Chapter 1 Basic Networking Chapter 1 Basic Networking:                      Data communication is the transfer of data from one device to another via some form of transmission medium. A data communications system must transmit data to the correct destination in an accurate and timely manner. The five components that make up a data communications system are the message, sender, receiver, medium, and protocol. Text, numbers, images, audio, and video are different forms of information. Data flow between two devices can occur in one of three ways: simplex, halfduplex, or full-duplex. A network is a set of communication devices connected by media links. In a point-to-point connection, two and only two devices are connected by a dedicated link. In a multipoint connection, three or more devices share a link. Topology refers to the physical or logical arrangement of a network. Devices may be arranged in a mesh, star, bus, or ring topology. A network can be categorized as a local area network (LAN), a metropolitan-area network (MAN), or a wide area network (WAN). A LAN is a data communication system within a building, plant, or campus, or between nearby buildings. A MAN is a data communication system covering an area the size of a town or city. A WAN is a data communication system spanning states, countries, or the whole world. An internet is a network of networks. The Internet is a collection of many separate networks. TCP/IP is the protocol suite for the Internet. There are local, regional, national, and international Internet service providers (ISPs). A protocol is a set of rules that governs data communication; the key elements of a protocol are syntax, semantics, and timing. Standards are necessary to ensure that products from different manufacturers can work together as expected. The ISO, ITU-T, ANSI, IEEE, and EIA are some of the organizations involved in standards creation. Forums are special-interest groups that quickly evaluate and standardize new technologies. A Request for Comment (RFC) is an idea or concept that is a precursor to an Internet standard.

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Chapter 1 Basic Networking Network Models         

The five-layer model provides guidelines for the development of universally compatible networking protocols. The physical, data link, and network layers are the network support layers. The application layer is the user support layer. The transport layer links the network support layers and the user support layer. The physical layer coordinates the functions required to transmit a bit stream over a physical medium. The data link layer is responsible for delivering data units from one station to the next without errors. The network layer is responsible for the source-to-destination delivery of a packet across multiple network links. The transport layer is responsible for the process-to-process delivery of the entire message. The application layer enables the users to access the network.

Signals                  Data must be transformed into electromagnetic signals prior to transmission across a network. Data and signals can be either analog or digital. A signal is periodic if it consists of a continuously repeating pattern. Each sine wave can be characterized by its amplitude, frequency, and phase. Frequency and period are inverses of each other. A time-domain graph plots amplitude as a function of time. A frequency-domain graph plots each sine wave’s peak amplitude against its frequency. By using Fourier analysis, any composite signal can be represented as a combination of simple sine waves. The spectrum of a signal consists of the sine waves that make up the signal. The bandwidth of a signal is the range of frequencies the signal occupies. Bandwidth is determined by finding the difference between the highest and lowest frequency components. Bit rate (number of bits per second) and bit interval (duration of 1 bit) are terms used to describe digital signals. A digital signal is a composite signal with an infinite bandwidth. Bit rate and bandwidth are proportional to each other. The Nyquist formula determines the theoretical data rate for a noiseless channel. The Shannon capacity determines the theoretical maximum data rate for a noisy channel. Attenuation, distortion, and noise can impair a signal. Page 4

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Chapter 1 Basic Networking       Attenuation is the loss of a signal’s energy due to the resistance of the medium. The decibel measures the relative strength of two signals or a signal at two different points. Distortion is the alteration of a signal due to the differing propagation speeds of each of the frequencies that make up a signal. Noise is the external energy that corrupts a signal. We can evaluate transmission media by throughput, propagation speed, and propagation time. The wavelength of a frequency is defined as the propagation speed divided by the frequency.

