Physical and Data Link Layers (Signals, Ethernet, CSMA-CD)

Context: FIT1047_MOC · the bottom two layers, implemented together on the NIC — bits become signals (physical), signals become governed frames (data link)

Quick Revision

• 🎯 Objective: physical = transmit bits as digital (voltage square waves) or analog (modulated waves) signals; data link = Media Access Control + frames + error detection + MAC addresses.
• 📦 Core Components: encodings (unipolar/NRZ/NRZI/Manchester) ➔ modulations (frequency/amplitude/phase) ➔ Ethernet CSMA/CD.
• ⚡ Key Constraint: CSMA/CD’s three parts — sense before sending, shared medium, jam + random wait on collision — recite all three.

📝 Core

1. Physical Layer

• NIC ➔ implements physical AND data link layers: socket/antenna, framing, error detection, encoding/modulation; on-board or USB/PCIe.
• Media ➔ electrical signals in copper (UTP most common, STP, coax legacy), light in optical fibre, radio through space.
• Cable standards ➔ 10BASE-T (10 Mbps, cat 3/5) → 100BASE-T (cat 5) → 1000BASE-T (cat 5e/6) → 10GbE (cat 5e–7 or fibre) → 40GbE (fibre).
• Digital encodings ➔ voltage square waves: unipolar (+3 V/0 V) · NRZ (±3 V) · NRZI (invert on 1) · Manchester (transition mid-bit: low→high vs high→low — self-clocking).
• Analog modulation ➔ vary a sinusoid’s frequency (FSK), amplitude (ASK), or phase (PSK); combining e.g. 4 amplitudes × 2 phases = 8 symbols ⟹ 3 bits/symbol — $3\times$ the data rate. Most networks combine AM+PM.

2. Data Link Layer

• Duties ➔ control access to the physical medium (MAC), frames↔signals, error detection, interface up to Network layer, identify interfaces via MAC addresses.
• Contention-based MAC ➔ anyone transmits anytime ⟹ collisions damage packets ⟹ carrier-sense + detect + retransmit.
• Ethernet (IEEE 802.3) ➔ THE LAN technology since 1973/1980; originally 10 Mbps shared coax bus, now switched 100 Mbps–1 Gbps UTP, fibre to 100 Gbps.
• CSMA/CD ➔ Carrier Sense (listen first) + Multiple Access (shared medium) + Collision Detection (jam signal, random backoff, retransmit).
• Hub trap ➔ a hub repeats every signal to all ports: physical star, logical shared bus — still one collision domain.
• Shared-Ethernet problems ➔ half-duplex, everything broadcast to everyone, CSMA/CD bounds network size ⟹ solved by switches (true logical star, next lectures).

⚠️ Common Mistakes

• 💡 MAC has two meanings here ➔ Media Access Control (the mechanism) and MAC address (the identifier) — expand the acronym the exam asks about.
• 💡 Physical vs logical topology ➔ hub wiring LOOKS like a star but behaves as a bus; the collision domain follows the logical shape.
• 💡 More symbols ≠ more Hz ➔ AM+PM packs more bits per symbol on the same wave — rate rises without new spectrum.

🧠 Active Recall

Walk through CSMA/CD when two devices transmit simultaneously.

Answer

• Short answer: Both sensed silence and sent; signals overlap → each detects a foreign signal → transmits a jam signal so ALL devices notice → each waits a random time → retransmits.
• Why: Random backoff breaks symmetry ➔ identical waits would re-collide forever.

Why is Manchester encoding worth halving the symbol efficiency?

Answer

• Short answer: The guaranteed mid-bit transition carries the clock with the data — receiver never drifts on long runs of 0s or 1s.
• Why: Self-clocking ➔ NRZ’s flat stretches give no timing edges; Manchester trades bandwidth for synchronisation.