Telecom Professional Qualification cheat sheet

OSI has 7 layers; TCP/IP model collapses them to 4 (Link, Internet, Transport, Application). Map by function, not by protocol name.

Circuit switching reserves a dedicated path for the call duration; packet switching multiplexes statistically, so it suits bursty data but needs congestion control.

Nyquist formula for a noiseless channel: C = 2B·log2(M) bit/s, where B is bandwidth in Hz and M is signal levels.

Shannon capacity: C = B·log2(1 + S/N). SNR in dB = 10·log10(S/N); 3 dB ≈ doubling of power.

Full-duplex uses separate directions simultaneously; half-duplex shares the medium, so effective throughput is reduced by turnaround and contention.

Access, metro, and core are the three structural tiers; core favors high capacity and resilience, access favors reach and cost per subscriber.

Anycast routes to the nearest instance of an address; unicast is one-to-one, multicast one-to-many, broadcast one-to-all on a segment.

Latency components: propagation (distance/speed), serialization (bits/rate), queuing (load-dependent), and processing delay.

PCM at 8 kHz sampling and 8 bits per sample yields 64 kbit/s per voice channel (DS0/E0); 32 channels form a 2.048 Mbit/s E1.

SDH hierarchy: STM-1 = 155.52 Mbit/s; STM-4 = 622.08 Mbit/s; STM-16 = 2.488 Gbit/s. Each step is exactly 4×.

WDM spacing rules: CWDM uses 20 nm grid over roughly 1270–1610 nm; DWDM uses 0.8 nm (100 GHz) or 0.4 nm (50 GHz) spacing near 1550 nm.

OTN frame: OPU carries client payload, ODU adds overhead and TCM, OTU adds FEC and framing; OTU2 = 10.7 Gbit/s, OTU4 = 111.8 Gbit/s.

Switching modes: circuit (time/space switching), packet (store-and-forward), and cell/ATM (fixed 53-byte cells, 48-byte payload).

Protection: 1+1 sends on both paths permanently; 1:1 keeps standby for extra traffic; MSP rings typically restore in under 50 ms.

Fiber loss windows: 1310 nm ≈ 0.35 dB/km, 1550 nm ≈ 0.20–0.25 dB/km; connectors ≈ 0.3–0.5 dB each, splices ≈ 0.1 dB.

Link budget: received power = transmit power − fiber loss − splice/connector loss − margin; must exceed receiver sensitivity.

IPv4 header is 20 bytes minimum; TTL decrements per router and prevents loops. IPv6 header is fixed 40 bytes with no header checksum.

Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. NAT translates these to public addresses at the edge.

Subnet host count = 2^(32−prefix) − 2; a /30 gives 2 usable hosts, a /24 gives 254.

OSPF is link-state with cost = reference bandwidth / interface bandwidth; IS-IS is also link-state; RIP is distance-vector with hop count limit 15.

BGP is the inter-domain path-vector protocol; it selects by weight, local preference, AS-path length, origin, MED, then eBGP over iBGP.

TCP three-way handshake is SYN, SYN-ACK, ACK; congestion control uses slow start, congestion avoidance, fast retransmit.

DNS resolves names hierarchically; A/AAAA give addresses, MX mail servers, CNAME aliases, PTR reverse lookups.

DHCP DORA sequence: Discover, Offer, Request, Acknowledge. Leases are renewed at 50% of lease time by default.

GSM uses 200 kHz carriers, 8 timeslots per carrier, and Gaussian MSK modulation; a full-rate traffic channel is 13 kbit/s speech coded to 22.8 kbit/s.

UMTS (3G) uses WCDMA with 5 MHz carriers; spreading factor determines chip rate of 3.84 Mcps per user rate.

LTE uses OFDMA downlink and SC-FDMA uplink; channel bandwidths are 1.4, 3, 5, 10, 15, and 20 MHz.

5G NR subcarrier spacing is 15·2^μ kHz (15, 30, 60, 120); FR1 is below 7.125 GHz, FR2 is millimeter wave.

5G core is service-based (SBA) with network slicing; gNB connects via NG interfaces, and CU/DU split separates central and distributed units.

