1. Chapter 8 Data and Network Communication Technology

2. Chapter 8 Data and Network Communication Technology
Chapter Outline
Communication Protocols
Encoding and Transmitting Bits
Transmission Media
Channel Organization
Focus – Serial and Parallel Storage Connections
Focus – Infiniband
Clock Synchronization
Error Detection and Correction
Focus – Wireless Network Standards
Focus - Upgrading Storage and Network Capacity (Part I)

3. Chapter Goals Explain communication protocols
Describe signals and the media used to transmit digital signals
Compare and contrast methods of encoding and transmitting data using analog or digital signals
Describe methods for efficiently using communication channels
Describe methods for detecting and correcting data transmission errors

4. Data and Network Communication Technology

5. Communication Protocol
Set of rules and conventions for communication
Message content and format
Bit encoding
Signal transmission
Transmission medium
Channel organization
Includes procedures for coordinating flow of data
Media access
Clock synchronization
Error detection and correction

6. Communication Protocol Components

7. Communication Protocol Components

8. Encoding and Transmitting Bits
Carrier waves
Modulation methods
Data bit encoding
analog signals
digital signals
Signals
Electrical, optical, or radio
Capacity and errors

9. Carrier Waves A sine wave with encoded bits
transports bits from one place to another
Characteristics of sine waves
amplitude
frequency
phase
Importance of waves in communications
Travel through space, wires, and fibers
Can have patterns encoded in them

10. Carrier Waves

11. Modulation Methods Techniques used to encode bits in sine waves
Frequency modulation (FM)
Amplitude modulation (AM)
Phase shift modulation
Multilevel coding

12. Amplitude Modulation (AM)
represents bit values as specific wave amplitudes

13. Frequency Modulation (FM)
represents bit values by varying carrier wave frequency while holding amplitude constant

14. Phase Shift Modulation
represents bit values by making a sudden shift in signal phase which can be detected and interpreted as data

15. embeds multiple bit values within a single wave characteristic
Multilevel coding
embeds multiple bit values
within a single wave characteristic

16. Analog Signals
Use full range of carrier wave characteristics to encode continuous data values
Can represent any data value within a continuum of values

17. Digital Signals Can contain one of a finite number of possible values
Pulse code modulation (PCM)
Binary data transmission via square waves
Square waves preferred over short distances

18. Pulse Code Modulation (PCM)

19. Digital Threshold
A digital signaling scheme defines a range of wave characteristic values to represent each bit value.

20. Signal Capacity and Errors
Digital signals make the opposite trade-off. They’re highly reliable because they’re insensitive to small changes in wave parameter values during signal transmission, but that reliability is achieved at the expense of data carrying capacity.
Digital signaling and analog signaling are often presented as completely different data transmission methods. A more accurate characterization is that they’re two ends of a continuum. Binary and analog signal encoding represent two extreme ends of that continuum. One end trades most transmission capacity for maximal accuracy, while the other trades absolute accuracy for maximal transmission capacity. Multi-level coding schemes represent points within the continuum extremes.

21. Signal Capacity and Errors
Analog signals compared with digital signals
Carry more information
Are more susceptible to transmission error

22. Transmission Media Communication path that transports signals
copper wire
optical fiber
Characteristics
Raw data transfer rate
speed
capacity
Bandwidth
Susceptibility
noise
distortion
external interference
and attenuation

23. Communication Channel
A communication channel consists of a sending device, receiving device, and the transmission medium that connects them.

24. Speed and Capacity Interdependent
jointly described as data transfer rate (raw versus effective data transfer rate)
Factors that account for transmission speed differences among media
Length of media
Ways in which multiple media segments are interconnected
Rate at which bits are encoded in signals and recognized by the receiver

25. Frequency and Bandwidth
Carrier wave frequency
Fundamental measure of data-carrying capacity (i.e., limits capacity)
Bandwidth
Difference between maximum and minimum frequencies of a signal
High-bandwidth channels can carry multiple messages simultaneously

26. The Electro-magnetic Spectrum
The range of electromagnetic frequencies
Subsets of that range are known as “bands”

27. Modem Protocols
Modulator-demodulator (modem) technology sends digital signals over voice-grade telephone channels by encoding them in an analog carrier wave.
Current rates are as high as 56,000 bps.

28. Signal-to-Noise (S/N) Ratio
Measure of the difference between noise power and signal power
Effective data transfer rate can be much lower than raw data transfer rate due to
Electromagnetic interference (EMI)
Attenuation
Distortion
Internal or external noise

29. In this channel, S/N ratio is positive for distances up to 5 kilometers

30. Electrical Cabling Transmits signals through copper wire Two types
Twisted pair
Relatively cheap
Limited in bandwidth, S/N ratio, and transmission speed
Axial (coaxial and twin-axial)
More expensive
Offers higher bandwidth, greater S/N ratio, and lower distortion
Resistant to EMI

31. Optical Cabling
Provides very high bandwidth, little internally generated noise and distortion, immunity to EMI
Requires amplifiers and repeaters for long distances to increase signal strength and remove noise and distortion
Two types
Multimode
Older
Surrounded by relective cladding
Single-mode (up to 10 Gbps)
Core density varies
forces signal to stay centered
Faster and much more expensive

32. Common Cable Connectors

33. Wireless Data Transmission
Transmit data through the atmosphere or space
Uses shortwave radio or infrared light waves
Advantages
Higher bandwidth than hard wire transmission
Avoids wired infrastructure
Simultaneous broadcast transmission
Disadvantages
Susceptibility to external interference
Cost
High demand for unused radio frequencies
Ability to be “overheard”

