Assam ASSEB HS Skill Telecom flashcards

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

Decibel formula

For power, dB = 10 log10(P2/P1); for voltage or current across equal impedance, dB = 20 log10(V2/V1). A 3 dB change halves or doubles power.

dBm reference

Absolute power relative to 1 milliwatt: dBm = 10 log10(P/1 mW). Thus 0 dBm is 1 mW, 30 dBm is 1 W, and −30 dBm is 1 microwatt.

Nyquist sampling theorem

A band-limited signal with bandwidth B must be sampled at a rate of at least 2B samples per second, otherwise aliasing folds high frequencies onto low ones.

Shannon capacity formula

Maximum error-free rate C = B log2(1 + S/N) bit/s. Capacity grows linearly with bandwidth but only logarithmically with signal-to-noise ratio.

Cat5e versus Cat6

Cat5e supports 1 Gbit/s over 100 m; Cat6 supports 1 Gbit/s to 100 m and 10 Gbit/s to about 55 m, with tighter crosstalk specifications. Both are 100 ohm.

Single-mode versus multimode fibre

Single-mode has an 8 to 10 micrometre core and carries one mode over tens of kilometres. Multimode has a 50 or 62.5 micrometre core and suits short building or campus spans.

OSI seven layers

Physical, Data Link, Network, Transport, Session, Presentation, Application. The TCP/IP model collapses these into link, internet, transport and application layers.

Circuit versus packet switching

Circuit switching reserves a dedicated end-to-end path for the call duration. Packet switching shares links, buffers packets and suits bursty data but adds delay and jitter.

Private IPv4 ranges

10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are non-routable on the public Internet and require network address translation to reach external hosts.

Subnet host count

Usable hosts equal 2 to the power of host bits, minus two. A /24 gives 254 usable addresses, while a /30 gives 2 for point-to-point links.

TCP three-way handshake

The client sends SYN, the server replies SYN-ACK, and the client sends ACK. This establishes sequence numbers before data transfer begins.

Cellular frequency reuse

Cells form a cluster of N cells that reuse the same frequencies. The reuse ratio Q = D/R = square root of 3N; larger N means less interference but fewer channels per cell.

Free-space path loss

FSPL in dB = 32.44 + 20 log10(distance in km) + 20 log10(frequency in MHz). Loss rises with the square of both distance and frequency.

GSM frame structure

GSM uses 200 kHz carriers and TDMA frames of 4.615 ms containing eight time slots, each carrying one user burst.

EIRP calculation

Effective isotropic radiated power equals transmitter power in dBm plus antenna gain in dBi minus feeder loss in dB. It defines the radiated signal strength in the main beam.

Fibre transmission windows

850 nm suits multimode LANs, 1310 nm gives zero dispersion in standard single-mode fibre, and 1550 nm gives the lowest attenuation, about 0.2 dB per kilometre.

GPON rates

Gigabit passive optical network delivers 2.488 Gbit/s downstream and 1.244 Gbit/s upstream, shared among users through passive splitters on one feeder fibre.

Fusion versus mechanical splice

Fusion splicing melts the fibre ends together and gives the lowest loss, typically 0.05 to 0.1 dB. Mechanical splices use an index-matching gel and show higher loss.

OTDR function

An optical time-domain reflectometer sends light pulses and plots backscattered power against distance, revealing splice loss, connector reflections, bends and breaks along the fibre.

Horizontal cabling limit

The permanent horizontal link from telecommunications room to outlet must not exceed 90 m; patch cords add up to 10 m, giving a 100 m channel.

T568A and T568B

These are the two standard RJ-45 pin assignments for UTP termination. A straight-through cable uses the same standard at both ends; a crossover cable uses different standards.

Fibre bend radius rule

Installed fibre should not be bent tighter than about ten times its cable diameter, and copper about four times, to avoid loss and mechanical damage.

Wire-map test

A cable tester verifies pin-to-pin continuity and reports opens, shorts, crossed pairs and split pairs, confirming the termination matches the intended standard.

Optical link loss

Link loss equals transmit power minus received power in dBm. It must stay within the power budget, which is transmit power minus receiver sensitivity.

TDR fault location

Time-domain reflectometry measures the round-trip delay of a reflection; distance equals propagation velocity times round-trip time divided by two.

Demarcation point

The boundary between the operator's network and the customer's premises wiring. Faults beyond it are the customer's responsibility and may attract a charge.

Call detail record

A record of a call or session containing calling and called numbers, start time, duration and charge. It is the evidence base for usage billing and disputes.

Prepaid versus postpaid billing

Prepaid deducts from a balance in real time and stops service at zero. Postpaid accumulates usage and issues a periodic invoice, requiring credit control.

Lockout and tagout

The safety procedure that isolates a circuit, locks the isolator and attaches a warning tag so equipment cannot be re-energised while work is in progress.

Laser eye safety

Never look into a fibre end or connector, even a dark one. Use an optical power meter to confirm whether light is present and label fibre links as laser sources.

Break-even quantity

Fixed costs divided by the difference between unit price and unit variable cost. Selling below this volume produces a loss even if each sale shows a margin.

Active listening

Concentrating fully on the speaker, not interrupting, and confirming understanding by restating the message. It reduces misunderstandings in fault reporting and customer handling.