Chapter 6
Multiple Radio Access
Outline




Introduction
Multiple Radio Access Protocols
Contention-based Protocols
 Pure ALOHA
 Slotted ALOHA
 CSMA (Carrier Sense Multiple Access)
 CSMA/CD (CSMA with Collision
Detection)
 CSMA/CA (CSMA with Collision
Avoidance)
Summary
2
Introduction
Multiple access control channels
 Each Mobile Station (MS) is attached to a
transmitter / receiver which communicates via a
channel shared by other nodes
 Transmission from any MS is received by other MSs
MS 3
MS 2
MS 1
Shared Multiple
Access Medium
MS 4
…

MS N
3
Introduction (Cont’d)

Multiple access issues



If more than one MS transmit at a time on the control
channel to BS, a collision occurs
How to determine which MS can transmit to BS?
Multiple access protocols


Solving multiple access issues
Different types:
 Contention protocols resolve a collision after it
occurs. These protocols execute a collision resolution
protocol after each collision
 Collision-free protocols ensure that a collision can
never occur
4
Channel Sharing Techniques
Static
Channelization
Medium Sharing
Techniques
Scheduling
Dynamic
Medium Access
Control
Random
Access
5
Classification of Multiple Access
Protocols
Multiple-access protocols
Contention-based
Random access
Conflict-free
Collision resolution
ALOHA,
TREE,
FDMA,
CSMA,
WINDOW,
etc.
TDMA,
BTMA,
ISMA, etc.
BTMA: Busy Tone Multiple Access
ISMA: Internet Streaming Media Alliance
CDMA,
Token Bus,
DQDB, etc.
DQDB: Distributed Queue Dual
Bus
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Contention-based Protocols


ALOHA
 Developed in the 1970s for a packet radio network by
Hawaii University
 Whenever a terminal (MS) has data, it transmits.
Sender finds out whether transmission was successful
or experienced a collision by listening to the
broadcast from the destination station. If there is a
collision, sender retransmits after some random time
Slotted ALOHA
 Improvement: Time is slotted and a packet can only
be transmitted at the beginning of one slot. Thus, it
can reduce the collision duration
7
Pure ALOHA
Node 1
Packet
1
Waiting a random time
Node 2
Packet
Retransmission
2
3
3
Retransmission
2
Time
Collision
Node 3
Packet
Collision mechanism in ALOHA
8
Throughput of Pure ALOHA
• The probability of successful transmission Ps is the probability no
other packet is scheduled in an interval of length 2T
Ps  P(no _ collision )
 e  2 gT
where g is the packet rate of the traffic
• The throughput Sth of pure Aloha as: S th  gTe 2 gT
• Defining G= gT to normalize offered load, we have
S th  Ge2G
• Differentiating Sth with respect to G and equating to zero gives
dSth
 2Ge 2G  e  2G  0
dG
• The Maximum throughput of ALOHA is
S max
1
  0.184
2e
9
Slotted ALOHA
Node 1
Packet
Nodes 2 & 3
Packets
No
transmission
1
2 3
Slot
Retransmission
2
Retransmission
3
Time
Collision
Collision mechanism in slotted ALOHA
10
Throughput of Slotted ALOHA
• The probability of successful transmission Ps is the probability no
n
other packet is scheduled
in an interval of length T
Ps  e  gT
where g is the packet rate of the traffic
 gT
• The throughput Sth of pure Aloha as: S th  gTe
• Defining G= gT to normalize offered load, we have
S th  Ge G
• Differentiating Sth with respect to G and equating to zero gives
dSth
 GeG  e G  0
dG
• The Maximum throughput of ALOHA is
S max 
1
 0.368
e
11
Throughput
0.5
S: Throughput
0.4
0.368
0.3
Slotted Aloha
0.2
0.1
00
0.184
Aloha
2
4
6
8
G = gT
12
Contention Protocols (Cont’d)

CSMA (Carrier Sense Multiple Access)


CSMA/CD (CSMA with Collision Detection)



Improvement: Stop ongoing transmission if a collision is
detected
CSMA/CA (CSMA with Collision Avoidance)


Improvement: Start transmission only if no transmission is
ongoing
Improvement: Wait a random time and try again when carrier
is quiet. If still quiet, then transmit.
CSMA/CA with ACK
CSMA/CA with RTS/CTS
13
CSMA (Carrier Sense Multiple
Access)



