# CMPE 150 Winter 2009

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1 CMPE 150 Winter 2009 Lecture 6 January 22, 2009 P.E. Mantey

2 CMPE Introduction to Computer Networks Instructor: Patrick Mantey / t / Office: Engr. 2 Room 595J Office hours: Tuesday 3-5 PM TA: Anselm Kia Web site: Text: Tannenbaum: Computer Networks (4 th edition available in bookstore, etc. )

3 Syllabus

4 Assignment #3 Available on the web site: Due Thursday, January 29, 2009

5 Today s Agenda Link Layer Services Framing kind, seq, ack, info Error Control (CRC) Flow Control Protocols Standards

6 Data Link Layer Design Issues Provide Services to Network Layer 1. Framing 2. Error Control 3. Flow Control

7 Cyclic Redundancy Check

8 Cyclic Redundancy d Check

9 Cyclic Redundancy Check At Transmitter, with M = , compute 2 r M= withg= T = 2 r M + R [note G starts and ends with 1 ] R = Transmit T=

10 Cyclic Redundancy Check At the Receiver, compute: Note remainder = 0 no errors detected

11 Polynomial Code M = x 5 + x 4 + x 3 + x + 1 = M(x) () G = x 3 + x = G(x) R(x) = remainder of [2 3 M(x)/G(x)] (recall r = 3) T(x) = 2 3 M(x) + R(x)

12 Error-Detecting Codes Calculation of the polynomial code checksum. Ref: Tannenbaum, Fig. 3-8

13 CRC Codes CRC-12 G(x) = x 12 +x 11 +x 3 +x 2 +x+1 + CRC-15 G(x) = x 16 + x 15 + x CRC-CCITT G(x) = x 16 + x 12 + x CRC-32 G(x) = 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

14 CRC Performance Errors go through undetected only if divisible by G(x) With suitably chosen G(x) CRC code detects: All single-bit errors All double-bit errors, as long as G(x) has at least three 1s Any odd number of error, as long as G(x) has a factor (x+1) Any burst error for which the length of the burst is less than the length of the G(x) (i.e. r ) Most larger burst errors For equally-probable errors, with burst of length r+1, probability bilit of an undetected t d error is 1/2 r-1 For longer burst, probability of undetected error is 1/2 r [adapted from Stallings, Chapter 7, page 205]

15 Logic implementation of Polynomial Encoder / Decoder Ref: Stallings, Fig. 7.6

16 Ref: Stallings, Fig. 7.6

17 Data Link Connects to (adjacent) computers (connected by a wire or equivalent) Serial -- Bits delivered in order sent

18 Data Link Layer - Functions Provide service interface to the network layer Dealing with transmission errors Regulating data flow Slow receivers not swamped by fast senders

19 Data Link Layer - Functions Relationship between packets and frames.

20 Services Provided to Network Layer ( ) Vi t l i ti (a) Virtual communication. (b) Actual communication.

21 Link Layer Services Unacknowledged Connectionless Service Acknowledged Connectionless Service Acknowledged Connection-oriented Service

22 Link Layer Services Unacknowledged Connectionless Service Sender just sends, no acknowledgments No attempt to resend lost frames Acknowledged Connectionless Service Acknowledged Connection-oriented Service

23 Link Layer Services Unacknowledged Connectionless Service Sender just sends, no acknowledgments No attempt to resend lost frames Used for Real-time traffic (voice, video) Highly reliable LANS When error control is done at higher layers (e.g. TCP) Acknowledged Connectionless Service Acknowledged Connection-oriented Service

24 Link Layer Services Unacknowledged Connectionless Service Sender just sends, no acknowledgments Acknowledged Connectionless Service No logical connection Frames individually acknowledged Tannenbaum says an option, not a requirement BUT: Acknowledged Connection-orientedoriented Service

25 Link Layer Services Unacknowledged Connectionless Service Sender just sends, no acknowledgments Acknowledged Connectionless Service No logical connection Frames individually acknowledged Tannenbaum says an option, not a requirement BUT: large packets become multiple (fixed size) frames Some frames get through, others lost, then resend of packet is very inefficient vs. resend of lost frames Acknowledged Connection-oriented Service

26 Link Layer Services Unacknowledged Connectionless Service Acknowledged Connectionless Service Acknowledged Connection-oriented Service Connection established before data sent Each frame numbered Data link guarantees error-free delivery Each frame received once and only once (vs. connectionless, where a packet can be received several time if errors or timeouts occur Provides reliable bit stream to Network Layer

