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

2. Prof. Rick Han, University of Colorado at Boulder 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

3. Prof. Rick Han, University of Colorado at Boulder 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

4. Prof. Rick Han, University of Colorado at Boulder 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 Ethernet (802.3) 802.11/Wireless Ethernet

5. Prof. Rick Han, University of Colorado at Boulder 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 Ethernet 802.11/Wireless Ethernet

6. Prof. Rick Han, University of Colorado at Boulder 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 Physical Layer MAC Layer Host A Data Link Layer

7. Prof. Rick Han, University of Colorado at Boulder 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) …… 231123123 Host 1 Host 2 Host 3 1 2 3 Not Eth. Router/ Mux 1 2 12

8. Prof. Rick Han, University of Colorado at Boulder 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 AM 500-1700 KHz FM 88-108 MHz Satellite GHz range Freq. (Hz) Host 1Host 2Host 3

9. Prof. Rick Han, University of Colorado at Boulder 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

10. Prof. Rick Han, University of Colorado at Boulder Code Division Multiple Access (CDMA) (2) Frequency hopping example 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 1 Host 2 Host 3 F1F2F3 Freq (Hz) F4 F1F2F3 …, F1, F3, F4, F2, F1, F3, F4, F2, … Possible hopping sequence F4

11. Prof. Rick Han, University of Colorado at Boulder 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

12. Prof. Rick Han, University of Colorado at Boulder 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

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

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

15. Prof. Rick Han, University of Colorado at Boulder 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?

16. Prof. Rick Han, University of Colorado at Boulder 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 Packet A time Host B listens Packet BPacket YPacket X Host B sends delay Collision still possible over long prop. delays

17. Prof. Rick Han, University of Colorado at Boulder 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) Packet A time Host B listens Packet B Packet YPacket X Host B sendsCollision 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

18. Prof. Rick Han, University of Colorado at Boulder 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

19. Prof. Rick Han, University of Colorado at Boulder 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 Packet A time Host B listens Packet B Host B sends Random Sleep

20. Prof. Rick Han, University of Colorado at Boulder 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 Packet A time Host B senses carrier Packet B delay Host B starts sending Packet B Host B detects collision And stops sending Not transmitted

21. Prof. Rick Han, University of Colorado at Boulder 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

22. Prof. Rick Han, University of Colorado at Boulder 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

23. Prof. Rick Han, University of Colorado at Boulder 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

24. Prof. Rick Han, University of Colorado at Boulder Random Access: Ethernet CSMA/CD (5) Ethernet CSMA/CD requires a minimum size to a frame: AB Ethernet Hosts A and B at opposite ends of the 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

25. Prof. Rick Han, University of Colorado at Boulder 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 AB Ethernet Hosts A & B at opposite ends of Ethernet t t+d B’s packet arrives at t+2d

26. Prof. Rick Han, University of Colorado at Boulder 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 Collision Host BHost C

27. Prof. Rick Han, University of Colorado at Boulder 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 Host A Host C Suppresses Its Data Host BHost C RTS CTS Data ACK