Encoding and Modulation                    Line coding is the process of converting binary data to a digital signal. The number of different values allowed in a signal is the signal level. The number of symbols that represent data is the data level. Bit rate is a function of the pulse rate and data level. Line coding methods must eliminate the dc component and provide a means of synchronization between the sender and the receiver. Line coding methods can be classified as unipolar, polar, or bipolar. NRZ, RZ, Manchester, and differential Manchester encoding are the most popular polar encoding methods. AMI is a popular bipolar encoding method. Block coding can improve the performance of line coding through redundancy and error correction. Block coding involves grouping the bits, substitution, and line coding. 4B/5B, 8B/10B, and 8B/6T are common block coding methods. Analog-to-digital conversion relies on PCM (pulse code modulation). PCM involves sampling, quantizing, and line coding. The Nyquist theorem says that the sampling rate must be at least twice the highest-frequency component in the original signal. Digital transmission can be either parallel or serial in mode. In parallel transmission, a group of bits is sent simultaneously, with each bit on a separate line. In serial transmission, there is only one line and the bits are sent sequentially. Serial transmission can be either synchronous or asynchronous. In asynchronous serial transmission, each byte (group of 8 bits) is framed with a start bit and a stop bit. There may be a variable-length gap between each byte. In synchronous serial transmission, bits are sent in a continuous stream without start and stop bits and without gaps between bytes. Regrouping the bits into meaningful bytes is the responsibility of the receiver.

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Chapter 1 Basic Networking

Analog Transmission  Digital-to-analog modulation can be accomplished using the following: *Amplitude shift keying (ASK)—the amplitude of the carrier signal varies. *Frequency shift keying (FSK)—the frequency of the carrier signal varies. *Phase shift keying (PSK)—the phase of the carrier signal varies. *Quadrature amplitude modulation (QAM)—both the phase and amplitude of the carrier signal vary. QAM enables a higher data transmission rate than other digital-to-analog methods. Baud rate and bit rate are not synonymous. Bit rate is the number of bits transmitted per second. Baud rate is the number of signal units transmitted per second. One signal unit can represent one or more bits. The minimum required bandwidth for ASK and PSK is the baud rate. The minimum required bandwidth (BW) for FSK modulation is BW =f c1 .f c0 + N baud , where f c1 is the frequency representing a 1 bit, f c0 is the frequency representing a 0 bit, and N baud is the baud rate. A regular telephone line uses frequencies between 600 and 3000 Hz for data communication. ASK modulation is especially susceptible to noise. Because it uses two carrier frequencies, FSK modulation requires more bandwidth than ASK and PSK. PSK and QAM modulation have two advantages over ASK: *They are not as susceptible to noise. *Each signal change can represent more than one bit. Trellis coding is a technique that uses redundancy to provide a lower error rate. The 56K modems are asymmetric; they download at a rate of 56 Kbps and upload at 33.6 Kbps. Analog-to-analog modulation can be implemented by using the following: Amplitude modulation (AM) Frequency modulation (FM) Phase modulation (PM) In AM radio, the bandwidth of the modulated signal must be twice the bandwidth of the modulating signal. In FM radio, the bandwidth of the modulated signal must be 10 times the bandwidth of the modulating signal.

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Multiplexing Multiplexing is the simultaneous transmission of multiple signals across a single data link.

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Chapter 1 Basic Networking             Frequency-division multiplexing (FDM) and wave-division multiplexing (WDM) are techniques for analog signals, while time-division multiplexing (TDM) is for digital signals. In FDM, each signal modulates a different carrier frequency. The modulated carriers are combined to form a new signal that is then sent across the link. In FDM, multiplexers modulate and combine signals while demultiplexers decompose and demodulate. In FDM, guard bands keep the modulated signals from overlapping and interfering with one another. Telephone companies use FDM to combine voice channels into successively larger groups for more efficient transmission. Wave-division multiplexing is similar in concept to FDM. The signals being multiplexed, however, are light waves. In TDM, digital signals from n devices are interleaved with one another, forming a frame of data (bits, bytes, or any other data unit). Framing bits allow the TDM multiplexer to synchronize properly. Digital signal (DS) is a hierarchy of TDM signals. T lines (T-1 to T-4) are the implementation of DS services. A T-1 line consists of 24 voice channels. T lines are used in North America. The European standard defines a variation called E lines. Inverse multiplexing splits a data stream from one high-speed line onto multiple lower-speed lines.

