1. Communication Networks NETW 501
Lecture 5
Medium Access Control
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

2. Outline Medium Access Protocols Channelization Summary Random Access
ALOHA
CSMA
CSMA/CD
Scheduling
Reservation
Polling
Token Passing
Summary
Channelization
FDMA
TDMA
CDMA

3. Multiple Access Protocols
Why do we need Medium Access Control?
Multiple Access Protocols
Random Access
Scheduling

4. Random Access

5. Electrical Engineering
ALOHA Protocols
Hawaii Islands 1970s
University of Hawaii needed a means to inter-connect terminals located at campuses on multiple islands to the host computer on main campus
Packet Radio
Messages are transmitted as soon as they become available
Electrical Engineering
Arts
Management

6. ALOHA Operation Collision
No Collision
Terminal A
Terminal A
Terminal A
Terminal B
Terminal B
Terminal B
Collision
Transmissions from two or more terminals overlap in time
ALOHA Operation
Messages transmitted once available
From time-to-time messages collide (treated as erroneous frames)
The terminal knows that its messages was corrupted when it receives no acknowledgements within 2tprop (i.e., a timeout is used)
Recovery by the use of retransmissions
Can we simply apply retransmission directly after timeouts?
NO! Retransmissions will collide again
SOLUTION: Retransmission after random intervals from timeouts (Backoff Algorithm)

7. Re-Transmission IF NECESSARY
ALOHA Operation
First Transmission
Re-Transmission IF NECESSARY
t0-X t0
t0+X
t0+X+2tpropo
t0+X+2tprop+B
Vulnerable Period
Backoff Period
Timeout
Vulnerable Period:
Collision will occur if any other terminal commences transmission within this period
Backoff Period
Random time set for retransmission

8. Slotted ALOHA Objective Reduce the probability of collisions HOW?
Transmission in synchronized slots
Avoid partial collisions
First Transmission
Re-Transmission IF NECESSARY
(k+2+B+ciel(2tprop/X))X kX
(k+1)X
(k+2)X +2tprop
Backoff Period
Vulnerable Period
Timeout

9. ALOHA Performance
System Load G: Average number of packets generated within packet transmission time X
Throughput S: Average number of packets transmitted successfully within packet transmission time X

10. Carrier Sense Multiple Access
Low throughput of ALOHA is due to waste of bandwidth due to collisions
CSMA:
Sense (i.e., Listen) the medium for presence of a carrier signal before transmission
A terminal transmit only if a sense an idle channel
Widely used in LAN with Bus Topology
Vulnerable period = tprop
Network Diameter A B C
Station A starts transmission at time t=0 A B C
At time tprop (for electrical energy to propagate from A to C) all network terminals would have listened to the transmission from A.
Terminal A is said to have captured the channel

11. Classes of CSMA
CSMA schemes vary with the behavior when the terminal has something to send and channel is sensed to be busy
1-Persistent CSMA
If channel is busy continue to sense channel
As soon as channel is sensed idle transmit frame
In case of collisions, a backoff algorithm is used to set a time to re-sense the channel
Non-Persistent CSMA
If channel is busy, immediately run the backoff algorithm to set a time to re-sense the channel
p-Persistent CSMA
If channel is busy continue to sense channel
As soon as channel is sensed idle transmit frame with probability p. With probability (1-p) wait for tprop before sensing the channel again
In case of collisions, a backoff algorithm is used to set a time to re-sense the channel

12. Performance of CSMA

13. CSMA with Collision detection (CSMA/CD)
TIME: t=0
Station A starts transmission
In both ALOHA and CSMA, when collision occurs we lose at least a full transmission time for nothing
CSMA/CD
Abort transmission when a collision is detected A B
TIME: t=tprop-ε
Station B sense an idle channel and starts transmission A B
TIME: t=tprop
Station B detects a collision and aborts transmission A B
TIME: t=2tprop- ε
Even though Station B has aborted transmission, its initial signal arrives at A. A detects a collision and aborts transmission A B

14. GENERALLY: AFTER N COLLISIONS
Exponential Backoff Algorithms
FIRST COLLISION
Wait 0 or 1 transmission frames (chosen randomly) before trying again
SECOND COLLISION
Wait 0, 1,2 or 3 transmission frames (chosen randomly) before trying again
THIRD COLLISION
Wait 0, 1,2,… or 7 transmission frames (chosen randomly) before trying again
GENERALLY: AFTER N COLLISIONS
Wait 0 to 2N-1 transmission frames if N≤10 before trying again. N>10, wait between 0 and 1023 (210-1)
Give up at N=16

15. Exponential Backoff Algorithms
Data Packet from
Upper Layer
NB=0, CW=2,
BE =macMinBE
Delay for Random
(2BE-1) Slots
Perform CCA
(1st or 2nd )
CCA
Success?
Yes No
CW=2, NB=NB+1,
BE =min(BE+1,macMaxBE)
CW=CW-1 No
NB> maxNB?
CW=0? No
Yes
Yes
Packet Discard
Packet Transmission

16. Exponential Backoff Algorithms
Tslot

17. References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy
“Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013
“Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International