NIELIT BCC cheat sheet

Signal bandwidth is the difference between the highest and lowest frequencies in a band: BW = f_high − f_min; measured in hertz.

Nyquist rate for a noiseless channel of bandwidth B with M signal levels: C = 2B·log2(M) bits per second.

Shannon capacity for a noisy channel: C = B·log2(1 + S/N), where S/N is a linear power ratio, not dB.

Conversion from dB to linear ratio: S/N = 10^(dB/10); a 20 dB ratio equals 100.

Modulation maps bits to symbols: bits per symbol = log2(M) for M-ary PSK/QAM; 16-QAM carries 4 bits per symbol.

In a half-duplex link only one side transmits at a time; full-duplex allows simultaneous two-way transmission.

IPv4 private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16; these are not routed on the public Internet.

A /24 IPv4 prefix has 256 addresses, of which 254 are usable for hosts after network and broadcast addresses.

1 byte = 8 bits; 1 KiB = 1024 bytes; 1 MiB = 1024 KiB; 1 GiB = 1024 MiB.

Binary to decimal: multiply each bit by 2^position from the right, starting at 2^0; 1101₂ = 13.

Two's complement negation: invert all bits and add 1; an n-bit signed range is −2^(n−1) to 2^(n−1) − 1.

Hexadecimal to binary: each hex digit maps to exactly 4 bits; F16 = 1111₂.

ROM is non-volatile and read-only in normal operation; RAM is volatile and loses contents on power loss.

Cache hierarchy order by speed: registers, L1, L2, L3 cache, RAM, SSD/HDD, tape.

A full adder sums three inputs (A, B, carry-in) and outputs Sum = A XOR B XOR Cin and Cout = majority(A, B, Cin).

ASCII is a 7-bit code covering 128 characters; extended ASCII uses 8 bits for 256 code points.

Spreadsheet relative references (A1) shift when copied; absolute references ($A$1) stay fixed; mixed references lock one axis.

Common spreadsheet functions: SUM, AVERAGE, COUNT (numbers only), COUNTA (non-empty cells), IF(condition, true_value, false_value).

A spreadsheet formula must begin with '='; text concatenation uses '&'; a range is written first:last, e.g. A1:A10.

In word processors, Ctrl+B/I/U toggle bold, italic, underline; Ctrl+Z undoes and Ctrl+Y redoes the last action.

Email fields: To is the primary recipient, Cc is a visible copy, Bcc is a hidden copy that other recipients cannot see.

URL structure: scheme://host:port/path?query#fragment; HTTPS defaults to port 443, HTTP to port 80.

HTTP status classes: 2xx success, 3xx redirection, 4xx client error (404 not found), 5xx server error.

A spreadsheet cell displaying #### means the column is too narrow for the value, not that the data is invalid.

Digital signature provides authenticity, integrity and non-repudiation; it does not by itself encrypt the message content.

Symmetric encryption uses one shared key (fast, e.g. AES); asymmetric encryption uses a public/private key pair (e.g. RSA).

Phishing is a social-engineering attack that tricks users into revealing credentials; checking the sender domain is a primary defence.

Strong password rule of thumb: length above complexity; multi-factor authentication adds a second independent factor.

Cloud service models: IaaS supplies virtual machines, PaaS supplies a runtime platform, SaaS supplies a finished application.

A blockchain is an append-only distributed ledger where each block stores the hash of the previous block, making tampering detectable.

UPI is an Indian instant payment interface operated by NPCI; a UPI ID has the form name@bankhandle.

Open-source software publishes its source code under a licence (e.g. GPL, MIT) that defines reuse and redistribution rights.