Digital Telecommunications and Networks - Essay Example

Part General Review of Signaling A Briefly explain how digital signals become more noise free than analog signals. Digital signals are relatively immune to noise, as opposed to analog signals. This is because with digital signals, it is not necessary to evaluate the exact amplitude, frequency, or phase to determine if its condition is either logic 1 or logic 0. It only involves evaluating transmitted pulses at precise time intervals, and determining whether the said pulse is above or equal to a set reference level (logic 1), or below it (logic 0). Digital transmission systems make use of signal regeneration as opposed to the signal amplification of analog transmission systems. This makes digital signals more resistant to additive noise. Additive noise is the noise produced by electronic circuits. It is named so because this type of noise accumulates with the addition of electronic components. In the signal amplification of analog systems, noise is amplified with the analog signal. Thus, the signal-to-noise ratio deteriorates for each time an analog signal is amplified. Digital regenerators, on the other hand, reproduce an entirely new digital signal from a sample noisy digital signal. The signal-to-noise ratio of this new digital signal has the same signal-to-noise ratio as the original signal. Thus, digital signals can be transmitted over longer distances than analog signals. B.) Briefly explain how Pulse Code Modulation (PCM) and Time Division Multiplexing (TDM) techniques are incorporated in telephony. Pulse Code Modulation is simply digitally coding analog signals. It consists of sampling analog information signals and then converting them to a serial n-bit binary code for transmission over a physical medium. With PCM, each code has the same number of bits and requires the same length of time for transmission. The presence or absence of a pulse within a specific time slot indicates either a logic condition of 1 or 0 respectively. Wayne Tomasi, in his book Electronic Communications Systems, outlined how a simplex PCM system is integrated in telephony: An analog input signal passes a band pass filter which limits it to the standard voice-band frequency range of 300Hz to 3000Hz. The sample and hold circuit periodically converts samples of the analog input signal to a multilevel Pulse Amplitude Modulation (PAM) signal. The analog-to-digital converter (ADC) converts these PAM signals to parallel PCM codes. These parallel PCM codes are then converted to serial binary data in the parallel-to-signal converter. Finally, the serial binary data are then outputted onto the transmission medium as serial pulses. At the receiver's end of the transmission medium, the serial pulses pass through the serial-to-parallel converter. The digital-to-analog converter then converts parallel PCM codes to parallel PAM signals. Finally, the hold circuit which is a low pass filter, converts parallel PAM signals to the original analog signal (408). Time Division Multiplexing is the transmission of information from multiple sources to one or more destinations using the same facility but at different transmission times. The following are the fundamentals of how Time Division Multiplexing is integrated in telephony as summarized by Wayne Tomasi: In a 2-channel PCM-TDM system, each channel's input is sampled and then converted to an eight-bit PCM code. While the PCM code of channel 1 is transmitted over the transmission medium, channel 2 analog input signals is sampled and converted to a PCM code. And when the PCM code of channel 2 is transmitted, it is channel 1 that then undergoes analog input sampling and conversion (453). C.) Describe the main operating principles of CDMA (Code Division Multiple Access) technology and GSM (Global System Mobile) technology in mobile communication. Code-Division Multiple Accessing is a cellular telephone system based on spread-spectrum technology. It uses a unique code rather than a frequency or time assignment to differentiate users from one another. With CDMA, the base station uses different spreading sequences for each mobile unit to transmit user data from all mobile units in a given cell. Thus, this system increases capacity and frequency reuse is optimized. Two commonly used techniques for spectrum spreading are frequency hopping and direct sequencing. Frequency hopping breaks a message into fixed-size data blocks and transmits these blocks in sequence but on different carrier frequencies. The order of the different carrier frequencies used is then known to both the transmitter and the receiver. Direct sequencing, on the other hand, adds a high-bit-rate pseudorandom code to a low-bit-rate information signal. Since the pseudorandom code is of a much higher bit rate than the information signal, the composite signal then more closely resembles the pseudorandom code than the information signal. Initially, the pseudorandom code must be known to both the receiver and the transmitter. When the receiver detects a composite signal resembling its pseudorandom code, it merely subtracts the code from the composite signal to extract the information signal. Both these spread spectrum techniques allow frequency sharing while minimizing interference. Global System for Mobile Communications is a second-generation telephone system intended to allow a subscriber to use a single telephone set throughout a wide geographical area. Wayne Tomasi defines the three principal subsystems of GSM as the Base Station Subsystems (BSS), Network Switching Subsystems (NSS), and Operational Support Subsystems (OSS) (826). The BSS provides and manages radio-frequency transmission between mobile units and the mobile switching center. Then, the base station controllers connect the mobile switching center to the NSS. The NSS is responsible for the switching functions of the system and it also allows communication with other telephone networks. Finally, the OSS is for the system operation and maintenance of the GSM network. D.) Explain how the following signals are generated in an exchange and telephone system. i) dial tone - A dial tone informs the caller that he has access to the switching machine. When the calling station goes off-hook, the loop closes and a dc current can flow on the loop. When a switching machine detects a dc current, it returns an audible dial tone to the calling station. ii) ringing tone - The ring-back tone informs the caller that the destination telephone number is available and is currently being rung. When the switching machine detects the first dialed number, it removes the dial tone. It then interprets the telephone number and locates the local loop that it corresponds to. Before ringing the destination station, it checks if it is on-hook (available) or off-hook (unavailable). When it indeed is available, the switching machine sends a ringing signal to activate the ringer in the destination telephone and at the same time, sends a ring-back tone to the calling station. iii) called subscriber reply tone - The busy tone informs the caller that the destination telephone number is unavailable. After locating the local loop the dialed number corresponds to, the switching machine checks if the destination is off hook or on hook. When it is