Question 9 of 9: Technical Terminology

Question 9: Technical Terminology (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

1. Parity bit. A single extra bit appended to a group of data bits so that the total number of 1-bits in the group (data + parity) is always even (even parity) or always odd (odd parity); the receiver recomputes the parity and flags an error if it disagrees, but this scheme can only reliably detect an ODD number of bit errors — two flipped bits cancel and go undetected, which is why it is used only for low-noise links or combined with stronger codes (e.g. CRC) elsewhere.
2. Bit Error Rate (BER). The ratio of the number of bit errors received to the total number of bits transmitted over a link in a given period; it is the standard measure of a digital transmission link's quality, driven by noise, interference, and attenuation on the physical channel.
3. Modem (modulator-demodulator). A device that converts a digital bit stream into an analog signal suitable for transmission over an analog medium (modulation), and converts the received analog signal back into digital data at the far end (demodulation); the classic example is a dial-up modem carrying digital computer data over the analog voice-grade telephone line, but the same modulate/demodulate role appears in DSL, cable, and radio modems.
4. Codec (coder-decoder). A device or algorithm that converts an analog signal (e.g. voice) into a digital bit stream for transmission/storage (coding) and reverses the process at the far end (decoding); e.g. a PCM codec used in digital telephony samples, quantizes, and encodes speech into 64 kbps digital channels.
5. Crosstalk. Unwanted coupling of a signal from one circuit or cable pair into an adjacent one, caused by electromagnetic (inductive/capacitive) coupling between conductors run close together; it is one of the impairments (with attenuation, delay distortion, and noise) that limits achievable data rate, and is mitigated by twisting wire pairs, shielding, and physical separation.
6. CSMA/CD (Carrier Sense Multiple Access with Collision Detection). The classic Ethernet medium-access method: a station listens before transmitting (carrier sense), and if two stations transmit at once anyway, both detect the resulting collision, stop, and each backs off a random time before retrying (using the binary exponential backoff algorithm) — used on shared (half-duplex/coaxial or hub-based) Ethernet segments; switched full-duplex Ethernet has no collisions and does not need it.
7. Decibel (dB). A logarithmic unit expressing the ratio of two power levels, $\text{dB}=10\log_{10}(P_1/P_2)$ (or $20\log_{10}(V_1/V_2)$ for voltage/amplitude ratios); it is used throughout communications because gains and losses along a chain of amplifiers/attenuators/cables become simple addition and subtraction in dB, instead of multiplication of raw ratios, and it compresses an enormous dynamic range into a manageable scale.
8. B-ISDN. Broadband Integrated Services Digital Network — a set of ITU-T standards extending narrowband ISDN to support high-bandwidth, integrated voice/video/data services over a single network, using Asynchronous Transfer Mode (ATM) as its underlying transport and switching technology.
9. TCP/IP. The Transmission Control Protocol / Internet Protocol suite that underlies the Internet: IP (network layer) provides best-effort, connectionless addressing and routing of packets between hosts, while TCP (transport layer) layered above it adds a reliable, connection-oriented, ordered, flow- and congestion-controlled byte stream on top of IP's unreliable delivery.
10. Packet. A formatted unit of data transmitted across a packet-switched network, consisting of a header (source/destination address, sequencing/control information) and a payload; large messages are broken into multiple packets, each routed independently (potentially over different paths) and reassembled at the destination.