE Std 802.11, 1999 Edition

Copyright © 1999 IEEE. All rights reserved.

41

b) The remainder after multiplication of the contents (treated as a polynomial) of the calculation fields

by

x

32

and then division by

G

(

x

).

The FCS field is transmitted commencing with the coefficient of the highest-order term.

As a typical implementation, at the transmitter, the initial remainder of the division is preset to all 1’s and is

then modified by division of the calculation fields by the generator polynomial

G

(

x

). The 1’s complement of

this remainder is transmitted, with the highest-order bit first, as the FCS field.

At the receiver, the initial remainder is preset to all 1’s and the serial incoming bits of the calculation fields

and FCS, when divided by

G

(

x

), results in the absence of transmission errors, in a unique nonzero remainder

value. The unique remainder value is the polynomial:

x

31

+

x

30

+

x

26

+

x

25

+

x

24

+

x

18

+

x

15

+

x

14

+

x

12

+

x

11

+

x

10

+

x

8

+

x

6

+

x5 + x4 + x3 + x + 1

7.2 Format of individual frame types

7.2.1 Control frames

In the following descriptions, “immediately previous” frame means a frame whose reception concluded

within the prior short interframe space (SIFS) interval.

The subfields within the Frame Control field of control frames are set as illustrated in Figure 15.

7.2.1.1 Request To Send (RTS) frame format

The frame format for the RTS frame is as defined in Figure 16.

The RA of the RTS frame is the address of the STA, on the WM, that is the intended immediate recipient of

the pending directed data or management frame.

The TA is the address of the STA transmitting the RTS frame.

Figure 15—Frame Control field subfield values within control frames

Figure 16—RTS frame

• 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

PLCPPhy. Layer Convergence Proto.

ISO/IEC 8802-11: 1999(E)ANSI/IEEE Std 802.11, 1999 Edition LOCAL AND METROPOLITAN AREA NETWORKS: WIRELESS LAN

196

Copyright © 1999 IEEE. All rights reserved.

15.1.3 Service specification method and notation

The models represented by figures and state diagrams are intended to be illustrations of functions provided.

It is important to distinguish between a model and a real implementation. The models are optimized for

simplicity and clarity of presentation; the actual method of implementation is left to the discretion of the

IEEE 802.11 DSSS PHY compliant developer.

The service of a layer or sublayer is a set of capabilities that it offers to a user in the next-higher layer (or

sublayer). Abstract services are specified here by describing the service primitives and parameters that char-

acterize each service. This definition is independent of any particular implementation.

15.2 DSSS PLCP sublayer

15.2.1 Overview

This clause provides a convergence procedure in which MPDUs are converted to and from PPDUs. During

transmission, the MPDU shall be prepended with a PLCP Preamble and Header to create the PPDU. At the

receiver, the PLCP Preamble and header are processed to aid in demodulation and delivery of the MPDU.

15.2.2 PLCP frame format

Figure 86 shows the format for the PPDU including the DSSS PLCP Preamble, the DSSS PLCP Header, and

the MPDU. The PLCP Preamble contains the following fields: Synchronization (Sync) and Start Frame

Delimiter (SFD). The PLCP Header contains the following fields: IEEE 802.11 Signaling (Signal), IEEE

802.11 Service (Service), LENGTH (Length), and CCITT CRC-16. Each of these fields is described in

detail in 15.2.3.

15.2.3 PLCP field definitions

The entire PLCP Preamble and Header shall be transmitted using the 1 Mbit/s DBPSK modulation described

in 15.4.7. All transmitted bits shall be scrambled using the feedthrough scrambler described in 15.2.4.

15.2.3.1 PLCP Synchronization (SYNC) field

The SYNC field shall consist of 128 bits of scrambled ones. This field shall be provided so that the receiver

can perform the necessary operations for synchronization.

Figure 86—PLCP frame format

• 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)

!"""#$%&'(()'*!%"+",,'+$-'.$/'$-0'#12',#"/!3!/$&!4-, ,56'789:((;988<

/=>?@ABC5'D'988<'!""":'$EE'@ABC5F'@GFG@HG6: !"!

!"#$#%&'()'&*+,-.&*/+-,0

#$%&'( )*+),-./0-,1.(,2/'341,2/', 1.(,5567,$89:&;$8%,1.(,<#6=,5>?5,@'(43A:(B,<#6=,5>?5,.(4;('B

5C67B,14$:,A$1-B,48;,@4;,A$1-D,E.(,5>?5,.(4;(',9/814$8-,1.(,2/::/0$8%,2$(:;-F,>GHIEJB,KLEGB,4,'(-('M(;

A$1B,48,(M(8,@4'$1N,A$1B,48;,1.(,CGKOP?G,2$(:;D,P8,1('3-,/2,3/;&:41$/8B,1.(,>GHIEJB,KLEGB,'(-('M(;,A$1B

48;,@4'$1N,A$1,Q0$1.,R,S('/,14$:,A$1-,4@@(8;(;T,9/8-1$1&1(,4,-(@4'41(,-$8%:(,<#6=,-N3A/:B,;(8/1(;,CPIHL>B

0.$9.,$-,1'48-3$11(;,0$1.,1.(,3/-1,'/A&-1,9/3A$841$/8,/2,U5CV,3/;&:41$/8,48;,4,9/;$8%,'41(,/2,K,W,)XYD

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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 offrames

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