Datalink Control
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Data Link Control (DLC)
ALOHA (“free for all”)
Stations transmit whenever they have data to send
Detect Collision or Wait for an acknowledgment
If no acknowledgment (or collision), try again after a random waiting time If no acknowledgment (or collision), try again after a random waiting time
Collision: If more than one node transmit at the same timeIf there is a collision, all nodes have to re-transmit packets
Vulnerable Window
For a given frame, the time when no other frame may be transmitted
if a collision is to be avoided. Assume all packets have same length (L) and require Tp seconds for
transmission
Each packet vulnerable to collisions for time Vp = ??
Suppose packet A sent at time to
If pkt B sent any time between to – Tp and to end of packet B collides If pkt B sent any time between to – Tp and to end of packet B collideswith beginning of packet A
If pkt C sent any time between to and to + Tp start of packet C will collidewith end of packet A
Total vulnerable interval for packet A is 2Tp
t
PacketC
PacketB Tp
PacketA
Tp
What is the throughput of Pure ALOHA?
Based on several assumptions:
1. Traffic Model: transmission attempts follows a Poisson distribution
2. Fixed packet size
Poisson Distribution:
Pr[i customers arrive in t time interval] =P[n(t) =i] = ((t) i e -t)/i!
Where = Average arrival rate ( no of arrival per unit tine)Where = Average arrival rate ( no of arrival per unit tine)
Assume, G = Average number of arrival in time interval T= T, where T =frame time
ALOHA Throughput
Let S = number of successful packet transmissions per frame time (equalschannel utilization)
G (=T in previous expression )= average number of attemptedtransmissions per packet time(user load+retransmissions).Then,with Poissondistribution traffic model
Average no of packet in 2T (Vulnerable window) time=
(2 )P[n(2T) =i] = ((2G) i e –2G)/i!
Now Throughput (S) is = offered load G * Prob that no frame suffers collision
S= G * Pr( that there is no transmission in 2T time interval
i.e Pr( n(2T)=0 ) )
Pr( n(2T)=0 )= e - 2G
S= G* e - 2G
ALOHA and Slotted ALOHAThroughput versus Load
0.15
0.2
0.25
0.3
0.35
0.4
Ge-G
0.184
0.368
S
Peaks at G=.5 ---> max(Throughput) = 1/2e ~ 0.18
ALOHA can achieve maximum throughput of 18.4%
dS/dG = e-2G – 2Ge-2G = 0
Gmax = 1/2 Smax = 1/(2e) ~ 0.184
0
0.05
0.1
0.15
0.0
1563
0.0
3125
0.0
625
0.1
25
0.2
5
0.5 1 2 4 8
Ge-2G
G
Slotted ALOHA
Time is divided into slots (i.e., slot = one packet transmission time at least)
Master station generates synchronization pulses for time-slots. (e.g., use “pip” from asatellite)
Station waits till beginning of slot to send packet.
Stations transmit ONLY at the beginning of a time slot
Collisions will occur because more than one frame can send in the slot n
But collision probability reduces as Vulnerability Window reduced from 2T to T;
Goodput doubles. Goodput doubles.Average no of packet in T (Vulnerable window) time=
P[n(T) =i] = ((G) i e –G)/i!
So S= G * Pr( that there is no transmission in 2T time interval
i.e Pr( n(T)=0 ) )
Pr( n(T)=0 )= e - G
S= G* e - G
Peaks at G=1max(Throughput) = 1/e ~ 0.36
dS/dG = e-G – Ge-G = 0
Gmax = 1.0 Smax = 1/e ~ 0.368
Aloha (contd..) Aloha performance :- not dependent on a
Reservation Aloha for Satellite ( Kesav 152, Gallgher313)
Observations:
ALOHA is an unstable protocol
If G increases to greater than 1 due to fluctuation in offered load, S willdecrease
Reduction in throughput means fewer successful packet transmissions and Reduction in throughput means fewer successful packet transmissions andmore collisions
Number of retransmissions increases, backlogging messages to betransmitted and traffic load G
This in turn decreases S
Results in operating point moving to right and S 0
Random access protocols can be made stable using backoff parameters
Comparison
1.0
0.9
0.8
0.5
0.6
0.7
S(throughputperpackettime)
0.1-persistent CSMA
0.5-persistentCSMA
Nonpersistent CSMA
0.01 persistent CSMA
With small p better s but longer delay
0.5
0.4
0.3
0.2
0.1
01 2 3 4 5 6 7 8 90
S(throughputperpackettime)
G (attempts per packet time)
PureALOHA
SlottedALOHA
1-persistent
CSMA
CSMA
Difference Between Wired and Wireless
A B C
A
B
C
Ethernet LAN Wireless LAN
If both A and C sense the channel to be idle at the same time,they send at the same time.
Collision can be detected at sender in Ethernet.
Half-duplex radios in wireless cannot detect collision at sender.
A and C cannot hear each other.
A sends to B, C cannot receive A.
C wants to send to B, C senses a “free” medium (CS fails)
Collision occurs at B.
Hidden Terminal Problem
BA C
Collision occurs at B.
A cannot receive the collision (CD fails).
A is “hidden” for C.
Exposed Terminal Problem
A B
CD
A starts sending to B.
C senses carrier, finds medium in use and has to wait for A->B toend.
D is outside the range of A, therefore waiting is not necessary.
