Chapitre 1
Chapter 01: Fondamentaux Réseau
Chapter 01: Fondamentaux Réseau
Chapter 01: Fondamentaux Réseau
1. Network Topologies
1.1 Bus Topology
In a bus topology, all devices share a single communication line (the "bus"). Data travels in both directions; each device checks if the data is addressed to it.
Diagramme en cours de génération...
Characteristics:
- Simple and inexpensive for small networks
- Single point of failure: the bus cable
- Limited length and device count
- Collisions common (CSMA/CD needed)
- Historical: 10BASE2 (ThinNet) and 10BASE5 (ThickNet)
Pros:
- Easy to implement
- Low cable cost
- Good for temporary setups
Cons:
- Difficult to troubleshoot
- Single break takes down entire network
- Performance degrades with many devices
- Nearly obsolete in modern networks
1.2 Star Topology
All devices connect to a central hub or switch. This is the dominant topology in modern networks.
Diagramme en cours de génération...
Characteristics:
- Central device (switch/hub) required
- Each device has dedicated connection
- Fault isolation is easy
- Most common topology today
Pros:
- Easy to install and manage
- Fault tolerant (one cable fails, rest work)
- Easy to add/remove devices
- High performance (switched)
Cons:
- Central device is single point of failure
- Higher cable cost
- Requires active electronics at center
1.3 Ring Topology
Each device connects to two neighbors, forming a ring. Data travels in one direction around the ring.
Diagramme en cours de génération...
Characteristics:
- Token passing (e.g., Token Ring, FDDI)
- Predictable performance
- Self-healing with dual rings (FDDI)
Pros:
- Fair access (token passing)
- Predictable performance under load
- Dual-ring variants provide redundancy
Cons:
- Single break can disrupt entire ring (without dual ring)
- Adding/removing devices disrupts the network
- Harder to troubleshoot than star
- Largely replaced by Ethernet
1.4 Mesh Topology
Every device connects to every other device (full mesh) or a subset (partial mesh).
Diagramme en cours de génération...
Characteristics:
- Full mesh: N(N-1)/2 links for N devices
- Partial mesh: strategic connectivity
- Maximum redundancy and reliability
Pros:
- Excellent fault tolerance
- No single point of failure
- Optimal routing possible
Cons:
- Very expensive (cabling cost scales with N²)
- Complex configuration
- Usually reserved for backbone/networks core
1.5 Hybrid Topologies
Most real networks use hybrid topologies combining multiple types.
Diagramme en cours de génération...
1.6 Topology Comparison
| Aspect | Bus | Star | Ring | Mesh |
|---|---|---|---|---|
| Cost | Low | Medium | Medium | High |
| Reliability | Low | Medium | Medium | Very High |
| Scalability | Poor | Good | Fair | Poor |
| Troubleshooting | Hard | Easy | Hard | Hard |
| Modern usage | Obsolete | Dominant | Niche (OT) | Core/DC |
2. Transmission Media
2.1 Copper Cabling
Twisted Pair (UTP/STP):
The most common cabling for local networks. Two conductors twisted together to reduce electromagnetic interference.
| Category | Max Frequency | Max Speed | Max Distance | Application |
|---|---|---|---|---|
| Cat 3 | 16 MHz | 10 Mbps | 100 m | Voice, 10BASE-T |
| Cat 5 | 100 MHz | 100 Mbps | 100 m | 100BASE-TX |
| Cat 5e | 100 MHz | 1 Gbps | 100 m | 1000BASE-T |
| Cat 6 | 250 MHz | 1 Gbps | 100 m | 1000BASE-T |
| Cat 6a | 500 MHz | 10 Gbps | 100 m | 10GBASE-T |
| Cat 7 | 600 MHz | 10 Gbps | 100 m | Shielded |
| Cat 7a | 1000 MHz | 40 Gbps | 50 m | Data centers |
| Cat 8 | 2000 MHz | 40 Gbps | 30 m | Data centers |
Pinout (T568A and T568B):
Diagramme en cours de génération...
Coaxial Cable:
- Used historically (10BASE2, 10BASE5)
- Still used for cable TV/internet (RG-6)
- Higher bandwidth than twisted pair
- Better shielding
2.2 Fiber Optic Cable
Fiber optics use light pulses transmitted through glass or plastic fibers.
Diagramme en cours de génération...
