Data Transmission Channels:
Data transmission channels refer to the physical medium through which data is sent from a sender to a receiver. They can be broadly categorized as:

Guided Media (Wired): The signal is guided along a solid physical path.
- Twisted-Pair Cable: Consists of two insulated copper wires twisted together. Used in telephone lines and LANs (e.g., Ethernet). It is inexpensive but susceptible to interference.
- Coaxial Cable: Has a central copper conductor surrounded by an insulator, a metallic shield, and an outer plastic sheath. Offers better bandwidth and less interference than twisted-pair. Used for cable TV and early Ethernet.
- Fiber-Optic Cable: Transmits data as pulses of light through thin strands of glass or plastic. Offers extremely high bandwidth, low attenuation, and immunity to electromagnetic interference. Used in long-distance communication and high-speed internet.
Un-guided Media (Wireless): The signal is broadcast through the air or space.
- Radio Waves: Omnidirectional waves used for AM/FM radio, television, and cordless phones.
- Microwaves: Unidirectional waves that require line-of-sight transmission. Used for cellular networks, satellite communication, and Wi-Fi.
- Infrared: Short-range communication that cannot penetrate walls. Used in TV remotes and wireless mice.

Synchronous vs. Asynchronous Transmission:

Feature	Asynchronous Transmission	Synchronous Transmission
Unit of Transfer	One character/byte at a time.	A block or frame of data.
Synchronization	Start and stop bits are added to each byte to synchronize the receiver.	A shared clock signal synchronizes the sender and receiver.
Timing Gap	Gaps between bytes can be irregular and of any duration.	No gaps between bytes within a frame. Timing is precise.
Efficiency	Less efficient due to the overhead of start/stop bits for each byte.	More efficient for large data transfers as overhead is smaller relative to the data block.
Speed	Slower	Faster
Example	Keyboard input, early dial-up modems.	High-speed network communication (e.g., data transfer between computers on a LAN).