Chapter Two Fundamentals of Data and Signals Data Communications and Computer Networks: A Business User's Approach Eighth Edition

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved. 2 Introduction Data are entities that convey meaning (computer files, music on CD, results from a blood gas analysis machine) Signals are the electric or electromagnetic encoding of data (telephone conversation, web page download) Computer networks and data/voice communication systems transmit signals Data and signals can be analog or digital

Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved. 3 Introduction (continued) Table 2-1 Four combinations of data and signals

4 Data and Signals Data are entities that convey meaning within a computer or computer system Signals are the electric or electromagnetic impulses used to encode and transmit data Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

5 Analog vs. Digital Data and signals can be either analog or digital Analog is a continuous waveform, with examples such as (naturally occurring) music and voice It is harder to separate noise from an analog signal than it is to separate noise from a digital signal (see the following two slides) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

6 Analog vs. Digital (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

7 Analog vs. Digital (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

8 Analog vs. Digital (continued) Digital is a discrete or non-continuous waveform Something about the signal makes it obvious that the signal can only appear in a fixed number of forms (see next slide) Noise in digital signal –You can still discern a high voltage from a low voltage –Too much noise – you cannot discern a high voltage from a low voltage Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

9 Analog vs. Digital (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

10 Analog vs. Digital (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

11 Analog vs. Digital (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

12 Fundamentals of Signals All signals have three components: –Amplitude –Frequency –Phase Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

13 Fundamentals of Signals – Amplitude Amplitude –The height of the wave above or below a given reference point –Amplitude is usually measured in volts Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

14 Fundamentals of Signals – Amplitude Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

15 Fundamentals of Signals – Frequency Frequency –The number of times a signal makes a complete cycle within a given time frame; frequency is measured in Hertz (Hz), or cycles per second (period = 1 / frequency) –Spectrum – Range of frequencies that a signal spans from minimum to maximum –Bandwidth – Absolute value of the difference between the lowest and highest frequencies of a signal –For example, consider an average voice The average voice has a frequency range of roughly 300 Hz to 3100 Hz The spectrum would be 300 – 3100 Hz The bandwidth would be 2800 Hz Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

16 Fundamentals of Signals – Frequency Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

17 Fundamentals of Signals – Phase Phase –The position of the waveform relative to a given moment of time or relative to time zero –A change in phase can be any number of angles between 0 and 360 degrees –Phase changes often occur on common angles, such as 45, 90, 135, etc. Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

18 Fundamentals of Signals – Phase Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

19 Fundamentals of Signals Phase –If a signal can experience two different phase angles, then 1 bit can be transmitted with each signal change (each baud) –If a signal can experience four different phase angles, then 2 bits can be transmitted with each signal change (each baud) –Note: number of bits transmitted with each signal change = log 2 (number of different phase angles) –(You can replace “phase angles” with “amplitude levels” or “frequency levels”) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

20 Data Codes The set of all textual characters or symbols and their corresponding binary patterns is called a data code There are three common data code sets: –EBCDIC –ASCII –Unicode Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

21 EBCDIC Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

22 ASCII Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

23 Unicode Each character is 16 bits A large number of languages / character sets For example: –T equals –r equals –a equals Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

24 Data and Signal Conversions In Action: Two Examples Let us transmit the message “Sam, what time is the meeting with accounting? Hannah.” This message leaves Hannah’s workstation and travels across a local area network Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

25 Data and Signal Conversions In Action: Two Examples (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

26 Data and Signal Conversions In Action: Two Examples (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

27 Data and Signal Conversions In Action: Two Examples (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

Chapter 6 Error Control Data Communications and Computer Networks: A Business User's Approach, Seventh Edition28

29 Error Control Once an error is detected, what is the receiver going to do? –Do nothing (simply toss the frame or packet) –Return an error message to the transmitter –Fix the error with no further help from the transmitter Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

30 Do Nothing (Toss the Frame/Packet) Seems like a strange way to control errors but some lower-layer protocols such as frame relay perform this type of error control For example, if frame relay detects an error, it simply tosses the frame –No message is returned Frame relay assumes a higher protocol (such as TCP/IP) will detect the tossed frame and ask for retransmission Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

31 Return A Message Once an error is detected, an error message is returned to the transmitter Two basic forms: –Stop-and-wait error control –Sliding window error control Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

32 Stop-and-Wait Error Control Stop-and-wait is the simplest of the error control protocols A transmitter sends a frame then stops and waits for an acknowledgment –If a positive acknowledgment (ACK) is received, the next frame is sent –If a negative acknowledgment (NAK) is received, the same frame is transmitted again Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

33 Stop-and-Wait Error Control (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

34 Sliding Window Error Control These techniques assume that multiple frames are in transmission at one time A sliding window protocol allows the transmitter to send a number of data packets at one time before receiving any acknowledgments –Depends on window size When a receiver does acknowledge receipt, the returned ACK contains the number of the frame expected next Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

35 Sliding Window Error Control (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

36 Sliding Window Error Control (continued) Older sliding window protocols numbered each frame or packet that was transmitted More modern sliding window protocols number each byte within a frame An example in which the packets are numbered, followed by an example in which the bytes are numbered: Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

37 Sliding Window Error Control (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

38 Sliding Window Error Control (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

39 Sliding Window Error Control (continued) Notice that an ACK is not always sent after each frame is received –It is more efficient to wait for a few received frames before returning an ACK How long should you wait until you return an ACK? Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

40 Sliding Window Error Control (continued) Using TCP/IP, there are some basic rules concerning ACKs: –Rule 1: If a receiver just received data and wants to send its own data, piggyback an ACK along with that data –Rule 2: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms for another packet If while waiting, another packet arrives, send the ACK immediately –Rule 3: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms No packet, send ACK Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

41 Sliding Window Error Control (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

42 Sliding Window Error Control (continued) What happens when a packet is lost? –As shown in the next slide, if a frame is lost, the following frame will be “out of sequence” The receiver will hold the out of sequence bytes in a buffer and request the sender to retransmit the missing frame Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

43 Sliding Window Error Control (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

44 Sliding Window Error Control (continued) What happens when an ACK is lost? –As shown in the next slide, if an ACK is lost, the sender will wait for the ACK to arrive and eventually time out When the time-out occurs, the sender will resend the last frame Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.

45 Sliding Window Error Control (continued) Data Communications and Computer Networks: A Business User's Approach, Eighth Edition © Cengage Learning. All Rights Reserved.