Chapter 5: The Data Link Layer Our goals: principles behind data - PDF document

Chapter 5: The Data Link Layer Our goals:  principles behind data link layer services:  error detection correction  error detection, correction  sharing a broadcast channel: multiple access  link layer addressing  reliable data transfer, flow control: done!  instantiation and implementation of various link layer technologies 11/17/2013 Data Link Layer (SSL) 5-1 Link layer, LANs 5.1 introduction, 5.5 link virtualization: services MPLS 5.2 error detection, , 5.6 data center correction networks 5.3 multiple access 5.7 a day in the life of protocols a web request 5.4 LANs (play animation in .ppt slides on your own) y )  addressing, ARP addressing, ARP  Ethernet  switches  VLANS 11/17/2013 Data Link Layer (SSL) 5-2 1

Link Layer: context  A link connects two  IP datagram transferred by adjacent IP nodes (layer 3) different link protocols over different along a path links which may provide different services 11/17/2013 Data Link Layer (SSL) 5-3 Link Layer: context  Link can be  unit of data: frame , wire which encapsulates an  IP datagram wireless   IP expects no service  IP expects no service LAN (layer 2) LAN (layer 2)   guarantee from links WAN (virtual link)  M application transport H t M network t k network k H n H t M data link protocol link link H l H n H t M H l H n H t M physical physical frame phys. link trailer adapter card 11/17/2013 Data Link Layer (SSL) 5-4 2

Link Layer Services  Framing  Encapsulate datagram with header and trailer  Error Detection  errors caused by signal attenuation, noise. d b i l tt ti i  receiver detects presence of errors  Error Correction  receiver identifies and corrects bit error(s) without resorting to retransmission  Link access  access protocol for shared channel access  “MAC” addresses used in frame headers to identify MAC addresses used in frame headers to identify  source, destination  different from IP addresses  why both MAC and IP addresses? 11/17/2013 Data Link Layer (SSL) 5-5 Link Layer Services (more)  Half-duplex and full-duplex  with half duplex, nodes at both ends of link can transmit, but not at same time  Flow Control  pacing between sender and receiver(s)  Reliable delivery between two physically connected devices  we learned how to do this already (chapter 3)  seldom used on low error-rate links (fiber some twisted  seldom used on low error-rate links (fiber, some twisted pair)  wireless links: high error rates Q: why both link-level and end-end reliability? 11/17/2013 Data Link Layer (SSL) 5-6 3

Link layer, LANs 5.1 introduction, 5.5 link virtualization: services MPLS 5.2 error detection, , 5.6 data center correction networks 5.3 multiple access 5.7 a day in the life of protocols a web request 5.4 LANs (play animation in .ppt slides on your own) y )  addressing, ARP addressing, ARP  Ethernet  switches  VLANS 11/17/2013 Data Link Layer (SSL) 5-7 Error Detection EDC= Error Detection and Correction bits (redundant check bits) D = Data protected by error checking, may include header fields • Error detection not 100% reliable • protocol may miss some errors • more check bits yield better detection and correction 11/17/2013 Data Link Layer (SSL) 5-8 4

Parity Checking Two Dimensional Bit Parity : Single Bit Parity: Detect and correct single bit errors Detect single bit errors error Even parity: total number of 1s total number of 1s is odd means there is a bit error or, 0 0 more precisely, an odd number of bit errors 11/17/2013 Data Link Layer (SSL) 5-9 Cyclic Redundancy Check (CRC) - sender  View data bits, D, as a  Goal : choose r CRC binary number bits, R, such that <D,R> is exactly divisible by is exactly divisible by G using modulo 2 arithmetic  Modulo 2 arithmetic  there is no carry in  Choose r+1 bit pattern addition, and no borrow (generator), G ( t ) G in subtraction  addition and subtraction same as bitwise exclusive OR (XOR) 11/17/2013 Data Link Layer (SSL) 5-10 5

