Matt A. Porter

Field Services Technician

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🧭 About Me

Matthew A. Porter
Former Intelligence Officer · Practical Technologist
TDS Field Services Technician · Founder, Amor Fati Labs

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📡 Fiber Optic Physics

Q: How is fiber optic internet physically different from copper/coax?

TLDR — light through glass beats electricity through metal.

Traditional telco infrastructure (DSL, POTS) transmits electrical signals over copper twisted-pair, while coaxial cable uses a copper core with shielding. Fiber optic cable transmits data as pulses of light through ultra-pure glass or plastic strands. Single-mode fiber (SMF) — what TDS deploys for FTTH — uses a 9 µm core diameter to carry a single light mode with virtually no modal dispersion, enabling multi-gigabit speeds over distances exceeding 40 km without amplification.

References

• TDS Fiber — What to Expect
• ITU-T G.652 — Single-Mode Optical Fibre and Cable
• IEEE 802.3 Ethernet Standards

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Total Internal Reflection — Why Light Stays in the Fiber

The critical angle $\theta_c$ at the core–cladding boundary is:

$$\theta_c = \arcsin!\left(\frac{n_2}{n_1}\right)$$

Symbol	Meaning	Typical value
$\theta_c$	Critical angle	~83.7° for SMF
$n_1$	Refractive index, core	1.468 (silica)
$n_2$	Refractive index, cladding	1.460 (silica cladding)

When light strikes the core–cladding boundary at an angle greater than $\theta_c$, it reflects entirely back into the core with zero energy lost to the cladding — the fundamental mechanism that makes fiber work.

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Numerical Aperture

$$\text{NA} = \sqrt{n_1^2 - n_2^2}$$

• Single-mode fiber: NA ≈ 0.12 — tight beam, one propagation mode, minimal dispersion
• Multi-mode fiber: NA ≈ 0.20–0.50 — wider acceptance cone, shorter distances

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Optical Signal Attenuation Along a Fiber Span

Signal power at distance $L$ (km) from the source:

$$P(L) = P_0 \cdot 10^{\displaystyle\left(\frac{-\alpha L}{10}\right)}$$

Or equivalently in dB, where the received power in dBm is:

$$P_{\text{dBm}}(L) = P_{0,\text{dBm}} - \alpha L$$

Symbol	Meaning	Typical value
$P_0$	Launch power	+3 to +7 dBm (OLT transmitter)
$\alpha$	Attenuation coefficient	0.35 dB/km @ 1310 nm · 0.20 dB/km @ 1550 nm
$L$	Fiber span length (km)	—

Example: A 20 km run at 1310 nm loses $0.35 \times 20 = 7\text{ dB}$ in fiber alone, before any connector or splice losses.

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Optical Splitter Insertion Loss

PON networks use passive 1:N optical splitters. Each split divides the optical power equally — but the dB loss does not divide equally: it adds logarithmically.

Ideal (theoretical) splitting loss for a 1:N splitter:

$$IL_{\text{split}} = 10 \log_{10}(N) \text{ dB}$$

Actual (excess loss included) insertion loss:

$$IL_{\text{actual}} = 10 \log_{10}(N) + IL_{\text{excess}}$$

Where $IL_{\text{excess}}$ accounts for manufacturing imperfections (typically 0.1–0.5 dB for a PLC splitter).

Split ratio $N$	Ideal $IL$	Typical actual $IL$
1:2	3.0 dB	3.4 dB
1:4	6.0 dB	6.7 dB
1:8	9.0 dB	9.8 dB
1:16	12.0 dB	13.0 dB
1:32	15.1 dB	16.0 dB
1:64	18.1 dB	19.2 dB

Cascaded splitters (e.g. a 1:4 feeding four 1:8s) add losses cumulatively:

$$IL_{\text{cascade}} = IL_1 + IL_2 = 10\log_{10}(N_1) + 10\log_{10}(N_2)$$

A 1:4 → 1:8 cascade gives an effective 1:32 split: $$IL = 6.0 + 9.0 = 15.0 \text{ dB (ideal)}$$

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Total Optical Power Budget

The full end-to-end loss from OLT to ONT must stay within the optical power budget (OPB) of the system:

$$\text{OPB} = P_{\text{tx}} - P_{\text{rx,min}}$$

$$L_{\text{total}} = \underbrace{\alpha_f \cdot d}{\text{fiber}} + \underbrace{\sum IL{\text{split}}}{\text{splitters}} + \underbrace{N_c \cdot \alpha_c}{\text{connectors}} + \underbrace{N_s \cdot \alpha_s}{\text{splices}} + \underbrace{M}{\text{margin}}$$

