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Chapter 6 Multiple Radio Access

Chapter 6 Multiple Radio Access. 曾志成 國立宜蘭大學 電機工程學系 tsengcc@niu.edu.tw. Introduction (1/2). Multiple access control channels Each Mobile Station (MS) has a transmitter/receiver that communicates with others via a shared channel. Transmission from any MS is received by other MSs. MS 3.

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Chapter 6 Multiple Radio Access

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  1. Chapter 6Multiple Radio Access 曾志成 國立宜蘭大學 電機工程學系 tsengcc@niu.edu.tw EE of NIU

  2. Introduction (1/2) • Multiple access control channels • Each Mobile Station (MS) has a transmitter/receiver that communicates with others via a shared channel. • Transmission from any MS is received by other MSs. MS 3 MS 4 MS 2 Shared Multiple Access Medium … MS 1 Node N EE of NIU

  3. Introduction (2/2) • Multiple access issues • If more than one MS transmit at the same time, a collision occurs. • How to determine which MS can transmit? • Multiple access protocols • Solving multiple access issues. • Different types: • Contention protocols resolve a collision after it occurs. These protocols execute a collision resolution protocol after each collision. • Collision-free protocols (e.g., a bit-map protocol and binary countdown) ensure that a collision can never occur. (will be discussed in Chapter 7) EE of NIU

  4. Static Channelization Scheduling Dynamic Medium Access Control Random Access Channel Sharing Techniques The channel assignment is done in a pre-specified way and does not change with time. Medium - Sharing Techniques The channel is allocated as needed and changes with time. EE of NIU

  5. Contention-based Conflict-free Collision resolution Random access FDMA, TDMA, CDMA, Token Bus, DQDB, etc. TREE, WINDOW, etc. ALOHA, CSMA, BTMA, ISMA, etc. DQDB: Distributed Queue Dual Bus Classification of Multiple Access Protocols for a Shared Channel Multiple Access Protocols • (will be discussed in Chap 7) BTMA: Busy Tone Multiple Access ISMA: Idle Signal Multiple Access EE of NIU

  6. Contention-Based Protocols --- ALOHA Family • Pure ALOHA • Developed in the 1970s for a packet radio network by Hawaii University • Whenever a terminal (MS) has data, it transmits. • Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. • If there is a collision, sender retransmits after some random time. • Slotted ALOHA • Time is slotted and a packet can only be transmitted at the beginning of a slot. • Slotted the time reduces the collision duration. EE of NIU

  7. Node 2 Packet Retransmission 2 2 3 Collision Node 3 Packet Pure ALOHA Node 1 Packet Waiting a random time Retransmission 1 3 Time t t+T t-T Collision mechanism in Pure ALOHA EE of NIU

  8. Throughput of Pure ALOHA • The probability of successful transmission Ps is the probability that no other packet is scheduled in an interval of length 2T(vulnerable period) where g is the packet rate of the traffic. • The throughput Sth of pure ALOHA as • Defining G= gT to normalize offered load, we have • Differentiating Sth with respect to G and equating to zero gives • The maximum throughput of pure ALOHA is obtained when G=1/2 and is EE of NIU

  9. Retransmission Retransmission 2 3 Slotted ALOHA Nodes 2 & 3 Packets Node 1 Packet 1 2 3 Time Collision Slot Collision mechanism in slotted ALOHA EE of NIU

  10. Throughput of Slotted ALOHA • The probability of successful transmission Ps is the probability no other packet is scheduled in an interval of length T where g is the packet rate of the traffic • The throughput Sth of slotted ALOHA as Sth=gTe-gT. • Defining G=gT to normalize offered load, we have Sth=Ge-G. • Differentiating Sth with respect to G and equating to zero gives • The maximum throughput of slotted ALOHA is obtained when G=1 and is EE of NIU

  11. 0.5 0.4 0.368 0.3 Slotted ALOHA S: Throughput 0.184 0.2 0.1 Pure ALOHA 0 0 2 4 6 8 G=gT Throughput EE of NIU

