Examkin

Question

Difficulty: HardPhysical Interface and Cabling Types

A network administrator is troubleshooting a physical connectivity issue between two Cisco Catalyst switches. A legacy patch cable constructed using only two twisted pairs (wired to pins 1, 2, 3, and 6) was used to connect the two GigabitEthernet interfaces. Both interfaces have been manually configured for speed 1000 and duplex full. A wire map tester confirms end-to-end continuity on pins 1, 2, 3, and 6, while pins 4, 5, 7, and 8 are open/unwired. Despite the ports being enabled, the link status remains down/down. Which statement accurately explains why the 1000BASE-T link fails to establish?

  1. 1000BASE-T requires all four wire pairs (eight conductors) to transmit and receive simultaneously using 5-level PAM signaling.Answer
  2. B
    Pins 4 and 5 are reserved for clock synchronization, while pins 7 and 8 carry link beat pulses required for Gigabit auto-negotiation.
  3. C
    1000BASE-T allocates pins 1, 2, 4, and 5 strictly for data transmit, and pins 3, 6, 7, and 8 strictly for data receive.
  4. D
    Manually forcing 1000 Mbps speed requires pins 7 and 8 to complete the hardware Auto-MDIX crossover detection circuit.

Answer

1000BASE-T requires all four wire pairs (eight conductors) to transmit and receive simultaneously using 5-level PAM signaling.
The 1000BASE-T standard (IEEE 802.3ab) requires Category 5e or Category 6 UTP cabling wired with all four pairs (eight conductors). Each pair carries a bi-directional data rate of 250 Mbps using 5-level Pulse Amplitude Modulation (PAM-5) and complex digital signal processing (DSP) to cancel echo and crosstalk. Because pins 4, 5, 7, and 8 are disconnected in a 2-pair cable, the 1000BASE-T physical layer cannot negotiate or establish a link.

Step-by-Step Solution

1
Analyze the physical media requirements for 100BASE-TX versus 1000BASE-T standards.
100BASE-TX operates using two pairs (pins 1, 2, 3, and 6). However, 1000BASE-T (Gigabit Ethernet over copper, IEEE 802.3ab) explicitly requires four pairs (all 8 pins).
Gigabit Ethernet achieves 1 Gbps by transmitting at 250 Mbps per pair across four pairs simultaneously using 5-level Pulse Amplitude Modulation (PAM-5) and hybrid cancellation circuits.
2
Evaluate the cable wiring described in the scenario against the 1000BASE-T requirements.
The legacy cable only has four conductors wired (pins 1, 2, 3, and 6), leaving pins 4, 5, 7, and 8 disconnected.
Without continuity across all eight pins (four pairs), physical Layer 1 signaling for 1000BASE-T cannot complete its multi-pair initialization, causing the link to remain in a down/down state.

Key Concept

1000BASE-T 4-Pair Cabling Requirement
Estimated Time:1m 30s
Rate this question

Topics

Physical Interface and Cabling Types

More questions from this topic