1.3 Physical Interfaces and Cabling
Key Takeaways
- Single-mode fiber uses a smaller core (8-10 microns) and reaches distances up to 100+ km.
- Multimode fiber uses a larger core (50-62.5 microns) and reaches up to 550m (OM4) at 10 Gbps.
- UTP copper cabling (Cat 5e/6/6a) supports up to 100 meters per segment.
- PoE (802.3af/at/bt) delivers power over Ethernet cables—essential for APs and IP phones.
- Use straight-through cables for unlike devices and crossover cables for like devices (though auto-MDIX handles this automatically on modern switches).
Physical Interfaces and Cabling
Understanding physical layer connectivity is essential for the CCNA exam. You must know the characteristics, distances, and appropriate use cases for each cable type.
Copper Cabling (Unshielded Twisted Pair — UTP)
UTP is the most common cabling in enterprise LANs. It uses twisted pairs of copper wires to transmit data and is terminated with RJ-45 connectors.
UTP Categories
| Category | Max Speed | Max Distance | Use Case |
|---|---|---|---|
| Cat 5 | 100 Mbps | 100 meters | Legacy (avoid) |
| Cat 5e | 1 Gbps | 100 meters | Common in existing installations |
| Cat 6 | 10 Gbps (up to 55m) | 100 meters | Modern installations |
| Cat 6a | 10 Gbps | 100 meters | Data centers, high-performance |
| Cat 7 | 10 Gbps | 100 meters | Shielded, specialized |
| Cat 8 | 25-40 Gbps | 30 meters | Data center switch-to-switch |
Key Rule: The maximum distance for UTP cabling is 100 meters (328 feet). This includes 90 meters of horizontal cabling plus 10 meters for patch cables. Beyond 100 meters, you must use fiber or a repeater.
Wiring Standards
Two wiring standards define which wires connect to which pins on an RJ-45 connector:
| Standard | Pin 1 | Pin 2 | Pin 3 | Pin 6 |
|---|---|---|---|---|
| T568A | Green/White | Green | Orange/White | Orange |
| T568B | Orange/White | Orange | Green/White | Green |
Straight-through cable: Same standard on both ends (T568B to T568B is most common)
- Used to connect unlike devices: PC to switch, router to switch
Crossover cable: Different standards on each end (T568A on one, T568B on the other)
- Used to connect like devices: switch to switch, router to router, PC to PC
Modern Note: Most modern Cisco switches support auto-MDIX (automatic medium-dependent interface crossover), which automatically detects and adjusts for the cable type. However, the exam still tests the traditional rules.
Fiber Optic Cabling
Fiber optic cables transmit data as pulses of light through glass or plastic strands. Fiber is immune to electromagnetic interference (EMI) and supports much longer distances than copper.
Single-Mode Fiber (SMF)
| Characteristic | Detail |
|---|---|
| Core diameter | 8-10 microns |
| Light source | Laser |
| Distance | Up to 100+ km |
| Bandwidth | Very high |
| Cost | Higher (cable and transceivers) |
| Color code | Yellow jacket (OS1/OS2) |
| Use case | Long-distance WAN links, campus backbone |
Single-mode fiber uses a very small core that allows only a single ray (mode) of light to propagate. This eliminates modal dispersion (spreading of the light signal), allowing much longer distances.
Multimode Fiber (MMF)
| Characteristic | Detail |
|---|---|
| Core diameter | 50 or 62.5 microns |
| Light source | LED or VCSEL |
| Distance | Up to 550m (OM4 at 10 Gbps) |
| Bandwidth | High |
| Cost | Lower than SMF |
| Color code | Orange (OM1/OM2) or Aqua (OM3/OM4) jacket |
| Use case | Building backbone, data center |
Multimode Fiber OM Standards
| Standard | Core | Max Distance (1 Gbps) | Max Distance (10 Gbps) |
|---|---|---|---|
| OM1 | 62.5 μm | 275m | 33m |
| OM2 | 50 μm | 550m | 82m |
| OM3 | 50 μm | 550m | 300m |
| OM4 | 50 μm | 550m | 400m |
| OM5 | 50 μm | 550m | 400m |
Fiber Connector Types
| Connector | Description | Use Case |
|---|---|---|
| LC | Small form-factor, push-pull latch | Most common for modern switches |
| SC | Square connector, push-pull | Data centers, older installations |
| ST | Round, bayonet-style twist-lock | Legacy installations |
| MPO/MTP | Multi-fiber (12 or 24 fibers) | High-density data center connections |
On the Exam: LC connectors are the most commonly tested. Know that fiber connections typically use two strands: one for transmit (Tx) and one for receive (Rx).
Power over Ethernet (PoE)
PoE allows network switches to deliver electrical power to connected devices over the same Ethernet cable used for data. This eliminates the need for separate power supplies for devices like access points, IP phones, and security cameras.
PoE Standards
| Standard | IEEE | Max Power (per port) | Common Name |
|---|---|---|---|
| PoE | 802.3af | 15.4W | PoE |
| PoE+ | 802.3at | 30W | PoE+ |
| UPoE / PoE++ | 802.3bt Type 3 | 60W | PoE++ |
| UPoE / PoE++ | 802.3bt Type 4 | 90W | PoE++ |
Common PoE-powered devices:
- Wireless access points (typically need 15-30W)
- IP phones (typically need 7-15W)
- Security cameras (typically need 12-25W)
- IoT sensors (typically need 5-13W)
PoE Power Budget
The power budget is the total amount of PoE power a switch can deliver across all ports. A 48-port switch might have a 370W or 740W power budget. If all ports need PoE, you must ensure the total power draw doesn't exceed the budget.
On the Exam: Know the PoE standards and their power levels. A common scenario: "Which PoE standard would support a device requiring 25 watts?" Answer: PoE+ (802.3at, 30W max).
Identifying Interface and Cable Issues
Common physical layer problems the CCNA tests:
| Issue | Symptoms | Solution |
|---|---|---|
| Cable too long (>100m copper) | Intermittent connectivity, CRC errors | Shorten cable or use fiber |
| Damaged cable | Interface errors, link flapping | Replace cable |
| Speed/duplex mismatch | Slow performance, collisions, late collisions | Set both sides to auto-negotiate or match manually |
| Wrong cable type | No link (link light off) | Use correct cable (or enable auto-MDIX) |
| EMI interference | CRC errors, intermittent drops | Route copper away from EMI sources, or use fiber |
| Bad SFP/transceiver | No link, interface errors | Replace transceiver module |
| Fiber Tx/Rx reversed | No link | Swap fiber strands |
What is the maximum distance for a single segment of Cat 6a UTP copper cabling?
Which type of fiber optic cable would you use for a 40 km WAN link between two buildings?
Which PoE standard provides up to 30 watts of power per port?