27 Connection-Oriented Service Protocol steps 1. establish connection 2. reliably deliver data stream Number frames Acknowledgments 3. free the connection ecto

28 Link Layer Connection of Routers in WAN Using Connection- Oriented Service Ref: Tannenbaum, Fig. 3-3

29 Framing (a) A frame delimited by flag bytes. ( ) y g y (b) Four examples of byte sequences before and after stuffing. Ref: Tannenbaum, Fig. 3-5

30 Framing: Bit Stuffing Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver s memory after destuffing. Ref: Tannenbaum, Fig. 3-6

31 Elementary Data Link Protocols Manage data flow at Link Layer An Unrestricted t Simplex Protocol A Simplex Stop-and-Wait Protocol A Simplex Protocol for a Noisy Channel

32 Protocol Definitions Continued Some definitions needed in the protocols to follow Some definitions needed in the protocols to follow. These are located in the file protocol.h.

33 Protocol Definitions (ctd.) Some definitions needed ddin the protocols to follow. These are located in the file protocol.h.

34 Unrestricted Simplex Protocol

35 From Stallings: Data and Computer Communications

36 Stop-and-Wait Simplest form of flow control. Transmitter sends frame and waits. Receiver receives frame and sends ACK. Transmitter gets ACK, sends other frame, and waits, until no more frames to send. Good when few frames. Problem: inefficient link utilization. In the case of high data rates or long In the case of high data rates or long propagation delays.

37 Simplex Stop-and- Wait Protocol

38 Animations

39 From Stallings: Data and Computer Communications

40 A Simplex Protocol for a Noisy Channel A positive acknowledgement with retransmission protocol. Continued

41 A Simplex Protocol for a Noisy Channel (ctd.) A positive acknowledgement with retransmission protocol.

42 Sliding Window Protocols

43 Sliding Window Protocols Supports bi-directional data transfer Full-duplex piggy backing of acks

44 Sliding Window Protocols A One-Bit Sliding Window Protocol A Protocol Using Go Back N A Protocol Using Selective Repeat

45 Sliding Window Protocols (2) A sliding window of size 1, with a 3-bit sequence number. (a) Initially. (b) After the first frame has been sent. (c) () After the first frame has been received. (d) After the first acknowledgement has been received. Tannenbaum Figure 3-13

46 Sliding Window Allows multiple frames to be in transit at the same time. Receiver allocates buffer space for n frames. Transmitter is allowed to send n (window size) frames without receiving ACK. Frame sequence number: labels frames.

47 Sliding Window Receiver ack s frame by including sequence number of next expected frame. Cumulative ACK: ack s multiple frames. Example: if receiver receives frames 2,3, and 4, it sends an ACK with sequence number 5, which h ack s receipt of 2, 3, and 4.

48 Sliding Window Sender maintains sequence numbers it s allowed to send; receiver maintains sequence number it can receive. These lists are sender and receiver windows. Sequence numbers are bounded; if frame reserves k-bit field for sequence numbers, then they can range from 0 2 k -1, modulo 2 k. Transmission window shrinks each time frame is sent, and grows each time an ACK is received.

49 Example: 3-bit sequence number and window size 7 A (sender) (receiver) B RR RR Stallings Figure 7-4

50 A One-Bit Sliding Window Protocol Continued

51 A One-Bit Sliding Window Protocol (ctd.)

52 1-bit Sequence Number Distinguish between original transmission and retransmission Sequence number will do this Extra long sequence field wasteful of header / channel Minimum size = 1 bit! Need to distinguish between frame m and frame m+1 Sender keeps trying to send frame m until it is acknowledged Sender won t send m+1 until m is acknowledged (Sender won t send m+2 until m+1 is acknowledged d which h means m was correctly received and acknowledged ) Receiver knows what sequence number to expect next Arriving frame with wrong sequence number is rejected as a duplicate Correctly received frame with right sequence number sent to network layer and expected sequence number incremented by 1 modulo 2.

53 One-Bit Sliding Window Protocol Two scenarios for protocol 4. (a) Normal case. (b) Abnormal case. The notation ti is (seq, ack, packet number). An asterisk indicates where a network layer accepts a packet. Tannenbaum Figure 3-15

54 Bit Length of a Link Assume link is fully occupied 1 st bit is just arriving at the receiver sender has sent continuously R = data rate (bps) d = distance (length of link) (meters) V = propagation velocity (meters / second) (~ 2-3 x 10 8 meters/second) Bit length = Rd / V bits

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