Max throughput achievable by slotted ALOHA
is 0.368
CSMA gives improved throughput compared
to Aloha protocols
Listens to the channel before transmitting a
packet (avoid avoidable collisions)
14
Collision Mechanism in CSMA
MS 5 sense
MS 1 Packet
MS 2 Packet
MS 3 Packet
1
2
Delay for MS 5
3
Delay for MS 4
4
5
Time
Collision
MS 4 senses
15
Kinds of CSMA
Nonpersistent
CSMA (no wait)
Unslotted Nonpersistent
CSMA
Slotted Nonpersistent CSMA
CSMA
Unslotted persistent CSMA
Persistent CSMA
(wait)
Slotted persistent CSMA
1-persistent
CSMA
p-persistent
CSMA
16
Nonpersistent CSMA Protocols

Nonpersistent CSMA Protocol:





Step 1: If the medium is idle, transmit immediately (same as p = 1)
Step 2: If the medium is busy, wait a random amount of time and
repeat Step 1
Random backoff reduces probability of collisions
Waste idle time if the backoff time is too long
For unslotted nonpersistent CSMA, the throughput is
given by:
Ge2T
Sth 
G(1  2 )  e G

For slotted nonpersistent CSMA, the throughput is given by:
Sth 
Ge2T
(1  e G   )
17
1-persistent CSMA Protocols
1-persistent CSMA Protocol:






Step 1: If the medium is idle, transmit immediately
Step 2: If the medium is busy, continue to listen until medium
becomes idle, and then transmit immediately
There will always be a collision if two nodes want to retransmit
(usually you stop transmission attempts after few tries)
For unslotted 1-persistent CSMA, the throughput is given
by:
G1  G  G(1  G  G / 2)e G (1 2 )
Sth 
G(1  2 )  (1  eG )  (1  G)e G (1 )
For slotted 1-persistent CSMA, the throughput is given by:
G(1    e G )e G (1 )
Sth 
(1   )(1  e G )  e G (1 )
18
p-persistent CSMA Protocols

p-persistent CSMA Protocol (time is slotted):



Step 1: If the medium is idle, transmit with probability
p, or delay the transmission with probability (1 - p)
until the next slot
Step 2: If the medium is busy, waits until the next slot
and continue to listen until medium becomes idle, then
go to Step 1
A good tradeoff between nonpersistent and 1persistent CSMA
19
Throughput
0.01-persistent CSMA
Nonpersistent CSMA
1.0
0.9
S: Throughput
0.8
0.7
0.1-persistent CSMA
0.5-persistent CSMA
0.6
0.5
1-persistent CSMA
0.4
0.3
0.2
Slotted
Aloha
Aloha
0.1
0
0
1
2
3
4
5
6
7
8
9
Traffic Load G
20
CSMA/CD (CSMA with Collision
Detection)



In CSMA, if 2 terminals begin sending packet at the same
time, each will transmit its complete packet (although
collision is taking place)
Wasting medium for an entire packet time
CSMA/CD
Step 1:
If the medium is idle, transmit
Step 2:
If the medium is busy, continue to listen until
the channel is idle then transmit
Step 3:
If a collision is detected during transmission,
cease transmitting (detection not possible by
wireless devices)
Step 4:
Wait a random amount of time and repeats
the same algorithm
21
CSMA/CD in Ethernet (Cont’d)
T0
A begins
transmission
T0+t-
B begins
transmission
T0+ttcd
T0+2t+tcd
+tcr -
A
B
(t is the propagation time)

A
B
A
B
A
B
B detects collision
A detects collision just
before end of transmission
Time
22
Throughput of slotted nonpersistent CSMA/CD
S: Throughput
1.2
α = t /T= 0
1
0.8
α = 0.01
0.6
α = 0.1
0.4
α=1
0.2
0
0.001
0.01
0.1
1
10
100
1000
Traffic load G
23
CSMA/CA (CSMA with collision
Avoidance) for wireless devices







