1. Data Link Layer IS250 Spring 2010 chuang@ischool.berkeley.edu

2. 802.3 Ethernet 802.11 WiFi 802.16 WiMax DSL Cable modem Cellular (3G, 2.5G, …) SONET, STS, OC … LAN: local area network MAN: metropolitan area network WAN: wide area network Backbone network Access network The “last mile” aka “first mile” aka “local loop” Network Technologies

3. Link Layer Functionalities • Framing • Error detection or correction • Media access control

4. Framing • Message transmitted over link as string of 0’s and 1’s • Sender and receiver has to agree where is the beginning and end of a message  framing • A frame is a link layer message unit • The prelude and/or postlude are special characters or character sequences that help establish the beginning and end of the frame • The header contains control information that is used by the network (e.g., network address; error detection) • The payload contains data that is meaningful only for the sender and receiver

5. Dealing with Errors • Data can be corrupted during transmission • Bits lost • Bit values changed • Frame includes additional information to help detect or correct errors • Set by sender; checked by receiver • Statistical guarantee

6. Error Detection M f • Message M • H = f(M) • Transmitted T = M || H • Received R = M’ || H’ • If H’ = f(M’) then no error H M H M’ H’

7. Error Detection Schemes • Parity • Send an additional parity bit (H) per character • Even parity: if # of 1’s in character is odd, H = 1; else H = 0 • Odd parity: if # of 1’s in character is odd, H = 0; else H = 1 • Cannot detect even numbers of bit errors • Checksum • Treat data as sequence of integers • Compute and send arithmetic sum (H) • Handles multiple bit errors, but not all errors • Cyclic Redundancy Check (CRC)

8. Checksum Examples • Checksum computed over data • Checksum appended to frame • 2nd bit reversed in each item, but checksum is the same

9. Cyclic Redundancy Check • CRC is a mathematical function of data, computed as the remainder from a division of two binary numbers, one representing the message M, and the other a fixed divisor P. • Example: CRC-CCITT: • P(X) = x16 + x12 + x5 + 1; or • P = 10001000000100001 • CRC-CCITT can detect: • all single/double bit errors • All odd-numbered bit errors • 100% of burst errors <= 16 bits • 99.997% of burst errors = 17 bits • 99.998% of burst errors >= 18 bits

10. CRC Algorithm • CRC can be computed and verified using binary long division • Numerical example • Use CRC scheme with P(x) = x5 + x4 + x + 1 • P: 110011 (6 bits) • Message M: 11100011 (8 bits) • Divide M by P; use the remainder as the CRC (what we call ‘H’ on slide 6) [Note: remainder one bit shorter than P] • Transmitted message T = M || H • Receiver performs CRC verification on received message T’: divide T’ by P will produce zero remainder if no error

11. Numerical Example        10110110 110011 ) 1110001100000 110011            101111 110011             111000 110011               101100 110011                111110 110011 11010 = H T = M || H = 1110001111010

12. Verification        10110110 110011 ) 1110001111010 110011            101111 110011             111001 110011               101010 110011                110011 110011 00000 No error since remainder is 0

13. Ethernet Wi-Fi Media Access Control • How do multiple, independent computers coordinate access to a shared communication medium?

14. Local Area Networks • LAN characteristics: • High throughput • Relatively low cost • Distance limitations • Often rely on shared media • Different topologies

15. Ethernet • Most widely deployed LAN technology • IEEE 802.3 standard • Several generations • Same frame format • Different data rates (10Mbps, 100Mbps, 1Gbps, 10Gbps) • Different media (coax, twisted pair, fiber)

16. Shared Medium • Shared medium used for all transmissions • Only one station transmit at any time • Stations take turns using medium • Media access control (MAC) policy ensures fairness

17. Data Transmission in Ethernet • Only one station transmit at any time • Signal propagates across entire cable • All stations receive transmission • CSMA/CD media access scheme

18. CSMA/CD • Multiple access (MA) • Multiple computers attach to shared media • Each uses same access algorithm • Carrier Sense (CS) • Wait until medium is idle • Begin to transmit frame • Simultaneous transmission possible

19. CSMA/CD (2) • Two simultaneous transmissions • Interfere with one another • Called a collision • CSMA plus collision detection (CD) • Listen to medium during transmission • Detect whether another station’s signal interferes • Back off from interference and try again

20. Example

21. Back-off after Collision • When collision occurs • Wait random time t1, 0 <= t1 <= d • Use CSMA and try again • If second collision occurs • Wait random time t2, 0 <= t2 <= 2d • Double range for each successive collision • Called exponential backoff

22. Wireless Ethernet (Wi-Fi) • Uses unlicensed spectrum (ISM band)

23. WLAN Media Access • Limited range • Not all stations receive all transmissions • Cannot use CSMA/CD • Example • Maximum transmission distance is d • Stations 1 and 3 do not receive each other’s transmissions • Known as the “hidden terminal” problem

24. CSMA/CA • Collision avoidance (CA) • upon sensing idle channel, waits for a random backoff duration before attempting to transmit • RTS/CTS Mechanism • Handshake before data transmission • Request to Send (RTS): “X is about to send to Y” • Clear to Send (CTS): “Y is about to receive from X” • Data frame sent from X to Y • Collisions of control messages possible; but control messages are much shorter than data frames

25. Addressing in Shared Medium • All stations on shared media receive all transmissions • Each frame contains address of intended recipient • Stations discard any frame addressed to another station • Shared media provide no confidentiality • Network analyzers can run in promiscuous mode • Designed for testing/debugging • Allows network interface to accept all packets

26. /Size Ethernet Addressing & Frame Format • Each station assigned unique 48-bit (6 byte) address • Known as Ethernet address, MAC address, or Physical address • Address assigned when network interface card (NIC) manufactured • Ethernet frame format:

27. Ethernet Evolution • Original Ethernet: bus topology • Modern Ethernet: star topology • Ethernet hub: • Propagates each incoming signal to all connections • noise and collisions also propagated • Ethernet switch: • Operates on frames • Does not forward noise or collisions • Understands addresses • Only forwards when necessary • Allows independent transmission on different segments

28. Layer 2 is not limited to LANs • Backbone operators (e.g., AT&T) deploy and operate long-haul copper-based or fiber-based digital circuits • SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) standards support framing, multiplexing, synchronization. OC-192 9.953Gbps OC-768 39.813Gbps