COSC 3213: Computer Networks I Instructor: Dr. Amir Asif Department of Computer Science York University Section B

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1 MAC: Scheduled Approaches 1. Reservation Systems 2. Polling Systems 3. Token Passing Systems Static Channelization: TDMA and FDMA COSC 3213: Computer Networks I Instructor: Dr. Amir Asif Department of Computer Science York University Section B Medium Access Control Garcia: Section 6.3,

2 Multiple Access Communications Multiple users share the communication channel so a scheme (medium sharing technique) must be devised to prevent collision of packets. Medium Sharing Techniques Static Channelization Dynamic Medium Access Control 1. Partition the medium into separate channels 2. Each channel is dedicated to a transmitting host 3. Useful for steady traffic conditions 4. TDMA, FDMA, CDMA Scheduling Random Access Attempts orderly access to transmission medium: Reservation, Polling, Token passing Allows dynamic sharing of transmission medium: ALOHA, Slotted ALOHA, CSMA, CSMA/CD Maximum throughput is limited 2

3 MAC Scheduling Approaches: Reservation Systems (1) 1. Reservation Systems Transmissions from stations are organized in cycles that have variable length Reservation interval Data transmissions Reservation interval Data transmissions r Station 2 Station 3 Station Station 3 Station Cycle n Cycle (n + 1) d time Each cycle consists of a reservation interval followed by the transmitted packets A station uses its minislot in the reservation interval to broadcast its intention for transmission = S1 S2 S3 S4 S5 S6 Each station has its own minislot for making reservations Activity 1: Show that the maximum efficiency of the above system with equal data frames is 1 / (1 + v) where v is the ratio between reservation frame length to data frame length? 3

4 MAC Scheduling Approaches: Reservation Systems (2) Modifications to Reservation Systems: a. Variable length frames be accommodated if the reservation slot for a station contains information on the frame length as well = S1 S2 frame length S3 frame length S4 S5 S6 Each active reservation minislot is followed by the length of data frame b. More than one frame can be transmitted by a station by modifying the reservation slot to indicate number of frames to be transmitted per station = S1 S2 No. of frames S3 No. of frames S4 S5 S6 Each active reservation minislot is followed by number of data frames Activity 2: Show that the maximum efficiency of system (b) is 1 / (1 + (v / k))) where v is the ratio between reservation frame length to a data frame length and k is the maximum number of frames allowed to be transmitted during one reservation by a station? 4

5 MAC Scheduling Approaches: Reservation Systems (3) c. Stations use ALOHA or slotted ALOHA to reserve minislots iduring the reservation interval Reservation interval Data transmissions Reservation interval Data transmissions indicates collison r 1 1 Data frame Data frame Data frame Data frame Data frame Cycle n Cycle (n + 1) d time Activity 3: Show that the maximum efficiency of system (c) with slotted ALOHA used for reservation is 1 / ( v) where v is the ratio between reservation frame length to a data frame length? Recall that the maximum throughput of slotted ALOHA is 1/e = 0.368, hence, successful reservation would require 1/0.368 = 2.71 minislots on average. Reservation MAC approaches are used when stations generate continuous data. Reservation MAC approaches are inefficient for low traffic systems as overhead due to reservation frames will be high. 5

6 MAC Scheduling Approaches: Polling Systems (1) 2. Polling Systems: consist of a primary station (central controller) and secondary stations. a. There are several versions of polling systems: bus topology, star topology, and no central controller. Bus topology consists of separate inbound and outbound lines Shared inbound line Central Controller Outbound line Controller uses the outbound line to send a polling message to secondary stations in a roundrobin fashion enquiring if they want to transmit When polled, station can send its inbound message (data) After transmitting the message, the polled station releases the inbound line by sending a goahead message to the controller Central controller can then continue polling other stations in a round-robin fashion or in some other pre-determined order. 6

7 MAC Scheduling Approaches: Polling Systems (2) b. Star topology with separate inbound and outbound frequency bands Central Controller Central controller uses radio transmission in a certain frequency range to transmit outbound message (downlink) while stations use a second frequency range to transmit inbound messages (uplink) Central controller polls a station (say 1) for transmission using the downlink frequency Station 1 sends its inbound message using the uplink frequency After transmitting the message, station 1 releases the uplink by sending a go-ahead message 7

8 MAC Scheduling Approaches: Polling Systems (3) c. Polling with no central controller: All stations can hear each other Stations develop a polling order list using some protocol Default station with the polling message transmits After completing its transmission, the station transmits the polling message to the next station on the polling order list 8

9 MAC Scheduling Approaches: Polling Systems (4) Options for polling stations: How many frames a station is allowed to transmit per poll? 1. Exhaustive Service: with the polled station transmitting all available frames. 2. Frame-limited Service: with the polled station restricted to transmit 1 frame per poll. 3. Time-limited Service: with the polled station restricted to transmit over a fixed interval of time. Definitions: 1. Walk Time: interval from when a station complete its transmission to when the next station transmits. 2. Cycle Time: interval between two consecutive polls of the same station. polling messages M 1 2 t Walk time (t ) packet transmissions Cycle time (T c ) 9

10 MAC Scheduling Approaches: Polling Systems (Analysis) polling messages M 1 2 t Walk time (t ) packet transmissions Cycle time (T c ) 1. Let λ/m frames/s be the average arrival rate of frames for transmission to a station 2. Assuming exhaustive service, average number of transmissions per stations 3. Average cycle time is given by E{ T c } = M E{ N [ E{ N } X + t ] c λ } = M E { } c T c E{ T c Mt } = 1 λx = 1 τ ρ 10