Single-Mode Fiber (SMF):
- Core diameter: 8-10 µm
- Light source: Laser (1310 nm, 1550 nm)
- Distance: Up to 100+ km
- Bandwidth: Essentially unlimited
- Color: Yellow jacket
- Standards: OS1, OS2
Multi-Mode Fiber (MMF):
- Core diameter: 50 µm (OM2-OM5) or 62.5 µm (OM1)
- Light source: LED or VCSEL (850 nm, 1300 nm)
- Distance: Up to 2 km (OM4/OM5)
- Bandwidth: Lower than SMF due to modal dispersion
- Color: Aqua (OM3/OM4), Lime (OM5)
- Standards: OM1, OM2, OM3, OM4, OM5
| Fiber Type | Core (µm) | Wavelength | Max Speed | Max Distance |
|---|---|---|---|---|
| OM1 | 62.5 | 850 nm | 1 Gbps | 275 m |
| OM2 | 50 | 850 nm | 1 Gbps | 550 m |
| OM3 | 50 | 850 nm | 100 Gbps | 100 m |
| OM4 | 50 | 850 nm | 100 Gbps | 150 m |
| OM5 | 50 | 850-950 nm | 100 Gbps | 150 m |
| OS2 (SMF) | 9 | 1310/1550 nm | 800 Gbps+ | 100 km+ |
Connector Types:
- LC: Lucent Connector (small form factor, dominant)
- SC: Subscriber Connector (push-pull)
- ST: Straight Tip (bayonet)
- MPO/MTP: Multi-fiber (12-24 fibers)
- FC: Ferrule Connector (screw-on)
2.3 Wireless / Radio
Frequency Bands:
| Band | Frequency | Range | Use |
|---|---|---|---|
| VLF | 3-30 kHz | Very long | Submarine |
| LF | 30-300 kHz | Long | Navigation |
| MF | 300-3000 kHz | Medium | AM radio |
| HF | 3-30 MHz | Short | Shortwave |
| VHF | 30-300 MHz | Short | TV, FM radio |
| UHF | 300-3000 MHz | Short | WiFi, cellular |
| SHF | 3-30 GHz | Line of sight | WiFi, 5G, satellite |
| EHF | 30-300 GHz | Very short | 6G, research |
WiFi Frequencies:
| Band | Range | Channels | Key Feature |
|---|---|---|---|
| 2.4 GHz | Longer range | 14 (3 non-overlap) | Better penetration |
| 5 GHz | Shorter range | 25+ non-overlap | Higher throughput |
| 6 GHz (WiFi 6E/7) | Short range | 59+ non-overlap | Ultra-wide spectrum |
2.4 Media Selection Criteria
| Criteria | Copper (UTP) | Fiber (SMF) | Wireless |
|---|---|---|---|
| Cost per meter | Low | Medium | N/A |
| Installation | Easy | Difficult | Easy |
| Bandwidth | Up to 40 Gbps | Unlimited | Up to 46 Gbps |
| Distance | 100 m | 100 km+ | 10-100 m |
| Interference | Susceptible | Immune | Susceptible |
| Security | Moderate | High | Low |
| Power over cable | PoE (802.3af/at/bt) | No | No |
3. Transmission Modes
3.1 Simplex (One Direction)
Data flows in only one direction. Like a radio broadcast.
Diagramme en cours de génération...
Examples:
- Radio/TV broadcast
- GPS satellite to receiver
- Pager system
3.2 Half-Duplex (Both Directions, One at a Time)
Data can flow both ways but only one direction at a time. Like a walkie-talkie.
Diagramme en cours de génération...
Examples:
- Walkie-talkies
- WiFi (half-duplex by nature)
- RS-485 bus
- Old Ethernet (hub-based, CSMA/CD)
3.3 Full-Duplex (Both Directions Simultaneously)
Data can flow both ways simultaneously. Like a telephone call.
Diagramme en cours de génération...
Examples:
- Telephone network
- Switched Ethernet (full-duplex)
- Fiber optic links (separate fibers for TX/RX)
- Cellular (FDD mode)
3.4 Auto-Negotiation
Ethernet devices negotiate speed and duplex mode automatically (IEEE 802.3ab).
Diagramme en cours de génération...
4. Bandwidth, Throughput, and Latency
4.1 Bandwidth
Theoretical maximum data rate of a medium. Unit: bits per second (bps).
- 1 kbps = 10³ bps
- 1 Mbps = 10⁶ bps
- 1 Gbps = 10⁹ bps
- 1 Tbps = 10¹² bps
4.2 Throughput
Actual data transfer rate achieved. Always ≤ bandwidth.