Transmission Media          Transmission media lie below the physical layer. A guided medium provides a physical conduit from one device to another. Twisted-pair cable, coaxial cable, and optical fiber are the most popular types of guided media. Twisted-pair cable consists of two insulated copper wires twisted together. Twisting allows each wire to have approximately the same noise environment. Twisted-pair cable is used in telephone lines for voice and data communications. Coaxial cable has the following layers (starting from the center): a metallic rodshaped inner conductor, an insulator covering the rod, a metallic outer conductor (shield), an insulator covering the shield, and a plastic cover. Coaxial cable can carry signals of higher frequency ranges than twisted-pair cable. Coaxial cable is used in cable TV networks and traditional Ethernet LANs. Fiber-optic cables are composed of a glass or plastic inner core surrounded by cladding, all encased in an outside jacket. Page 7

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Chapter 1 Basic Networking              Fiber-optic cables carry data signals in the form of light. The signal is propagated along the inner core by reflection. Fiber-optic transmission is becoming increasingly popular due to its noise resistance, low attenuation, and high-bandwith capabilities. Signal propagation in optical fibers can be multimode (multiple beams from a light source) or single-mode (essentially one beam from a light source). In multimode step-index propagation, the core density is constant and the light beam changes direction suddenly at the interface between the core and the cladding. In multimode graded-index propagation, the core density decreases with distance from the center. This causes a curving of the light beams. Fiber-optic cable is used in backbone networks, cable TV networks, and Fast Ethernet networks. Unguided media (usually air) transport electromagnetic waves without the use of a physical conductor. Wireless data is transmitted through ground propagation, sky propagation, and line-of-sight propagation. Wireless data can be classifed as radio waves, microwaves, or infrared waves. Radio waves are omnidirectional. The radio wave band is under government regulation. Microwaves are unidirectional; propagation is line of sight. Microwaves are used for cellular phone, satellite, and wireless LAN communications. The parabolic dish antenna and the horn antenna are used for transmission and reception of microwaves. Infrared waves are used for short-range communications such as those between a PC and a peripheral device.

Error Detection and Correction Access Method        Errors can be categorized as a single-bit error or a burst error. A single-bit error has one bit error per data unit. A burst error has two or more bit errors per data unit. Redundancy is the concept of sending extra bits for use in error detection. Three common redundancy methods are parity check, cyclic redundancy check (CRC), and checksum. An extra bit (parity bit) is added to the data unit in the parity check. The parity check can detect only an odd number of errors; it cannot detect an even number of errors. In the two-dimensional parity check, a redundant data unit follows n data units. CRC, a powerful redundancy checking technique, appends a sequence of redundant bits derived from binary division to the data unit.

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Chapter 1 Basic Networking      The divisor in the CRC generator is often represented as an algebraic polynomial. Errors are corrected through retransmission and by forward error correction. The Hamming code is an error correction method using redundant bits. The number of bits is a function of the length of the data bits. In the Hamming code, for a data unit of m bits, use the formula 2 r >= m +r +1 to determine r, the number of redundant bits needed. By rearranging the order of bit transmission of the data units, the Hamming code can correct burst errors.

Data Link Controls and Protocols  Flow control is the regulation of the sender’s data rate so that the receiver buffer does not become overwhelmed.  Error control is both error detection and error correction.  In Stop-and-Wait ARQ, the sender sends a frame and waits for an acknowledgment from the receiver before sending the next frame.  In Go-Back-N ARQ, multiple frames can be in transit at the same time. If there is an error, retransmission begins with the last unacknowledged frame even if subsequent frames have arrived correctly. Duplicate frames are discarded.  In Selective Repeat ARQ, multiple frames can be in transit at the same time. If there is an error, only the unacknowledged frame is retransmitted.  Flow control mechanisms with sliding windows have control variables at both sender and receiver sites.  Piggybacking couples an acknowledgment with a data frame.  The bandwidth-delay product is a measure of the number of bits a system can have in transit.  HDLC is a protocol that implements ARQ mechanisms. It supports communication over point-to-point or multipoint links.  HDLC stations communicate in normal response mode (NRM) or asynchronous balanced mode (ABM).  HDLC protocol defines three types of frames: the information frame (I-frame), the supervisory frame (S-frame), and the unnumbered frame (U-frame).  HDLC handle data transparency by adding a 0 whenever there are five consecutive 1s following a 0. This is called bit stuffing.

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