~3 دقیقه مطالعه • بروزرسانی ۵ مرداد ۱۴۰۴
1. What Is a Network?
A network enables devices to share data, printers, and communication tools like video conferencing. LANs (Local Area Networks) physically connect devices via Ethernet cables to switches or hubs.
2. Communication in LANs
Devices use MAC addresses for direct communication. Example:
Bob broadcasts: "Sally, where are you?"
Sally responds with her IP and MAC address
Bob retrieves MAC using ARP
Direct communication begins between MAC addresses
3. Ethernet Overview
Ethernet is a contention-based technology operating at Layer 1 (Physical) and Layer 2 (Data Link) of the OSI model. It’s popular for its simplicity and fault isolation.
4. Collision & Broadcast Domains
- Collision Domain: Multiple simultaneous transmissions cause interference
- Broadcast Domain: All devices hear a broadcast (MAC FF:FF:FF:FF:FF:FF)
5. CSMA/CD Protocol
1. Device senses media (is it free?)
2. If free, it sends data
3. If collision detected → send jam signal
4. All devices stop transmitting
5. After random backoff → retry
Collisions result in delay, congestion, and reduced throughput.
6. Baseband vs Broadband
| Type | Description | Example |
|---|---|---|
| Baseband | Single digital signal across the medium | Ethernet |
| Broadband | Multiple signals via frequency modulation | TV, Cable Internet |
7. Bit Rate vs Baud Rate
- Bit Rate: Bits transferred per second (e.g. 56,000 bps)
- Baud Rate: Number of signal changes per second
8. Wavelength & Fiber Optics
Wavelength measures the distance between peaks in EM waves. WDM allows multiple signals to travel via fiber optic cables at different wavelengths.
9. Half-Duplex vs Full-Duplex
| Mode | Description | Efficiency |
|---|---|---|
| Half-Duplex | Send or receive, not simultaneously | 30–40% |
| Full-Duplex | Send and receive at same time | Near 100% |
10. MAC Addressing in Ethernet
MAC Address Format: 00:14:22:BE:18:3B
- 48-bit unique identifier
- OUI: First 24 bits → vendor ID
- Device ID: Last 24 bits → specific NIC
- I/G bit: 0 = Unicast, 1 = Multicast/Broadcast
- L/G bit: 0 = Global, 1 = Locally modified
11. Ethernet Frame Structure (802.3)
Preamble: 7 bytes for synchronization
SOF: 1 byte start frame delimiter
Destination MAC (DA): 6 bytes
Source MAC (SA): 6 bytes
Type/Length: 2 bytes
Data Payload: 46 to 1500 bytes
FCS: 4 bytes (CRC for error detection)
12. Physical Ethernet Standards (IEEE 802.3)
| Standard | Speed | Medium | Distance |
|---|---|---|---|
| 10BaseT | 10 Mbps | UTP Cat 3 | 100 m |
| 100BaseTX | 100 Mbps | UTP Cat 5 | 100 m |
| 1000BaseT | 1 Gbps | UTP Cat 5e/6 | 100 m |
| 10GBaseT | 10 Gbps | UTP Cat 6A | 100 m |
| 40GBaseT | 40 Gbps | UTP Cat 8 | 30 m |
13. Ethernet Beyond Cables
- Powerline (IEEE 1901): Data transmission via electrical lines up to 500 Mbps
- HDMI Ethernet: Up to 100 Mbps over HDMI cables
14. Wavelength Multiplexing (WDM)
| Type | Channels | Distance | Range |
|---|---|---|---|
| CWDM | ≤18 | 70 km | 1310/1550 nm |
| DWDM | ≥80 | 1000 km+ | Dense spectrum |
15. Numeric Conversions
IP: 192.168.10.15
Binary: 11000000.10101000.00001010.00001111
Decimal: 204.51.170.85
Hexadecimal: CC.33.AA.55
16. Conclusion
Ethernet technologies—including MAC addressing, duplex modes, collision management, and physical cabling—are essential for building resilient networks. Understanding these principles ensures readiness for certifications like Network+, and builds the foundation for secure and scalable infrastructure design.
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