Network Devices

Modem 

Computers store digital data, but telephone lines transmit analog data. Modems convert digital signals to analog for transmission and back to digital for reception—this process is called modulation and demodulation. Modems enable internet access and tasks like email and data transfer, but they have limited bandwidth. Modem speeds are measured in bits per second and are classified as asynchronous or synchronous. Asynchronous modems transmit data without a clock using start bits for timing and are ideal for PC-to-PC communication. Synchronous modems use clocks and synch signals, allowing for the transmission of various data types with error checking via CRC. 

Hubs 

Hubs are central connection points for network cabling, with three types: passive, active, and switching. Passive hubs combine signals without processing them, while active hubs amplify and regenerate signals using electronic components. Switching hubs, the most advanced, quickly route signals between ports and can replace routers. Hubs are essential for extending networks, particularly those not using coaxial cables. Intelligent hubs add capabilities like network management and intelligent path selection in addition to signal amplification. 

Repeaters 

Repeaters are simple, inexpensive devices used to extend network cabling by regenerating signals. They do not filter or interpret data but simply repeat signals from one port to another. There are two types: amplifying repeaters, which boost the entire signal, and signal-regenerating repeaters, which create an exact duplicate of the original signal. By adding repeaters, the network length can be expanded beyond its predefined limit. 

Routers 

Routers are intelligent devices used for internetwork connectivity, connecting two or more independent networks, such as Ethernet and token-ring. They use logical network addresses to efficiently route data packets, employing algorithms to determine the best path for delivery. Routers can divide large LANs into smaller segments or connect LANs to WANs. They support protocols like IP, IPX, and DDP for routing. Routers can be static, requiring manual path specification, or dynamic, automatically determining routes. A router maintains a table of available routes and uses algorithms to select the most efficient one.

A modem is a device with both modulator and demodulator circuits. It converts binary signals into analog signals for transmission over telephone lines and then converts them back to binary at the receiver end, enabling digital communication via telephone lines. 

Steps in digital communication using Modem 

1. The user at computer 'A' sends data in digital form.
2. Computer 'A' converts the digital data into analog form using a modulation technique.
3. The modulated signal, within telephone frequency range, is transmitted over the telephone line using the correct dial code. 
4. Computer 'B' is selected by its dial code.
5. The modem of computer 'B' converts the analog modulated signal back into the original digital data through demodulation.
6. The demodulated digital information is processed and displayed on the screen of terminal 'B'.