All terminals listen to the same medium as CSMA/CD
Terminal ready to transmit senses the medium
If medium is busy it waits until the end of current
transmission
It again waits for an additional predetermined time period
DIFS (Distributed inter frame Space)
Then picks up a random number of slots (the initial value
of backoff counter) within a contention window to wait
before transmitting its frame
If there are transmissions by other MSs during this time
period (backoff time), the MS freezes its counter
It resumes count down after other MSs finish transmission
+ DIFS. The MS can start its transmission when the
counter reaches to zero
24
CSMA/CA (Cont’d) for
Wireless Devices
MS B’s frame
MS A’s frame
Delay: B
MS C’s frame
Time
Delay: C
MSs B & C sense
the medium
MSs B resenses the
medium and transmits
its frame
MSs C starts
transmitting
MSs C resenses the
medium but defers to
MS B
25
CSMA/CA Explained
Contention
DIFS window
DIFS
Contention window
Next Frame
Medium Busy
Defer access
Time
Slot
Backoff after defer
DIFS – Distributed Inter Frame Spacing
26
Example of Backoff Intervals
(2)
DIFS
DIFS
(3)
Backoff=9
DIFS Backoff=4
Backoff=2
DIFS (5)
busy
Station 1
Packet arrival at MAC
Backoff=5
busy
Station 2
(1)
busy
Station 3
Backoff=7
Backoff=2
(4)
busy
Station 4
(1)
After packet arrival at MAC, station 3 senses medium free for DIFS, so it starts transmission
immediately (without backoff interval).
(2)
For station 1, 2, and 4, their DIFS intervals are interrupted by station 3. Thus, backoff
intervals for station 1, 2, and 4, are generated randomly (i.e. 9, 5, and 7, respectively).
(3)
After transmission of station 2, the remaining backoff interval of station 1 is (9-5)=4.
(4)
After transmission of station 2, the remaining backoff interval of station 4 is (7-5)=2.
(5)
After transmission of station 4, the remaining backoff interval of station 1 is (4-2)=2.
Random Backoff Time

Backoff time = CW* Random() *
Slot time
 CW = starts at CWmin, and
doubles after each failure until
reaching CWmax (e.g., CWmin
= 7, CWmax = 255)
 Random() = (0,1)
 Slot Time = Transmitter turn on
delay + medium propagation
delay + medium busy detect
response time
CWmax
8
63
255 255
127
31
CWmin
7
15
第三次重送
初始值
第二次重送
第一次重送
Priority Scheme in MAC


Priorities of frames are distinguished by the IFS (interframe spacing) between two consecutive frames.
4 IFS's:

SIFS: the highest priority


PIFS (PCF-IFS): 2nd highest


by PCF to send any of the Contention Free Period frames.
DIFS (DCF-IFS): 3nd highest


ACK, CTS, the second or subsequent MPDU of a fragmented burst,
and to respond to a poll from the PCF.
by the DCF to transmit MPDUs or MMPDUs
EIFS (Extended-IFS): lowest

by the DCF to transmit a frame when previous frame was not
received correctly
CSMA/CA with ACK for Ad Hoc
Networks




Immediate Acknowledgements from receiver
upon reception of data frame without any need
for sensing the medium
ACK frame transmitted after time interval
SIFS (Short Inter-Frame Space) (SIFS < DIFS)
Receiver transmits ACK without sensing the
medium
If ACK is lost, retransmission done
30
CSMA/CA/ACK
DIFS
Source
MS
BS
Time
Data
SIFS
ACK
DIFS Contention window
Other
MSs
Next Frame
Defer access
Backoff after defer
SIFS – Short Inter Frame Spacing
31
Hidden Terminal Problem
Radio transmission range
R
R
A
R
B
C
A and C are hidden with respect to each other
32
CSMA/CA with RTS/CTS for
Hidden Terminal Problem




Transmitter sends an RTS (request to send)
after medium has been idle for time interval
more than DIFS
Receiver responds with CTS (clear to send)
after medium has been idle for SIFS
Then Data is exchanged
RTS/CTS is used for reserving channel for
data transmission so that the collision can only
occur in control message
33
CSMA/CA with RTS/CTS
DIFS
Source
MS
Destination
MS
SIFS
Time
Data
RTS
SIFS
SIFS
CTS
ACK
DIFS
Contention window
Next Frame
Other
MSs
Defer access
Backoff
34
RTS/CTS
MS A
MS B
Propagation delay
This helps avoid
hidden terminal
problem in Ad hoc
networks
35
Exposed Terminal Problem
Radio Transmission range
R
R
A
A
R
B
C
R
D
Transmission at B forces C (Exposed) to stop
transmission to D
36
Homework


Problems: 6.7, 6.12, 6.17 (Due Nov. 1)
Practice at home: 6.8, 6.10, 6.16, 6.21
37