  12. Contention-based Protocols --- CSMA Family • CSMA (Carrier Sense Multiple Access) • Improvement: Start transmission only if no transmission is ongoing. • CSMA/CD (CSMA with Collision Detection) • Improvement: Stop ongoing transmission if a collision is detected. • CSMA/CA (CSMA with Collision Avoidance) • Improvement: Wait a random time and try again when carrier is quiet. If still quiet, then transmit. • CSMA/CA with ACK • CSMA/CA with RTS/CTS EE of NIU

  13. CSMA (Carrier Sense Multiple Access) • Maximal throughput achievable by slotted ALOHA is 0.368 • CSMA gives improved throughput compared to ALOHA protocols • Listens to the channel before transmitting a packet (avoid avoidable collisions) EE of NIU

  14. MS 5 sense MS 2 Packet Delay for MS 5 MS 3 Packet 2 3 5 Delay for MS 4 Collision due to hidden terminal problem MS 4 senses Collision Mechanism in CSMA MS 1 Packet 1 4 Time EE of NIU

  15. Hidden Terminal Problem Radio transmission range R R R A B C A and C are hidden with respect to each other EE of NIU

  16. Exposed Terminal Problem Radio Transmission range R R R R A B C D Transmission at B forces C (Exposed) to stop transmission to D EE of NIU

  17. Unslotted Nonpersistent CSMA Nonpersistent CSMA Slotted Nonpersistent CSMA Unslotted persistent CSMA Persistent CSMA Slotted persistent CSMA 1-persistent CSMA p-persistent CSMA Types of CSMA Protocols CSMA EE of NIU

  18. Nonpersistent CSMA Protocols • The MS senses the medium first whenever it has a packet to send • If the medium is idle, transmits immediately. • If the medium is busy, waits a random amount of time and senses the medium again. • If collision occurs, waits a random amount of time and starts all over again. • Random backoff reduces probability of collisions. • Waste idle time if the backoff time is too long. EE of NIU

  19. Throughput of Nonpersistent CSMA • For unslottednonpersistent CSMA, the throughput is • For slotted nonpersistent CSMA, the throughput is • G is the offered traffic rate • T is the packet transmission time • t is the propagation delay • a=t/T is the normalized time unit EE of NIU

  20. 1-persistent CSMA Protocols • The MS senses the medium first whenever it wants to send • If the medium is idle, the MS transmits immediately. (This is the reason why it is called 1-persistent.) • If the medium is busy, the MS keeps listening until medium becomes idle, and then transmits immediately. • There will always be a collision if more than one node have ready packets and wait for the channel to become idle. • For unslotted 1-persistent CSMA, the throughput is given by: • For slotted 1-persistent CSMA, the throughput is given by: EE of NIU

  21. p-persistent CSMA Protocols • In this protocol, the time is slotted. • The size of slot is the contention period, i.e., the round trip propagation time. • The MS senses the medium first whenever it has a packet to send • If the medium is idle, the MS transmits with probability p, or defer transmission by one time slot with probability (1-p). • If the medium is busy, the MS waits until the next slot and checks the medium again. • If a collision occurs, the MS waits for a random amount of time and starts all over again. • A good tradeoff between nonpersistent (p=0) and 1-persistent (p=1) CSMA. EE of NIU

  22. How to Select The Probability p? • Assume that N is the number of nodes have a packet to send at a time and the medium is busy. • Then,Np is the expected number of nodes that will attempt to transmit once the medium becomes idle. • If Np > 1, then a collision is expected to occur. Therefore, network must make sure that Np≤ 1 to avoid collision. EE of NIU

  23. Throughput ofp-persistent CSMA Protocol (1/2) • Throughput S as: where G is the offered traffic rate, a=t/T=propagation delay/packet transmission time and EE of NIU

