Formula Sheets: Media Access Control (MAC)

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MA C La y er F orm ula Sheet
MA C La y er Ov erview
• F unction : Con trols access to shared medium, resolv es con ten tion, and ensures fair transmission.
• F rame A ddressing : Uses 48-bit MA C addresses (6 b ytes).
• Throughput : S =
L
data
T
total
, where L
data
is pa yload size, T
total
includes transmission and o v erhead
times.
Ethernet (IEEE 802.3)
• F rame F ormat : Pream ble (7 b ytes) + SFD (1 b yte) + Dest MA C (6 b ytes) + Src MA C (6 b ytes)
+ T yp e (2 b ytes) + Data (46-1500 b ytes) + CR C (4 b ytes).
• Minim um F rame Size : L
min
= 64 b ytes, ensures collision detection within 2·T
prop
.
• Throughput E?iciency : ? =
L
data
L
data
+L
o v erhead
+T
IFS
·R
, where T
IFS
is in terframe space (9.6 µ s at 10
Mbps), R is data rate.
• CSMA/CD (Carrier Sense Multiple A ccess with C ollision Detection) :
– E?iciency: ?˜
1
1+5a
, where a =
T prop
T trans
, T
trans
=
L
R
.
– Collision Windo w: T
collision
= 2·T
prop
, require s L
min
= 2·T
prop
·R .
– Binary Exp onen tial Bac k off: After k -th collision, w ait random time in [0,2
k
-1] slots,k= 10 ,
slot time = 51.2 µ s (10 Mbps).
Wi-Fi (IEEE 802.11)
• CSMA/CA (Collision A v oidance) :
– Con ten tion Windo w: CW ? [CW
min
,CW
max
] , doubles after collision (e.g., CW
min
= 15 ,
CW
max
= 1023 for 802.11b).
– Bac k off Time: T
bac k off
= random(0,CW)·T
slot
, whereT
slot
is slot time (e.g., 20µ s for 802.11b).
– Throughput: S =
L
data
T
data
+T
DIFS
+T
SIFS
+T
R TS
+T
CTS
+T
A CK
, where DIFS/SIFS are in terframe spaces.
– R TS/CTS Ov erhead: T
R TS/CTS
=T
R TS
+T
CTS
+2·T
SIFS
.
• Hidden T erminal Solution : R TS/CTS reduces collision probabilit y , P
collision
?
1
N stations
.
• F rame Error Rate : P
FER
= 1-(1-P
bit
)
L
, where P
bit
is bit error rate, L is frame length.
Blueto oth (IEEE 802.15.1)
• Time Division Duplex (TDD) :
– Slot Duration: T
slot
= 625µ s.
– F rame T ransmission Time: T
frame
=N
slots
·T
slot
, where N
slots
= 1,3,5 .
• Piconet Throughput : S =
L
data
T
frame
+T
guard
, where T
guard
is in ter-slot gap.
• F requency Hopping Spread Sp ectrum (FHSS) :
– Hop Rate: 1600 hops/s, T
hop
= 625µ s.
– Bandwidth: 79 c hannels, 1 MHz eac h, total 79 MHz in 2.4 GHz band.
• Max Devices : N
activ e
= 7 p er piconet (1 master, 7 sla v es).
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MA C La y er F orm ula Sheet
MA C La y er Ov erview
• F unction : Con trols access to shared medium, resolv es con ten tion, and ensures fair transmission.
• F rame A ddressing : Uses 48-bit MA C addresses (6 b ytes).
• Throughput : S =
L
data
T
total
, where L
data
is pa yload size, T
total
includes transmission and o v erhead
times.
Ethernet (IEEE 802.3)
• F rame F ormat : Pream ble (7 b ytes) + SFD (1 b yte) + Dest MA C (6 b ytes) + Src MA C (6 b ytes)
+ T yp e (2 b ytes) + Data (46-1500 b ytes) + CR C (4 b ytes).
• Minim um F rame Size : L
min
= 64 b ytes, ensures collision detection within 2·T
prop
.