off hook, it sends a station busy signal to the calling station. Part 2: General Review of Switching A.) State the 3 main types of switching occurred in a telephone switching system. Briefly explain. Circuit switching involves connection establishment, information transfer and connection termination. Circuit switching uses a dedicated transmission path. Once a call is established, all the circuits and switches utilized in the network to establish the aforesaid call are allotted to a single user only for the duration of the call. The network circuits and switches can be available to another user only when the present call is terminated. Because information transfer between both ends of the call is continuous, it operates in real time. Blocking can occur with circuit switching since there are only a finite number of circuits and switching paths available at a time. Message switching involves data transmission, data storage and data forwarding. This type of switching does not have a dedicated transmission path. Data is transmitted and is stored in network switch. There is only a transfer of data between switches only when it is convenient to do so. Thus, data can be held in a switch at a certain length of time and thus might constitute a delay. Blocking, however, cannot occur with message switching, but the duration of the delay can be as long as 24 hours (Tomasi 965). Message switching supports speed and code conversion. In other words, once information has entered the network it is converted to a format best suited for transmission. Packet switching involves dividing data into packets, transmission of packets, packet holding and packet forwarding. As with message switching, packet switching does not have a dedicated transmission type. Information is divided into smaller segments called packets. These packets are then transmitted and held in network switches. Since the hold time of packets are very short, packet switching operates very near to real time. Packets travel independently from each other. Thus, packet transmission delay is possible. Blocking cannot occur in this type of switching. Packet switching also supports speed and code conversion. B.) List three main differences between public switching and local switching system. A local switching system is permanently associated with a particular subscriber's station, whereas public switching involves a common-usage connection. Local switching monitors the activity at the subscriber's end, whereas public switching involves monitoring the performance of the trunks. Local switching provides switching to subscriber lines connected to it, whereas public switching provides switching to central offices connected to it. C.) Compare and contrast the following: i) 2-wire switching and 4-wire switching In four-wire switching, two wires carry the signals in one direction and another pair of wires carries them in the opposite direction. On the other hand, in two-wire switching only 1 pair of wires facilitate the transmission of a single telephone call in both directions. Four-wire switching allows amplification while two-wire switching does not. If the distance between the subscribers is substantial, the amplifiers are necessary to compensate the attenuation and since amplifiers are unidirectional, four-wire transmission is necessary. For local exchange, two -wire switching is used since the line from subscriber to local office are two-wire in operation. Four-wire circuits are generally less noisy, have less crosstalk, and provide more isolation between the two directions of signal propagation. Alternatively, two-wire circuits require les wire, less circuitry and thus, less costly than four-wire circuits. ii) Single stage switching and multiple stage switching Single stage switching can only be used to interconnect one particular inlet-outlet pair. Multiple stage switching on the other hand provides alternative paths between inlets and outlets. In addition, building exchanges in multiple stages decreases the number of crosspoints. This makes multiple stage switching more efficient than single stage switching because a large number of crosspoints on each inlet and outlet line imply a large amount of capacitive loading on the message paths (Gnanasivam 98). iii) Space division switching and time division switching Both space division switching and time division switching aim to setup and release connection between subscribers. In space division switching, the paths in the circuit are separated from each other spatially while in time division switching they are not. Space division switching does not allow sharing of crosspoints. Once a connection is established, the crosspoint of a multistage space switch assigned to the said connection is dedicated to the connection for its duration. Time division switching, on the other hand, allows sharing of crosspoints for shorter periods of time. It uses time division multiplexing to assign a switching element to many inlet-outlet pairs for few microseconds. Therefore, in time division switching, greater savings in crosspoints can be achieved. Part 3: Demonstration of Frequency-Division Multiplexing A.) Put the correct terms in the misleading parts. B.) Summarize your understanding about FDM. Frequency Division Multiplexing is an analog transmission technique that allows simultaneous transmissions of multiple signals in a broad frequency band over a single transmission medium. With FDM, multiple signals that originally occupy the same frequency spectrum are converted into different but adjacent frequency bands. The following is an outline of how frequency division multiplexing operates in a telephone channel: An information signal passes through a low pass filter to remove any high frequency components it cotains. This information signal then modulates carrier signals separated by 4kHz. It then passes through a linear summer and is transmitted together with other adjacent frequency bands. At the FDM receiver, the broadband signal passes through different band pass filters to avoid interference between each information signal it contains. Band pass filters are used to restrict each signal to the allocated 4kHz band. Thus BPF outputs restricted narrowband composite (information plus carrier) signals. The narrowband composite signal then passes through the demodulator to extract the information signal from the composite signal. Finally the information signal passes through the low pass filter to eliminate high frequency components. C.) Indicate the most important part of the FDM. The most important part of frequency division multiplexing is the modulator. In a modulator, many relatively narrowband sources modulate carrier frequencies with defined separations. This allows the integration of these formed composite signals into the frequency subdivisions of a given broad bandwidth. D.) What types of purposes are served by the Channel in FDM Some applications that use frequency division multiplexing are commercial FM and television broadcasting, high-volume telephone network and data communications systems, and cable television and data distribution networks. Each of these aforesaid applications use any of the following: point to point microwave radios, coaxial cable wire line system and fiber optic transmission. Read More