  24. Throughput ofp-persistent CSMA Protocol (2/2) EE of NIU

  25. Throughput 0.01-persistent CSMA 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Nonpersistent CSMA 0.1-persistent CSMA 0.5-persistent CSMA 1-persistent CSMA S: Throughput Slotted ALOHA ALOHA 0 1 2 3 4 5 6 7 8 9 Traffic Load G EE of NIU

  26. CSMA/CD (CSMA with Collision Detection) (1/3) • In CSMA, if 2 terminals begin sending packet at the same time, each will transmit its complete packet (although collision is taking place) • Wasting medium for an entire packet time • CSMA/CD • Step 1: If the medium is idle, transmit. • Step 2: If the medium is busy, continue to listen until the channel is idle then transmit . • Step 3: If a collision is detected during transmission, ceases the transmission (detection is impossible by wireless devices) and waits a random amount of time and repeats the same algorithm. EE of NIU

  27. CSMA/CD (CSMA with Collision Detection) (2/3) EE of NIU

  28. T0+t- B begins transmission A B T0+t+tcd B detects collision A B T0+2t+tcd+tcr- A detects collision just before end of transmission A B CSMA/CD in Ethernet (2/2) T0 A begins transmission cd: time required for a terminal to detect a collision detection cr: time required to send the jamming signal to achieve the consensus reinforcement procedure A B (tis the propagation time)  Time EE of NIU

  29. 1.2 α =t /T= 0 1 0.8 α = 0.01 S: Throughput 0.6 0.4 α = 0.1 α = 1 0.2 0 0.001 0.01 0.1 1 10 100 1000 Traffic load G Throughput of Slotted Nonpersistent CSMA/CD EE of NIU

  30. CSMA/CA (CSMA with Collision Avoidance) for Wireless Devices • Each terminal ready to transmit senses the medium. • If medium is busy, it waits until the end of current transmission. • It again waits for an additional predetermined time period DIFS (Distributed Inter Frame Space). • Then, picks up a random number of slots (the initial value of backoff counter) within a contention window to wait before transmitting its frame. • If there are transmissions by other MSs during this time period (backoff time), the MS freezes its counter. • After the channel has been free longer than DIFS, the MS resumes count down. The MS starts its transmission when the counter reaches to zero. EE of NIU

  31. Medium Busy Defer access Slot Backoff after defer The Basic Mechanism of CSMA/CA Contention window DIFS DIFS Next Frame Time DIFS – Distributed Inter Frame Spacing EE of NIU

  32. Example of Backoff Mechanism (1/2) EE of NIU

  33. Example of Backoff Mechanism (2/2) EE of NIU

  34. CSMA/CA with ACK for Ad Hoc Networks • Immediate Acknowledgements from receiver upon reception of data frame without any need for sensing the medium • ACK frame transmitted after time interval SIFS (Short Inter-Frame Space) (SIFS <DIFS) • Receiver transmits ACK without sensing the medium • If ACK is lost, retransmission done EE of NIU

  35. DIFS SIFS DIFS Contention window Backoff after defer CSMA/CA/ACK Data Time Source MS ACK BS Next Frame Other MSs Defer access SIFS – Short Inter Frame Spacing EE of NIU

  36. CSMA/CA with RTS/CTS for Ad Hoc Networks • Transmitter sends an RTS (request to send) after medium has been idle for time interval more than DIFS • Receiver responds with CTS (clear to send) after medium has been idle for SIFS • Then Data is exchanged • RTS/CTS is used for reserving channel for data transmission so that the collision can only occur in control message EE of NIU

  37. DIFS SIFS SIFS SIFS CTS DIFS Contention window Backoff Defer access CSMA/CA with RTS/CTS Time RTS Data Source MS ACK Destination MS Next Frame Other MSs EE of NIU

  38. RTS Propagation delay CTS Data ACK RTS/CTS MS A MS B This helps avoid hidden terminal problem in ad hoc networks EE of NIU

  39. Homework • P6.2 • P6.6 • P6.7 • P6.8 • P6.11 EE of NIU

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