1. Communication Networks NETW 501
Lecture 6
Medium Access Control – Part 2
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

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

3. Scheduling

4. Scheduling Why do we need Scheduling? Solution:
Under high loads, random access may suffer from high levels of collisions reducing the throughput
Collisions and retransmission also mean increased delay in packet delivery
Solution:
Scheduling schemes attempt to provide an orderly access to the transmission medium
Requires signaling

5. Scheduling We are going to learn about several scheduling techniques:
Reservation
Polling
Token Passing

6. 1. Reservation Controller Process
I need to send 3 frames
I need to send 2 frames
Controller
Process A C
I need to send 4 frames
I need to send 1 frame B D
Processing
Timeslot 1 2 3 4 5 6 7 8 9 10
Reserving Terminal B A D C
Request messages are sent over signaling channels

7. 1. Reservation Controller Process Timeslot 1 2 3 4 5 6 7 8 9 10
Use slots 2,7,10
Use slots 6, 9
Controller
Process A C
Use slots 3,4,5,8
Use slot 1 B D
Timeslot 1 2 3 4 5 6 7 8 9 10
Reserving Terminal B A D C

8. 2. Polling Controller Process
Do you have something to send?
Do you have something to send?
Controller
Process A C
Do you have something to send?
Do you have something to send? B D
At any given time only one terminal has the right to transmit over the medium
The central station could poll in
Round Robin fashion
Some other pre-determined order
Polling messages are send over signaling channels

9. 2. Polling : System Options
Service Limits
How much is a station allowed to transmit per poll?
- Exhaustive : until station’s data buffer is empty (including new frame arrivals)
Gated : all data in buffer when poll arrives
Frame-Limited : one frame per poll
Time-Limited : up to some maximum time
Priority mechanisms
More bandwidth & lower delay for stations that appear multiple times in the polling list
Issue polls for stations with message of priority k or higher

10. 3. Token Passing Reservation and polling require a central controller
In Token Passing the coordination for accessing the medium is distributed
A Token is either
Free (No terminal is transmitting)
Busy (Some terminal is transmitting) A C B D A
Token Ring Operation
A station that has something to send and sees a free token, changes the state of token to busy and inserts its address in the sender filed of the header D A
Free Token
Busy Token
Data Frame
Sender Field in Header
Dest. Field in Header

11. 3. Token Passing Reservation and polling require a central controller
In Token Passing the coordination for accessing the medium is distributed
A Token is either
Free (No terminal is transmitting)
Busy (Some terminal is transmitting) A C B D C
Token Ring Operation
A station that sees the token as busy and its name is not in the sender or destination fields, simply acts as repeater than forwards the frame D A D A
Free Token
Busy Token
Data Frame
Sender Field in Header
Dest. Field in Header

12. 3. Token Passing Reservation and polling require a central controller
In Token Passing the coordination for accessing the medium is distributed
A Token is either
Free (No terminal is transmitting)
Busy (Some terminal is transmitting) A C B D D
Token Ring Operation
Send data frame to upper layer
A station that sees the token as busy and its name is in the destination field, sends the data frame to the upper layer before acting as a repeater D A D A
Free Token
Busy Token
Data Frame
Sender Field in Header
Dest. Field in Header

13. 3. Token Passing Reservation and polling require a central controller
In Token Passing the coordination for accessing the medium is distributed
A Token is either
Free (No terminal is transmitting)
Busy (Some terminal is transmitting) A C B D A
Token Ring Operation
A station that sees the token as busy and its name is in the sender fields, changes the state of the token to free and removes the frame from the ring D A
Free Token
Busy Token
Data Frame
Sender Field in Header
Dest. Field in Header

14. Channelization

15. Single Shared Channel: Is it enough?
NO! … WHY?
Services demands are smaller than total available system resources
It is sometimes desirable to allow simultaneous communications from multiple users
Channelization
Sub-dividing the available bandwidth (i.e., resources) into several (orthogonal) channels
Channelization Techniques
FDMA : Frequency Division Multiple Access
TDMA : Time Division Multiple Access
CDMA : Code Division Multiple Access
Medium access protocols are still needed to regulate use of available channels

16. Channelization Techniques
FDMA
Channel bandwidth divided into frequency bands
At any given instant each band should be used by only one user

17. Channelization Techniques
TDMA
System resources are divided in to time slots
Each user uses the entire bandwidth but not all the time

18. Channelization Techniques
CDMA
Each user is allocated a unique code to use for communication
Users may transmit simultaneously over the same frequency band
Used in wireless systems

19. Channelization Techniques Summary

20. Medium Access Control Summary
Medium Access Protocols
Rules to access shared channels
Medium Access Techniques
Random (e.g., ALOHA, CSMA, CSMA/CD)
Scheduling (e.g., Reservation, Polling, Token Passing)
Channelization
Sub-dividing the system bandwidth in to multiple channels
Medium access techniques can be extended to define rules for access to available channel

21. 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