Communication Networks NETW 501 Lecture 6 Medium Access

Communication Networks NETW 501 Lecture 6 Medium Access Control – Part 2 Course Instructor: Dr. -Ing. Maggie Mashaly maggie. ezzat@guc. edu. eg C 3. 220 1

Outline ØMedium Access Protocols • Random Access › ALOHA › CSMA/CD ØChannelization • FDMA • TDMA • CDMA • Scheduling › Reservation › Polling › Token Passing 2

Scheduling 3

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

Scheduling We are going to learn about several scheduling techniques: 1. Reservation 2. Polling 3. Token Passing 5

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

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

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

2. Polling : System Options ØService 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 ØPriority 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 9

3. Token Passing ØReservation and polling require a central controller ØIn Token Passing the coordination for accessing the medium is distributed ØA Token is either A C B D – Free (No terminal is transmitting) – Busy (Some terminal is transmitting) 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 A D A Free Token Busy Token Data Frame Sender Field in Header Dest. Field in Header 10

3. Token Passing ØReservation and polling require a central controller ØIn Token Passing the coordination for accessing the medium is distributed ØA Token is either A C B D – Free (No terminal is transmitting) – Busy (Some terminal is transmitting) 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 C D A Free Token Busy Token Data Frame Sender Field in Header Dest. Field in Header 11

3. Token Passing ØReservation and polling require a central controller ØIn Token Passing the coordination for accessing the medium is distributed ØA Token is either A C B D – Free (No terminal is transmitting) – Busy (Some terminal is transmitting) Token Ring Operation 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 Send data frame to upper layer D A Free Token Busy Token Data Frame Sender Field in Header Dest. Field in Header 12

3. Token Passing ØReservation and polling require a central controller ØIn Token Passing the coordination for accessing the medium is distributed ØA Token is either A C B D – Free (No terminal is transmitting) – Busy (Some terminal is transmitting) 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 A D A Free Token Busy Token Data Frame Sender Field in Header Dest. Field in Header 13

Channelization 14

Single Shared Channel: Is it enough? ØNO! … WHY? – Services demands are smaller than total available system resources – It is sometimes desirable to allow simultaneous communications from multiple users ØChannelization – Sub-dividing the available bandwidth (i. e. , resources) into several (orthogonal) channels ØChannelization Techniques – FDMA : Frequency Division Multiple Access – TDMA : Time Division Multiple Access – CDMA : Code Division Multiple Access ØMedium access protocols are still needed to regulate use of available channels 15

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

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

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

Channelization Techniques Summary 19

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

References • NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy • “Communication Networks 2 nd Edition”, A. Leon-Garcia and I. Widjaja, Mc. Graw Hill, 2013 • “Computer Networks 4 th Edition”, A. S. Tanenbaum, Pearson International 21