NIELIT ACC cheat sheet
NIELIT ACC · Keep the key ideas within reach.
IECT Fundamentals: Signals, Number Systems and Digital Logic
Signal bandwidth: a channel of bandwidth B Hz supports a maximum symbol rate of 2B baud (Nyquist); with M discrete levels the bit rate is 2B·log2(M) bit/s. Shannon capacity: C = B·log2(1 + S/N) bit/s, where S/N is a linear power ratio, not dB.
SNR in dB = 10·log10(S/N); amplitude ratio in dB = 20·log10(V1/V2). Doubling power adds 3 dB; tenfold power adds 10 dB.
Number systems: 1 byte = 8 bits; 1 KiB = 1024 bytes, 1 MiB = 2^20 bytes. Binary 1011 0110 = 0xB6 = 182 decimal. Two's complement of an n-bit number x is 2^n − x; an 8-bit signed range is −128 to +127.
Logic identities: De Morgan - (A·B)' = A' + B' and (A+B)' = A'·B'; XOR is 1 when inputs differ and equals A'B + AB'. NAND and NOR are each functionally complete.
Modulation: AM bandwidth = 2·fm; FM uses a much wider band (Carson's rule: BW ≈ 2(Δf + fm)). QAM carries log2(M) bits per symbol; 16-QAM = 4 bits/symbol, 64-QAM = 6 bits/symbol.
Transmission media: twisted pair (short haul, low cost, crosstalk-limited), coaxial cable (higher bandwidth, better shielding), optical fibre (very high bandwidth, immune to EMI, low attenuation).
Multiplexing decision rule: FDM splits the frequency band, TDM splits time slots, WDM is optical FDM, and CDMA separates users by orthogonal spreading codes.
Error detection: parity detects odd numbers of bit errors only; CRC with an n-bit polynomial detects all single-bit errors and all bursts of length ≤ n; Hamming distance d corrects floor((d−1)/2) errors.
IECT Fundamentals: Electronics, Computers and Communication Networks
Semiconductor basics: a diode conducts when forward biased above ~0.7 V (Si) or ~0.3 V (Ge); a Zener diode regulates in reverse breakdown; a BJT has β = IC/IB and operates in cutoff, active, or saturation.
Op-amp rules: with negative feedback the input terminals are at equal potential (virtual short) and draw no current; inverting gain = −Rf/Rin, non-inverting gain = 1 + Rf/Rin.
OSI seven layers, bottom up: Physical, Data Link, Network, Transport, Session, Presentation, Application. TCP/IP maps to four: Network Access, Internet, Transport, Application.
Protocol-to-layer rule: IP and ICMP at Network; TCP and UDP at Transport; HTTP, FTP, SMTP, DNS and DHCP at Application; Ethernet and PPP at Data Link.
Addressing: IPv4 is 32-bit dotted decimal; IPv6 is 128-bit hexadecimal. A /24 prefix leaves 8 host bits, giving 254 usable hosts. Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
Port numbers: FTP 20/21, SSH 22, Telnet 23, SMTP 25, DNS 53, HTTP 80, POP3 110, HTTPS 443, IMAP 143. TCP is connection-oriented and reliable; UDP is connectionless and best-effort.
Boolean/CPU facts: a half adder gives Sum = A⊕B and Carry = A·B; a full adder adds a carry-in. CPU fetch–decode–execute cycle; cache (L1/L2) is faster than RAM, which is faster than secondary storage.
Wireless and access: GSM uses TDMA/FDMA, LTE and 5G NR use OFDMA; Wi-Fi (IEEE 802.11) shares unlicensed ISM bands; fibre access (GPON) and DSL differ in that DSL runs over existing copper pairs.