Data Communication

2.1 Core concepts

• Data Communication defined: The term comprises "Data" (text, image, audio, video, multimedia files) and "Communication" (act of sending or receiving data). Data communication refers to the exchange of data between two or more networked or connected devices capable of sending and receiving data over a communication medium. (NCERT §11.1, p. 203)
• Five components of data communication: Every data communication system has exactly five components — Sender, Receiver, Message, Communication medium (also called transmission media), and Protocol. (NCERT §11.2, p. 204)
• Sender: A computer or device capable of sending data — e.g., computer, mobile phone, smartwatch, walkie-talkie, video recording device. (NCERT §11.2, p. 204)
• Receiver: A computer or device capable of receiving data — e.g., computer, printer, laptop, mobile phone, television. Sender and receiver are called nodes in a network. (NCERT §11.2, p. 204)
• Message: The data or information to be exchanged — text, number, image, audio, video, multimedia. (NCERT §11.2, p. 205)
• Communication medium: The path through which the message travels between source and destination; also called medium or link; can be wired or wireless (e.g., television cable, telephone cable, ethernet cable, satellite link, microwaves). (NCERT §11.2, p. 205)
• Protocol: A set of rules that communicating parties must follow for successful and reliable communication. Examples: Ethernet, HTTP. (NCERT §11.2, p. 205)
• Bandwidth (§11.3.1): The capacity of a channel is the range of frequencies available for data transmission. Higher bandwidth means higher data transfer rate. Bandwidth = maximum frequency − minimum frequency in composite signals. Unit: Hertz (Hz). 1 KHz = 1000 Hz; 1 MHz = 1000 KHz = 1,000,000 Hz. (NCERT §11.3.1, p. 205)
• Data Transfer Rate (§11.3.2): Number of bits transmitted between source and destination in one second; also called bit rate. Measured in bits per second (bps). Higher units: 1 Kbps = 2^10 bps = 1024 bps; 1 Mbps = 2^20 bps = 1024 Kbps; 1 Gbps = 2^30 bps = 1024 Mbps; 1 Tbps = 2^40 bps = 1024 Gbps. Note: MBps = Megabyte per second; Mbps = Megabit per second. (NCERT §11.3.2, p. 205)
• Types of Data Communication (§11.4): Data communication happens in the form of signals between two or more nodes over point-to-point or multipoint channels. Three broad types: Simplex, Half-duplex, Full-duplex. (NCERT §11.4, p. 206)
• Simplex: One-way/unidirectional. One device is always sender, other is always receiver. Full link capacity used in one direction. Examples: keyboard input, audio sent to speaker, IoT home appliance control. (NCERT §11.4.1, p. 206)
• Half-duplex: Two-way/bidirectional but NOT simultaneous. Both devices can send and receive, but not at the same time. The direction of transmission can be switched. Example: walkie-talkie with push-to-talk button. (NCERT §11.4.2, p. 206–207)
• Full-duplex: Two-way/bidirectional AND simultaneous. Both devices send and receive at the same time. Examples: mobile phones, landline telephones. Achieved via two simplex lines or shared channel capacity. (NCERT §11.4.3, p. 207)
• Switching Techniques (§11.5): An alternative to dedicated connections; data is routed through various nodes, forming a temporary route. Two main techniques: (NCERT §11.5, p. 208)
• Circuit Switching: A dedicated path is identified before communication starts. This path is a connected sequence of links between network nodes. All packets follow the same path. Example: traditional telephone call. (NCERT §11.5.1, p. 208)
• Packet Switching: Messages broken into smaller pieces called packets, each with a header (destination address) and message part. Packets transmitted independently; different packets may take different routes. Reassembled at destination. Channel occupied only during packet transmission — efficient for shared networks. Example: Internet data transmission. (NCERT §11.5.2, p. 208)
• Transmission Media (§11.6): A transmission medium carries signals/data between source (transmitter) and destination (receiver). In data communication, transmission media are the links that carry messages between communicating devices. Classified as: (NCERT §11.6, p. 209)
• Guided (wired): Physical link made of wire/cable — signals propagated between nodes. Examples: twisted pair cable, coaxial cable, fiber-optic cable. Also called wired media. (NCERT §11.6, p. 209)
• Unguided (wireless): Data travels in air as electromagnetic waves using an antenna. Also called wireless media. (NCERT §11.6, p. 209)
• Wired Transmission Media (§11.6.1): Three commonly used types: (NCERT §11.6.1, p. 210)
• Twisted Pair Cable: Two copper wires twisted like a DNA helical structure, insulated with plastic. Twisted configuration minimises electrical interference. Most commonly used in telephone lines and LANs. Two types: UTP (Unshielded Twisted Pair) and STP (Shielded Twisted Pair). (NCERT §11.6.1(A), p. 210–211)
