EITF25 Internet Techniques and Applications L3: Data Link layer. Stefan Höst

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1 EITF25 Internet Techniques and Applications L3: Data Link layer Stefan Höst

it can be seen as transmitting a series of binary

2 Communication on physical layer To transmit on the physical medium use signals At each computer it can be seen as transmitting a series of binary data (0s and 1s) Convenient with finite sequences 2

3 Packages Data is divided into packets before transmission. header data trailer Header and trailer contains control information needed for the transmission. 3

4 Application Protocols information Application protocol Application bits Application information bits PHY signals Physical link PHY 4

5 HTTP, an application protocol HTTP = Hyper Text Transfer Protocol HTTP request HTTP reply Web server 5

6 Link layer protocol The sender and receiver uses a link layer protocol that provides error control for data that is sent on a physical link. application data Link bits PHY Application protocol Link protocol Physical protocol application data Link bits PHY 6

7 Framing The Physical layer protocol sends a bitstream. The Link layer protocol needs to separate the packets. Therefore, the link layer protocol at the sender packetize data in frames so that the receiver can distinguish one frame from another. 7

8 Bit stuffing Avoid having the flag pattern ( ) in the data: Transmitter After five consecutive 1s insert a 0 Receiver After five consecutive 1s: Next bit = 0: delete next bit Next bits = 10: Stop flag Next bits = 11: Abort 8

9 Bitstuffing Example 9

10 Error control The data is assumed to be error free by higher layers But at physical layer errors do occur. Some typical patterns: Bit errors Individual bits are inverted (0 1 or 1 0) Burst errors Several bits close to each other are inverted 10

11 Bit error In a single-bit error, only 1 bit in the data unit has changed. 11

12 Burst errors A burst error means several bits in the data unit have changed. 12

13 To find bit errors To detect and/or correct errors, extra (redundant) bits is added to the data message in a controlled way. Encoding Data Extra bits The value of the extra bits depends on the data and the coding scheme. 13

14 Error detection process 14

15 Block coding In block coding, divide the message into blocks of k bits, called datawords. Add r redundant bits to each block to make the length n = k + r. The resulting n-bit blocks are called codewords. The code rate is R = k/n 15

16 Error control In error detection the aim is to detect errors The transmission protocol decides what to do about erroneous packages In error correction the aim is to correct errors Roughly half as many errors can be corrected as can be detected. In most communication systems both error detection and error correction occur 16

17 Error detection schemes Simple parity-check code Cyclic Redundancy Check (CRC) Checksum 17

18 Simple Parity-Check Code A k-bit dataword is changed to an n-bit codeword, where n = k+1. In (Even) Parity-Check the extra bit is selected to make the total number of 1s in the codeword even = A simple parity-check code can detect an odd number of errors. 18

19 Simple Parity-check code 19

20 Cyclic Redundancy Check (CRC) In CRC, the sender and receiver uses a predefined shared divisor to calculate the codeword. 20

21 Dataword representation The dataword of k bits be represented by a polynomial, d(x). 21

22 Encoder process The objective is to find a codeword, c(x), of n bits, with Data vector, k bits: d(x) with deg=k Redundant vector, n-k bits: r(x) with deg=n-k Codeword: c(x)=d(x)x n-k + r(x) The codeword is determined by a generator polynomial (divisor), g(x), of degree n-k. g(x) =x n k + g n k 1 x n k g 1 x +1 22

23 CRC Derivation The CRC polynomial is the reminder of the division d(x)x n k I.e. denote g(x) r(x) =R g(x) d(x)x n k 23

24 CRC example Use three CRC bits for the dataword Use the generator Data poly: d(x) = x 3 +1 Generator poly: g(x) = x 3 +x+1 Dividend: d(x)x 3 = (x 3 +1)x 3 = x 6 +x 3 CRC poly: r(x) = R g(x) (x 6 +x 3 ) Codeword poly: c(x) = d(x)x 3 + r(x) 24

25 CRC example 25

26 CRC, some theory The notation r(x) =R g(x) d(x)x n k is equivalent to d(x)x n k = g(x)z(x)+r(x) Or, equivalently, c(x) =d(x)x n k + r(x) =g(x)z(x) 26

27 CRC, receiver decision A polynomial c(x) with deg<n is a codeword if and only if g(x) divides c(x), i.e. R g(x) (c(x))=0. The received polynomial: y(x) = c(x)+e(x) The syndrom s(x) = R g(x) (y(x)) = R g(x) (e(x)) Detection rule: s(x) =0 s(x) 6= 0 : Assume correct transmission : Detect an error 27

28 Error detection capabilities Single errors: e(x)=x i is not divisable by g(x) Double error: e(x)=x j +x i =x i (x j-i +1) Use primitive polynomial p(x) with deg=l. Then if n-1<2 L -1 it is not divisable and all double errors will be detected If x+1 g(x) all odd error patterns will be detected In practice, set g(x)=(x+1)p(x) 28

