Link layer, LANs: outline. Chapter 5-1 Link Layer. Link layer: introduction. Link layer services

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1 Chapter 5 Link Layer Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 Link layer, LANs: outline 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LANs addressing, ARP Ethernet switches VLANS 5.5 data center networking 5.6 a day in the life of a web request Link Layer 5 Link Layer 5-2 Link layer: introduction terminology: hosts and routers: nodes communication channels that connect adjacent nodes along communication path: links wired links wireless links layer-2 packet: frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link global ISP Link Layer 5-3 Link layer services framing, link access: encapsulate datagram into frame, adding header, trailer channel access if shared medium MAC addresses used in frame headers to identify source, dest different from IP address! reliable delivery between adjacent nodes we learned how to do this already (chapter 3)! seldom used on low bit-error link (fiber, some twisted pair) wireless links: high error rates Link Layer 5-4

2 Link layer services (more) error detection: errors caused by signal attenuation, noise. receiver detects presence of errors: signals sender for retransmission or drops frame error correction: receiver identifies and corrects bit error(s) without resorting to retransmission flow control: pacing between adjacent sending and receiving nodes Where is the link layer implemented? in each and every host link layer implemented in adaptor (aka network interface card NIC) or on a chip Ethernet card, card; Ethernet chipset implements link, physical layer attaches into host s system buses combination of hardware and software (firmware) application transport network link link physical cpu controller physical transmission memory host bus (e.g., PCI) network adapter card Link Layer 5-5 Link Layer 5-6 Adaptors communicating Link layer, LANs: outline datagram controller sending host frame sending side: encapsulates datagram in frame adds error checking bits, rdt, flow control, etc. datagram datagram controller receiving host receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to upper layer at receiving side 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LANs addressing, ARP Ethernet switches VLANS 5.5 data center networking 5.6 a day in the life of a web request Link Layer 5-7 Link Layer 5-8

3 Error control digital transmission systems introduce errors applications require certain reliability level data applications require error-free transfer voice & video applications tolerate some errors error control used when transmission system does not meet application requirement error control ensures a data stream is transmitted to a certain level of accuracy despite errors two basic approaches: error detection & retransmission (ARQ) forward error correction (FEC) Principle of error detection transmitter: for a given bit stream M, additional bits (called errordetecting code) are calculated as a function of M and appended to the end of M receiver: for each incoming frame, perform the same calculation and compares the two results. A detected error occurs if there is a mismatch Link Layer 5-9 Link Layer 50 Error detection Parity checks - single bit parity check Sender Data E= f(data) Data 1. Calculate E from received data 2. Compare E and E if identical, no error assumed. otherwise, error detected E Legend: E,E = Error detecting codes f = Error detecting function Data E = f(data) E Compare Receiver add a single bit(parity bit) to each character so that the total number of ones is even (even parity) or odd (odd parity) if di = i-th data bit, then parity bit =d1d2... dn (even parity) d1 d2 d3 d4 d5 d6 d (even parity) single-bit parity checks can only detect odd number of errors Link Layer 51 Link Layer 52

4 Odd-parity : example data parity 1 error 2 errors 3 errors Two dimensional parity check horizontal and vertical parity checks (row and column) Character 1 Data bits d11 d12 ¼ d1n f1r Character parity bits data parity Error detected data parity Error not detected data parity Error detected Single-bit parity check can only detect odd number of errors Character 2 Character m Block check character d21 d22 ¼ d2n f2r ¼ dm1 dm2 ¼ dmn fmr fc1 fc2 ¼ fcn fcr Link Layer 53 Link Layer 54 Error-detecting capability Cyclic Redundancy Check (CRC) One error Three errors Two errors Four errors (undetectable) 1, 2, or 3 errors can always be detected; Not all patterns >4 errors can be detected powerful error detection, easily implemented by hardware logic principle: given a k-bit message (M), the transmitter generates an n-bit sequence (R), so that the resulting frame (T) is exactly divisible by some predetermined (n+1)-bit number (G) use modulo-2 arithmetic: no carries/borrows; add subtract XOR M(k bits) message digits T(k+n bits) R(n bits) check digits CRC : frame Arrows indicate failed check bits Link Layer 55 Link Layer 56

