JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year COMPUTER NETWORKS Previous Question Paper (September - 2021 - R15)

Transmission media are the physical paths between transmitters and receivers in a communication system. They are broadly classified into guided and unguided media.

Guided Media use physical conductors to direct electromagnetic signals along a specific path. Examples include:

• Twisted Pair: Consists of two insulated copper wires twisted together to reduce electromagnetic interference. It is widely used in telephone systems and local area networks.
• Coaxial Cable: Features a central conductor surrounded by an insulating layer and a metallic shield. It offers higher bandwidth and better noise immunity than twisted pair, commonly used for cable television and broadband internet.
• Fiber Optic Cable: Uses light pulses to transmit data through glass or plastic strands. It provides extremely high bandwidth, low attenuation, and complete immunity to electromagnetic interference.

Unguided Media transmit electromagnetic waves through free space (air, vacuum, or water) without physical conductors. Examples include:

• Radio Waves: Omnidirectional waves used for AM/FM radio, television, and mobile communications.
• Microwaves: Unidirectional waves requiring line-of-sight propagation, used for satellite communication and point-to-point terrestrial links.
• Infrared: High-frequency waves used for short-range communication, such as remote controls and wireless keyboards.

Automatic Repeat Request (ARQ) is an error-control method that uses acknowledgments and timeouts to achieve reliable data transmission. The basic block diagram consists of a Sender (with a buffer and timer) and a Receiver (with a buffer and error detection unit) connected by a communication channel.

In Selective Repeat ARQ, the sender transmits a window of frames. If a frame is damaged or lost, the receiver does not discard subsequent correctly received frames. Instead, it accepts and buffers the out-of-order frames while sending a Negative Acknowledgment (NACK) specifically for the missing or corrupted frame.

Key features include:

• The sender only retransmits the specific frame that was NACKed or timed out, rather than resending the entire window.
• The receiver uses a buffer to reorder the frames before passing them to the network layer.
• This method is more efficient than Go-Back-N in noisy channels because it avoids unnecessary retransmissions, though it requires more complex logic and buffer space at both ends.