Factors affecting throughput:
- Protocol overhead
- Congestion
- Errors/retransmissions
- Hardware limitations
4.3 Latency
Delay from source to destination. Components:
Diagramme en cours de génération...
- Processing Delay: Time to process packet header (microseconds)
- Queueing Delay: Time waiting in output queue (variable)
- Transmission Delay: Time to push bits onto wire = packet_size / bandwidth
- Propagation Delay: Time for signal to travel = distance / propagation_speed
Propagation speed:
- Copper: ~0.67c (~2 × 10⁸ m/s)
- Fiber: ~0.67c (~2 × 10⁸ m/s)
- Free space: c (3 × 10⁸ m/s)
4.4 Bandwidth-Delay Product (BDP)
BDP = Bandwidth × RTT (Round Trip Time)
The amount of data "in flight" in the network. Critical for TCP window sizing.
Example: 10 Gbps link, 100 ms RTT:
- BDP = 10 × 10⁹ × 0.1 = 1 × 10⁹ bits = 125 MB
4.5 Link Budget Calculation
Diagramme en cours de génération...
Calculation:
- Transmit power: -3 dBm
- Fiber loss: 2 km × 0.5 dB/km = -1 dB
- Splice loss: -0.1 dB
- Patch panel: -0.5 dB
- Received power: -3 - 1 - 0.1 - 0.5 = -4.6 dBm
- Margin: -4.6 - (-20) = 15.4 dB
5. Multiplexing Techniques
5.1 Frequency Division Multiplexing (FDM)
Each signal uses a different frequency range (carrier wave). Used in:
- Radio/TV broadcast
- Fiber optic (WDM is a form of FDM)
- DSL (frequency division on phone line)
Diagramme en cours de génération...
5.2 Time Division Multiplexing (TDM)
Each signal uses the entire bandwidth but in time slots.
Diagramme en cours de génération...
Types:
- Synchronous TDM (STDM): Fixed time slots, even if idle
- Statistical TDM: Dynamic allocation based on demand
5.3 Wavelength Division Multiplexing (WDM)
FDM applied to fiber optics. Each signal uses a different wavelength (color) of light.
Diagramme en cours de génération...
| WDM Type | Channels | Spacing | Typical Capacity |
|---|---|---|---|
| CWDM | 18 | 20 nm | 200 Gbps |
| DWDM | 80+ | 0.4/0.8 nm | 8 Tbps+ |
| Flex-Grid | Variable | 12.5 GHz | 50 Tbps+ |
5.4 Code Division Multiple Access (CDMA)
Each signal uses a unique code (spread spectrum). Used in 3G cellular.
6. Physical Layer Standards
6.1 Ethernet Naming Convention
[Speed][Type][PHY] or [Speed]BASE-[Medium][-Additional]
| Name | Speed | Medium | Max Length | Encoding |
|---|---|---|---|---|
| 10BASE-T | 10 Mbps | Cat 3+ | 100 m | Manchester |
| 100BASE-TX | 100 Mbps | Cat 5 | 100 m | MLT-3 |
| 1000BASE-T | 1 Gbps | Cat 5e | 100 m | PAM-5 |
| 10GBASE-T | 10 Gbps | Cat 6a | 100 m | PAM-16 |
| 10GBASE-SR | 10 Gbps | MMF (OM3) | 300 m | 64B/66B |
| 10GBASE-LR | 10 Gbps | SMF | 10 km | 64B/66B |
| 10GBASE-ER | 10 Gbps | SMF | 40 km | 64B/66B |
| 100GBASE-SR10 | 100 Gbps | MMF | 150 m | 10×10G |
| 100GBASE-LR4 | 100 Gbps | SMF | 10 km | 4×25G WDM |
| 400GBASE-DR4 | 400 Gbps | SMF | 500 m | 4×100G PAM4 |
| 400GBASE-LR8 | 400 Gbps | SMF | 10 km | 8×50G WDM |
| 800GBASE-DR8 | 800 Gbps | SMF | 500 m | 8×100G PAM4 |
6.2 WiFi 802.11 Physical Layers
| PHY Standard | Band | Modulation | Max Rate |
|---|---|---|---|
| DSSS (802.11b) | 2.4 GHz | DSSS | 11 Mbps |
| OFDM (802.11a/g) | 5/2.4 GHz | OFDM | 54 Mbps |
| HT-OFDM (802.11n) | 2.4/5 GHz | OFDM + MIMO | 600 Mbps |
| VHT-OFDM (802.11ac) | 5 GHz | OFDM + MU-MIMO | 3.5 Gbps |
| HE-OFDM (802.11ax) | 2.4/5/6 GHz | OFDMA | 9.6 Gbps |
| EHT-OFDM (802.11be) | 2.4/5/6 GHz | 4096-QAM, 16× | 46 Gbps |
7. Networking Hardware
7.1 Network Interface Card (NIC)
Every network device needs a NIC. Modern NICs are integrated into motherboards.
