Communication Networks NETW 501 Lecture 6 Medium Access Control – Part 2 Course Instructor: Dr.-Ing. Maggie Mashaly maggie.ezzat@guc.edu.eg C3.220

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

Scheduling

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

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

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

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

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

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

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

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

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

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

Channelization

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

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

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

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

Channelization Techniques Summary

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

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