Unit 5: Data Communication

Data communication is the exchange of data between two devices through a transmission medium, forming the backbone of every network. Before the individual technologies make sense, fix the shared vocabulary and the physical constraints they all obey.

• Sender and receiver: the source device that generates the message and the destination that consumes it; each needs an agreed protocol.
• Message: the information communicated — text, numbers, images, audio, video — all reduced to binary (0/1) for machine handling.
• Medium: the physical path (copper, fibre, air) carrying the signal; classed as guided (cable) or unguided (wireless).
• Protocol: a rule set governing format, timing, sequencing and error handling; without a common protocol two devices cannot interpret each other.
• Bandwidth and data rate: bandwidth is the range of frequencies a medium supports (Hz); data rate is throughput in bits per second (bps, Kbps, Mbps, Gbps).
• Directional modes: simplex (one way), half-duplex (both ways, one at a time, like a walkie-talkie), full-duplex (both ways simultaneously, like a phone call).

II. Local and Global Reach of the Network

Networks are classified by the geographic area they span, which in turn dictates ownership, speed and technology.

A. Local reach — LAN and its neighbours

The point: short-distance networks owned by a single organisation, prized for high speed and low cost per link.

• LAN (Local Area Network): covers a room, building or campus, typically under 1–2 km; data rates of 100 Mbps to 10 Gbps over Ethernet cabling or Wi-Fi.
• Privately owned: an office or lab installs and maintains its own cabling and switches, so there are no recurring carrier charges.
• PAN (Personal Area Network): the smallest scale — a few metres around one person, e.g. a phone paired with earbuds over Bluetooth.
• MAN (Metropolitan Area Network): spans a city (up to ~50 km), linking several LANs, e.g. a cable-TV network or a university's multiple campuses.

B. Global reach — WAN and the Internet

The point: long-distance networks that cross cities, countries and oceans, usually leased from carriers rather than owned.

• WAN (Wide Area Network): spans countries or continents using leased telephone lines, satellite links and undersea fibre; individual links are slower and costlier per bit than LAN links.
• The Internet: the largest WAN — a global "network of networks" tied together by the TCP/IP protocol suite and routers that forward packets between autonomous networks.
• Backbone and last mile: high-capacity fibre backbones carry aggregated traffic between regions; the last mile is the final link to the home or office, often the slowest segment.
• Scaling trade-off: as reach grows from PAN to WAN, speed and control fall while cost, latency and dependence on third-party carriers rise.

III. Digital and Analog Transmission

Signals travel either as continuous waves or as discrete pulses, and the choice shapes quality, distance and equipment.

A. Analog Transmission

The point: representing data as a continuously varying physical quantity such as voltage or wave amplitude.

• Continuous waveform: the signal takes infinitely many values over time, like the sine wave of a human voice on a traditional phone line.
• Three parameters: amplitude (signal strength), frequency (cycles per second, Hz) and phase (position in the cycle) — any can be varied to carry information.
• Amplifiers and noise: long analog lines use amplifiers to boost the signal, but amplifiers also boost accumulated noise, so quality degrades with distance.

B. Digital Transmission

The point: representing data as discrete voltage levels, almost always binary 0 and 1.

• Discrete pulses: a fixed voltage represents 1 and another (often 0 V) represents 0, giving a square-wave pattern.
• Repeaters, not amplifiers: digital links use repeaters that regenerate a clean new pulse rather than amplifying noise, so long-distance quality stays high.
• Advantages: better noise immunity, easy encryption, and direct compatibility with computers, which are inherently digital.
• Attenuation and distortion: both signal types weaken (attenuate) over distance; digital's regeneration is why it now dominates long-haul networks.

C. Conversion between the two

The point: mismatched signal-medium pairs require conversion at each end.

• Modulation (digital → analog): encodes bits onto an analog carrier for media like old telephone lines; done by a modem.
• Digitisation (analog → digital): samples an analog wave and quantises it into binary, e.g. converting voice for a digital phone network (codec).

IV. Data Communication with Standard Telephone Lines and Modems

The public telephone network, built for analog voice, was the first widespread path for computer data, bridged by the modem.

