wireless data networks link layer - university of colorado...
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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
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Link layer — 17
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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
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