Chapter 2 Multiple Access Protocols

1. Chapter 2Multiple Access Protocols Professor Rick Han University of Colorado at Boulder rhan@cs.colorado.edu

2. Announcements • Email/Hand in HW #1 in class Thursday Jan. 29 • Programming assignment #1 is online and is due Feb. 10 • Dora submission • No TA office hours this week • Next, Chapter 2, Media Access Protocols Prof. Rick Han, University of Colorado at Boulder

3. Recap of Previous Lecture • Stop-and-Wait • Sliding Window protocols – keep the bit pipe full • Go-Back-N • Window-based Flow Control • Selective Repeat Protocol • Preview of Shared-media protocols Prof. Rick Han, University of Colorado at Boulder

4. Shared-Media or Broadcast Networks • N senders and receivers connected by a shared medium (copper wire, atmosphere, water) • Shared local access to the same media • Local Area Network (LAN) • Ethernet, Fast Ethernet, Gigabit Ethernet, … • Wireless Ethernet, or 802.11 a/b/g, or WiFi 802.11/Wireless Ethernet Ethernet (802.3) Prof. Rick Han, University of Colorado at Boulder

5. Multiple Access Protocols • Determine which host is allowed to transmit next to a shared medium • Channel reservation: TDMA, FDMA, CDMA, Token Ring, … • Random access: ALOHA, CSMA/CD, CSMA/CA 802.11/Wireless Ethernet Ethernet Prof. Rick Han, University of Colorado at Boulder

6. Multiple Access Protocols (2) • Also called Medium-Access Control (MAC) protocols • Before data link-layer packets can be sent, a sender has to gain access to the media • MAC layer is often placed in the stack between layer 2 and layer 1 Host A Data Link Layer MAC Layer Physical Layer Prof. Rick Han, University of Colorado at Boulder

7. … … 1 2 3 1 2 3 1 2 1 3 2 1 3 1 2 2 Host 1 Host 3 Router/ Mux Host 2 Not Eth. Time Division Multiple Access (TDMA) • Divide time into multiple slots • Each host sends in a pre-determined slot • Out-of-band reservation mechanism • Compare to Time Division Multiplexing (TDM) Prof. Rick Han, University of Colorado at Boulder

8. Frequency Division Multiple Access (FDMA) • Divide spectrum into frequency bins • Each host sends in a pre-determined frequency bin • Out-of-band reservation mechanism (FCC) • Also called Frequency Division Multiplexing (FDM) • Example: AM/FM radio, TV Host 1 Host 2 Host 3 Freq. (Hz) AM 500-1700 KHz FM 88-108 MHz Satellite GHz range Prof. Rick Han, University of Colorado at Boulder

9. Code Division Multiple Access (CDMA) • Use multiple orthogonal codes to partition a range of spectrum • Each host sends using a pre-determined code • Also called “spread spectrum” • Two forms spread spectrum: • Direct-Sequence Spread Spectrum– DSSS • Chipping sequences spread the signal’s spectrum • CDMA is often used as synonym for DSSS • Examples: 802.11b, cell • Frequency-hopping spread spectrum– FHSS • Example: Bluetooth • Advantage: simple, but not as efficient Prof. Rick Han, University of Colorado at Boulder

10. Possible hopping sequence F1 F2 F3 F4 …, F1, F3, F4, F2, F1, F3, F4, F2, … Code Division Multiple Access (CDMA) (2) • Frequency hopping example Freq (Hz) F1 F2 F3 F4 Host 1 Host 2 Bluetooth Host 1’s Code: 1342, Host 2’s Code: 3214, Host 3’s Code: 4123 Note that all 3 codes are orthogonal: at each instant in time, each host is on a different frequency Host 3 Prof. Rick Han, University of Colorado at Boulder

11. Random Access/MAC Protocols • Multiple users share the same frequency band and/or same time and/or same code • Analogy: conversation in a crowded room • What protocol steps do people use to talk in the same room (shared media)? • Important factors: • Wait for silence • Then talk • Listen while talking. • What do we do if there’s 2 talkers? Backoff. • Repeat • Protocols also add a random increasing timeout Prof. Rick Han, University of Colorado at Boulder

12. Random Access: ALOHA Protocol • Developed at University of Hawaii in 1971 by Abramson • Ground-based UHF radios connect computers on several island campuses to main university computer on Oahu • “pure” ALOHA: hosts transmit whenever they have information to send – form of random access • Collision will occur when two hosts try to transmit packets at the same time • Hosts wait a timeout=1 RTT for an ACK. • If no ACK by timeout, then wait a randomly selected delay to avoid repeated collisions, then retransmit Prof. Rick Han, University of Colorado at Boulder

13. Packet A Packet C Packet B Random Access: ALOHA Protocol (2) • Collision of packets can occur when a packet overlaps another packet time T0 Wasted Time Due to a Collision = 2 packet intervals Collision Collision Wasted Time Colliding with B Prof. Rick Han, University of Colorado at Boulder

14. Packet A Packet C Packet B Random Access: Slotted ALOHA • Rather than sending a packet at any time, send along time slot boundaries • Collisions are confined to one time slot time T0 Wasted Time Due to a Collision = 1 packet interval No Collision Collision Prof. Rick Han, University of Colorado at Boulder

15. Random Access: Slotted ALOHA (2) • How do hosts synchronize to begin transmitting along time slot boundaries? • One central station transmits a synchronization pulse or beacon • Slotted ALOHA is more efficient than ALOHA because when there is a collision, the wasted time is confined to one time slot • Assuming Poisson packet arrivals (memoryless), can compute the maximum throughput of ALOHA to be 18%. • Maximum throughput of Slotted ALOHA is 37% • Why are ALOHA & slotted ALOHA so inefficient? Prof. Rick Han, University of Colorado at Boulder