11 MAC Scheduling Approaches: Polling Systems (Analysis) polling messages M 1 2 t Walk time (t ) packet transmissions Cycle time (T c ) 1. Assuming Frame-limited service, then maximum cycle time is given by 2. Maximum normalized throughput is given by [ X + t ] max{ T c } = M η max = MX MX + τ 11

12 MAC Scheduling Approaches: Token Ring (1) 3. Token Passing Systems: decenteralized approach with no central controller In ring topology, each station is connected in a ring using an interface Interface operates in two modes listen mode transmit mode input from ring delay output to ring delay to device from device 12

13 MAC Scheduling Approaches: Token Ring (2) Interface operates in two modes listen mode transmit mode input from ring delay output to ring delay to device from device 1. Each bit is reproduced on the ring with a delay 2. Delay is a multiple of (one bit duration) 3. Delay allows to check for certain bit patterns 1. Station transmits a message bit by bit on ring 2. Station receives a message bit by bit from ring 3. No forwarding of bits is done 13

14 MAC Scheduling Approaches: Token Ring (3) When no station is transmitting, there is a free token floating on the ring Token Frame Format SD AC ED Starting delimiter J K 0 J K J, K non-data symbols (line code) Access control P P P T M R R R PPP Priority; T Token bit M Monitor bit; RRR Reservation Ending delimiter J K 1 J K 1 I E I intermediate-frame bit E error-detection bit When a free token is received (T = 0), the interface changes the passing token bit (T = 1) and starts transmitting 14

15 MAC Scheduling Approaches: Token Ring (4) When a free token is received (T = 0), the interface changes the passing token bit (T = 1) and starts transmitting Data Frame Format or 6 2 or Destination Source SD AC FC Information Address Address FCS ED Each transmitted bit is removed by the destination station or by the source station After the transmission is complete, the source station inserts the free token back onto the ring with (T = 0) 1 FS Token Frame Format SD AC ED Access control P P P T M R R R PPP Priority; T Token bit M Monitor bit; RRR Reservation 15

16 MAC Scheduling Approaches: Token Ring (5) Ring Latency: Maximum number of bits in transition around the ring If frame size > ring latency, a complete frame cannot be on the ring at one time If frame size < ring latency, complete frame is on transition in the ring. Ring Latency in seconds = τ + Mb/R Ring Latency in bits = (τ + Mb/R)R where τ is total propagation delay around the ring, M is the number of stations in the ring, b is the number of bit-delays in an interface. Approaches to Token Reinsertion: 1. Single token operation (delayed token release): in which the token is released only after a complete frame is received by the transmitting station. Suitable when frame size is nearly equal to ring latency. 2. Multiple token operation (early token release): in which token is released after the transmission of a frame is completed by the transmitting station. Suitable when frame size is less than ring latency. 16

17 MAC Scheduling Approaches: Token Ring Analysis 1. Single token operation: Frame transmission time = X Time duration between the start instants of two frames = X + max(x,τ ) + τ /M η max = X + X max( X, τ ) + τ / M Time to pass tokens 2. Multiple token operation: Frame transmission time = X Time duration between the start instants of two frames = X + τ /M η max X = X + τ / M 17

18 MAC Scheduling Approaches: Token Ring Analysis M = 50 Maximum throughput M = 10 Single frame operation M = 50 M = 10 Multiple token operation Single token operation a = τ / X! a <<1, any token reinsertion strategy acceptable! a 1, single token reinsertion strategy acceptable! a >1, multitoken reinsertion strategy necessary 18

19 MAC Scheduling Approaches: Channelization Channelization: Semi-static bandwidth allocation of portion of shared medium to a given user Highly efficient for constant-bit rate traffic Preferred approach in Cellular telephone networks and Terrestrial / satellite broadcast radio television transmission. 1. Frequency Division Multiple Access (FDMA) Frequency band allocated to users Broadcast radio & TV, analog cellular phone 2. Time Division Multiple Access (TDMA) Periodic time slots allocated to users Telephone backbone, GSM digital cellular phone 3. Code Division Multiple Access (CDMA) Code allocated to users Cellular phones 3G cellular 19

20 MAC Scheduling Approaches: Channelization (FDMA) 1. Divide channel into M frequency bands 2. Each station transmits and listens on assigned bands 3. Stations can transmit at the same time Frequency W Hz (R bps) 1 2 M 1 M Guard bands Time 4. Each station transmits at most R/M bps 5. Good for stream traffic; Used in connection-oriented systems 6. Inefficient for bursty traffic 20

21 MAC Scheduling Approaches: Channelization (TDMA) 1. Divide time in transmission cycles and slots 2. Dedicate 1 slot per station in transmission cycles 3. Stations transmit data burst at full channel bandwidth within their own slot Frequency Guard time W Hz (R bps) M 1 One cycle Time 4. Each station transmits at R bps 1/M of the time within a cycle 5. Good for stream traffic; Used in connection-oriented systems 6. Inefficient for bursty traffic 21

Multiple Access (3) Required reading: Garcia 6.3, 6.4.1, CSE 3213, Fall 2010 Instructor: N. Vlajic

1 Multiple Access (3) Required reading: Garcia 6.3, 6.4.1, 6.4.2 CSE 3213, Fall 2010 Instructor: N. Vlajic 2 Medium Sharing Techniques Static Channelization FDMA TDMA Attempt to produce an orderly access