1. Lecture 4 Wireless Medium Access Control
Lecture 4 Wireless Medium Access Control
Prof. Shamik Sengupta
Office 4210 N
Fall 2010
Lecture 1

2. Medium Access Control (MAC)
Base Station
Forward link
Reverse link
Mobile Station
Mobile Station
Mobile Station
Mobile Station

3. Earlier MAC Protocols: A quick overview
Channel Partitioning: TDMA, FDMA
divide channel into “pieces” (time slots, frequency)
allocate piece to node for exclusive use C B A
Time f
Frequency A B C
Frequency
Time f 2 1
Channel Partitioning: adv., disadv.
Share channel efficiently at high load
inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!

4. Earlier MAC Protocols: A quick overview
Packet Radio (PR) Access Technique:
Users attempt to access a single channel in an uncoordinated or random manner
Random Access: Aloha, Slotted Aloha
allow collisions
“recover” from collisions
Random access MAC protocols
efficient at low load: single node can fully utilize channel
high load: collision overhead

5. Pure (unslotted) ALOHA
Devised by Norman Abramson and his colleagues
University of Hawaii
Simple, no synchronization
when frame first arrives
transmit immediately
collision probability increases:
frame sent at t0 collides with other frames sent in [t0-1,t0+1]

6. Pure Aloha efficiency
What is the efficiency?

7. Slotted ALOHA
Assumptions:
all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot, all nodes detect collision
Operation:
when node obtains fresh frame, transmits in next slot
if no collision: node can send new frame in next slot
if collision: node retransmits frame in each subsequent slot with prob. p until success

8. Slotted ALOHA
Pros
single active node can continuously transmit at full rate of channel
highly decentralized: only slots in nodes need to be in sync
simple
Cons
collisions, wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization

9. Slotted Aloha efficiency
At best: channel
used for useful
transmissions 37%
of time! !
5: DataLink Layer

10. Why Aloha protocols were disadvantageous?
Aloha protocols do not listen to the channel before transmission
Do not exploit info about other users
Listening to the channel if any user is transmitting is key to the efficient wireless access
This was the basic of CSMA protocols
Carrier Sense Multiple Access Protocol

11. Carrier Sense Multiple Access (CSMA) Protocol
Two imp parameters in CSMA
Detection delay
Propagation delay
A function of the receiver hardware
Time reqd for a terminal to sense whether or not the channel is idle
Relative measure of how fast a packet travels from one station to another station (BS or AP)
Systems must be built taking this parameter significantly in account
High propagation delay impact efficiency
E.g., two extreme transmitting users may get into collision again and again due to high propagation delay

12. Variations of CSMA 1-persistent CSMA p-persistent CSMA CSMA/CD
Listens to the channel, if idle transmit
p-persistent CSMA
Listens to the channel, if idle, transmit with prob p in the first slot or (1-p) in the next slot
CSMA/CD
Further improvement over earlier CSMA
Not only listens to channel before transmissions but also during transmissions
If collision is detected, transmissions are aborted immediately
Saves valuable resources from wastage
Combines “listen before talk” and “listen while talk”
Happens in Ethernet (because of full-duplex radios)

13. CSMA in wireless The concept of CSMA/CD is interesting
How about applying it in wireless medium access control?
Problems in wireless networks
signal strength decreases proportional to the square of the distance
the sender would apply CS and CD, but the collisions happen at the receiver
a sender cannot “hear” the collision at the same time of transmission, because transmission power suppresses receiving power
i.e., CD does not work
furthermore, CS might not work if, e.g., a terminal is “hidden”
Wireless MAC use variants of CSMA
CSMA/CA (collision avoidance protocol)
Does not make collision zero, just tries to reduce it
Very popular in IEEE (WLAN)

14. infrastructure network
IEEE802.11
infrastructure network
ad-hoc network AP
wired network
AP: Access Point

