CCNA Network Cabling: Fiber, Copper, Ethernet Standards & Errors

Fiber optic cables carry light instead of electrical signals, eliminating electromagnetic interference and enabling much longer distances. The two types differ in the size of the glass core that carries the light.

Single-mode fiber (SMF) has a very small core (around 9 μm diameter) that allows only one light mode to propagate. This eliminates modal dispersion, enabling distances of 10 km to 100 km or more depending on the transceiver. SMF uses laser light sources (expensive transceivers) and is the choice for WAN links, metropolitan area networks, and any long-haul run.

Multimode fiber (MMF) has a larger core (50 μm or 62.5 μm) that allows multiple light modes simultaneously. Modal dispersion — different modes arriving at slightly different times — limits the usable distance. OM3/OM4/OM5 multimode cables support 100 GbE at distances up to 100–400 m. MMF uses lower-cost LED or VCSEL sources and is the data center and campus short-run choice.

Unshielded Twisted Pair (UTP) is the most common LAN cable. The twists in each pair cancel out electromagnetic interference — higher-category cables have more twists per foot. Cat 5e supports Gigabit Ethernet at up to 100 m. Cat 6 supports 10 GbE at up to 55 m and Gigabit at 100 m. Cat 6A supports 10 GbE at the full 100 m. For new installs, Cat 6A is the standard choice.

Shielded Twisted Pair (STP) adds metallic shielding around the pairs, providing additional EMI rejection for environments with heavy electrical interference (factory floors, near motors). It costs more and requires grounding.

Straight-through cables have both ends wired identically (T568A-T568A or T568B-T568B). They connect dissimilar devices: switch to router, switch to PC. Crossover cables swap the transmit and receive pairs (T568A on one end, T568B on the other). They connect like devices: switch to switch, PC to PC. Modern devices with Auto-MDIX automatically detect and correct the crossover, so cable type rarely matters on current hardware.

Coaxial cable (coax) is rarely used in new LAN installations but appears in cable modem/broadband connections and some legacy environments. For CCNA, know it exists but focus on UTP and fiber.

The `show interfaces` command on a Cisco switch or router reveals error counters. Understanding what each counter indicates is a key CCNA troubleshooting skill.

Runts are frames smaller than 64 bytes. Giants are frames larger than 1518 bytes (or the configured MTU). Both indicate corrupted or malformed frames. Input errors is the aggregate of all receive errors including runts, giants, and CRC errors. CRC errors indicate that the frame's checksum doesn't match — usually caused by a bad cable, a bad port, or electromagnetic interference.

A duplex mismatch is one of the most common real-world causes of poor Ethernet performance. If one side is set to full-duplex and the other auto-negotiates to half-duplex (or is manually set to half), the half-duplex side uses CSMA/CD and sees the full-duplex side's transmissions as collisions. The result: late collisions on the half-duplex side. Late collisions are collisions that occur after the first 64 bytes of the frame — impossible in a properly operating Ethernet segment — and are the classic indicator of a duplex mismatch.

Speed mismatch is simpler: if ports auto-negotiate to different speeds, the link simply won't come up. The interface will show line protocol down.

Cisco devices use Small Form-factor Pluggable (SFP) transceivers for fiber connections. SFP supports up to 1 GbE; SFP+ supports 10 GbE; QSFP supports 40 GbE; QSFP28 supports 100 GbE. The transceiver determines whether single-mode or multimode fiber is used and what distance is supported.

Common fiber connectors: LC connectors are the small, square-body duplex connectors found on SFP modules. SC connectors are larger and also common. ST connectors use a bayonet lock and are found in older installations. For CCNA, know that LC is the standard SFP/SFP+ connector.