NIC Components:
- PHY (Physical layer transceiver)
- MAC (Media Access Control) controller
- MAC address (48-bit, burned-in)
- Transceiver/connector
- DMA engine (for PCIe NICs)
Diagramme en cours de génération...
7.2 Transceivers (SFP, QSFP, OSFP)
Small form-factor pluggable modules for media conversion.
| Form Factor | Width | Channels | Max Speed |
|---|---|---|---|
| SFP | 1 | 1 | 4.25 Gbps |
| SFP+ | 1 | 1 | 16 Gbps |
| SFP28 | 1 | 1 | 28 Gbps |
| QSFP+ | ~1.5× | 4 | 40 Gbps |
| QSFP28 | ~1.5× | 4 | 100 Gbps |
| QSFP56 | ~1.5× | 4 | 200 Gbps |
| QSFP-DD | ~2× | 8 | 800 Gbps |
| OSFP | ~2× | 8 | 1.6 Tbps |
Diagramme en cours de génération...
7.3 Cabling Standards
Structured Cabling (TIA/EIA-568):
Diagramme en cours de génération...
Cable Types:
- Straight-through: Pin 1→1, 2→2, etc. (PC to switch)
- Crossover: Pin 1→3, 2→6, etc. (PC to PC, switch to switch)
- Rollover: Cisco console cable
7.4 Power over Ethernet (PoE)
Delivers power over Ethernet cabling alongside data.
| Standard | Power per Port | Pins Used | Year |
|---|---|---|---|
| 802.3af (PoE) | 15.4 W | 4 (2 pairs) | 2003 |
| 802.3at (PoE+) | 30 W | 4 (2 pairs) | 2009 |
| 802.3bt Type 3 (PoE++) | 60 W | 8 (4 pairs) | 2018 |
| 802.3bt Type 4 | 90 W | 8 (4 pairs) | 2018 |
Applications: IP phones, cameras, WiFi APs, IoT devices
8. Signal Encoding
8.1 Digital Encoding Schemes
| Scheme | Description | Used In |
|---|---|---|
| NRZ | 1 = high, 0 = low | Fast Ethernet |
| NRZI | 1 = transition, 0 = no transition | USB |
| Manchester | XOR with clock | 10BASE-T |
| MLT-3 | Three-level | 100BASE-TX |
| PAM-5 | Five-level (2 bits + FEC) | 1000BASE-T |
| PAM-4 | Four-level (2 bits) | 400GBASE-DR4 |
| 64B/66B | Block encoding | 10GBASE |
8.2 Line Coding
Diagramme en cours de génération...
9. Error Detection
9.1 CRC (Cyclic Redundancy Check)
- Used in Ethernet, WiFi, and many other protocols
- Frame Check Sequence (FCS) at end of Ethernet frame
- 32-bit CRC (CRC-32) standard for Ethernet
9.2 Forward Error Correction (FEC)
- Used in high-speed optics (PAM4 requires FEC)
- RS-FEC (Reed-Solomon)
- FC-FEC (Fire Code)
- Enables higher data rates with acceptable BER
10. Summary and Key Formulas
Key formulas every network engineer must know:
| Formula | Description |
|---|---|
| Throughput = WindowSize / RTT | TCP throughput limit |
| BDP = Bandwidth × RTT | Bandwidth-delay product |
| Latency = D_processing + D_queueing + D_transmission + D_propagation | Total latency |
| Light in fiber ≈ 0.67c | Propagation speed in glass |
| Signal loss (dB) = 10 × log10(P_out / P_in) | Power ratio in dB |
11. Exercises
Exercise 1: Topology Identification
Given a network diagram, identify the topology type and calculate the number of links needed for full mesh.
Exercise 2: Link Budget
Calculate the received power for: Tx = 0 dBm, 10 km SMF (0.3 dB/km), 2 splices (0.1 dB each), 2 connectors (0.5 dB each). Rx sensitivity = -16 dBm. Is there enough margin?
Exercise 3: Transmission Delay
Calculate the transmission delay for a 1500-byte packet on a 10 Gbps link.
Exercise 4: BDP Calculation
Calculate the BDP for a 400 Gbps link with 50 ms RTT.