Data Communication

Every message that crosses a network — a web page, an e-mail, a video call — needs a sender, a receiver, something to travel through and rules both ends agree on. This chapter is about those pieces: how much a channel can carry, which directions data may flow in, how a network sets up the path a message takes, the wired and wireless media that carry signals, and the protocols that govern particular jobs such as fetching a web page, transferring a file or sending mail. The paper asks mostly for matches — a medium with its property, a protocol with its job — so learn each one with what makes it different.

Components and Measures of Data Communication

Data communication is the exchange of data between two or more networked devices. It has five components: the sender, which creates and sends the message; the receiver, which accepts it; the message itself — text, numbers, images, sound or video; the communication medium, the path it travels along, wired or wireless; and the protocol, the set of rules both sides follow so that the message is understood.

  • Bandwidth is the range of frequencies a channel can carry, measured in hertz (Hz) — kHz, MHz, GHz. A wider band can carry more data.
  • Data transfer rate is the number of bits sent per second, measured in bits per second (bps) — Kbps, Mbps, Gbps, Tbps.
Modes of data flow
ModeDirectionExample
Simplexone way onlykeyboard to computer; a radio broadcast
Half-duplexboth ways, but only one way at a timewalkie-talkie
Full-duplexboth ways at the same timea telephone call

Switching Techniques

In a large network a message passes through many intermediate devices, and switching decides how its path is set up. In circuit switching, a dedicated path between sender and receiver is established before any data is sent, reserved for the whole conversation and released at the end — the traditional telephone call. Its resources sit idle during silences, but once the circuit is up, data arrives in order with no waiting. In packet switching, the message is broken into small packets, each carrying the destination address and a sequence number; packets travel independently, possibly by different routes, sharing the links with other traffic, and are reassembled in order at the destination. No path is reserved, so links are used efficiently; the Internet works this way.

Transmission Media

Guided (wired) media carry signals along a physical path. Twisted-pair cable has pairs of insulated copper wires twisted together, the twisting reducing interference from outside and between pairs; it is cheap and used for telephone lines and Ethernet LANs with RJ45 connectors, in unshielded (UTP) and shielded (STP) forms. Coaxial cable has a copper core, insulation, a braided metal shield and an outer cover; the shield gives it better protection from interference and more bandwidth than twisted pair, and it is used for cable TV. Optical fibre carries data as light through a glass or plastic core surrounded by cladding, the light staying inside by total internal reflection; it offers very high bandwidth over long distances and is immune to electromagnetic interference, but it is costly and harder to install and join, so it forms network backbones.

Unguided (wireless) media
MediumApproximate bandBehaviourUsed for
Radio waves3 kHz – 1 GHzomnidirectional; travel long distances and pass through wallsAM and FM radio, cordless phones
Microwaves1 GHz – 300 GHzunidirectional, line of sight; cannot pass through solid obstacles, so towers are placed highsatellite links, mobile networks, long-distance links between towers
Infrared300 GHz – 400 THzvery short range, line of sight; blocked by walls, so it stays inside a roomTV remote controls, short links between nearby devices

Mobile telecommunication has moved through generations. 1G carried analog voice; 2G made voice digital and added SMS; 3G brought mobile Internet and video calls; 4G brought high-speed mobile broadband; and 5G adds much higher speeds and very low delay, suited to connecting large numbers of devices.

Protocols

A protocol is a set of rules governing how data is formatted, sent and received, so that devices built by different makers can communicate. The book describes five:

The protocols and their jobs
ProtocolJob
HTTP — HyperText Transfer Protocoltransfers web pages: the browser (client) sends a request and the web server sends a response
FTP — File Transfer Protocoltransfers files between a client and a server — uploading and downloading
PPP — Point-to-Point Protocolcarries data over a direct link between two nodes, such as a computer dialling in to its Internet service provider
SMTP — Simple Mail Transfer Protocolsends e-mail from the sender's client to the mail server and between mail servers (reading mail from a mailbox uses other protocols, such as POP3)
TCP/IP — Transmission Control Protocol / Internet Protocolthe Internet's core suite: TCP breaks data into packets, checks that all arrive and puts them back in order; IP addresses each packet and routes it to its destination
⚠️ TCP and IP do different halves
Reliability and order belong to TCP; addressing and routing belong to IP. A question that credits IP with re-sending lost packets, or TCP with choosing the route, has swapped them.

Key takeaways

  • Data communication needs a sender, a receiver, a message, a medium and a protocol.
  • Bandwidth is measured in hertz; data transfer rate in bits per second.
  • Simplex is one way (keyboard), half-duplex one way at a time (walkie-talkie), full-duplex both ways at once (telephone).
  • Circuit switching reserves a path before sending; packet switching sends independent packets that may take different routes.
  • Twisted pair is cheap, coaxial better shielded, optical fibre uses light by total internal reflection and ignores electromagnetic interference.
  • Radio waves are omnidirectional and pass walls; microwaves and infrared need line of sight, infrared only over a short range.
  • HTTP for web pages, FTP for files, PPP for a direct two-node link, SMTP for sending mail, TCP/IP for reliable addressed delivery.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. On a walkie-talkie, both people can talk, but only one can speak at a time. Which mode of data communication is this?

