Chapter 2 Multiple Access Protocols Professor Rick Han

Chapter 2 Multiple Access Protocols Professor Rick Han University of Colorado at Boulder rhan@cs. colorado. edu 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 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 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 Wi. Fi 802. 11/Wireless Prof. Rick Han, University of Ethernet (802. 3) 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 802. 11/Wireless Prof. Rick Han, University of Ethernet Colorado at Boulder

Multiple Access Protocols (2) Host A Data Link Layer MAC Layer • 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 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) … 1 2 3 Host 1 1 2 3 Host 3 1 2 1 3 Host 2 2 Not Eth. Prof. Rick Han, University of Colorado at Boulder … 3 2 Router/ Mux 1

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

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” • Direct-Sequence Spread Spectrum– DSSS • Chipping sequences spread the signal’s spectrum • CDMA is often used as synonym for DSSS • Examples: 802. 11 b, cell Frequency-hopping spread spectrum– FHSS • Example: Bluetooth • Advantage: simple, but not as efficient • Two forms spread spectrum: • Prof. Rick Han, University of Colorado at Boulder

Code Division Multiple Access (CDMA) (2) • Frequency hopping example Freq (Hz) F 1 F 2 F 3 F 4 Possible F 2 F 3 F 4 hopping F 1 sequence …, F 1, F 3, F 4, F 2, … 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 Prof. Rick Han, University of Colorado at Boulder Host 3

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 • 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 • What protocol steps do people use to talk in the same room (shared media)? • 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 Prof. Rick Han, University of retransmit Colorado at Boulder

Random Access: ALOHA Protocol (2) • Collision of packets can occur when a packet overlaps another packet Packet A time T 0 Packet B Packet C Collision Wasted Time Colliding with B Prof. Rick Han, University of Colorado at Boulder Wasted Time Due to a Collision = 2 packet intervals

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 time T 0 Packet B Packet C Collision No Collision Prof. Rick Han, University of Colorado at Boulder Wasted Time Due to a Collision = 1 packet interval

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

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

Random Access: 1 -Persistent CSMA • If channel is busy, • • A host listens continuously When channel becomes free, a host transmits packet immediately (with probability 1) Packet A Packet B Host B listens Host B sends • Collision scenarios • • • Packet X Y time Collision 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

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

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

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

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

Random Access: Ethernet CSMA/CD (5) • Ethernet CSMA/CD requires a minimum size to a frame: Hosts A and B at opposite ends of the Ethernet A B Ethernet t B’s packet arrives at t+2 d 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 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 • Minimum frame size >= 2*(prop. delay)*BW • Therefore, to detect a collision: A Hosts A & B at opposite ends of Ethernet B Ethernet t t+d Prof. Rick Han, University of Colorado at Boulder B’s packet arrives at t+2 d

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

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 Data ACK Host C Suppresses Its Data Prof. Rick Han, University of Colorado at Boulder