Term	Meaning	Typical value
$\alpha_f$	Fiber attenuation	0.35 dB/km (1310 nm)
$d$	Fiber distance (km)	—
$IL_{\text{split}}$	Splitter insertion loss	See table above
$\alpha_c$	Connector loss per pair	0.3–0.5 dB
$\alpha_s$	Splice loss per joint	0.02–0.1 dB
$M$	System margin (ageing, temp)	1–3 dB

XGS-PON (ITU-T G.9807.1) class N2: OPB = 29 dB — sufficient for a 1:32 split over ~20 km of feeder fiber.

References

• ITU-T G.9807.1 — XGS-PON Standard
• Calix AXOS Platform
• Broadband Forum Resources

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Shannon–Hartley — Maximum Theoretical Throughput

$$C = B \log_2!\left(1 + \frac{S}{N}\right)$$

Symbol	Meaning
$C$	Channel capacity (bits/second)
$B$	Bandwidth (Hz)
$S/N$	Signal-to-noise ratio (linear)

Fiber's near-zero noise floor and multi-THz optical bandwidth give it a theoretical capacity orders of magnitude above copper. A single SMF strand with dense wavelength-division multiplexing (DWDM) can carry >100 Tbps in laboratory conditions.

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Q: Why does fiber have symmetrical upload and download speeds?

Copper-based technologies (DSL, cable) are engineered asymmetrically because electrical signal transmission over legacy plant degrades upstream frequencies faster than downstream. Fiber carries light — upload and download travel on separate wavelengths (1310 nm upstream / 1490 nm downstream on GPON, or time-division multiplexed on XGS-PON) with identical physical characteristics in both directions, yielding true symmetrical throughput.

Q: What is latency like on fiber vs. copper?

Light travels through silica fiber at approximately $2 \times 10^8$ m/s (roughly ⅔ the speed of light in vacuum). Propagation delay for a 100 km fiber span is ~0.5 ms one-way. DSL adds not only propagation delay but also interleaving delay (up to 20 ms on ADSL2+) and the latency of active amplification stages. Typical TDS Fiber round-trip latency to regional infrastructure is under 5 ms.

Q: Do I need special equipment to use TDS Fiber?

TDS provides and installs an Optical Network Terminal (ONT) at your premises — a small device that converts the optical signal to Ethernet. You connect your router to the ONT's Ethernet port. No phone filters, DSL modems, or coax splitters needed.

References

• What is an ONT? — TDS Support
• Wi-Fi 6 Certified — Wi-Fi Alliance
• FCC Types of Broadband Connections

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⚛️ Quantum Physics of Fiber Optics & Telecom Security

Fiber optic networks are fundamentally quantum-mechanical — every bit transmitted is carried by photons whose behavior is governed by quantum physics. This section covers the quantum principles directly relevant to fiber optic signal transmission, quantum-secured telecommunications, and the physical limits of optical detection.

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Photon Energy in Optical Fiber

Every signal in a fiber optic network is carried by photons. The energy of a single photon at the operating wavelength is:

$$E = h\nu = \frac{hc}{\lambda}$$

Symbol	Meaning	Typical value
$h$	Planck's constant	$6.626 \times 10^{-34}$ J·s
$\nu$	Photon frequency	~229 THz (1310 nm) · ~194 THz (1550 nm)
$c$	Speed of light in vacuum	$3 \times 10^8$ m/s
$\lambda$	Wavelength	1310 nm (upstream) · 1550 nm (downstream)

At 1550 nm, each photon carries approximately $1.28 \times 10^{-19}$ J. A typical GPON transmitter at +5 dBm (~3.2 mW) emits roughly $2.5 \times 10^{16}$ photons per second — but after splitter and fiber losses, an ONT receiver may detect only $10^{6}$–$10^{9}$ photons per second, approaching regimes where quantum noise becomes significant.

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Photon Coherence & Polarization in Fiber

Photons in fiber exhibit quantum properties that directly affect signal quality:

Coherence length determines how far a photon's wave packet maintains a stable phase relationship — critical for coherent detection in advanced PON and DWDM systems:

$$L_c = \frac{c}{\Delta\nu}$$

where $\Delta\nu$ is the spectral linewidth of the laser source. Narrow-linewidth lasers used in coherent optical systems have $L_c > 100$ km, enabling phase-sensitive detection over long fiber spans.