A. Standard telephone lines

The point: the ordinary voice line (PSTN) carries a limited analog band that constrains data speed.

• Voice band: the line passes roughly 300–3400 Hz, tuned for speech, not the wide band data ideally wants.
• PSTN: the Public Switched Telephone Network uses circuit switching — a dedicated end-to-end path is reserved for the call's duration.
• Dial-up access: the computer literally "dials" an ISP's number, occupying the voice line so the phone cannot be used for calls simultaneously.

B. Modems

The point: a modem (MOdulator-DEModulator) lets digital computers speak over an analog voice line.

• Modulate on send: converts outgoing bits into audible analog tones suited to the voice band.
• Demodulate on receive: converts incoming tones back into bits for the computer.
• Speed ceiling: classic dial-up modems top out around 56 Kbps, limited by the narrow voice band and noise.
• Modulation techniques used:
  • ASK (Amplitude Shift Keying): varies amplitude to represent bits.
  • FSK (Frequency Shift Keying): switches between two frequencies for 0 and 1.
  • PSK (Phase Shift Keying): shifts the wave's phase; more robust and used for higher rates.

V. Using Digital Data Connections

The point: connections that carry data digitally end-to-end, avoiding the analog voice-band bottleneck and delivering far higher, always-on speeds.

A. DSL (Digital Subscriber Line)

• Same copper, higher frequencies: reuses the existing phone wire but transmits data on frequencies above the voice band, so phone and internet work at once.
• ADSL: the common asymmetric variant gives more download than upload bandwidth (e.g. several Mbps down, less up), matching typical browsing.

B. Cable and fibre connections

• Cable modem: shares the coaxial TV network's high bandwidth; speed can dip when many neighbours are active because the segment is shared.
• Leased line (e.g. T1/E1): a dedicated always-on digital circuit rented by businesses; a T1 delivers 1.544 Mbps with guaranteed capacity.
• Fibre optic (FTTH): carries data as pulses of light through glass, offering the highest bandwidth, immunity to electrical noise and Gbps-class speeds.

C. ISDN

• Integrated Services Digital Network: an early fully digital dial-up standard combining B channels (64 Kbps bearer, for data/voice) and a D channel (signalling); Basic Rate offers 2B+D.

D. Advantages over dial-up

• Always on: no dialling delay and the phone line stays free.
• Higher throughput: Mbps to Gbps versus 56 Kbps, enabling streaming and large transfers.
• Digital end-to-end: fewer conversions means less noise and error.

VI. Wireless Networks

The point: networks using unguided media — radio waves, microwaves, infrared — to remove the constraint of physical cabling.

A. Principle and media

• Unguided transmission: signals radiate through air; no cable is laid, so devices stay mobile.
• Spectrum bands: radio waves (broad coverage, penetrate walls), microwaves (line-of-sight, high capacity), infrared (very short range, e.g. remote controls).

B. Wi-Fi (WLAN)

• IEEE 802.11: the standard for wireless LANs; devices connect to an access point that bridges to the wired network.
• Range and rate: tens of metres indoors with speeds from tens of Mbps to over 1 Gbps in newer standards, sharing the medium among users.

C. Bluetooth

• Short-range PAN: links devices within about 10 m over the 2.4 GHz band, for keyboards, headsets and file transfer at low power.

D. Cellular / mobile networks

• Cell-based coverage: the region is divided into cells, each served by a base station, allowing handover as a user moves.
• Generations: 3G, 4G/LTE and 5G progressively raise data rates and cut latency, with 5G reaching Gbps speeds.

E. Satellite communication

• Wide reach: a satellite relays microwave signals to cover remote areas without ground infrastructure.
• Propagation delay: distant (geostationary) satellites add noticeable latency, a drawback for real-time use.

F. Advantages and limitations

• Advantages: mobility, quick deployment, and reach where cabling is impractical.
• Limitations: shared medium lowers per-user speed, signals suffer interference and attenuation, and open airwaves demand encryption (e.g. WPA) for security.