Chapter 6 The Data Link layer 6.1 introduction, 6.5 link - PowerPoint PPT Presentation

Chapter 6 The Data Link layer 6.1 introduction, 6.5 link virtualization: services MPLS 6.2 error detection, 6.6 data center correction networks 6.3 multiple access 6 3 l i l 6 7 6.7 a day in the life of d i h lif f protocols a web request 6 4 LAN 6.4 LANs (play animati n in ppt (play animation in .ppt slide on your own)  addressing, ARP  Ethernet  Ethernet  layer-2 switches  VLANS VLANS 12/5/2017 Data Link Layer (SSL) 6-1 1

Link Layer: context  A link connects two adjacent IP nodes (layer 3) along a path along a path  IP datagram transferred by  IP datagram transferred by different link protocols over different  An Ethernet switch links which may provide different (layer 2) is considered to services ser ces be part of a link be part of a link 12/5/2017 Data Link Layer (SSL) 6-2 2

Link Layer: context  Link can be  Link can be  unit of data: frame , wire which encapsulates an  IP datagram g wireless wireless   IP expects no service LAN (layer 2)  guarantee from links WAN (virtual link)  application application M M transport M H t network network M H n H t data link protocol p t l link l k link M H l H n H t M H l H n H t physical physical frame phys. link trailer trailer adapter card 12/5/2017 Data Link Layer (SSL) 6-3 3

Link Layer Services L nk Layer Serv ces  Framing  Encapsulate datagram with header and trailer  E  Error Detection D t ti n  errors caused by signal attenuation, noise.  receiver detects presence of errors  Error Correction  E C cti n  receiver identifies and corrects bit error(s) without resorting to retransmission  Link access  Link access access protocol for shared channel access   “MAC” addresses used in frame headers to identify source, destination , o different from IP addresses o why both MAC and IP addresses? 12/5/2017 Data Link Layer (SSL) 6-4 4

Link Layer Services (more) L nk Layer Serv ces (more)  Half-duplex and full-duplex  with half duplex (shared channel), nodes at both ends of p ( ), link can transmit, but not at same time  Flow Control  pacing between sender and receiver(s)  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 pair) pair)  wireless links: high error rates Q: why both link-level and end-end reliability? 12/5/2017 Data Link Layer (SSL) 6-5 5

Chapter 6 The Data Link layer 6.1 introduction, 6.5 link virtualization: services MPLS 6.2 error detection, 6.6 data center correction networks 6.3 multiple access 6 3 l i l 6 7 6.7 a day in the life of d i h lif f protocols a web request 6 4 LAN 6.4 LANs (play animati n in ppt (play animation in .ppt slide on your own)  addressing, ARP  Ethernet  Ethernet  layer-2 switches  VLANS VLANS 12/5/2017 Data Link Layer (SSL) 6-6 6

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 G using modulo 2 arithmetic arithmetic  Modulo 2 arithmetic  there is no carry in  Choose r+1 bit pattern addition, and no borrow (generator), G in subtraction  addition and subtraction same as bitwise exclusive OR (XOR) 12/5/2017 Data Link Layer (SSL) 6-7 7

Cyclic Redundancy Check (CRC) - receiver  Receiver knows G,  Bit string <D,R> sent performs division. If p is is exactly divisible by x tl di isibl b non-zero remainder, G error detected !  can detect all burst n d t t ll b st  errors less than r+1 bits;  longer burst errors are detectable with probability 1 (0 5) r probability 1-(0.5) 12/5/2017 Data Link Layer (SSL) 6-8 8

CRC Theory and Example Want: (D*2r) XOR R = nG add R to both sides: dd R t b th id D*2 r XOR R XOR R = (nG) XOR R Equivalently Equivalently: the remainder from dividing D*2r by G is equal to R; to R; the desired CRC bit string is D*2r R = remainder[ ] G 12/5/2017 Data Link Layer (SSL) 6-9 9

Chapter 6 The Data Link layer 6.1 introduction, 6.5 link virtualization: services MPLS 6.2 error detection, 6.6 data center correction networks 6.3 multiple access 6 3 l i l 6 7 6.7 a day in the life of d i h lif f protocols a web request 6 4 LAN 6.4 LANs (play animati n in ppt (play animation in .ppt slide on your own)  addressing, ARP  Ethernet  Ethernet  layer-2 switches  VLANS VLANS 12/5/2017 Data Link Layer (SSL) 6-10 10