5 CRC calculation M CRC (n bits) (=2 n M) 2 n M/G = Q + R/G Q: quotient R: remainder M CRC (=R) method: 1) append n 0 s to the right of M (initializes CRC bits with zeros) 2) divide the extended message 2 n M by G 3) let CRC be equal to R Link Layer 57 CRC calculation: example generator: (1,0,1,1) G(x) = x 3 + x + 1 message: (1,1,0,0) M(x) = x 3 + x 2 CRC bits: 3 bits (to be calculated) initialization: (1,1,0,0,0,0,0) x 3 M(x) = x 6 + x ) therefore, CRC bits = 010 transmitted codeword: T= (1,1,0,0,0,1,0) T(x) = x 6 + x 5 + x x 3 + x 2 + x x 3 + x + 1 ) x 6 + x 5 x 6 + x 4 + x 3 x 5 + x 4 + x 3 x 5 + x 3 + x 2 x 4 + x 2 x 4 + x 2 + x x Link Layer 58 Undetectable error patterns (transmitter) T(x) (channel) + (receiver) E(x) error polynomial T (x)=t(x)+e(x) E(x) has 1s in error locations & 0s elsewhere receiver divides the received polynomial T (x) by G(x) blindspot: if E(x) is a multiple of G(x), that is, E(x) is a nonzero codeword, then T (x) = T(x) + E(x) = Q(x)G(x) + Q (x)g(x) If an error polynomial is divisible by the generator polynomial, then the error pattern will be undetectable. choose the generator polynomial so that selected error patterns can be detected. Errors detected by CRC all single-bit errors if G(x) has more than one term all double-bit errors, as long as G(x) has a factor with at least three terms any odd number of errors, as long as G(x) contains a factor (x+1) any burst error of length n bits or less, n=degree of the polynomial G(x) Link Layer 59 Link Layer 5-20

6 Standard generator polynomials CRC-8: = x 8 + x 2 + x + 1 CRC6: = x 16 + x 15 + x = (x + 1)(x 15 + x + 1) CCITT6: = x 16 + x 12 + x CCITT-32: ATM Bisync HDLC IEEE 802 = x 32 + x 26 + x 23 + x 22 + x 16 + x 12 + x 11 + x 10 + x 8 +x 7 + x 5 + x 4 + x 2 + x + 1 Internet checksum several Internet protocols use check bits to detect errors in the header (e.g. IP, UDP) or the header and data (e.g. TCP). in the IP protocol, a checksum is calculated for header contents and included in a special field. checksum recalculated at every router, so algorithm selected for ease of implementation in software. Link Layer 5-21 Link Layer 5-22 Internet checksum (more) sender side: 1. message is divided into 16-bit words. 2. the value of the checksum word is set to all words including the checksum are added using one s complement addition. 4. the sum is complemented and becomes the checksum. 5. the checksum is sent with the data. receiver side: 1. message (including checksum) is divided into 16-bit words. 2. all words are added using one s complement addition. 3. the sum is complemented and becomes the new checksum. 4. if the value of checksum is 0, the message is accepted; otherwise, it is rejected. Link Layer 5-23 Internet checksum: example This column should be added to the partial sum - Partial sum calculation carry from 16th column carry from 15th column carry from 3rd column carry from 2nd column carry from 1st column partial sum Link Layer 5-24

7 Internet checksum: example - Sum and checksum calculation if there is no carry from the last column, the partial sum is the sum. however, if there are extra columns, these are added to the partial sum to obtain the sum (carry from the last column wraps around). 1 carry from 1 st column partial sum sum checksum Link Layer 5-25 Checksum of IP header: example ,5, and and and sum checksum Link Layer 5-26 Link layer, LANs: outline 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LANs addressing, ARP Ethernet switches VLANS 5.5 data center networking 5.6 a day in the life of a web request Link Layer 5-27 Multiple access links, protocols two types of links : point-to-point dial-up line point-to-point link between Ethernet switch, host broadcast (shared wire or medium) traditional cabled Ethernet upstream HFC wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) Link Layer 5-28

8 Multiple access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: collision if node receives two or more signals at the same time multiple access control (MAC) protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination An ideal MAC protocol given: broadcast channel of rate R bps desiderated: 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, slots 4. simple Link Layer 5-29 Link Layer 5-30 MAC protocols: taxonomy three categories of MAC protocols: channel partitioning partition medium into separate channels (time slot, frequency band, code) allocate channel to node for exclusive use random access no coordination among nodes send, wait, and retry if collision scheduling: taking turns: nodes take turns, but nodes with more to send can take longer turns reservation: node wishing to transmit makes reservations for time slots in advance Channel partitioning: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 6-slot frame 6-slot frame Link Layer 5-31 Link Layer 5-32