IEEE 802.11 channel facts: 2.4 GHz has 3 non-overlapping channels (1, 6, 11); 5 GHz offers many more; 6 GHz adds the Wi-Fi 6E band.

Cellular handover types: intra-frequency, inter-frequency, and inter-system (e.g., 5G to 4G fallback via EPS fallback or VoNR).

Link budget for radio: received power = EIRP − path loss + receive gain − margins; path loss grows with log(distance) and log(frequency).

PON shares one feeder fiber via a passive splitter; a 1:32 split means each subscriber shares the OLT's downstream capacity.

GPON: 2.488 Gbit/s downstream, 1.244 Gbit/s upstream, 1490 nm down / 1310 nm up, 1550 nm for video overlay.

XGS-PON: symmetric 10 Gbit/s, 1270 nm upstream and 1577 nm downstream; NG-PON2 adds tunable wavelength options.

PON downstream uses broadcast with GEM encapsulation; upstream uses TDMA with dynamic bandwidth allocation (DBA) grants.

ADSL2+ uses DMT with up to 2.2 MHz band; VDSL2 extends to 17a (17 MHz) or 35b (35 MHz) profiles, with vectoring to cancel crosstalk.

FTTx topologies: FTTH (fiber to home), FTTB (building), FTTC (curb/cabinet), each trading fiber reach against copper drop length.

Optical return loss and reflectance matter in PON because reflections degrade upstream burst reception; splitters add roughly 3.5 dB per 1:2 stage.

Power budget check: OLT launch power minus fiber/splitter/connector loss must stay within the ONU receiver sensitivity window.

Spectrum assignment models: beauty contest, comparative selection, auction (e.g., SMRA), and administrative assignment for public safety.

Licensing types: individual (site-specific), class/general authorization (e.g., Wi-Fi), and license-exempt under technical conditions.

ITU Radio Regulations divide the world into three regions; Region 1 covers Europe/Africa, Region 2 the Americas, Region 3 Asia-Pacific.

Regulatory duties typically include interconnection obligations, number portability, universal service funding, and tariff transparency.

Spectrum refarming moves bands from legacy to new uses (e.g., 700 MHz digital dividend, 3.5 GHz for 5G); coordination avoids harmful interference.

Licence conditions usually specify power limits, out-of-band emission masks, coverage obligations, and roll-out milestones.

Interconnection rules: cost-oriented termination rates, non-discrimination, and reference interconnection offers (RIO).

Numbering administration covers E.164 plans, number portability databases, and short codes; regulators audit allocation fairness.

AAA triad: authentication (identity), authorization (permissions), accounting (usage logging). RADIUS and Diameter are common carriers.

IPsec: AH provides integrity only; ESP provides confidentiality plus integrity. IKE negotiates SAs; tunnel mode hides inner headers.

TLS handshake establishes a session key via asymmetric exchange, then encrypts with symmetric ciphers; certificates bind identity to a public key.

DDoS mitigation layers: volumetric scrubbing, SYN cookies, rate limiting, and anycast dispersion across scrubbing centers.

Service assurance KPIs: availability = MTBF/(MTBF+MTTR); SLA breach triggers credits; monitoring uses SNMP, NetFlow, and synthetic probes.

Fault management loop: detect, isolate, correlate alarms, dispatch, restore, and verify; root-cause analysis reduces alarm storms.

Segmentation controls lateral movement: VLANs, VRFs, and zero-trust micro-segmentation limit blast radius after compromise.

Logging and forensics require synchronized time (NTP) and retention policies; hashing preserves evidence integrity.

SDN separates control plane from data plane; OpenFlow is one southbound protocol; the controller holds a global network view.

NFV decouples network functions from proprietary hardware; VNFs run on NFVI, orchestrated by MANO (NFVO, VNFM, VIM).

Cloud service models: IaaS (infrastructure), PaaS (platform), SaaS (application); deployment models: public, private, hybrid, community.

Edge computing places compute near users to cut latency; MEC in 5G hosts applications at the RAN edge.

IoT protocol stack options: MQTT (pub/sub, lightweight), CoAP (UDP-based REST), LoRaWAN (long range, low rate), NB-IoT (cellular LPWA).