Polarization — each photon carries a polarization state that can be represented on the Poincaré sphere. Fiber birefringence rotates this state unpredictably (polarization mode dispersion, PMD), causing signal degradation:

$$\Delta\tau_{\text{PMD}} = D_{\text{PMD}} \sqrt{L}$$

Symbol	Meaning	Typical value
$\Delta\tau_{\text{PMD}}$	Differential group delay	—
$D_{\text{PMD}}$	PMD coefficient	$\leq 0.1$ ps/$\sqrt{\text{km}}$ (modern SMF)
$L$	Fiber length (km)	—

PMD is a quantum-mechanical effect — it arises from photon polarization states coupling differently to stress-induced birefringence in the fiber.

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Quantum Noise Limits on Fiber Optic Detection

The ultimate sensitivity of any fiber optic receiver is set by quantum shot noise — the irreducible noise arising from the discrete, random arrival of photons at the detector.

Shot noise power in a photodetector:

$$\sigma_{\text{shot}}^2 = 2qI_pB$$

Symbol	Meaning
$q$	Electron charge ($1.602 \times 10^{-19}$ C)
$I_p$	Photocurrent (proportional to received optical power)
$B$	Electrical bandwidth of the receiver

Quantum-limited signal-to-noise ratio for direct detection:

$$\text{SNR}_{\text{quantum}} = \frac{\eta P}{h\nu B}$$

where $\eta$ is the photodetector quantum efficiency and $P$ is the received optical power. This sets the quantum limit — no classical receiver can do better without squeezed light or other quantum-enhanced techniques.

Sensitivity at the quantum limit — the minimum number of photons per bit required for a given bit error rate (BER):

$$\bar{n}_p = -\ln(\text{BER})$$

For $\text{BER} = 10^{-9}$ (standard telecom target), $\bar{n}_p \approx 20$ photons per bit. Real receivers require 10–100× more due to thermal noise and amplifier noise, but this quantum limit defines the theoretical floor.

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Coherent Detection — Homodyne & Heterodyne

Modern high-capacity fiber systems (100G+ DWDM) use coherent detection, which mixes the received signal with a local oscillator (LO) laser to recover both amplitude and phase. This is inherently quantum-mechanical — it measures the quadratures of the optical field.

Homodyne detection (LO frequency = signal frequency) measures one quadrature:

$$I_{\text{out}} \propto E_{\text{sig}} E_{\text{LO}} \cos(\phi_{\text{sig}} - \phi_{\text{LO}})$$

Heterodyne detection (LO offset by intermediate frequency $\Delta f$) recovers both quadratures simultaneously:

$$I_{\text{out}} \propto E_{\text{sig}} E_{\text{LO}} \cos(2\pi \Delta f \cdot t + \phi_{\text{sig}} - \phi_{\text{LO}})$$

Coherent receivers achieve near-quantum-limited sensitivity and enable advanced modulation formats (QPSK, 16-QAM) that multiply fiber capacity. The quantum noise floor for coherent detection is one photon per mode — the standard quantum limit (SQL).

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Quantum Key Distribution (QKD) Over Fiber

QKD is the primary application of quantum mechanics to telecom security. It uses the quantum properties of photons transmitted through fiber to establish provably secure encryption keys.

BB84 Protocol — the foundation of fiber-based QKD:

1. Alice sends single photons through fiber, each randomly prepared in one of four polarization states across two bases:
   • Rectilinear basis ($+$): $|H\rangle$ or $|V\rangle$ (horizontal / vertical)
   • Diagonal basis ($\times$): $|D\rangle$ or $|A\rangle$ (diagonal / anti-diagonal)
2. Bob randomly chooses a measurement basis for each received photon
3. They publicly compare bases (not results) and keep only matching-basis measurements
4. The no-cloning theorem guarantees that any eavesdropper (Eve) attempting to intercept and re-send photons introduces detectable errors

Quantum Bit Error Rate (QBER):

$$\text{QBER} = \frac{N_{\text{error}}}{N_{\text{sifted}}}$$

QBER threshold	Meaning
$\text{QBER} < 11%$	Secure key extraction possible (BB84)
$\text{QBER} \geq 11%$	Eavesdropper likely present — key discarded

The QBER arises from fiber imperfections (polarization drift, dark counts in single-photon detectors), and any eavesdropping adds to it — this is the security guarantee.

