Data communication

Data communication is the exchange of data between two devices through some transmission medium. Its effectiveness depends on delivery (to the right destination), accuracy, timeliness and low jitter. This note covers the communication process, speed and modes of transmission, media, modems, network types and topologies, protocols and core networking terms.

Key concepts

Communication process – five components

1. Message – the information (text, numbers, images, audio, video).
2. Sender – device that sends.
3. Receiver – device that receives.
4. Transmission medium – physical path.
5. Protocol – rules that govern the communication.

Data transmission speed

• Bandwidth – range of frequencies a channel carries (Hz) or, informally, its capacity in bits per second (bps, kbps, Mbps, Gbps).
• Baud rate – signal changes per second; bit rate = baud × bits per signal element.
• Channel classes: narrowband (e.g. telegraph, ≤ 300 bps), voice band (telephone, up to ~56 kbps), broadband (high speed – DSL, cable, fibre, Mbps–Gbps).
• Throughput and latency describe actual performance.

Communication types (modes)

Mode	Direction	Example
Simplex	One way only	Radio/TV broadcast, keyboard to computer
Half-duplex	Both ways, one at a time	Walkie-talkie
Full-duplex	Both ways simultaneously	Telephone, mobile call
Also: serial (one bit at a time, long distance – USB) vs parallel (several bits at once, short distance), and asynchronous (start/stop bits per character) vs synchronous (blocks with clock sync).

Data transmission media – guided: twisted pair, coaxial, optical fibre; unguided: radio, microwave, satellite (geostationary at about 35 786 km above the equator), infrared, Bluetooth.

Modem and its working – MOdulator–DEModulator. At the sender it modulates digital signals into analog form for telephone/cable lines; at the receiver it demodulates analog back to digital. Techniques: amplitude (ASK), frequency (FSK), phase (PSK) and QAM. Types: internal/external, dial-up, DSL/ADSL, cable, wireless (3G/4G dongles). Characteristics – data rate (bps), modulation technique, error correction and compression support (V.42, V.42bis), full/half duplex, synchronous/asynchronous, auto-dial/auto-answer, Hayes AT command set.

Types of networks

Type	Coverage	Example
PAN	A few metres around one person	Bluetooth devices
LAN	Room/building	Office, college lab
CAN	Campus (several LANs)	University
MAN	City	Cable TV network, city Wi-Fi
WAN	Country/world	Internet, bank networks
By architecture: peer-to-peer (all equal) vs client–server (central servers). Also intranet (private, internal), extranet (partners), Internet (public).

LAN topologies

Topology	Description	Pros	Cons
Bus	All nodes on one backbone cable with terminators	Cheap, little cable	Cable fault stops all; collisions
Star	All nodes to a central hub/switch	Easy to manage, fault isolation	Central device is single point of failure
Ring	Each node connected to two neighbours in a loop	Orderly (token) access	One break can stop the ring (unless dual ring)
Mesh	Every node connected to every other	Very reliable, redundant	Costly; needs $n(n-1)/2$ links
Tree	Hierarchy of stars	Scalable	Depends on root
Hybrid	Mixture	Flexible	Complex

Computer protocols – TCP/IP (Internet suite), HTTP/HTTPS (web), FTP (files), SMTP/POP3/IMAP (e-mail), Telnet/SSH (remote login), DHCP (automatic IP assignment), DNS (name resolution), Ethernet, Wi-Fi (802.11).

Concepts relating to networking – IP address (logical; IPv4 32-bit, IPv6 128-bit), MAC address (48-bit physical), domain name and DNS, URL, ISP, bandwidth, packet switching vs circuit switching, server, client, node, workstation, protocol stack, firewall, VPN, Wi-Fi hotspot. Nepal's telecom regulator is the Nepal Telecommunications Authority (NTA).

High-yield points

• Five components: message, sender, receiver, medium, protocol.
• Simplex = one way (TV); half-duplex = walkie-talkie; full-duplex = telephone.
• Bit rate = baud × bits per signal element.
• Mesh needs $n(n-1)/2$ links and $n-1$ ports per device.
• Geostationary satellites orbit about 35 786 km above the equator.

Formulas

Mesh links: $L = \dfrac{n(n-1)}{2}$; ports per device $= n - 1$. Transmission time $= \dfrac{\text{data size (bits)}}{\text{data rate (bps)}}$.

Worked example

A fully connected mesh of 8 computers – how many cables and ports per computer?

• Links $= 8 \times 7 / 2 = 28$ cables; each computer needs $7$ ports. How long to send a 5 MB file over a 10 Mbps link (ignore overhead)? $5 \times 8 = 40$ Mb → $40/10 = 4$ s.

Exam tips

• Walkie-talkie is half-duplex; telephone is full-duplex; TV broadcast is simplex.
• Modem converts digital ↔ analog; it does not route packets.
• Star topology failure of the hub stops the network; a single node failure does not.
• Mesh gives maximum reliability but highest cost.
• LAN < MAN < WAN in geographic coverage; the Internet is the largest WAN.

Exam-style questions

From Calmon's question bank, which has 25 questions on this topic.

1. In data communication, the set of rules that governs how data is transmitted and received between devices is called a:

• A. Message
• B. Protocol
• C. Medium
• D. Signal

Answer: B. A protocol defines what is communicated, how and when (syntax, semantics and timing); the other elements are sender, receiver, message and medium.

2. A telephone conversation, in which both parties can talk and listen at the same time, is an example of:

• A. Full-duplex mode
• B. Broadcast mode
• C. Half-duplex mode
• D. Simplex mode

Answer: A. Full-duplex communication allows simultaneous transmission in both directions.

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