    1. Simplex
    2. Full-duplex
    3. Half-duplex
    4. Broadcast
    Show answer

    Answer: C — Half-duplex

    Data flows in both directions, but never in both at once, which is half-duplex. Simplex allows one direction only, like a keyboard sending to a computer. Full-duplex allows both at the same time, as on a telephone call. Broadcast describes one sender reaching many receivers, not the direction of an exchange.

  2. In which switching technique is a dedicated path between sender and receiver set up before any data is transmitted, and kept for the whole session?

    1. Circuit switching
    2. Packet switching
    3. Hashing
    4. Routing by DNS
    Show answer

    Answer: A — Circuit switching

    Circuit switching establishes and reserves the whole path first, as a traditional telephone call does, and releases it only when the session ends. Packet switching is the opposite approach: no path is reserved and each packet finds its own way. Hashing is a searching technique, and DNS translates names into addresses rather than setting up paths.

  3. Which statement about packet switching is correct?

    1. All packets of a message must travel along one reserved route
    2. The message is sent whole, without being divided
    3. Packets may take different routes and are reassembled in order at the destination
    4. A circuit must be established before the first packet is sent
    Show answer

    Answer: C — Packets may take different routes and are reassembled in order at the destination

    Packet switching divides the message into numbered packets that travel independently, possibly by different routes, sharing the links with other traffic; the sequence numbers let the destination put them back in order. A reserved route and a circuit set up in advance both describe circuit switching, and the whole point of the technique is that the message is divided.

  4. Which property makes optical fibre especially suitable for running beside heavy electrical machinery?

    1. It is the cheapest cable to buy and install
    2. It carries light, so it is immune to electromagnetic interference
    3. Its braided copper shield blocks all interference
    4. It uses radio waves that pass through walls
    Show answer

    Answer: B — It carries light, so it is immune to electromagnetic interference

    Optical fibre carries data as light kept inside the core by total internal reflection, and light is not disturbed by the electromagnetic fields that machinery produces. It is the most expensive of the three cables, not the cheapest. A braided shield belongs to coaxial cable, which reduces interference rather than escaping it, and radio waves are a wireless medium.

  5. A TV remote works only when pointed at the television and does not operate a TV in the next room. Which medium does it use?

    1. Infrared
    2. Radio waves
    3. Coaxial cable
    4. Microwaves from a satellite
    Show answer

    Answer: A — Infrared

    Infrared needs a line of sight over a short distance and is stopped by walls, which is exactly why the remote must be pointed and cannot reach the next room. Radio waves are omnidirectional and pass through walls, so a radio remote would work from another room. A remote has no cable, and satellite microwaves cover long distances.

  6. Which protocol is used to send an e-mail from the sender's mail client to the mail server?

    1. FTP
    2. PPP
    3. HTTP
    4. SMTP
    Show answer

    Answer: D — SMTP

    SMTP, the Simple Mail Transfer Protocol, carries outgoing mail from the client to the server and between mail servers. FTP transfers files, HTTP transfers web pages, and PPP carries data over a direct link between two nodes. A webmail page is fetched with HTTP, but the mail itself is still handed on by SMTP.

  7. Match each protocol with its job. A. HTTP B. FTP C. PPP D. TCP — 1. Transfers files between client and server 2. Ensures packets arrive completely and in order 3. Transfers web pages 4. Carries data over a direct link between two nodes

    1. A-3, B-1, C-2, D-4
    2. A-1, B-3, C-4, D-2
    3. A-3, B-4, C-1, D-2
    4. A-3, B-1, C-4, D-2
    Show answer

    Answer: D — A-3, B-1, C-4, D-2

    HTTP fetches web pages, FTP moves files, PPP runs a direct point-to-point link such as a computer to its service provider, and TCP is the half of TCP/IP responsible for complete, ordered delivery. A-3, B-1, C-2, D-4 swaps PPP and TCP, both "connection" ideas; A-1, B-3, C-4, D-2 swaps HTTP and FTP, both "transfer" protocols.

  8. Assertion (A): The bandwidth of a communication channel is measured in hertz. Reason (R): The data transfer rate of a channel is measured in bits per second.

    1. Both A and R are true, and R is the correct explanation of A
    2. Both A and R are true, but R is not the correct explanation of A
    3. A is true, but R is false
    4. A is false, but R is true
    Show answer

    Answer: B — Both A and R are true, but R is not the correct explanation of A

    Bandwidth is a range of frequencies, so it is measured in hertz, and data transfer rate is a count of bits per second — both statements are true. But they describe two different quantities, and the unit of one says nothing about why the other uses hertz. Choosing "R explains A" treats bandwidth and data rate as the same thing, which is the confusion the pair is set to catch.