Secure key rate for a decoy-state BB84 QKD system over fiber:

$$R_{\text{key}} = q \left[ Q_1 \left(1 - H_2(e_1)\right) - Q_\mu f_e H_2(E_\mu) \right]$$

Symbol	Meaning
$q$	Protocol efficiency factor (1/2 for BB84)
$Q_1$	Single-photon gain
$e_1$	Single-photon error rate
$Q_\mu$	Overall gain at signal intensity $\mu$
$E_\mu$	Overall QBER at signal intensity $\mu$
$f_e$	Error correction efficiency (~1.16)
$H_2(x)$	Binary Shannon entropy: $-x\log_2 x - (1-x)\log_2(1-x)$

Practical fiber QKD performance:

Parameter	Typical value
Operating wavelength	1550 nm (C-band, lowest fiber loss)
Maximum fiber distance (standard BB84)	~100 km
Maximum fiber distance (twin-field QKD)	~600 km
Secure key rate (50 km fiber)	~1–100 kbit/s
Single-photon detector	InGaAs APD or SNSPD

Fiber attenuation limits QKD range. Since QKD requires single photons (which cannot be amplified without destroying their quantum state), the range is directly limited by fiber loss:

$$P_{\text{received}} = P_0 \cdot 10^{-\alpha L / 10}$$

At 1550 nm ($\alpha = 0.2$ dB/km), a 100 km span attenuates by 20 dB — reducing the photon count by 99%. This is why QKD distance is fundamentally constrained by fiber attenuation, unlike classical signals that can be optically amplified.

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Entanglement-Based QKD (E91 Protocol) Over Fiber

An alternative QKD approach uses entangled photon pairs distributed through fiber:

1. A source generates polarization-entangled photon pairs (e.g., via spontaneous parametric down-conversion) and sends one photon to Alice and one to Bob through separate fibers
2. Alice and Bob each randomly measure in one of three bases
3. Bell's inequality violation confirms entanglement was preserved through the fiber — proving no eavesdropper intercepted the photons

The Bell inequality for the CHSH variant:

$$S = |E(a,b) - E(a,b') + E(a',b) + E(a',b')| \leq 2$$

Quantum mechanics predicts $S \leq 2\sqrt{2} \approx 2.83$. Violation of $S > 2$ confirms entanglement and guarantees security.

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Post-Quantum Cryptography for Telecom Networks

While QKD secures the key exchange channel using physics, the broader telecom infrastructure must also prepare for the threat quantum computers pose to existing public-key cryptography used in network protocols (TLS, IPsec, MPLS signaling).

Shor's algorithm can factor large integers in polynomial time, breaking RSA and ECC:

$$O!\left((\log N)^2 (\log \log N) (\log \log \log N)\right)$$

This directly threatens the cryptographic foundations of telecom signaling, authentication, and session establishment. In response, NIST standardized post-quantum cryptographic algorithms (2024) for deployment across telecom infrastructure:

Algorithm	Type	Telecom application
ML-KEM (CRYSTALS-Kyber)	Lattice-based key encapsulation	TLS key exchange in network management
ML-DSA (CRYSTALS-Dilithium)	Lattice-based digital signature	Certificate authentication for network elements
SLH-DSA (SPHINCS+)	Hash-based signature	Firmware signing for ONTs and OLTs

References

• ITU-T Y.3800 — Framework for Quantum Key Distribution Networks
• ETSI QKD Industry Specification Group
• NIST Post-Quantum Cryptography Standards
• Introduction to Quantum Computing — LinkedIn Learning Certificate
• Toshiba QKD Systems

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📜 Certifications & Credentials

Full certificate index: certs/README.md

Selected credentials:

Certificate	Provider
Introduction to Quantum Computing	LinkedIn Learning
CompTIA Security+ SY0-701 Prep	LinkedIn Learning
AI in RAN — Radio Access Network	LinkedIn Learning
Understanding Copper & Fiber Optic Systems	LinkedIn Learning
Introduction to Telecom Standards, Networks & Innovations	LinkedIn Learning
Linux System Engineer: Networking & SSH	LinkedIn Learning
ArcGIS Python Scripting	LinkedIn Learning
InfraWorks & ArcGIS AEC Collaboration	LinkedIn Learning
GitHub Actions	LinkedIn Learning
Project Management Foundations	LinkedIn Learning

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Views expressed here are my own and do not represent TDS Telecom.