15. 802.11 infrastructure mode Station (STA) Basic Service Set (BSS)
terminal with access mechanisms to the wireless medium and radio contact to the access point
Basic Service Set (BSS)
group of stations using the same radio frequency
Access Point
station integrated into the wireless LAN and the distribution system
Portal
bridge to other (wired) networks
Distribution System
interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS
Distribution System
Portal
802.x LAN
Access
Point
LAN
BSS2
BSS1
STA1
STA2
STA3
ESS 9

16. 802.11: ad-hoc mode Direct communication within a limited range
Station (STA): terminal with access mechanisms to the wireless medium
Basic Service Set (BSS): group of stations in range and using the same radio frequency
LAN
STA1
BSS1
STA3
STA2
BSS2
STA5
STA4
LAN

17. infrastructure network
IEEE standard
mobile terminal
access point
server
fixed terminal
application
TCP
PHY
MAC IP
802.3 MAC
802.3 PHY
LLC
infrastructure network

18. How does the medium access work in WLAN?
Access methods
DCF CSMA/CA (mandatory)
collision avoidance via exponential backoff
Minimum distance (IFS) between consecutive packets
ACK packet for acknowledgements (not for broadcasts)
DCF with RTS/CTS (optional)
Distributed Foundation Wireless MAC
avoids hidden terminal problem
PCF (optional)
access point polls terminals according to a list
Contention
Based
Contention
Free
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)

19. direct access if medium is free  DIFS
– MAC
Priorities
defined through different inter frame spaces
SIFS (Short Inter Frame Spacing)
highest priority, for ACK, CTS, polling response
PIFS (PCF IFS)
medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS)
lowest priority, for asynchronous data service, competing stations
aSIFSTime = aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime.
aSlotTime = aCCATime + aRxTxTurnaroundTime + aAirPropagationTime + aMACProcessingDelay.
PIFS = aSIFSTime + aSlotTime
DIFS = aSIFSTime + 2*aSlotTime
EIFS = aSIFSTime + (8 ´ ACKSize) + aPreambleLength + aPLCPHeaderLngth+ DIFS
For DSSS:
aSlotTime 20 µs
aSIFSTime 10 µs
aCCATime < 15 µs
aRxTxTurnaroundTime <5 µs
SIFS =
PIFS =
DIFS =
DIFS
DIFS
PIFS
SIFS
medium busy
contention
next frame t
direct access if medium is free  DIFS

20. WLAN CSMA/CA access method
contention window
(randomized back-off mechanism)
DIFS
DIFS
medium busy
next frame
direct access if medium is free  DIFS t
slot time
Station ready to send
starts sensing the medium (Carrier Sense)
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, then the station must additionally wait a random back-off time
collision avoidance, multiple of slot-time
If another station occupies the medium during the back-off time of the station, the back-off timer freezes 12

21. WLAN access scheme details
Sending unicast packets
station has to wait for DIFS before sending data
receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)
automatic retransmission of data packets in case of transmission errors
DIFS
data
sender
SIFS
ACK
receiver
DIFS
data
other
stations t
waiting time
contention

22. Contention for channel
When the other stations find the channel idle, they would like to transmit their own packets
Contention for channel
If all the waiting stations attempt at once, this will surely result in collision
Some CA scheme is necessary
Backoff intervals can be used to reduce collision probability

23. B1 and B2 are backoff intervals
When transmitting a packet, choose a backoff interval in the range [0,cw]
cw is contention window
Count down the backoff interval when medium is idle
Count-down is suspended if medium becomes busy
When backoff interval reaches 0, transmit packet
B1 = 25
B2 = 20
B1 = 5
B2 = 15
data
wait
data
wait
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
Assume cw = 31

24. Backoff Interval
The time spent counting down backoff intervals is a part of MAC overhead
Choosing a large cw leads to large backoff intervals and can result in larger overhead
Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously)
Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed
IEEE DCF: contention window cw is chosen dynamically depending on collision occurrence
Follows Binary exponential backoff algorithm

25. Binary Exponential Backoff (BEB) in DCF
Even before the first collision, nodes follow BEB
Initial backoff interval (before 1st collision)
[0,7]
If still packets collide, double the collision interval
[0,15], [0,31] and so on…
Express this binary exponential backoff interval as a function of collision number

26. Numerical example #1
Two nodes, A and C both waiting for a busy channel to be idle so that they can proceed with their first transmission. After the channel becomes idle, what is the probability of A and C colliding in their first transmissions?