Links and Multiple Access Protocols Two types of “links”:  point-to-point p p  fiber optic link  link between Ethernet switch and host  broadcast (shared wire or medium)  broadcast (shared wire or medium)  old-fashioned Ethernet  shared coax cable in HFC (hybrid fiber cable), e.g., Spectrum  wireless (802.11 LAN and others), etc. humans at a party humans at a party shared cable (e.g., sh d bl ( (shared air, acoustics) shared RF shared RF old Ethernet) (e.g., 802.11 WiFi) (satellite) 12/5/2017 Data Link Layer (SSL) 6-11 11

Mult ple Access protocols Multiple Access protocols single shared broadcast channel  two or more simultaneous transmissions by nodes may i l i i b d interfere with each other  collision if a node receives two or more signals at the same g time  N  Need a protocol to determine when nodes can transmit d t l t d t mi h d s t smit  no out-of-band channel for coordination 12/5/2017 Data Link Layer (SSL) 5-12 12

MA Protocols: a taxonomy Three broad classes:  Channel Partitioning  Channel Partitioning (e.g., cell phones) (e g cell phones)  divide channel into smaller “pieces” (frequency bands, time slots, codes)  allocate a piece to each node for exclusive use ll t i t h d f l i  Random Access (e.g., early Ethernet, 802.11 wifi)  shared channel shared channel , collisions allowed collisions allowed  “recover” from collisions  does not provide QoS p Q  “Taking turns” (e.g., token-ring LAN, FDDI)  nodes take turns  a node with more to send can take a longer turn d ith t s d t k l t 12/5/2017 Data Link Layer (SSL) 6-13 13

Channel Partitioning protocols FDMA: frequency division multiple access * FDMA f di i i l i l *  each station assigned a fixed frequency band (note: MIMO antenna can use multiple frequencies) antenna can use multiple frequencies)  unused transmission time in frequency bands go idle uency s bands frequ FDM cable * multiple transmitters p 12/5/2017 Data Link Layer (SSL) 6-14 14

Channel Partitioning protocols TDMA: time division multiple access*  each station gets fixed length slot (length = pkt g g ( g p trans time) in each frame  requires time synchronization  unused slots go idle  unused slots go idle 6 slot 6-slot frame 3 3 4 1 4 1 * multiple transmitters 12/5/2017 Data Link Layer (SSL) 6-15 15

Random Access Protocols  When node has packet to send  transmit at full channel data rate  no a priori coordination among nodes i i di ti d  two or more transmitting nodes ➜ “collision”  random access MA protocol specifies:  random access MA protocol specifies:  how to detect collision  how to recover from collision (e.g., via delayed retransmissions) retransmissions)  examples (chronological):  ALOHA  slotted ALOHA  CSMA, CSMA/CD, CSMA/CA 12/5/2017 Data Link Layer (SSL) 6-16 16

Slotted Aloha  time is divided into equal size slots (pkt trans. times)  requires time synchronization  node with new arriving pkt: transmit at beginning of  node with new arriving pkt: transmit at beginning of next slot  if collision: retransmit pkt in a future slot with p probability p (or one of K slots at random), until successful. Success (S), Collision (C), Empty (E) slots 12/5/2017 Data Link Layer (SSL) 6-17 17

Slotted Aloha efficiency L Long-term fraction of time slots that are f i f i l h successful? Suppose N nodes have packets to send 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] S = Prob [one of N nodes transmits] = N p (1-p) (N-1) Channel occupied Channel occupied by useful … choosing optimum p, let N -> infinity transmissions < = 1/e = .37 as N -> infinity = 1/e = 37 as N > infinity 37% of time 37% of time 12/5/2017 Data Link Layer (SSL) 6-18 18

S ∂ ∂ [NP (1 [NP (1 P) P) N 1 ] ] − = = − P P ∂ ∂ S ∂ NP (1 ( P) ) N 1 (1 ( P) ) N 1 N − − = − + − P P ∂ ∂ NP (N 1) (1 P) N(1 P) N 2 N 1 − − = − − − + − N(1 N(1 P) P) N 2 N 2 { P(N { P(N 1) 1) 1 1 P } P } − = − − − + − 0 N(1 P) { NP P 1 P } N 2 − P = − − + + − 0 1.0 S 1 ∂ 0 when P to maximize S = = ∂ P P N N ∂ My terminology : “Probability Division Multiplex” Division of probability does not have to be fair, i.e., p y P 1 +P 2 + … +P N = 1 is condition for maximum 12/5/2017 Data Link Layer (SSL) 6-19 19