DATA COMMUNICATIONSDATA COMMUNICATIONS

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

Four fundamental characteristics:1. Delivery: correct destination2. Accuracy: correct data3. Timeliness: fast enough4. Jitter: uneven delay

1-1 DATA COMMUNICATIONS1-1 DATA COMMUNICATIONS

Figure 1.1 Five components of data communication

Components

Data Representation

TextEmail, articles, etcCoding (Unicode, ASCII)

NumbersDirect conversion

ImagesPixels, resolution, gray scale, RGB, YCM

AudioContinuous, signal conversion

VideoMovie, continuous/discrete

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

Data Flow (Transmission Modes)

NETWORKSNETWORKS

A A networknetwork is a set of devices (often referred to as is a set of devices (often referred to as nodesnodes) ) connected by communication connected by communication linkslinks. .

Figure 1.3 Types of connections: point-to-point and multipoint

Type of Connections (line configurations)

Figure 1.4 Categories of physical topology

Physical topology

Mesh

Figure 1.5 A fully connected mesh topology (five devices)

1

2

3

4

5

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 INTERNETTHE INTERNET

History of the Internet1. ARPA2. ARPANET3. Transmission control Protocol (TCP)4. Internetworking Protocol (IP)

Internet todayInternet service providers (ISPs)

1-4 PROTOCOLS AND STANDARDS1-4 PROTOCOLS AND STANDARDS

Protocols (rules)1. Why do we need protocols?2. Key elements of protocols

a) Syntaxb) Semanticsc) Timing

Standards1. De facto vs. De jure2. Organizations3. 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

Network support layers

User support layers

Figure 2.4 An exchange using the OSI model

LAYERS IN THE OSI MODELLAYERS IN THE OSI MODEL

Figure 2.5 Physical layer

The physical layer is responsible for movements ofindividual 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 SUITETCP/IP PROTOCOL SUITE

1.1. The layers in the The layers in the TCP/IP protocol suiteTCP/IP protocol suite do not do not exactly match those in the OSI model. The original exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four TCP/IP protocol suite was defined as having four layers: layers: host-to-networkhost-to-network, , internetinternet, , transporttransport, and , and applicationapplication. .

2.2. However, when TCP/IP is compared to OSI, we can However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five say that the TCP/IP protocol suite is made of five layers: layers: physicalphysical, , data linkdata link, , networknetwork, , transporttransport, and , and applicationapplication..

3.3. Topics covered:Topics covered:1. Physical and Data Link Layers2. Network Layer3. Transport Layer4. 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 DIGITALANALOG AND DIGITAL

To be transmitted, data must be transformed to electromagnetic signals.To be transmitted, data must be transformed to electromagnetic signals.

Data can be Data can be analoganalog or or digitaldigital. . Analog dataAnalog data refers to information that is refers to information that is continuous; continuous; digital datadigital data refers to information that has discrete states. refers to information that has discrete states. Analog data take on continuous values. Digital data take on discrete values.Analog data take on continuous values. Digital data take on discrete values.

Signals can be Signals can be analoganalog or or digitaldigital. . Analog signalsAnalog signals can have an infinite can have an infinite number of values in a range; number of values in a range; digital signalsdigital signals can have only a limited number can have only a limited number of values.of values.

Analog and Digital DataAnalog and Digital SignalsPeriodic and Nonperiodic Signals

Topics discussed in this section:Topics discussed in this section:

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 patternsPeriodic signals: repeat patternsNonperiodic signals: no patternsNonperiodic signals: no patterns

PERIODIC ANALOG SIGNALSPERIODIC ANALOG SIGNALS

•Periodic analog signals can be classified as Periodic analog signals can be classified as simplesimple or or compositecomposite. . •A A simple periodic analog signalsimple periodic analog signal, a , a sine wavesine wave, cannot be , cannot be decomposed into simpler signals. decomposed into simpler signals. •A A composite periodic analog signalcomposite periodic analog signal is composed of multiple is composed of multiple sine waves.sine waves.