Cyclic Redundancy Check (CRC) - receiver  Receiver knows G,  Bit string <D,R> sent performs division. If is exactly divisible by non-zero remainder, non zero remainder, G G error detected !  can detect all burst errors less than r+1 bits;  longer burst errors are detectable with are detectable with probability 1-(0.5) r 11/17/2013 Data Link Layer (SSL) 5-11 CRC Theory and Example Want: (D*2r) XOR R = nG add R to both sides: D*2r = (nG) XOR R r Equivalently: the remainder from dividing D*2r by G is equal to R; the desired CRC bit string is st i is D*2r R = remainder[ ] G 11/17/2013 Data Link Layer (SSL) 5-12 6

Link layer, LANs 5.1 introduction, 5.5 link virtualization: services MPLS 5.2 error detection, , 5.6 data center correction networks 5.3 multiple access 5.7 a day in the life of protocols a web request 5.4 LANs (play animation in .ppt slides on your own) y )  addressing, ARP addressing, ARP  Ethernet  switches  VLANS 11/17/2013 Data Link Layer (SSL) 5-13 Multiple Access Links and Protocols Two types of “links”:  point-to-point  fiber optic link f p  link between Ethernet switch and host  broadcast (shared wire or medium)  old-fashioned Ethernet  upstream channel of HFC (hybrid fiber cable)  wireless (802.11 LAN and others), etc. humans at a party shared cable (e.g., (shared air, acoustics) shared RF shared RF old Ethernet) (e.g., 802.11 WiFi) (satellite) 11/17/2013 Data Link Layer (SSL) 5-14 7

Multiple Access protocols single shared broadcast channel  two or more simultaneous transmissions by nodes may interfere with each other  collision if a node receives two or more signals at the same time  Need a protocol to determine when nodes can transmit  no out-of-band channel for coordination 11/17/2013 Data Link Layer (SSL) 5-15 Ideal Multiple Access Protocol Broadcast channel of rate R bps 1 when one node wants to transmit it can send at 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized:  no special node to coordinate transmissions  no synchronization of clocks, time slots h i ti f l k ti l t 4. simple 11/17/2013 Data Link Layer (SSL) 5-16 8

MA Protocols: a taxonomy Three broad classes:  Channel Partitioning  di id h  divide channel into smaller “pieces” (time slots, frequency l i t sm ll “ i s” (tim sl ts f bands, codes)  allocate a piece to each node for exclusive use  Random Access  shared channel, collisions allowed  “recover” from collisions  “Taking turns”  nodes take turns  a node with more to send can take a longer turn 11/17/2013 Data Link Layer (SSL) 5-17 Channel Partitioning protocols TDMA: time division multiple access*  each station gets fixed length slot (length = pkt trans time) in each frame tran t m ) n ach fram  requires time synchronization  unused slots go idle 6-slot frame 3 1 3 4 1 4 * there are multiple transmitters 11/17/2013 Data Link Layer (SSL) 5-18 9

Channel Partitioning protocols FDMA: frequency division multiple access  each station assigned a fixed frequency band  unused transmission time in frequency bands go idle frequency bands FDM cable 11/17/2013 Data Link Layer (SSL) 5-19 Random Access Protocols  When node has packet to send  transmit at full channel data rate  no a priori coordination among nodes  two or more transmitting nodes ➜ “collision”  random access MA protocol specifies:  how to detect collision  how to recover from collision (e.g., via delayed retransmissions)  examples:  slotted ALOHA  ALOHA  CSMA, CSMA/CD, CSMA/CA 11/17/2013 Data Link Layer (SSL) 5-20 10

Slotted Aloha  time is divided into equal size slots (pkt trans. times)  requires time synchronization  node with new arriving pkt: transmit at beginning of next slot t l t  if collision: retransmit pkt in a future slot with probability p (or one of K slots at random), until successful. Success (S), Collision (C), Empty (E) slots 11/17/2013 Data Link Layer (SSL) 5-21 Slotted Aloha efficiency Long-term fraction of time slots that are successful? Suppose N nodes have packets to send  each transmits in slot with probability p  prob. successful transmission S is by a particular node: S = p (1-p) (N-1) by any of N nodes: S = Prob [one of N nodes transmits] = N p (1-p) (N-1) Channel occupied by useful … choosing optimum p, let N -> infinity transmissions < 37% of time = 1/e = .37 as N -> infinity 11/17/2013 Data Link Layer (SSL) 5-22 11