16. Packet A Packet B Packet Y Packet X time Host B listens delay Collision still possible over long prop. delays Host B sends Random Access: CSMA • ALOHA & slotted ALOHA are inefficient because hosts don’t take into account what other hosts are doing before they transmit • “Talk-before-listen” protocols • Example: at party, everyone speaks whenever they want to, regardless of whether another person is speaking • Instead, “listen before you talk” = Carrier Sense Multiple Access (CSMA) • Sense for “carriers” (see if anyone else is transmitting) before you begin transmitting Prof. Rick Han, University of Colorado at Boulder

17. Packet A Packet B Packet Y Packet X time Host B listens Host B sends Collision Random Access: 1-Persistent CSMA • If channel is busy, • A host listens continuously • When channel becomes free, a host transmits its packet immediately (with probability 1) • Collision scenarios • Hosts A and B are far apart (long prop. delay). A’s signal takes a long time to reach B. So, B thinks channel is free, and begins transmitting. • Hosts B and C transmit as soon as A finishes • Still, CSMA is more efficient than ALOHA variants Prof. Rick Han, University of Colorado at Boulder

18. Random Access: p-Persistent CSMA • Generalization of 1-persistent CSMA • Typically applied to slotted channels • Slot length is chosen as maximum propagation delay • A host senses the channel, and • If slot is idle, transmit with probability p, or defer with probability q=1-p • If next slot is idle, transmit with probability p, or defer with probability 1-p, repeat… • If channel is busy, then sense channel continuously until it becomes free, begin again Prof. Rick Han, University of Colorado at Boulder

19. Packet A Packet B time Host B listens Random Sleep Host B sends Random Access: Non-Persistent CSMA • Host does not sense channel continuously • Instead, if channel is busy, • Wait/sleep a random interval before sensing again • As with 1-persistent CSMA, as soon as channel is idle, then send a packet • Random interval reduces collisions • Higher throughput than 1-persistent CSMA when many senders Prof. Rick Han, University of Colorado at Boulder

20. Packet B Packet A Not transmitted delay Host B detects collision And stops sending Host B starts sending Packet B Random Access: Ethernet CSMA/CD • Ethernet uses CSMA/CD, i.e. CSMA with Collision Detection (CD) • “Listen-while-talk” protocol • A host listens even while it is transmitting, and if a collision is detected, stops transmitting time Host B senses carrier Prof. Rick Han, University of Colorado at Boulder

21. Random Access: Ethernet CSMA/CD (2) • Can abort transmission sooner than end-of-packet if there is a collision • Can happen if prop. delays are long • Better efficiency than pure CSMA • CSMA/CD doesn’t require explicit acknowledgement • Unlike CSMA, which requires an ACK or timeout to detect a collision • Collision detection is built into the transmitter • When collision detected, begin retransmission Prof. Rick Han, University of Colorado at Boulder

22. Random Access: Ethernet CSMA/CD (3) • Exponential backoff strategy • When a collision is detected, a host waits for some randomly chosen time, then retransmits a packet • If a second collision is detected, a host doubles the original wait time, then retransmits the packet • Each time there is another collision, the wait time is doubled before retransmission • Variants: • At each retransmission, choose a random value from the exponentially increasing wait time. • At each retransmission, choose randomly from among a discrete set of values within exponentially increasing wait time • Retransmit a finite # of times Prof. Rick Han, University of Colorado at Boulder

23. Random Access: Ethernet CSMA/CD (4) • CSMA/CD can be used with nonpersistent, 1-persistent, or p-persistent variants of CSMA • Ethernet is synonymous with the IEEE 802.3 standard • Initial work on Ethernet at Xerox in early 70’s • Ethernet specifies 1-persistent CSMA/CD • To extend an Ethernet, repeaters are placed. • Start to run into propagation delay issues and noise amplification issues • Ethernet keeps its maximum length to 2500 m to keep prop. delays tight, so that CSMA/CD responds well Prof. Rick Han, University of Colorado at Boulder

24. Hosts A and B at opposite ends of the Ethernet A B Ethernet t t+d, d=prop. delay B transmits @ time t+d Just before A’s packet arrives B sees Collision at t+d, Transmits a “runt” packet B’s packet arrives at t+2d Random Access: Ethernet CSMA/CD (5) • Ethernet CSMA/CD requires a minimum size to a frame: Prof. Rick Han, University of Colorado at Boulder

25. A B Hosts A & B at opposite ends of Ethernet Ethernet t t+d B’s packet arrives at t+2d Random Access: Ethernet CSMA/CD (6) • If B’s packet arrives at A and A is no longer transmitting, then Host A will • Fail to detect the collision • thinks its packet got through • Thinks the incoming packet is a new packet • Therefore, to detect a collision: • Minimum frame size >= 2*(prop. delay)*BW Prof. Rick Han, University of Colorado at Boulder

26. Random Access: 802.11 “Wireless” Ethernet • Employs CSMA/CA, i.e. CSMA with Collision Avoidance (CA) • Hidden terminal effect • Example: B can hear A and C, but A and C can’t hear each other. If A is sending B, C thinks channel is clear and starts sending => collision! • Doesn’t happen in wired Ethernet, because hosts can hear each other Host A Host B Host C Collision Prof. Rick Han, University of Colorado at Boulder

27. Random Access: 802.11 “Wireless” Ethernet (2) • How to handle the hidden terminal effect? • Host A sends a Request-To-Send (RTS) • Host B sends a Clear-To-Send (CTS) • Host C hears the CTS, and does not interrupt transmission between A and B • This helps implement Collision Avoidance RTS Host A Host B Host C CTS CTS Data Host C Suppresses Its Data ACK Prof. Rick Han, University of Colorado at Boulder