Sine WaveWavelengthTime and Frequency DomainComposite SignalsBandwidth

Topics discussed in this section:Topics discussed in this section:

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 periodRecall: 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.

•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.

Sine Waves and Composite Signals

Figure 3.12 The bandwidth of periodic and nonperiodic composite signals

DIGITAL SIGNALSDIGITAL SIGNALS

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

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

Frequency Vs. Bit Rate

Frequency: the number of periods in 1s.

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

Wavelength is the distance an analog signal can travel in one period.

Bit length is the distance one bit occupies on the transmission medium.

Wavelength Vs. Bit length

Figure 3.17 The time and frequency domains of periodic and nonperiodic digital signals

A digital signal is a composite analog signal with an infinite bandwidth.

Figure 3.18 Baseband transmission

Baseband transmission: sending a digital signal over a channel without changing the digital signal to an analog signal.

Figure 3.19 Bandwidths of two low-pass channels

Figure 3.20 Baseband transmission using a dedicated medium

Baseband transmission of a digital signal that preserves the shape of the digital signal is possible

only if we have a low-pass channel with an infinite or very wide bandwidth.

Figure 3.21 Rough approximation of a digital signal using the first harmonic for worst case

Figure 3.22 Simulating a digital signal with first three harmonics

Table 3.2 Bandwidth requirements

Figure 3.23 Bandwidth of a bandpass channel

Broadband transmission: changing the digital signal to an analog signal for transmission.

Bandpass channel: a channel with a bandwidth that does not start from zero.

TRANSMISSION IMPAIRMENTTRANSMISSION IMPAIRMENT

Signals travel through transmission media, which are not Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of What is sent is not what is received. Three causes of impairment are impairment are attenuationattenuation, , distortiondistortion, and , and noisenoise..

Figure 3.26 Attenuation

Decibel: measures the relative strengths of two signals or one signal at two different points.

It’s negative if a signal is attenuated and positive if a signal is amplified.

1

210log10P

PdB

Sometimes the decibel is used to measure signal power in milliwatts. In this case, it is referred to as dBm and is calculated as dBm = 10 log10 Pm , where Pm is the power in milliwatts.

Example 3.29

Figure 3.28 Distortion

Figure 3.29 Noise

SNR = average signal power/average noise power•Thermal•Induced•Crosstalk•Impulse

SNRSNRdB 10log10

The values of SNR and SNRdB for a noiseless channel are

Example 3.32

We can never achieve this ratio in real life; it is an ideal.

Figure 3.30 Two cases of SNR: a high SNR and a low SNR

DATA RATE LIMITSDATA RATE LIMITS

A very important consideration in data communications A very important consideration in data communications is how fast we can send data, in bits per second, over a is how fast we can send data, in bits per second, over a channel. Data rate depends on three factors:channel. Data rate depends on three factors: 1.1. The bandwidth available The bandwidth available 2.2. The level of the signals we use The level of the signals we use 33. The quality of the channel (the level of noise). The quality of the channel (the level of noise)

Noiseless Channel: Nyquist Bit RateNoisy Channel: Shannon CapacityUsing Both Limits

Topics discussed in this section:Topics discussed in this section:

Increasing the levels of a signal may reduce the reliability of the system.

Nyquist bit rate = 2 x bandwidth x log2L

In reality we can not have a noiseless channel.

Shannon capacity = bandwidth x log2(1+SNR)

For practical purposes, when the SNR is very high, we can assume that SNR + 1 is almost the same as SNR. In these cases, the theoretical channel capacity can be simplified to

Example 3.40

For example, we can calculate the theoretical capacity of the previous example as

The Shannon capacity gives us the upper limit; the Nyquist formula tells us

how many signal levels we need.

PERFORMANCEPERFORMANCE

BandwidthThroughputLatency (Delay)Bandwidth-Delay Product

In networking, we use the term bandwidth in two contexts.

❏ The first, bandwidth in hertz, refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass.

❏ The second, bandwidth in bits per second, refers to the speed of bit transmission in a channel or link.

Latency (Delay):•Propagation time•Transmission time•Queuing time•Processing delay