Engineer Information Communications flashcards

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

Shannon capacity formula

C = B·log2(1 + S/N) bits per second, giving the maximum error-free rate over an AWGN channel of bandwidth B with linear signal-to-noise power ratio S/N.

Nyquist sampling theorem

A signal band-limited to fmax must be sampled at least 2·fmax times per second to be reconstructed without aliasing; below this rate, spectral overlap corrupts the signal irreversibly.

SQNR per bit

For uniform quantization of a full-scale sinusoid, SQNR ≈ 6.02n + 1.76 dB, so each additional bit improves signal-to-quantization-noise ratio by about 6 dB.

Source entropy

H = −Σ p_i log2 p_i bits per symbol measures average information; lossless coding cannot compress below H, and Huffman coding stays within one bit per symbol of it.

BPSK bit error rate

In AWGN, BPSK BER = Q(√(2Eb/N0)); the curve falls steeply as energy per bit rises, and QPSK has the same BER for the same Eb/N0 while carrying twice the bits.

OSI layer order

Physical, Data Link, Network, Transport, Session, Presentation, Application. Data units are bits, frames, packets, and segments respectively, with each layer adding its own header.

TCP three-way handshake

Client sends SYN, server replies SYN-ACK, client sends ACK. Sequence numbers are synchronized so both sides can detect loss, reordering, and duplicate segments.

TCP slow start

Congestion window starts small and doubles each round-trip time until ssthresh, after which congestion avoidance increases it by roughly one segment per RTT.

Subnet usable hosts

For a prefix with h host bits, usable addresses equal 2^h − 2 because the all-zeros network address and all-ones broadcast address cannot be assigned to hosts.

IPv4 TTL field

An 8-bit hop counter decremented by each router; a packet reaching zero is discarded and an ICMP Time Exceeded message is returned, preventing endless forwarding loops.

Free-space path loss

FSPL in dB = 32.44 + 20log10(d in km) + 20log10(f in MHz). Loss rises 6 dB per doubling of distance or frequency in free space with isotropic antennas.

Doppler shift and coherence time

Doppler shift fd = v·fc/c·cosθ; coherence time is approximately 1/(4·fd), the interval over which the channel response stays roughly constant for a moving receiver.

Cellular frequency reuse factor

Cluster size N = i² + ij + j²; reuse distance D = R·√(3N). Larger N raises co-channel signal-to-interference ratio but reduces capacity per cell.

OFDM cyclic prefix

A guard interval copied from the end of each OFDM symbol; when longer than the channel delay spread it removes inter-symbol interference, at a cost of spectral efficiency.

Rayleigh versus Rician fading

Rayleigh fading has no dominant line-of-sight path and its envelope is Rayleigh distributed; Rician fading adds a strong deterministic component described by the K-factor.

Receiver sensitivity calculation

Sensitivity = −174 dBm/Hz + 10log10(B) + NF + required SNR. Thermal noise floor plus receiver noise figure plus demodulation threshold sets the minimum detectable signal.

Optical fiber loss windows

Standard single-mode fiber has lowest loss near 1550 nm (~0.2 dB/km) and zero material dispersion near 1310 nm; 850 nm is used for short-reach multimode links.

Chromatic dispersion

Wavelength-dependent propagation delay measured in ps/(nm·km); it broadens pulses and limits reach at high bit rates, requiring dispersion-compensating fiber or coherent DSP.

DWDM channel grid

Dense WDM places channels on ITU-T grids around 1550 nm with 100 or 50 GHz spacing, letting many wavelengths share one fiber and one amplifier band.

SDH STM-1 rate

Synchronous Digital Hierarchy STM-1 carries 155.52 Mbit/s; STM-4 is 622.08 Mbit/s and STM-16 is 2.488 Gbit/s, scaling by factors of four.

OSNR in amplified spans

Optical signal-to-noise ratio degrades with each EDFA because amplified spontaneous emission accumulates; more spans or higher noise figure lower OSNR at the receiver.

AES key and block sizes

AES uses a fixed 128-bit block with 128, 192, or 256-bit keys and 10, 12, or 14 rounds respectively; it replaced DES, whose 56-bit key is brute-forceable.

AES-GCM mode properties

Galois/Counter Mode provides authenticated encryption: confidentiality plus a tag verifying integrity and authenticity, so tampered ciphertext is rejected without a separate MAC.

Hash collision resistance

For an n-bit hash, finding a collision costs about 2^(n/2) by the birthday bound, while preimage resistance costs 2^n; SHA-1 and MD5 are no longer collision-resistant.

Digital signature purpose

Signing a hash with a private key, verified by the public key, proves integrity, origin authenticity, and non-repudiation; it does not by itself provide confidentiality.

TLS 1.3 forward secrecy

Ephemeral (EC)DHE key exchange gives each session a unique key, so compromise of the server's long-term private key cannot decrypt previously recorded sessions.

Algorithm complexity ordering

Growth from fastest to slowest: O(1), O(log n), O(n), O(n log n), O(n²), O(2ⁿ). Binary search is O(log n); merge sort is O(n log n) in the worst case.

Race condition

A defect where output depends on unsynchronized timing of concurrent accesses to shared state; fixed with mutual exclusion, atomic operations, or message passing.

ACID transaction properties

Atomicity, Consistency, Isolation, Durability. Isolation levels trade concurrency for protection against dirty reads, non-repeatable reads, and phantom rows.

ITU-T versus ITU-R

ITU-T produces telecommunications standards such as G.652 and G.709; ITU-R manages radio spectrum and radiocommunication recommendations; ITU-D handles development.

IEEE 802.11 and 802.3

IEEE 802.11 specifies wireless LAN medium access and PHY layers; IEEE 802.3 specifies wired Ethernet. Both are adopted worldwide and referenced by national regulations.

3GPP release mapping

Release 8 introduced LTE, Release 10 LTE-Advanced, and Release 15 the first 5G NR specification; later releases add features while keeping backward compatibility.