2. Data Link Layer: Medium Access Control

Scheduling

© Tallal Elshabrawy 3 Why Scheduling? 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 Scheduling schemes attempt to provide an orderly access to the transmission medium Requires signaling

© Tallal Elshabrawy 4 Reservation Controller Process A C I need to send 3 frames Timeslot Reserving TerminalBADDDCADCA B I need to send 1 frame I need to send 2 frames D I need to send 4 frames Processing Request messages are sent over signaling channels

© Tallal Elshabrawy 5 Reservation Controller Process A C Use slots 2,7,10 Timeslot Reserving TerminalBADDDCADCA B Use slots 6, 9 D Use slots 3,4,5,8 Use slot 1

© Tallal Elshabrawy 6 Polling Controller Process A C B D Do you have something to send? 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

© Tallal Elshabrawy 7 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

© Tallal Elshabrawy 8 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 Free Token Busy Token Data Frame 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 Sender Field in Header Dest. Field in Header DA Token Ring Operation

© Tallal Elshabrawy 9 Token Passing A C B D C Free Token Busy Token Data Frame 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 Sender Field in Header Dest. Field in Header Token Ring Operation DA DA 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

© Tallal Elshabrawy 10 Token Passing A C B D D Free Token Busy Token Data Frame 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 Sender Field in Header Dest. Field in Header Token Ring Operation DA DA Send data frame to upper layer 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

© Tallal Elshabrawy 11 Token Passing A C B D A Free Token Busy Token Data Frame 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 Sender Field in Header Dest. Field in Header Token Ring Operation DA 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

Channelization

© Tallal Elshabrawy 13 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 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 A Single Shared Channel is Enough?

© Tallal Elshabrawy 14 Channelization Techniques FDMA Channel bandwidth divided in to frequency bands At any given instant each band should be used by only one user TDMA System resources are divided in to time slots Each user uses the entire bandwidth but not all the time 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

© Tallal Elshabrawy 15 Orthogonality and FDMA Divide channel into M frequency bands Each station transmits and listens on assigned bands A guard band is allocated to ensure channel separation Frequency Guard bands Time W 1 2 M M–1 …

© Tallal Elshabrawy 16 Orthogonality and TDMA Dedicate time slots where assigned stations occupy the whole channel bandwidth A guard time is necessary to prevent collisions that might occur due to any problems in clock synchronization between stations 1 Time Guard time One cycle M W Frequency...

© Tallal Elshabrawy 17 Orthogonality and CDMA chip 1 1 xxxxxxxx xxxxxxxx Orthognality of Two Transmissions: Interference is zero Two DIFFERENT frequencies are orthogonal (or constitute negligible interference) Two DIFFERENT Timeslots are orthogonal (or constitute negligible interference) CDMA: Users share frequency and time Orthoganlity achieved by assigning each user a specific code Code Characteristics Mean sum of multiplication of any two different codes is zero or very low Mean sum of multiplication of a code with itself is unity

© Tallal Elshabrawy 18 Channelization Techniques Summary

© Tallal Elshabrawy 19 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