KENYA METHODIST UNIVERSITY

BBIT 242
TELECOMMUNICATION NETWORKS

OKWACHI OJIAMBO NICHOLAS
BIT-1-4628-3/2012

1.
PSTN refers to a Public Switched Telephone Network. It carries phone traffic between PBX devices and is based on circuit switching, while PSDN refers to a Public Switched Data Network that carries data traffic between company networks and across the internet1.Public Switched Telephone Network is the normal telephone network, based on circuit switching while Packet Switched Data Network is the electronic telephone network that allows you to send voice and data together.
2.
Circuit-switching; it is a network switching technique in which a physical path is obtained for and dedicated to a single connection between two end-points in the network for the duration of the connection. Full bandwidth is dedicated to the channel during the duration of the communication session. It is employed in ordinary voice phone service. The telephone company reserves a specific physical path to the number you are calling for the duration of your call. During that time, no one else can use the physical lines involved.
Packet Switching; this technique entails packaging data in specially formatted units (packets) that are typically routed from source to destination using network switches and routers. Each packet contains address information that identifies the sending computer and intended recipient. Using these addresses, network switches and routers determine how best to transfer the packet between hops (network switches and routers) on the path to its destination. This technique is an alternative to circuit switching and there is more efficient use of overall network bandwidth due to flexibility in routing the smaller packets over shared links. However there is potential for network security risks due to the use of shared physical links; and this requires protocols and other related elements on packet switching networks to be designed with the appropriate security precautions.
Message Switching (Store and Forward Networks); it was succeeded by the packet switching mode. In this technique if a station wishes to send a message to another station, it first adds the destination address to the message. There is no dedicated path between the sender and receiver. Each message is treated as an independent unit and each complete message is then transmitted from device to device from the source node to intermediate node. The actual path taken by the message to its destination is dynamic as the path is established as it travels along. The intermediate node stores the complete message temporarily, inspects it for errors and transmits the message to the next node based on an available free channel and its routing information; hence called Store and Forward networks. When the message reaches a node, the channel on which it came is released for use by another message.

3.
Predictive Encoding is a system employed in telephone networks. The system operates at the transmitter terminal by removing redundant speech samples during talk bursts as well as during silence intervals. The receiver remembers the most recent sample transmitted through each channel. As long as a new sample is not transmitted, the receiver repeats the sample value stored in its memory. Each sample from each incoming trunk is examined every fraction of a time to determine which samples must be transmitted and which are redundant. Such a system includes the Speech Predictive Encoding Communications (SPEC) systems and the Time-Assignment Speech Interpolation (TASI). The SPEC system operates at high speed and predicts redundant or missing samples at the receiver.
4.
Checksums; are an error-checking technique in which the number of bits in a unit of data is summed, transmitted along with the data, and checked by the receiving computer. If the sum differs, an error probably occurred in transmission and the transmission is repeated.
Cyclic Redundancy Checks;
This is a calculation method used to check the accuracy of a digital transmission over a communication link. The sending computer uses one of several formulas to calculate a value from the information contained in the data, and this value is appended to the message block before it is sent. The receiving computer performs the same calculation on the same data and should drive the same number. If the two CRC’s do not match, indicating that a transmission error has occurred, the receiving computer asks the sending computer to retransmit the data.
The procedure is known as redundancy check because each transmission includes extra or redundant error-checking values as well as the data itself. A CRC is generated by dividing the total number of bits in the block of data being sent by a predetermined binary number. The reminder is then added to the packet and the packet is transmitted. On the receiving end, the reverse mathematical operation is performed to verify the packet contents. If the computation is successful, the packet is passed to the next step. If it fails, the issuing node is notified and the entire packet is re-transmitted.
Cryptographic Hash Functions;
It is a type of security mechanism that produces a hash value, message digest or checksum value for a specific data object. They are implemented in information security to evaluate the integrity of data, authentication control and other security mechanisms. Cryptographic hash functions work by generating the checksum value of a data object. If the data is intentionally or unintentionally modified, the checksum value is changed. Thus, a data object’s integrity may be evaluated by comparing and verifying previous and current checksums.

5. Microwaves are high frequencies of between 1GHz to 30GHz. They are used in both satellite to parabolic reflector communication and point to point communication.
In satellite communication microwave signals are transmitted from a microwave relay station from one part of the world through a parabolic reflector to a satellite in space. The signal is received and retransmitted to another ground microwave relay station and received via another parabolic reflector.
In point-to-point communication signals are transmitted from transmission masts located in high leveled surfaces in order to maintain a line of sight for transmission to be complete from sender to receiver.

Microwaves provide for communication in remote areas, especially where fibre optics cannot reach and hence a huge advantage. They also have high bandwidth capacity meaning they can transmit high amounts of data at one particular time due to their high frequencies.
Use of microwaves in communication is however expensive and cost of communication is passed on to the end user. Microwaves are also prone to interference like weather, and its line-of-sight nature limits its ability to receive and transmit data.
6.
In carrier sense multiple access/collision avoidance: CSMA/CA, the host first checks the network/wire to see if it’s not in use. If it is free a sample data will be sent to detect any possible collisions. If it does not the host will assume that the real data will not collide when sent.
Token passing in a network is employed as both Collision Avoidance and Collision detection do not guarantee complete absence of data collisions. The host holds the token only when it is empty, marks it as being used, fill it with data, and sends it to the network.