• Throughput E?iciency : ? =
L
data
L
data
+L
o v erhead
+T
IFS
·R
, where T
IFS
is in terframe space (9.6 µ s at 10
Mbps), R is data rate.
• CSMA/CD (Carrier Sense Multiple A ccess with C ollision Detection) :
– E?iciency: ?˜
1
1+5a
, where a =
T prop
T trans
, T
trans
=
L
R
.
– Collision Windo w: T
collision
= 2·T
prop
, require s L
min
= 2·T
prop
·R .
– Binary Exp onen tial Bac k off: After k -th collision, w ait random time in [0,2
k
-1] slots,k= 10 ,
slot time = 51.2 µ s (10 Mbps).
Wi-Fi (IEEE 802.11)
• CSMA/CA (Collision A v oidance) :
– Con ten tion Windo w: CW ? [CW
min
,CW
max
] , doubles after collision (e.g., CW
min
= 15 ,
CW
max
= 1023 for 802.11b).
– Bac k off Time: T
bac k off
= random(0,CW)·T
slot
, whereT
slot
is slot time (e.g., 20µ s for 802.11b).
– Throughput: S =
L
data
T
data
+T
DIFS
+T
SIFS
+T
R TS
+T
CTS
+T
A CK
, where DIFS/SIFS are in terframe spaces.
– R TS/CTS Ov erhead: T
R TS/CTS
=T
R TS
+T
CTS
+2·T
SIFS
.
• Hidden T erminal Solution : R TS/CTS reduces collision probabilit y , P
collision
?
1
N stations
.
• F rame Error Rate : P
FER
= 1-(1-P
bit
)
L
, where P
bit
is bit error rate, L is frame length.
Blueto oth (IEEE 802.15.1)
• Time Division Duplex (TDD) :
– Slot Duration: T
slot
= 625µ s.
– F rame T ransmission Time: T
frame
=N
slots
·T
slot
, where N
slots
= 1,3,5 .
• Piconet Throughput : S =
L
data
T
frame
+T
guard
, where T
guard
is in ter-slot gap.
• F requency Hopping Spread Sp ectrum (FHSS) :
– Hop Rate: 1600 hops/s, T
hop
= 625µ s.
– Bandwidth: 79 c hannels, 1 MHz eac h, total 79 MHz in 2.4 GHz band.
• Max Devices : N
activ e
= 7 p er piconet (1 master, 7 sla v es).
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Switc h ing and Bridging
• Learning Bridge :
– MA C T able Up date: T
learn
=O(1) p er fram e.
– T able Size: S
table
=N
en tries
·(S
MA C
+S
p ort
) , where S
MA C
= 6 b ytes, S
p ort
is p ort ID size.
• Spanning T ree Proto col (STP) :
– P ath Cost: C =
?
C
link
, where C
link
?
1
Bandwidth
(e.g., 100 for 10 Mbps, 19 for 100 Mbps).
– Con v ergence Time: T
STP
=T
listen
+T
learn
˜ 30-50 s.
• F orw arding Dela y : T
forw ard
=T
queue
+T
switc h
, where T
switc h
?
L
frame
R
.
Media A ccess Con trol Proto cols
• Pure Aloha :
– Throughput: S =Ge
-2G
, where G is offered load (frames p er frame time).
– Max Throughput: S
max
= 0.184 at G = 0.5 .
• Slotted Aloha :
– Throughput: S =Ge
-G
.
– Max Throughput: S
max
= 0.368 at G = 1 .
• Collision Probabilit y : P
collision
?
N stations
T con ten tion
, where T
con ten tion
is con ten tion p erio d.
P e rformance Metrics
• Latency : T
latency
=T
prop
+T
trans
+T
queue
+T
pro c
, where T
trans
=
L
R
.
• Throughput : S =
L
effectiv e
T
total
, where L
effectiv e
excludes o v erhead.
• Channel Utilization : U =
T
data
T
data
+T
o v erhead
, where T
o v erhead
includes bac k off, IFS, and collisions.
• Error Rate : P
error
= 1-(1-P
bit
)
L
, where P
bit
is bit error rate.
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