A and C are “exposed” terminals
CSMA: Distributed, Packet mode scheme
Carrier Sense and its variants:
Use of carrier sensing capability to know if someone else is using themedium
1 persistent
If medium busy, keep sensing
If medium Idle send immediately
p persistent p persistent
If medium busy, keep sensing
If medium becomes Idle after continuous sensing,
– send with probability p, (wait IFS time for 802.11, then do a random back-off)
– in case of no-send (1-p), wait for 1 time slot, and begin medium sensingagain
If medium is free for IFS period, transmit packet.
Non persistent
If medium busy, wait for random time before sensing again
If medium Idle send immediately
Summary of CSMA schemes (Diagramfrom Keshav)
802.11 - MAC layer
Traffic services Asynchronous Data Service (mandatory) – DCF
Time-Bounded Service (optional) - PCF
Access methods DCF CSMA/CA (mandatory)
collision avoidance via randomized back-off mechanism
ACK packet for acknowledgements (not for broadcasts)
DCF w/ RTS/CTS (optional)
avoids hidden/exposed terminal problem, provides reliability
PCF (optional)
access point polls terminals according to a list
t
medium busy
DIFSDIFS
next frame
contention window(randomized back-offmechanism)
802.11 - CSMA/CA
station which has data to send starts sensing the medium (Carrier Sense
slot time
direct access ifmedium is free DIFS
station which has data to send starts sensing the medium (Carrier Sensebased on CCA, Clear Channel Assessment)
if the medium is free for the duration of an Inter-Frame Space (IFS),the station can start sending (IFS depends on service type)
if the medium is busy, the station has to wait for a free IFS plus anadditional random back-off time (multiple of slot-time)
if another station occupies the medium during the back-off time of thestation, the back-off timer stops (fairness)
802.11 DCF – basic access
If medium is free for DIFS time, station sends data
receivers acknowledge at once (after waiting for SIFS) if the packetwas received correctly (CRC)
automatic retransmission of data packets in case of transmission errors
DIFS
t
SIFS
DIFS
data
ACK
waiting time
otherstations
receiver
senderdata
DIFS
contention
Solution to Hidden/Exposed Terminals
A first sends a Request-to-Send (RTS) to B
On receiving RTS, B responds Clear-to-Send (CTS)
Hidden node C overhears CTS and keeps quiet Transfer duration is included in both RTS and CTS
Exposed node overhears a RTS but not the CTS Exposed node overhears a RTS but not the CTS D’s transmission cannot interfere at B
A B C
RTS
CTS CTS
DATA
D
RTS
802.11 - Reliability
Use acknowledgements When B receives DATA from A, B sends an ACK
If A fails to receive an ACK, A retransmits the DATA
Both C and D remain quiet until ACK (to prevent collision ofACK)
Expected duration of transmission+ACK is included in RTS/CTSpacketspackets
A B C
RTS
CTS CTS
DATA
D
RTS
ACK
802.11 –RTS/CTS
If medium is free for DIFS, station can send RTS with reservation parameter(reservation determines amount of time the data packet needs the medium)
acknowledgement via CTS after SIFS by receiver (if ready to receive)
sender can now send data at once, acknowledgement via ACK
other stations store medium reservations distributed via RTS and CTS
DIFSdataRTS
t
SIFS
data
ACK
defer access
otherstations
receiver
senderdata
DIFS
contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
802.11 - Carrier Sensing
In IEEE 802.11, carrier sensing is performed
at the air interface (physical carrier sensing), and
at the MAC layer (virtual carrier sensing)
Physical carrier sensing
detects presence of other users by analyzing all detected packets
Detects activity in the channel via relative signal strength fromother sourcesother sources
Virtual carrier sensing is done by sending MPDU durationinformation in the header of RTS/CTS and data frames
Channel is busy if either mechanisms indicate it to be
Duration field indicates the amount of time (in microseconds) requiredto complete frame transmission
Stations in the BSS use the information in the duration field to adjusttheir network allocation vector (NAV)
802.11 - Collision Avoidance
If medium is not free during DIFS time..
Go into Collision Avoidance: Once channel becomes idle,wait for DIFS time plus a randomly chosen backoff timebefore attempting to transmit
For DCF the backoff is chosen as follows: When first transmitting a packet, choose a backoff interval in the When first transmitting a packet, choose a backoff interval in the
range [0,cw]; cw is contention window, nominally 31
Count down the backoff interval when medium is idle
Count-down is suspended if medium becomes busy
When backoff interval reaches 0, transmit RTS
If collision, then double the cw up to a maximum of 1024
Time spent counting down backoff intervals is part ofMAC overhead
Example - backoff
wait
B1 = 5B1 = 25
data
data
B2 = 15B2 = 20
wait
B1 and B2 are backoff intervalsat nodes 1 and 2
cw = 31
B2 = 10
802.11 - Priorities
defined through different inter frame spaces – mandatory idle timeintervals between the transmission of frames
SIFS (Short Inter Frame Spacing)
highest priority, for ACK, CTS, polling response
SIFSTime and SlotTime are fixed per PHY layer (10 s and 20 srespectively in DSSS)respectively in DSSS)
PIFS (PCF IFS)
medium priority, for time-bounded service using PCF
PIFSTime = SIFSTime + SlotTime
DIFS (DCF IFS)
lowest priority, for asynchronous data service
DCF-IFS: DIFSTime = SIFSTime + 2xSlotTime
802.11 - Congestion Control
Contention window (cw) in DCF: Congestion controlachieved by dynamically choosing cw
large cw leads to larger backoff intervals
small cw leads to larger number of collisions
Binary Exponential Backoff in DCF: Binary Exponential Backoff in DCF: When a node fails to receive CTS in response to its RTS, it
increases the contention window
cw is doubled (up to a bound cwmax =1023)
Upon successful completion data transfer, restore cw to cwmin=31