Data Link Layer IS250 Spring 2010

2 Network Technologies  Ethernet  WiFi  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”

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  Message M  H = f(M)  Transmitted T = M || H  Received R = M’ || H’  If H’ = f(M’) then no error M H f MH 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  2 nd 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) = x 16 + x 12 + x 5 + 1; or -P =  CRC-CCITT can detect: -all single/double bit errors -All odd-numbered bit errors -100% of burst errors <= 16 bits % of burst errors = 17 bits % 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) = x 5 + x 4 + x + 1 -P: (6 bits) -Message M: (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 ) = H T = M || H =

12 Verification ) No error since remainder is 0

13 CRC Implementation  CRC can be efficiently computed in hardware using XOR gates and shift registers  Example: 16 bit CRC -Registers initialized to zeros -Bits of message shifted in -CRC found in value of registers Before shift After shift

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

15 Local Area Networks  LAN characteristics: -High throughput -Relatively low cost -Distance limitations -Often rely on shared media  Different topologies

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

17 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

18 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

19 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

20 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

21 Example

22 Back-off after Collision  When collision occurs -Wait random time t 1, 0 <= t 1 <= d -Use CSMA and try again  If second collision occurs -Wait random time t 2, 0 <= t 2 <= 2d  Double range for each successive collision  Called exponential backoff

23 Wireless Ethernet (Wi-Fi)  Uses unlicensed spectrum (ISM band)

24 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

25 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

26 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

27 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: /Size

28 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

29 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 Gbps OC Gbps