• Coaxial Cable: Copper wire core surrounded by insulating material, further surrounded by an outer conductor (copper mesh), all wrapped in plastic cover. Better shielded and higher bandwidth than twisted pair. Used for higher frequency signals over longer distances. (NCERT §11.6.1(B), p. 211)
• Optical Fibre: Carries data as light signals. Thin transparent glass strand at centre covered with less dense glass cladding, outer jacket of PVC or Teflon. Uses refraction to direct light. Light-weight, high bandwidth, high data transfer rate, immune to electromagnetic noise, used in backbone networks. Drawback: expensive and unidirectional — two cables needed for full duplex. (NCERT §11.6.1(C), p. 212)
• Wireless Transmission Media (§11.6.2): Electromagnetic spectrum range 3 KHz to 900 THz. Divided into 4 categories — Radio Waves, Microwaves, Infrared Waves, and Visible/Light Waves. Three useful for wireless communication: (NCERT §11.6.2, p. 212–213)
• Radio Waves: 3 KHz–1 GHz; omni-directional; susceptible to interference; 3–300 KHz can penetrate walls; used in AM/FM radio, television, cordless phones.
• Microwaves: 1 GHz–300 GHz; unidirectional; require line-of-sight; cannot penetrate walls; used in radar and satellite; very high information-carrying capacity.
• Infrared Waves: 300 GHz–400 THz; cannot penetrate walls; used for short-distance point-to-point communication (e.g., TV remote, mobile-to-printer, Bluetooth devices). (NCERT Table 11.1, p. 213)
• Wireless Technologies (§11.6.3): (NCERT §11.6.3, p. 213–215)
• Bluetooth: Short-range wireless; uses unlicensed 2.4 GHz frequency band; range 10 metres; speed 1–2 Mbps. Devices within 10 m form a piconet in master-slave configuration; master can communicate with up to 7 active slaves; supports up to 255 devices in the network (8 active at a time). (NCERT §11.6.3(A), p. 213–214)
• Wireless LAN (Wi-Fi): IEEE standard 802.11; uses Access Points (APs) connected to a wired network; devices connect to the Internet through an AP. Benefits: extends/replaces wired infrastructure, increases mobile access, easy internet access in public places. (NCERT §11.6.3(B), p. 214–215)
• Mobile Telecommunication Technologies (§11.7): Architecture of mobile networks evolved through generations: (NCERT §11.7, p. 215–216)
• 1G (~1982): Analog signals; voice calls only.
• 2G (~1991): Digital signals; improved call quality; encryption; SMS and MMS introduced.
• 3G (~2001 commercially): Digital voice + data; Internet access via radio towers; faster data transfer.
• 4G: Much faster than 3G; supports interactive multimedia, voice, video, wireless internet; very different technology from 3G.
• 5G (under development at time of writing): Expected to support M2M (Machine to Machine) communications; data transfer in Gbps; supports IoT and connected vehicles. (NCERT §11.7, p. 215–216)
• Protocol (§11.8): A set of standard rules that sender, receiver, and all intermediate devices must follow. Needed for flow control (speed mismatch between sender/receiver), access control (which node accesses shared channel), addressing, data format conversion, packet ordering, and ensuring data integrity. (NCERT §11.8, p. 216)
• Key Protocols (§11.8.2–11.8.6):
• HTTP: HyperText Transfer Protocol; primary protocol to access the World Wide Web; developed by Tim Berners-Lee at CERN in 1989 with IETF and W3C; request-response (client-server) protocol running over TCP; formats and transmits hypertext between browser and web server. (NCERT §11.8.2, p. 217)
• FTP: File Transfer Protocol; used for transferring files from one machine to another; client-server model; handles differences in file naming conventions, data representation formats, and directory structures. (NCERT §11.8.3, p. 218)
• PPP: Point-to-Point Protocol; establishes dedicated direct connection between two communicating devices; defines authentication and direct link establishment. Example: two routers with direct connection; home computers connecting to ISP through a modem. Requires duplex mode; maintains data integrity. (NCERT §11.8.4, p. 218)
• SMTP: Simple Mail Transfer Protocol; used for email services; uses message header (like envelope) containing email addresses of recipients; sender program takes mails from outgoing queue and transmits to destination(s); receiver program accepts mail and places in appropriate user mailbox. (NCERT §11.8.5, p. 219)
• TCP/IP: Transmission Control Protocol/Internet Protocol; set of standardised rules using client-server model. IP assigns unique IP address to every node on the Internet. TCP breaks data into IP packets, routes them through the network, and guarantees delivery and sequential ordering at the destination. (NCERT §11.8.6, p. 219–220)

