DATA COMMUNICATIONS

DATA COMMUNICATIONS • Telecommunication: communication at a distance. • Data: information presented in whatever form is agreed upon by the parties creating and using the data. • Data communications: the exchange of data between two devices via some form of transmission medium such as a wire cable.

1-1 DATA COMMUNICATIONS • Four fundamental characteristics: • Delivery: correct destination • Accuracy: correct data • Timeliness: fast enough • Jitter: uneven delay

Components Figure 1.1 Five components of data communication

Data Representation • Text Email, articles, etc Coding (Unicode, ASCII) • Numbers Direct conversion • Images Pixels, resolution, gray scale, RGB, YCM • Audio Continuous, signal conversion • Video Movie, continuous/discrete

Data Flow (Transmission Modes) Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

NETWORKS A network is a set of devices (often referred to as nodes) connected by communication links.

Type of Connections (line configurations) Figure 1.3 Types of connections: point-to-point and multipoint

Physical topology Figure 1.4 Categories of physical topology

Mesh 1 2 4 5 3 Figure 1.5 A fully connected mesh topology (five devices)

Star Figure 1.6 A star topology connecting four stations

Bus Figure 1.7 A bus topology connecting three stations

Ring Figure 1.8 A ring topology connecting six stations

Network models • Local area network (LAN) • Wide area network (WAN) • Metropolitan area networks

Figure 1.12 A heterogeneous network made of four WANs and two LANs

THE INTERNET • History of the Internet • ARPA • ARPANET • Transmission control Protocol (TCP) • Internetworking Protocol (IP) • Internet today • Internet service providers (ISPs)

1-4 PROTOCOLS AND STANDARDS • Protocols (rules) • Why do we need protocols? • Key elements of protocols • Syntax • Semantics • Timing • Standards • De facto vs. De jure • Organizations • Internet standards (Internet draft & RFC)

Chapter 2 Network Models

Figure 2.2 Seven layers of the OSI model

Figure 2.3 The interaction between layers in the OSI model User support layers Network support layers

Figure 2.4 An exchange using the OSI model

LAYERS IN THE OSI MODEL Figure 2.5 Physical layer The physical layer is responsible for movements of individual bits from one hop (node) to the next.

Figure 2.6 Data link layer The data link layer is responsible for moving frames from one hop (node) to the next.

Figure 2.8 Network layer The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Figure 2.10 Transport layer The transport layer is responsible for the delivery of a message from one process to another.

Figure 2.12 Session layer The session layer is responsible for dialog control and synchronization.

Figure 2.13 Presentation layer The presentation layer is responsible for translation, compression, and encryption.

Figure 2.14 Application layer The application layer is responsible for providing services to the user.

Figure 2.15 Summary of layers

TCP/IP PROTOCOL SUITE • The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. • However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application. • Topics covered: • Physical and Data Link Layers • Network Layer • Transport Layer • Application Layer

Figure 2.16 TCP/IP and OSI model

Figure 2.18 Relationship of layers and addresses in TCP/IP

Chapter 3 Data and Signals

ANALOG AND DIGITAL To be transmitted, data must be transformed to electromagnetic signals. Data can be analog or digital. Analog data refers to information that is continuous; digital data refers to information that has discrete states. Analog data take on continuous values. Digital data take on discrete values. Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values. Topics discussed in this section: Analog and Digital DataAnalog and Digital SignalsPeriodic and Nonperiodic Signals

Figure 3.1 Comparison of analog and digital signals

In data communications, we commonly use periodic analog signals and nonperiodic digital signals. Periodic signals: repeat patterns Nonperiodic signals: no patterns

PERIODIC ANALOG SIGNALS • Periodic analog signals can be classified as simple or composite. • A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. • A composite periodic analog signal is composed of multiple sine waves. Topics discussed in this section: Sine WaveWavelengthTime and Frequency DomainComposite Signals Bandwidth

Figure 3.2 A sine wave Peak amplitude Frequency Phase

Frequency and period are the inverse of each other.

Table 3.1 Units of period and frequency

Frequency is the rate of change with respect to time. Change in a short span of time means high frequency. Change over a long span of time means low frequency. If a signal does not change at all, its frequency is zero. If a signal changes instantaneously, its frequency is infinite.

Figure 3.5 Three sine waves with the same amplitude and frequency, but different phases

Figure 3.6 Wavelength and period Wavelength is the distance a simple signal can travel in one period. Wavelength (w) = signal velocity x period Recall: period = 1 / frequency

Figure 3.7 The time-domain and frequency-domain plots of a sine wave A complete sine wave in the time domain can be represented by one single spike in the frequency domain.

Figure 3.8 The time domain and frequency domain of three sine waves The frequency domain is more compact and useful when we are dealing with more than one sine wave.

Sine Waves and Composite Signals • A single-frequency sine wave is not useful in data communications; we need to send a composite signal. • A composite signal is made of many simple sine waves. • According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases. • If the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies; if the composite signal is nonperiodic, the decomposition gives a combination of sine waves with continuous frequencies.

Figure 3.12 The bandwidth of periodic and nonperiodic composite signals

DIGITAL SIGNALS In addition to being represented by an analog signal, information can also be represented by a digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level.

Frequency Vs. Bit Rate Frequency: the number of periods in 1s. Bit rate: the number of bits sent in 1s, expressed in bits per second (bps).

Figure 3.16 Two digital signals: one with two signal levels and the other with four signal levels

Wavelength Vs. Bit length Wavelength is the distance an analog signal can travel in one period. Bit length is the distance one bit occupies on the transmission medium.

DATA COMMUNICATIONS