Network slicing creates isolated logical networks over shared infrastructure with per-slice SLAs and QoS.

Containers versus VMs: containers share the host kernel and start faster; VMs provide stronger isolation via hypervisor.

Zero-touch provisioning and intent-based networking automate configuration from declarative policies rather than device-by-device CLI.

Revenue models: subscription (ARPU), usage-based, wholesale/access, and value-added services; churn measures subscriber loss rate.

Tariff structures: flat-rate, tiered, bucket/data-cap, and peak/off-peak pricing; bundling raises ARPU and reduces churn.

Key metrics: ARPU = revenue / subscribers; CAC vs LTV determines payback period; EBITDA margin indicates operating efficiency.

Cost structure splits into CAPEX (network build) and OPEX (operations, maintenance, power, licenses); depreciation spreads CAPEX.

Customer operations: order-to-cash, trouble ticketing, first-call resolution, and SLA-driven field dispatch.

Roaming and interconnection settlements use TAP files and wholesale rates; fraud controls include SIM-box and IRSF detection.

Regulated retail tariffs may apply to universal service or dominant operators; price transparency rules require clear contract terms.

Billing accuracy requires mediation between network records (CDRs) and the billing system, plus dispute and credit-note handling.

Standards bodies: ITU-T (telecom protocols), ITU-R (radio), 3GPP (cellular), IEEE (LAN), IETF (Internet RFCs), ETSI (European).

QoS classes: conversational (voice, <150 ms one-way latency), streaming, interactive, background; DSCP markings map to per-hop behavior.

ITU-T G.114 recommends one-way voice latency under 150 ms; jitter buffers trade delay against packet loss concealment.

MOS scales voice quality 1–5; E-model computes R-factor from delay, loss, and codec impairment.

Availability targets: five nines = 5.26 minutes downtime per year; four nines = 52.6 minutes.

Professional ethics: protect subscriber privacy, avoid conflicts of interest, report accurate test data, and follow licensing terms.

Net neutrality principles: no blocking, no throttling, transparency; regulators may allow reasonable traffic management.

Interoperability conformance testing uses standards-based test suites; certification marks indicate compliance with technical requirements.

OSI has 7 layers; TCP/IP model collapses them to 4 (Link, Internet, Transport, Application). Map by function, not by protocol name.

Circuit switching reserves a dedicated path for the call duration; packet switching multiplexes statistically, so it suits bursty data but needs congestion control.

Nyquist formula for a noiseless channel: C = 2B·log2(M) bit/s, where B is bandwidth in Hz and M is signal levels.

Shannon capacity: C = B·log2(1 + S/N). SNR in dB = 10·log10(S/N); 3 dB ≈ doubling of power.

Full-duplex uses separate directions simultaneously; half-duplex shares the medium, so effective throughput is reduced by turnaround and contention.

Access, metro, and core are the three structural tiers; core favors high capacity and resilience, access favors reach and cost per subscriber.

Anycast routes to the nearest instance of an address; unicast is one-to-one, multicast one-to-many, broadcast one-to-all on a segment.

Latency components: propagation (distance/speed), serialization (bits/rate), queuing (load-dependent), and processing delay.

PCM at 8 kHz sampling and 8 bits per sample yields 64 kbit/s per voice channel (DS0/E0); 32 channels form a 2.048 Mbit/s E1.

SDH hierarchy: STM-1 = 155.52 Mbit/s; STM-4 = 622.08 Mbit/s; STM-16 = 2.488 Gbit/s. Each step is exactly 4×.

WDM spacing rules: CWDM uses 20 nm grid over roughly 1270–1610 nm; DWDM uses 0.8 nm (100 GHz) or 0.4 nm (50 GHz) spacing near 1550 nm.

OTN frame: OPU carries client payload, ODU adds overhead and TCM, OTU adds FEC and framing; OTU2 = 10.7 Gbit/s, OTU4 = 111.8 Gbit/s.

Switching modes: circuit (time/space switching), packet (store-and-forward), and cell/ATM (fixed 53-byte cells, 48-byte payload).

Protection: 1+1 sends on both paths permanently; 1:1 keeps standby for extra traffic; MSP rings typically restore in under 50 ms.