27. Numerical example #2
Two nodes, X and Y intend to transmit frames of 10 and 5 timeslots. Initially after waiting for DIFS, X and Y both generate random backoff number, rX and rY as 2. In the next stage, X generates rX =1 and Y generates rY =3. What will be the time (slots) taken to complete both transmissions and receive acks?
Assume, SIFS=1 timeslot, DIFS=2 timeslots

28. Avoiding collisions (more)
idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames
sender first transmits small request-to-send (RTS) packets to BS using CSMA
RTSs may still collide with each other (but they’re short)
BS broadcasts clear-to-send CTS in response to RTS
CTS heard by all nodes
sender transmits data frame
other stations defer transmissions
avoid data frame collisions completely
using small reservation packets!

29. Collision Avoidance: RTS-CTS exchangeB AP
RTS(A)
RTS(B)
reservation collision
RTS(A)
CTS(A)
DATA (A)
ACK(A)
defer
time

30. 802.11 access scheme details – RTS/CTS
Sending unicast packets
station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium)
ack via CTS after SIFS by receiver (if ready to receive)
sender can now send data at once, acknowledgement via ACK
other stations store reservations distributed via RTS and CTS
DIFS
RTS
data
sender
SIFS
SIFS
SIFS
CTS
ACK
receiver
DIFS
NAV (RTS)
data
other
stations
NAV (CTS) t
defer access
contention

31. All backlogged nodes choose a random number, R Each node counts down R
Steps – RTS/CTS
All backlogged nodes choose a random number, R
Each node counts down R
Continue carrier sensing while counting down
Once carrier busy, freeze countdown
Whoever reaches ZERO transmits RTS
Neighbors freeze countdown, decode RTS
RTS contains (CTS + DATA + ACK) duration = T_comm
Neighbors set NAV = T_comm
Remains silent for NAV time

32. Receiver replies with CTS
Steps – RTS/CTS
Receiver replies with CTS
Also contains (DATA + ACK) duration.
Neighbors update NAV again
Tx sends DATA, Rx acknowledges with ACK
After ACK, everyone initiates remaining countdown
Tx chooses new R = rand (0, CW)
If RTS or DATA collides (i.e., no CTS/ACK returns)
Indicates collision
RTS chooses new random no. following BEB

33. Numerical example #3
Two nodes, X and Y intend to transmit frames of 10 and 5 timeslots. Initially after waiting for DIFS, X and Y both generate random backoff number, rX and rY as 2. In the next stage, X generates rX =1 and Y generates rY =3. What will be the time (slots) taken to complete both transmissions and receive acks?
Assume, SIFS=1 timeslot, DIFS=2 timeslots
RTS threshold = 8.

34. Another special access – with Fragmentation
DIFS
RTS
frag1
frag2
sender
SIFS
SIFS
SIFS
SIFS
SIFS
CTS
ACK1
ACK2
receiver
NAV (RTS)
NAV (CTS)
DIFS
NAV (frag1)
data
other
stations
NAV (ACK1) t
contention

35. Point Coordination Function
SuperFrame
medium busy
PIFS
SIFS
SIFS D1 D2
point
coordinator
SIFS
SIFS U1 U2
wireless
stations
stations‘
NAV
NAV

36. Point Coordination Function
PIFS
SIFS D3 D4
CFend
point
coordinator
SIFS U4
wireless
stations
stations‘
NAV
NAV
contention free period t
contention
period