9 Channel partitioning: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle FDM cable frequency bands Channel partitioning: CDMA CDMA: code division multiple access unique code assigned to each user; i.e., code set partitioning all users share same frequency, but each user has own chipping sequence (i.e., code) to encode data allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence Link Layer 5-33 Link Layer 5-34 CDMA encode/decode CDMA: two-sender interference sender data bits code d 1 = d 0 = slot 1 slot 0 Z i,m = d i. cm channel output Z i,m slot 1 channel output slot 0 channel output Sender 1 Sender 2 channel sums together transmissions by sender 1 and 2 received input receiver code slot 1 slot 0 M D i = Z. i,m cm m=1 M d 1 = slot 1 channel output d 0 = 1 slot 0 channel output using same code as sender 1, receiver recovers sender 1 s original data from summed channel data! Link Layer 5-35 Link Layer 5-36

10 Random access protocols when node has packet to send transmit at full channel data rate R. no a priori coordination among nodes two or more transmitting nodes collision, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) examples of random access MAC protocols: ALOHA CSMA, CSMA/CD, CSMA/CA ALOHA ALOHA protocol was developed for a radio (wireless) LAN at the University of Hawaii in early 1970 talk when you please scheme original ALOHA protocol is called pure ALOHA each station whenever it has a new frame to send: 1. immediately transmits the frame 2. waits for a round-trip interval for ACK 3. if ACK is not received, waits for a random amount of time and repeats the step 1 Link Layer 5-37 Link Layer 5-38 Collisions in pure ALOHA Vulnerable time in pure ALOHA Link Layer 5-39 Link Layer 5-40

11 Slotted ALOHA In 1972, Robert published a method for doubling the capacity of an ALOHA system time is divided into fixed slots corresponding to one frame for each slot nodes can transmit only at the beginning of the next slot nodes need to be synchronized Collisions in slotted ALOHA Link Layer 5-41 Link Layer 5-42 Vulnerable time in slotted ALOHA Carrier Sense Multiple Access (CSMA) carrier sensing is useful to reduce the possibility that a new transmission will collide with an ongoing transmission listen before talk scheme: If channel sensed idle, transmit entire frame If channel sensed busy, defer transmission CSMA can improve performance over the ALOHA because no station begins to transmit when it senses the channel busy three CSMA schemes: Non-persistent CSMA 1-persistent CSMA p-persistent CSMA Link Layer 5-43 Link Layer 5-44

12 Collisions in CSMA collisions can still occur: propagation delay means two nodes may not hear each other s transmission collision: entire frame transmission time wasted Vulnerable time in CSMA B s C s Link Layer 5-45 Link Layer 5-46 Three CSMA schemes CSMA with Collision Detection (CSMA/CD) listen while talk scheme: listen before transmission until the channel is free additionally continue to monitor channel during transmission if collision is detected, immediately abort transmission and then, retransmit after a random amount of time collisions can be detected by looking at the power of the received signal and comparing it to the transmitted signal CSMA/CD is used on Ethernet (IEEE 802.3) LANs Link Layer 5-47 Link Layer 5-48

13 Collision and abortion in CSMA/CD Frame size for collision detection in the worst case, a station cannot detect a collision during 2, where is the propagation delay from end to end. minimum frame size >= 2 + safety margin. Link Layer 5-49 Link Layer 5-50 Ethernet CSMA/CD algorithm Taking turns MAC protocols 1. NIC receives datagram from network layer, creates frame 2. If NIC senses channel idle, starts frame transmission. If NIC senses channel busy, waits until channel idle, then transmits. 3. If NIC transmits entire frame without detecting another transmission, frame transmission is done! 4. If NIC detects another transmission while transmitting, aborts and sends jam sequence 5. After aborting, NIC enters binary (exponential) backoff: after mth collision, NIC chooses K at random from {0,1,2,, 2 m }. NIC waits K 512 bit times, returns to Step 2 longer backoff interval with more collisions polling: master node invites slave nodes to transmit in turn typically used with dumb slave devices concerns: polling overhead latency single point of failure (master) data slaves data poll master Link Layer 5-51 Link Layer 5-52

14 Taking turns MAC protocols token passing: control token passed from one node to next sequentially. token message concerns: token overhead latency single point of failure (token) (nothing to send) T T Summary of MAC protocols channel partitioning, by time, frequency or code TDMA, FDMA, CDMA random access (dynamic) ALOHA, S-ALOHA, CSMA, CSMA/CD collision detection: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet CSMA/CA used in (WiFi) taking turns polling from central site token passing: FDDI, token ring (802.5) data Link Layer 5-53 Link Layer 5-54

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