29 Some standard CRC polynomials 29

30 Checksum The checksum is used in the Internet by several protocols although not at the data link layer. The main principle is to divide the data into segments of n bits. Then add the segments and use the sum as redundant bits. 30

31 Checksum process 31

32 Checksum example Message: Checksum using 4 bits: => =29 => is more than 4 bits. Use one s complement addition: => = 1110 The checksum is complemented to help the receiver. 32

33 Checksum (another example)

34 Forward Error Correction (FEC) Two simple examples Repetition code Concatenated parity check 34

35 Repetition code Transmitter For each bit, transmit three copies Receiver Decode acording to majority decision If one error uccured this will be corrected 35

36 Repetition code, Example Transmitter d =0) c = 000 Channel One error: e = 010 ) y = 010 Receiver y = 010 ) ĉ = 000 ) ˆd =0 36

37 Vertical and horisontal parity Encoding Let d be a binary matrix. Add parity bits for each row and column Decoding If one error, it can be found from the parity bits Two or three errors can be detected (but not always corrected) 37

38 V+H parity, Example Encoding d = Channel One error y = ) c =

39 V+H parity, Example Decoding Two parity bits wrong. Points at one position. y = ) ĉ = ) ˆd =

40 The need for Flow control The receiver must be able to handle all recieved frames. If the transmission rate is too high, the receiever may become overloaded and drop frames due to full buffers. 40

41 Error and flow control The basic principle in error and flow control is that the receiver acknowledges all correctly received packets. Data ACK 41

42 Automatic Repeat Request (ARQ) schemes Stop-and-wait ARQ Go-back-N ARQ Selective Repeat ARQ Unfortunately, Fouruzan 5th edition has become rather confused on these schemes with descriptions in two different chapters. Please follow the lecture slides for a more compact view. 42

43 Stop-and-wait ARQ The receiver sends an ACK for every data packet that is correctly received. The sender transmits the next packet when it has received an ACK for the previous one. The sender uses a time-out for each packet. If the time-out expires (i.e. no ACK has arrived), the packet is retransmitted. Packets are identified with a sequence number, alternating between 0 and 1. ACK is labeled with the next packet 43

44 Stop-and-Wait, example 44

45 Go-back-N ARQ The sender can transmit several packets without an ACK from the receiver. The receiver ACKs all packets that are correctly received in the right order. The sender can transmit new packets when ACKs are received for previous packets. This is called pipelining. 45

46 Sequence numbers The packets are identified with a seqence number. If the header allows m bits for the sequence number, the numbers can range from 0 to 2 m -1. This means that the maximum number of outstanding packets (transmitted packets that have not been ACKed) is 2 m -1. A sliding window defines the range of sequence numbers that currently concerns the sender and receiver. 46

47 Sliding window at sender 47

48 Importance of window size 48

49 Go-back-N, example (lost ACK) 49

50 Go-back-N, example (lost data) 50

51 Selective repeat ARQ Works as Go-back-N as long as there are no lost packets. The receiver sends a negative acknowledgment (NAK) when it detects a lost packet. Only the lost packets are retransmitted. 51

52 Send and receive windows 52

53 Selective repeat, example 53

54 Piggyback Often the traffic goes both ways. Use the transmitted packages to send ACK Let S n and R n be transmitted and received sequense numbers. Use frame as below. Flag S n R n Data CRC Flag 54

Point-to-Point Protocol (PPP) Point-to-point protocol (PPP) is one of the classical Internet protocols, and it is in this course used as an example of the

55 Point-to-Point Protocol (PPP) Point-to-point protocol (PPP) is one of the classical Internet protocols, and it is in this course used as an example of the function of a link layer protocol. It is a byte-oriented (character-oriented) protocol, that is, everything in the frame is treated in bytes. Frame format:

56 Byte stuffing When the flag appears in the data field, it needs to be escaped. Since PPP is a byte-oriented protocol, byte stuffing is used. Every time the flaglike pattern appears in the data field, the escape byte is stuffed to tell the receiver that the next byte is not a flag.

57 Error and flow control PPP uses a 2 or 4 byte CRC for error detection. However, no further error or flow control is provided. 57

58 Transition phases A PPP connection goes through a number of phases:

59 Help protocols PPP uses a set of other protocols to manage the connection: Link Control Protocol (LCP) is responsible for establishing, maintaining, configuring, and terminating links. Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) are used in the authentication process. A Network Control Protocol (NCP) configures the link for the network protocol. One example is Internet Protocol Control Protocol (IPCP) for Internet.

60 Other protocols in PPP

EITF25 Internet- - Techniques and Applica8ons Stefan Höst. L4 Data link (part 1)

EITF25 Internet- - Techniques and Applica8ons Stefan Höst L4 Data link (part 1) Previously on EITF25 (or digital signal) 2 Data Link Layer Medium Access Control Access to network Logical Link Control Node-