34. Communication Protocol Components

35. Communication Protocol Components

36. Channel Organization Configuration and organization issues
Number of transmission wires or bandwidth assigned to each channel
Assignment of those wires or frequencies to carry specific signals
Sharing, or lack thereof, of channels among multiple senders and receivers
Three types: simple, half-duplex, full duplex

37. Channel Organization Type Function Example Simple
Uses one optical fiber or copper wire pair to transmit data in one direction only
Broadcast
Radio & TV
Half-duplex
Identical to a simplex channel but sends a control signal to reverse transmission direction
CB Radios
Full duplex
Uses two fibers or wire pairs to support simultaneous transmission in both directions
Telephone

38. Channel Organization Simple

39. Channel Organization Half-duplex

40. Channel Organization Full Duplex

41. Parallel and Serial Transmission
Uses multiple lines to send several bits simultaneously
Unreliable over distances greater than a few meters due to skew and crosstalk
Provides higher channel throughput
Relatively expensive
Uses a single line to send one bit at a time
Reliable over much longer distances
Cheaper to implement; uses fewer wires or wireless channels

42. Parallel Transmission of a Byte

43. Serial Transmission of a Byte

44. Channel Sharing
Uses available capacity by combining traffic of multiple users
For use when no single user or application needs a continuous supply of data transfer data capacity
Techniques
Circuit switching
Packet switching
Frequency division multiplexing (FDM)

45. Channel Sharing Techniques
Circuit switching
Allocates an entire channel to a single user for duration of one data transfer operation
Only used where data transfer delay and available data transfer capacity must be within precise and predictable limits (e.g., telephone service)
Packet switching
Allocates time on the channel by dividing many message streams into smaller units (packets) and intermixing them during transmission
FDM
Divides a broadband channel into several baseband channels (e.g., cable television)

46. Packet Switching, or TDM (Time Division Multiplexing)
Packets are sent to their destination
as channel capacity becomes available.

47. Channel Sharing, or FDM (Frequency Division Multiplexing)
Signals are transmitted within each subchannel at a fixed frequency or narrow frequency range.

48. Technology Focus Infiniband
A data interconnection standard developed by a consortium of digital communications companies.
Uses a switched fabric which interconnects multiple devices with multiple transmission pathways and a mesh of switches
A matrix-like interconnection
Allows any sender and any receiver to connect
Supports many simultaneous connections

49. Communication Protocol Components

50. Communication Protocol Components

51. Communication Protocol Components

52. Clock Synchronization
Ensures that sender/receiver use same time periods and boundaries to encode/decode bit values
Asynchronous transmission
Relies on specific start and stop signals to indicate beginning and end of a message unit
Synchronous transmission
Ensures that sender/receiver clocks are always synchronized by sending continuous data streams

53. Unsynchronized Clocks
If bit time boundaries are misaligned, the receiver is unable to interpret bits correctly because they contain two different signal levels.

54. Synchronous Transmission
Messages are transmitted in fixed-size byte groups called blocks.

55. Asynchronous Transmission

56. Communication Protocol Components

57. Error Detection and Correction
A form of redundant transmission
Increasing redundancy increases chances of error detection at the expense of reducing channel throughput
Common error detection methods
Parity checking
Block checking
Cyclical redundancy checking

58. How Methods of Error Detection and Correction Vary
Size and content of redundant transmission
Efficient use of the communication channel
Probability that an error will be detected
Failure is a Type I error
potentially disasterous
Probability that an error-free message will be identified as an error
Failure is a Type II error
wasteful
Complexity of the error detection method

59. Parity Checking Also called VRC Vertical Redundancy Checking
Can be based on even or odd bit counts
Has a high Type I error rate
Reliability issues
Unreliable in channels subject to error bursts affecting many adjacent bits
More reliable in channels with rare errors that are usually confined to widely spaced bits

60. Parity Checking

61. Block Checking Horizontal Redundancy Checking
Also called Longitudinal Redundancy Checking (LRC)
Sending device counts number of 1-valued bits at each bit position within a block
Sender combines parity bits for each position into a Block Check Character (BCC) and appends it to the end of the block
Receiver counts 1-valued bits in each position and derives its own BCC to compare with that transmitted by sender

62. Block Checking Longitudinal Redunancy Checking
An even parity bit is computed for each position of a block of 8 bytes.
The set of parity bits forms a BCC that is appended to the block for error detection.

63. CRC Cyclic Redundancy Checking
Most widely used error detection technique
Produces a BCC usually more than 8 bits long; can be as large as 128 bits
Much lower Type I and Type II error rates than VRC (parity) checking and LRC checking

64. Technology Focus – 802.11 Wireless Network Standards
The first standard defined two RF transmission methods
FHSS Frequency Hopping Spread Spectrum
DSSS Direct Sequence Spread Spectrum
The a standard divides the bands between 5 and 6 GHz into 22 channels and uses OFDM to encode data.
The b standard enhances DSSS by creating more high bandwidth channels
The g standard updates the b standard with narrower bands using OFDM.

65. Business Focus – Upgrading Network and Storage Capacity
Bradley Advertising Agency
The trade-off between short and long-range benefits of copper and fiber optic wiring
Copper is installed in most buildings, works well for current needs, and can be upgraded
Current technology pushes copper to its maximum
Fiber optic cable has far greater theoretical capacity than copper
Current optical products are expensive and not yet perfected
Fiber optic cable is the future
But when is it cost effective for a particular organization or need?

66. Summary Communication protocols
How bits are represented and transported among computer systems and hardware components
Transmission media
Channel organization
Clock synchronization
Detecting and correcting errors in data transmission, reception, or interpretation

67. Chapter Goals Explain communication protocols
Describe signals and the media used to transmit digital signals
Compare and contrast methods of encoding and transmitting data using analog or digital signals
Describe methods for efficiently using communication channels
Describe methods for detecting and correcting data transmission errors