Fiber loss windows: 1310 nm ≈ 0.35 dB/km, 1550 nm ≈ 0.20–0.25 dB/km; connectors ≈ 0.3–0.5 dB each, splices ≈ 0.1 dB.

Link budget: received power = transmit power − fiber loss − splice/connector loss − margin; must exceed receiver sensitivity.

IPv4 header is 20 bytes minimum; TTL decrements per router and prevents loops. IPv6 header is fixed 40 bytes with no header checksum.

Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. NAT translates these to public addresses at the edge.

Subnet host count = 2^(32−prefix) − 2; a /30 gives 2 usable hosts, a /24 gives 254.

OSPF is link-state with cost = reference bandwidth / interface bandwidth; IS-IS is also link-state; RIP is distance-vector with hop count limit 15.

BGP is the inter-domain path-vector protocol; it selects by weight, local preference, AS-path length, origin, MED, then eBGP over iBGP.

TCP three-way handshake is SYN, SYN-ACK, ACK; congestion control uses slow start, congestion avoidance, fast retransmit.

DNS resolves names hierarchically; A/AAAA give addresses, MX mail servers, CNAME aliases, PTR reverse lookups.

DHCP DORA sequence: Discover, Offer, Request, Acknowledge. Leases are renewed at 50% of lease time by default.

GSM uses 200 kHz carriers, 8 timeslots per carrier, and Gaussian MSK modulation; a full-rate traffic channel is 13 kbit/s speech coded to 22.8 kbit/s.

UMTS (3G) uses WCDMA with 5 MHz carriers; spreading factor determines chip rate of 3.84 Mcps per user rate.

LTE uses OFDMA downlink and SC-FDMA uplink; channel bandwidths are 1.4, 3, 5, 10, 15, and 20 MHz.

5G NR subcarrier spacing is 15·2^μ kHz (15, 30, 60, 120); FR1 is below 7.125 GHz, FR2 is millimeter wave.

5G core is service-based (SBA) with network slicing; gNB connects via NG interfaces, and CU/DU split separates central and distributed units.

IEEE 802.11 channel facts: 2.4 GHz has 3 non-overlapping channels (1, 6, 11); 5 GHz offers many more; 6 GHz adds the Wi-Fi 6E band.

Cellular handover types: intra-frequency, inter-frequency, and inter-system (e.g., 5G to 4G fallback via EPS fallback or VoNR).

Link budget for radio: received power = EIRP − path loss + receive gain − margins; path loss grows with log(distance) and log(frequency).

PON shares one feeder fiber via a passive splitter; a 1:32 split means each subscriber shares the OLT's downstream capacity.

GPON: 2.488 Gbit/s downstream, 1.244 Gbit/s upstream, 1490 nm down / 1310 nm up, 1550 nm for video overlay.

XGS-PON: symmetric 10 Gbit/s, 1270 nm upstream and 1577 nm downstream; NG-PON2 adds tunable wavelength options.

PON downstream uses broadcast with GEM encapsulation; upstream uses TDMA with dynamic bandwidth allocation (DBA) grants.

ADSL2+ uses DMT with up to 2.2 MHz band; VDSL2 extends to 17a (17 MHz) or 35b (35 MHz) profiles, with vectoring to cancel crosstalk.

FTTx topologies: FTTH (fiber to home), FTTB (building), FTTC (curb/cabinet), each trading fiber reach against copper drop length.

Optical return loss and reflectance matter in PON because reflections degrade upstream burst reception; splitters add roughly 3.5 dB per 1:2 stage.

Power budget check: OLT launch power minus fiber/splitter/connector loss must stay within the ONU receiver sensitivity window.

Spectrum assignment models: beauty contest, comparative selection, auction (e.g., SMRA), and administrative assignment for public safety.

Licensing types: individual (site-specific), class/general authorization (e.g., Wi-Fi), and license-exempt under technical conditions.

ITU Radio Regulations divide the world into three regions; Region 1 covers Europe/Africa, Region 2 the Americas, Region 3 Asia-Pacific.

Regulatory duties typically include interconnection obligations, number portability, universal service funding, and tariff transparency.

Spectrum refarming moves bands from legacy to new uses (e.g., 700 MHz digital dividend, 3.5 GHz for 5G); coordination avoids harmful interference.

