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Data Link Layer IS250 Spring 2010 chuang@ischool.berkeley.edu
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2 Network Technologies 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”
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3 Link Layer Functionalities Framing Error detection or correction Media access control
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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
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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
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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’
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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)
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8 Checksum Examples Checksum computed over data Checksum appended to frame 2 nd bit reversed in each item, but checksum is the same
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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 = 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
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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: 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
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11 Numerical Example 10110110 110011 ) 1110001100000 110011 101111 110011 111000 110011 101100 110011 111110 110011 11010 = H T = M || H = 1110001111010
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12 Verification 10110110 110011 ) 1110001111010 110011 101111 110011 111001 110011 101010 110011 00000 No error since remainder is 0
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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
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14 Media Access Control How do multiple, independent computers coordinate access to a shared communication medium? Ethernet Wi-Fi
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15 Local Area Networks LAN characteristics: -High throughput -Relatively low cost -Distance limitations -Often rely on shared media Different topologies
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16 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)
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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
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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
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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
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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
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21 Example
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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
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23 Wireless Ethernet (Wi-Fi) Uses unlicensed spectrum (ISM band)
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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
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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
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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
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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
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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
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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-192 9.953Gbps OC-768 39.813Gbps
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