1.3 Physical Interfaces & Cabling
Layer 1 of the OSI model. Master the copper Ethernet standards, fiber optic classifications, and the exact physical pinouts required to terminate and troubleshoot network cables.
IEEE Ethernet Copper Standards
| Speed | Common Name | IEEE Standard | Informal Name | Max Length |
|---|---|---|---|---|
| 10 Mbps | 10BASE-T | 802.3i | Ethernet | 100m |
| 100 Mbps | 100BASE-TX | 802.3u | Fast Ethernet (FE) | 100m |
| 1 Gbps | 1000BASE-T | 802.3ab | Gigabit Ethernet (GigE) | 100m |
| 10 Gbps | 10GBASE-T | 802.3an | 10 GigE | 100m (Cat6a) / 55m (Cat6) |
Inside Unshielded Twisted Pair (UTP)
Why are they twisted?
Electrical currents running through wires generate magnetic fields, which can cause Electromagnetic Interference (EMI) and bleed over into adjacent wires, causing Crosstalk.
By twisting the pairs together, the magnetic fields of the two wires are exactly opposite and cancel each other out. Furthermore, each color pair (Blue, Orange, Green, Brown) is twisted at a slightly different rate (twists per inch) to prevent crosstalk between the pairs themselves.
Transmission Operations
- 10 / 100BASE-TOnly uses 2 pairs (4 wires total). One pair is strictly dedicated to Transmitting (TX), and the other pair is strictly dedicated to Receiving (RX).
- 1000BASE-TUses all 4 pairs (8 wires total). It utilizes advanced encoding so that every single pair is bi-directional, transmitting and receiving simultaneously.
RJ-45 Pinouts: T568A vs T568B
T568B is the standard for most commercial MSP environments. A Straight-Through cable has the same standard on both ends. A Crossover cable has T568A on one end and T568B on the other (swapping the TX and RX pins).
T568B Standard (Most Common)
| Pin | Color | 10/100 Function | Gigabit Function |
|---|---|---|---|
| 1 | White/Orange | Transmit (TX+) | Bi-directional (BI_DA+) |
| 2 | Orange | Transmit (TX-) | Bi-directional (BI_DA-) |
| 3 | White/Green | Receive (RX+) | Bi-directional (BI_DB+) |
| 4 | Blue | Unused / PoE | Bi-directional (BI_DC+) |
| 5 | White/Blue | Unused / PoE | Bi-directional (BI_DC-) |
| 6 | Green | Receive (RX-) | Bi-directional (BI_DB-) |
| 7 | White/Brown | Unused / PoE | Bi-directional (BI_DD+) |
| 8 | Brown | Unused / PoE | Bi-directional (BI_DD-) |
T568A Standard
| Pin | Color | 10/100 Function |
|---|---|---|
| 1 | White/Green | Transmit (TX+) |
| 2 | Green | Transmit (TX-) |
| 3 | White/Orange | Receive (RX+) |
| 4 | Blue | Unused / PoE |
| 5 | White/Blue | Unused / PoE |
| 6 | Orange | Receive (RX-) |
| 7 | White/Brown | Unused / PoE |
| 8 | Brown | Unused / PoE |
Historically, you had to use a crossover cable to connect a switch to a switch, or a PC to a PC, so the TX pins aligned with the RX pins. Modern Cisco switches and NICs feature Auto-MDIX, which electronically crosses the pins for you. In the real world, you will almost exclusively use straight-through T568B cables.
Fiber Optic Fundamentals
Single-Mode Fiber (SMF)
- Core Size9 Microns (Very narrow)
- Light SourceLaser
- Jacket ColorUsually Yellow
- Max DistanceMiles / Kilometers
- Connecting campus buildings, long-haul ISP runs, and connecting cities.
Multi-Mode Fiber (MMF)
- Core Size50 or 62.5 Microns (Wider)
- Light SourceLED
- Jacket ColorAqua or Orange
- Max DistanceUp to 550 Meters
- Connecting switches within the same server room or datacenter.
IEEE Fiber Optic Standards
| Speed | Standard Name | IEEE Spec | Cable Type | Informal / Range | Max Distance |
|---|---|---|---|---|---|
| 1 Gbps | 1000BASE-SX | 802.3z | MMF | Short Range | Up to 550m |
| 1 Gbps | 1000BASE-LX | 802.3z | SMF | Long Range | Up to 5km |
| 10 Gbps | 10GBASE-SR | 802.3ae | MMF | 10G Short Range | Up to 400m |
| 10 Gbps | 10GBASE-LR | 802.3ae | SMF | 10G Long Range | Up to 10km |
| 10 Gbps | 10GBASE-ER | 802.3ae | SMF | 10G Extended Range | Up to 40km |