Licence conditions usually specify power limits, out-of-band emission masks, coverage obligations, and roll-out milestones.

Interconnection rules: cost-oriented termination rates, non-discrimination, and reference interconnection offers (RIO).

Numbering administration covers E.164 plans, number portability databases, and short codes; regulators audit allocation fairness.

AAA triad: authentication (identity), authorization (permissions), accounting (usage logging). RADIUS and Diameter are common carriers.

IPsec: AH provides integrity only; ESP provides confidentiality plus integrity. IKE negotiates SAs; tunnel mode hides inner headers.

TLS handshake establishes a session key via asymmetric exchange, then encrypts with symmetric ciphers; certificates bind identity to a public key.

DDoS mitigation layers: volumetric scrubbing, SYN cookies, rate limiting, and anycast dispersion across scrubbing centers.

Service assurance KPIs: availability = MTBF/(MTBF+MTTR); SLA breach triggers credits; monitoring uses SNMP, NetFlow, and synthetic probes.

Fault management loop: detect, isolate, correlate alarms, dispatch, restore, and verify; root-cause analysis reduces alarm storms.

Segmentation controls lateral movement: VLANs, VRFs, and zero-trust micro-segmentation limit blast radius after compromise.

Logging and forensics require synchronized time (NTP) and retention policies; hashing preserves evidence integrity.

SDN separates control plane from data plane; OpenFlow is one southbound protocol; the controller holds a global network view.

NFV decouples network functions from proprietary hardware; VNFs run on NFVI, orchestrated by MANO (NFVO, VNFM, VIM).

Cloud service models: IaaS (infrastructure), PaaS (platform), SaaS (application); deployment models: public, private, hybrid, community.

Edge computing places compute near users to cut latency; MEC in 5G hosts applications at the RAN edge.

IoT protocol stack options: MQTT (pub/sub, lightweight), CoAP (UDP-based REST), LoRaWAN (long range, low rate), NB-IoT (cellular LPWA).

Network slicing creates isolated logical networks over shared infrastructure with per-slice SLAs and QoS.

Containers versus VMs: containers share the host kernel and start faster; VMs provide stronger isolation via hypervisor.

Zero-touch provisioning and intent-based networking automate configuration from declarative policies rather than device-by-device CLI.

Revenue models: subscription (ARPU), usage-based, wholesale/access, and value-added services; churn measures subscriber loss rate.

Tariff structures: flat-rate, tiered, bucket/data-cap, and peak/off-peak pricing; bundling raises ARPU and reduces churn.

Key metrics: ARPU = revenue / subscribers; CAC vs LTV determines payback period; EBITDA margin indicates operating efficiency.

Cost structure splits into CAPEX (network build) and OPEX (operations, maintenance, power, licenses); depreciation spreads CAPEX.

Customer operations: order-to-cash, trouble ticketing, first-call resolution, and SLA-driven field dispatch.

Roaming and interconnection settlements use TAP files and wholesale rates; fraud controls include SIM-box and IRSF detection.

Regulated retail tariffs may apply to universal service or dominant operators; price transparency rules require clear contract terms.

Billing accuracy requires mediation between network records (CDRs) and the billing system, plus dispute and credit-note handling.

Standards bodies: ITU-T (telecom protocols), ITU-R (radio), 3GPP (cellular), IEEE (LAN), IETF (Internet RFCs), ETSI (European).

QoS classes: conversational (voice, <150 ms one-way latency), streaming, interactive, background; DSCP markings map to per-hop behavior.

ITU-T G.114 recommends one-way voice latency under 150 ms; jitter buffers trade delay against packet loss concealment.

MOS scales voice quality 1–5; E-model computes R-factor from delay, loss, and codec impairment.

Availability targets: five nines = 5.26 minutes downtime per year; four nines = 52.6 minutes.

Professional ethics: protect subscriber privacy, avoid conflicts of interest, report accurate test data, and follow licensing terms.

Net neutrality principles: no blocking, no throttling, transparency; regulators may allow reasonable traffic management.

Interoperability conformance testing uses standards-based test suites; certification marks indicate compliance with technical requirements.