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 11.1 (p. 203): Simple network of computing devices — shows central server, switch, sharing printer connected to switch, and computers forming a network. Illustrates the concept of communication media connecting devices.
• Figure 11.2 (p. 204): Components of data communication — shows Sender → Transmission Media → Receiver with message depicted above. The five-component model is a frequent exam question.
• Figure 11.3 (p. 206): Simplex communication — dish antenna transmitting one direction to a television set; illustrates unidirectional flow.
• Figure 11.4 (p. 207): Half-duplex communication — two walkie-talkies; at any moment only one direction arrow is active; the other is crossed.
• Figure 11.5 (p. 207): Full-duplex communication — arrows in both directions simultaneously between two computers A and B.
• Figure 11.7 (p. 210): Classification of communication media tree — Communication Media splits into Wired Media (Twisted Pair, Co-axial, Fiber-Optic) and Wireless Media / Electromagnetic Waves (Radio Waves, Micro Waves, Infrared Waves). This tree diagram is a very common MCQ target.
• Figure 11.9 (p. 211): UTP vs STP cables — UTP has only a plastic cover; STP has an additional metal shield inside the plastic cover.
• Figure 11.10 (p. 211): Coaxial cable cross-section — copper core → insulating material → braided outer conductor → protective plastic covering.
• Figure 11.11 (p. 212): Fiber optic cable — sender and receiver at ends; light travels by refraction inside the glass fibre; cladding of less dense material surrounds the glass core.
• Figure 11.12 (p. 213): Electromagnetic waves spectrum (3 KHz to 900 THz) — Radio Waves | Microwaves | Infrared | Light Waves in ascending frequency order.
• Figure 11.13 (p. 214): Access point creating a wireless LAN — Access Point connected to wired network; Tablet, Laptop, Smartphone, Desktop connecting wirelessly.
• Figure 11.14 (p. 217): Speed mismatch between computers A (1024 Mbps) and B (512 Mbps) resulting in packet loss — illustrates why flow control protocol is needed.
• Table 11.1 (p. 213): Classification of transmission waves — Radio Waves (3 KHz–1 GHz, omni-directional), Microwaves (1–300 GHz, unidirectional, line-of-sight), Infrared (300 GHz–400 THz, short-distance) — memorise frequency ranges and key properties for each.

2.4 Common confusions / NTA trap points

• Half-duplex vs Full-duplex: Students often confuse half-duplex (bidirectional but NOT simultaneous — walkie-talkie) with full-duplex (bidirectional AND simultaneous — phone call). NTA distractors will swap the examples (e.g., list a mobile phone as half-duplex).
• Mbps vs MBps: The NCERT explicitly notes that MBps = Megabyte per second while Mbps = Megabit per second (p. 205). NTA options often mix upper/lower case to create a trap.
• Bandwidth unit vs Data Transfer Rate unit: Bandwidth is measured in Hz (frequency range); data transfer rate is measured in bps. Students conflate the two. NTA may ask "what is the unit of bandwidth?" and offer bps as a distractor.
• Bluetooth piconet numbers: The NCERT states up to 255 devices in the network, 8 can communicate at the same time (1 master + 7 active slaves). NTA distractors often swap 7 and 8, or state that the master communicates with 8 slaves.
• Circuit switching vs Packet switching — channel occupancy: In circuit switching, the channel is dedicated for the entire duration of the connection. In packet switching, the channel is occupied only during packet transmission and released afterwards — this distinction is a classic NTA trap.
• Optical fibre is unidirectional (NCERT §11.6.1(C), p. 212). Two cables needed for full-duplex.
• HTTP is the WWW protocol; SMTP is for email (NCERT §11.8.2, §11.8.5). Don't confuse the two.
• Bluetooth uses 2.4 GHz unlicensed (NCERT §11.6.3(A), p. 213). Not 5 GHz, not licensed.
• Microwave needs line-of-sight (NCERT Table 11.1, p. 213). Cannot bend; satellite/radar use it.
• TCP ensures sequencing, IP routes (NCERT §11.8.6, p. 219). Distinct roles inside TCP/IP suite.
• Packet switching is more efficient for shared networks (NCERT §11.5.2, p. 208). Channel free between packets.
• Twisted pair: UTP vs STP (NCERT §11.6.1(A), p. 211). STP adds a metal shield.