1. Distinguish between PSTN and PSDN technologies of data transmission.(3 marks) Public Switched Telephone Network (PSTN) The public switched telephone network (PSTN) is the aggregate of the world’s circuit-switched telephone networks that are operated by national, regional, or local telephony operators, providing infrastructure and services for public telecommunication. The PSTN consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all interconnected by switching centers, thus allowing any telephone in the world to communicate with any other.

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Packet-switched data network (PSDN) public switched data network (PSDN) is a publicly available packet-switched network, distinct from the PSTN.A method of network data transmission, in which small blocks of data, or packets, are transmitted over a channel which, for the duration of the packet’s transmission, is dedicated to that packet alone and is not interrupted to transmit other packets. This strategy is used in transmitting data over the Internet and often over a LAN, and it capitalizes on the increase in efficiency that is obtained when there are many paths available and there is a large volume of traffic over these paths.

2. Discuss the switching techniques employed in telecommunications. (9 marks)

Switching Techniques – In large networks there might be multiple paths linking sender and receiver. Information may be switched as it travels through various communication channels. There are four typical switching techniques available for digital traffic.

* Circuit Switching
Circuit switching is a A networking technology that provides a temporary, but dedicated, connection between two stations no matter how many switching devices the data are routed through. Circuit switching was originally developed for the analog-based telephone system in order to guarantee steady, consistent service for two people engaged in a phone conversation. Analog circuit switching (FDM) has given way to digital circuit switching (TDM), and the digital counterpart still maintains the connection until broken (one side hangs up). This means bandwidth is continuously reserved and “silence is transmitted” just the same as digital audio

* Packet Switching
A digital network technology that breaks up a message into smaller chunks (packets)

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for transmission. Unlike circuit switching in traditional telephone networks, which requires the establishment of a dedicated point-to-point connection, each packet in a packet-switched network contains a destination address. Thus, all packets in a single message do not have to travel the same path. As traffic conditions change, they can be dynamically routed via different paths in the network, and they can even arrive out of order. The destination computer reassembles the packets into their proper sequence.

* Message Switching
A computer system used to switch data between various points. Computers have always been ideal switches due to their input/output and compare capabilities. It inputs the data, compares its destination with a set of stored destinations and routes it accordingly. Note: A “message” switch is a generic term for a data routing device, but a “messaging” switch converts mail and messaging protocols.

3. Describe predictive encoding and name one standard defined by it. (2 marks) Predictive coding is a combination of machine-learning technology and work flow processes that use keyword search, filtering and sampling to automate portions of an e-discovery document review. Several standards have been defined such as GSM (13 kbps), G.729 (8 kbps), and G.723.3 (6.4 or 5.3 kbps). Detailed discussions of these techniques are beyond the scope of this book.

4. Describe three common error detection techniques, giving an example of how each technique is used in telecommunication networks. (7 marks) Error detection refers to the techniques used to detect noise or other impairments introduced into data while it is transmitted from source to destination. Error detection ensures reliable delivery of data across vulnerable networks. Three common error detection techniques include:

1. Parity Check

The simplest and oldest error detection method, in communications, parity checking refers to the use of parity bits to check that data has been transmitted accurately. The parity bit is added to every data unit (typically seven or eight bits) that is transmitted. The parity bit for each unit is set so that all bytes have either an odd number or an even number of set bits.

2. Cyclic Redundancy Check CRC

CRC error detection method treats the packet of data to be transmitted as a large polynomial .Transmitter takes the message polynomial and using polynomial arithmetic, divides it by a given generating polynomial .Quotient is discarded but the remainder is “attached” to the end of the message

3. Checksum
Checksums are used to ensure the integrity of data portions for data transmission or storage. A checksum is basically a calculated summary of such a data portion. Network data transmissions often produce errors, such as toggled, missing or duplicated bits. As a result, the data received might not be identical to the data transmitted, which is obviously a bad thing. Because of these transmission errors, network protocols very often use checksums to detect such errors. The transmitter will calculate a checksum of the data and transmits the data together with the checksum. The receiver will calculate the checksum of the received data with the same algorithm as the transmitter. If the received and calculated checksums don’t match a transmission error has occurred.

5. Briefly describe how microwaves are used in data transmission and list two advantages and two disadvantages of this technology. (6 marks) Microwave transmission refers to the technology of transmitting information or energy by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeter; these are called microwaves.

Wires are not needed as they travel through air, thus, a cheaper form of communication. Some are reflected off the ionosphere, so can travel around the Earth. Can carry a message instantaneously over a wide area

Travel in a straight line, so repeater stations may be needed. Need special aerials to receive them. The range of frequencies that can be accessed by existing technology is limited, so there is a lot of competition amongst companies for the use of the frequencies.

6. A medium access control protocol is part of the software that allows a workstation to place data onto a LAN. Depending on the network’s topology, several types of protocols may be applicable. Explain the operation of CSMA/CA and token passing. (3 marks)

CSMA/CA: The Carrier-Sense Multiple Access/Collision Avoidance (CSMA/CA) access method, as the name indicates, has several characteristics in common with CSMA/CD. The difference is in the last of the three components: Instead of detecting data collisions, the CSMA/CA method attempts to avoid them altogether. On a network that uses the CSMA/CA access method, when a computer has data to transmit, its NIC first checks the cable to determine if there is already data on the wire. So far, the process is identical to CSMA/CD. However, if the NIC senses that the cable is not in use, it still does not send its data packet. Instead, it sends a signal of intent–indicating that it is about to transmit data out onto the cable.

Token passing: The token passing access method is a non-contention method that works very differently from the contention methods previously discussed. Token passing is a more orderly way for a network to conduct its business. A signal called a token goes from one computer to the next. In a Token Ring network, the token goes around the ring; in a token bus network, it goes down the line of the bus. If a computer has data to transmit, it must wait until the token reaches it; then that computer can capture the token and transmit data. Token passing is the second most popular access method in use on LANs today, after CSMA/CD

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