lecture 4 wireless medium access control
DESCRIPTION
Prof. Shamik Sengupta Office 4210 N [email protected] http://jjcweb.jjay.cuny.edu/ssengupta/ Fall 2010. Lecture 4 Wireless Medium Access Control. Medium Access Control (MAC). Base Station. Forward link. Reverse link. Mobile Station. Mobile Station. Mobile Station. - PowerPoint PPT PresentationTRANSCRIPT
Lecture 4 Lecture 4 Wireless Medium Access ControlWireless Medium Access Control
Prof. Shamik Sengupta
Office 4210 N
http://jjcweb.jjay.cuny.edu/ssengupta/
Fall 2010
Mobile Station
Base Station
Mobile StationMobile Station
Mobile Station
Forward link
Reverse link
Medium Access Control (MAC)
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 C B A C B A C B A
C
AB
Time
f0 Freq
uenc
y
A A
B B
C C Freq
uenc
y
Time
f2
f1
f0
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!
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
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]
Pure Aloha efficiency
What is the efficiency?
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
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
5: DataLink Layer 5-9
Slotted Aloha efficiency
Efficiency : 37%
At best: channel
used for useful
transmissions 37%
of time!!
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
Carrier Sense Multiple Access (CSMA) Protocol
Two imp parameters in CSMA– Detection delay
– Propagation delay
Detection delay– A function of the receiver hardware
– Time reqd for a terminal to sense whether or not the channel is idle
Propagation delay– 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
Variations of CSMA
1-persistent CSMA– 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)
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 802.11 (WLAN)
IEEE802.11
infrastructure network
ad-hoc network
APAP
AP
wired network
AP: Access Point
802.11 infrastructure mode
Station (STA)– 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
802.11 LAN
BSS2
802.11 LAN
BSS1
Access
Point
STA1
STA2 STA3
ESS
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
802.11 LAN
BSS2
802.11 LAN
BSS1
STA1
STA4
STA5
STA2
STA3
IEEE standard 802.11
mobile terminal
access point
server
fixed terminal
application
TCP
802.11 PHY
802.11 MAC
IP
802.3 MAC
802.3 PHY
application
TCP
802.3 PHY
802.3 MAC
IP
802.11 MAC
802.11 PHY
LLC
infrastructure network
LLC LLC
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)
802.11 – 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
t
medium busySIFS
PIFS
DIFSDIFS
next framecontention
direct access if medium is free DIFS
t
medium busy
DIFSDIFS
next frame
contention window
(randomized back-offmechanism)
WLAN CSMA/CA access method
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
slot timedirect access if
medium is free DIFS
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
t
SIFS
DIFS
data
ACK
waiting time
other
stations
receiver
senderdata
DIFS
contention
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
Backoff Interval
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
data
wait
B1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervals
at nodes 1 and 2Assume cw = 31
B2 = 10
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 802.11 DCF: contention window cw is chosen dynamically
depending on collision occurrence
– Follows Binary exponential backoff algorithm
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
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?
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
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!
Collision Avoidance: RTS-CTS exchange
AP
A B
time
RTS(A)
RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
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
t
SIFS
DIFS
data
ACK
defer access
other
stations
receiver
senderdata
DIFS
contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
31
802.11 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
802.11 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
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.
Another special access – with Fragmentation
t
SIFS
DIFS
data
ACK1
other
stations
receiver
sender frag1
DIFS
contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
NAV (frag1)NAV (ACK1)
SIFSACK2
frag2
SIFS
Point Coordination Function
PIFS
stations‘
NAV
wireless
stations
point
coordinator
D1
U1
SIFS
NAV
SIFSD2
U2
SIFS
SIFS
SuperFramet0
medium busy
t1
Point Coordination Function
tstations‘
NAV
wireless
stations
point
coordinator
D3
NAV
PIFSD4
U4
SIFS
SIFSCFend
contention
period
contention free period
t2 t3 t4