NIELIT ACC flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Nyquist bit rate formula

For a noiseless channel of bandwidth B hertz with M signal levels, the maximum bit rate is 2B log2(M) bits per second. The 2B term is the maximum symbol (baud) rate.

Shannon channel capacity

C = B log2(1 + S/N) bits per second, where B is bandwidth in hertz and S/N is the linear signal-to-noise power ratio. Capacity sets an upper bound that no coding can exceed.

Decibel conversion

Power ratio in dB equals 10 log10(P1/P2); voltage or current ratio equals 20 log10(V1/V2). Thus 3 dB is about double the power and 10 dB is ten times the power.

Two's complement representation

The negative of an n-bit binary value is formed by inverting all bits and adding one, equivalent to 2^n minus the value. An 8-bit signed range is −128 to +127.

De Morgan's theorems

The complement of a product equals the sum of the complements, and the complement of a sum equals the product of the complements: (A·B)' = A' + B' and (A+B)' = A'·B'.

XOR gate function

The exclusive-OR output is 1 only when its inputs differ, and 0 when they are equal. Its Boolean expression is A'B + AB', and it acts as a controlled inverter.

AM bandwidth requirement

An amplitude-modulated signal with modulating frequency fm occupies twice that frequency, so the bandwidth is 2fm. Both the upper and lower sidebands carry identical information.

QAM bits per symbol

In M-ary quadrature amplitude modulation each symbol carries log2(M) bits. 16-QAM carries 4 bits per symbol and 64-QAM carries 6 bits per symbol.

Optical fibre advantages

Fibre carries very high bandwidth over long distances with low attenuation and is immune to electromagnetic interference. It also resists tapping, but joining fibres requires precision splicing.

FDM versus TDM

Frequency division multiplexing assigns each user a distinct sub-band continuously in time. Time division multiplexing gives each user the full band in assigned, recurring time slots.

CRC error detection

A cyclic redundancy check divides the data by a generator polynomial and appends the remainder. An n-bit polynomial detects all single-bit errors and all burst errors of length n or less.

Hamming distance capability

The Hamming distance between two codewords is the number of bit positions in which they differ. Distance d detects d−1 errors and corrects floor((d−1)/2) errors.

Zener diode regulation

A Zener diode is operated in reverse breakdown, where its voltage stays nearly constant despite current changes. It therefore provides a simple shunt voltage reference or regulator.

BJT current gain beta

Beta equals collector current divided by base current in the active region. A large beta means a small base current controls a much larger collector current, enabling amplification.

Op-amp virtual short

With negative feedback, an ideal op-amp drives its two inputs to nearly the same voltage and draws negligible input current. This yields inverting gain −Rf/Rin and non-inverting gain 1 + Rf/Rin.

OSI layer order

From layer 1 upward: Physical, Data Link, Network, Transport, Session, Presentation, Application. The lower three handle delivery; the upper four handle user-oriented processing.

TCP versus UDP

TCP is connection-oriented, acknowledges segments, retransmits losses and preserves order. UDP is connectionless and best-effort, with low overhead, suited to voice, video and DNS queries.

Private IPv4 address ranges

Non-routable private blocks are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Hosts using them reach the internet through NAT on a border router.

Subnet host count

A /24 prefix leaves 8 host bits, so 2^8 − 2 = 254 usable addresses after removing the network and broadcast addresses. Each prefix bit borrowed halves the host space.

Well-known port numbers

Common TCP/UDP ports: FTP 20 and 21, SSH 22, Telnet 23, SMTP 25, DNS 53, HTTP 80, POP3 110, IMAP 143 and HTTPS 443. Ports below 1024 are well-known.

Half adder logic

A half adder adds two bits and outputs Sum = A XOR B and Carry = A AND B. A full adder extends this with a carry-in input to chain stages.

CPU fetch-decode-execute cycle

The processor fetches an instruction from memory at the program counter, decodes the opcode, executes the operation, and writes results back before repeating.

Cache memory hierarchy

Registers are fastest, then L1, L2 and L3 cache, then main RAM, then secondary storage. Cache holds recently used data to reduce average memory access time.

GSM multiple access

GSM combines FDMA for separating radio channels with TDMA for sharing each channel among eight users in rotating time slots. It is a second-generation digital cellular standard.

OFDMA in LTE and 5G

Orthogonal frequency division multiple access splits the channel into many narrow orthogonal subcarriers and assigns subsets to users. It gives high spectral efficiency and robustness to multipath.

Wi-Fi standard family

IEEE 802.11 defines wireless LANs operating in unlicensed 2.4 and 5 GHz ISM bands. Later amendments such as 802.11n, ac and ax raise throughput using MIMO and wider channels.

ICMP purpose

The Internet Control Message Protocol carries error and diagnostic messages between routers and hosts, such as destination unreachable and echo request. The ping utility relies on it.

DNS resolution

The Domain Name System maps human-readable names to IP addresses through a hierarchy of root, top-level and authoritative servers, using port 53 for queries and replies.

DHCP function

The Dynamic Host Configuration Protocol automatically supplies an IP address, subnet mask, default gateway and DNS server to a host when it joins a network.

NAT operation

Network address translation rewrites private source addresses to a public address at the router and tracks port mappings so replies return to the right internal host.

Fourier bandwidth relation

A periodic signal's spectrum contains the fundamental and its harmonics. Sharper edges require higher harmonics, so faster digital pulses need greater bandwidth to avoid distortion.

Asynchronous versus synchronous transmission

Asynchronous transmission frames each character with start and stop bits, tolerating timing drift. Synchronous transmission sends long frames against a shared clock for higher efficiency.