Comparing 100BASE-TX and 100BASE-FX: Understanding the Basics and Differences

The evolution of Ethernet technology has been continuous, adapting to the growing demand for higher performance. Starting from the original 10 megabits per second (Mbps) Ethernet, the industry has progressed through Fast Ethernet and Gigabit Ethernet, and currently, 10 Gigabit Ethernet is in the spotlight. As performance requirements have surged, new Ethernet specifications have emerged. This article focuses on various Fast Ethernet standards, notably 100BASE-T, 100BASE-TX, and 100BASE-FX.

Terms About 100Base-TX/T4/FX

Breaking down the term “100Base-TX/T4/FX” helps in understanding its components:

  • 100: The “100” in 100Base refers to 100 Megahertz (MHz) or 100 million cycles per second. This translates to a data rate of 100 megabits per second (Mbps). Although the theoretical capacity is 100 Mbps, real-world performance might be closer to 4 Mbps.
  • Base: “Base” signifies Baseband communication, the method used by Ethernet networks. It indicates that the cable dedicates all available bandwidth for data transmission. This differs from broadband, where bandwidth is shared among users. While Ethernet users may experience occasional slowdowns due to congestion, the impact is generally less compared to broadband.
  • 100Base-TX: The “TX” or “T” represents Twisted Pair, a type of cable carrying the signal. “TX” indicates that CAT5 Unshielded Twisted Pair (UTP) cables are utilized, using two pairs of copper wires to support a speed of 100 Mbps. These cables can sustain data speeds up to 100 meters and should have a minimum recommended distance of 2.5 meters between two cables.
  • 100Base-FX: In “FX,” the cable employs two pairs of fiber optics to achieve speeds exceeding 100 Mbps. This type of cable can cover distances of up to 2 kilometers, making it suitable for long-distance applications.


What is 100BASE-T?

100BASE-T defines PHYs supporting 100 Mbps networks over twisted pair cables, a 10x improvement over 10BASE-T Ethernet. It employs Collision Detection / Carrier Sense Multiple Access (CD/CSMA) to avoid collisions.

IEEE 802.3 Standards (1995):
IEEE 802.3 introduces 100Mbps Ethernet PHYs: 100BASE-TX, 100BASE-T4, and 100BASE-T2.

100BASE-T2 (IEEE 802.3y):
Utilizes two copper cable pairs (category 3), distinct from 100BASE-TX using category 5 cables.


  • Early Fast Ethernet variant employing four copper pairs for communication.
  • Uses category 3 twisted pair wires.
  • One pair handles signals; two pairs are reserved.

Synonymous Terminology:
Due to limited deployment of 100BASE-T2 and 100BASE-T4, 100BASE-T often equates to 100BASE-TX.

What is 100BASE-TX?

100BASE-TX is a widespread term for Fast Ethernet, operating over two wire pairs within a category 5 or better cable. Each network segment has a maximum cable length of 100 meters (328 feet).

Cable Configuration:

  • Utilizes pairs of wires terminated on pins 1, 2, 3, and 6, similar to 10BASE-T.
  • A standard category 5 cable can support two 100BASE-TX links with a wiring adaptor, benefiting from its four pairs.

Cabling Standards:
Follows TIA/EIA-568-B termination standards, T568A or T568B, for conventional wiring.

What is 100BASE-FX?

100BASE-FX is a Fast Ethernet standard that operates using fiber optic cables. “FX” represents fiber, enabling a 100 Mbps data speed over optical fiber.

Optical Fiber Configuration:

  • Uses two strands of optical fiber: one for transmitting (TX) and one for receiving the light signal.
  • Conversion from 100BASE-TX to 100BASE-FX fiber signal extends the maximum distance to 120km over single-mode fiber.

The industry introduced specific terms within the 100BASE-FX framework, such as 100BASE-SX, 100BASE-LX, 100BASE-EX, 100BASE-ZX, and 100BASE-EZX. Note that these terms are non-standard within IEEE, tailored to industry practices.

Exploring 100BASE-FX Variations:


  • Fast Ethernet over multimode fiber for short reach.
  • Supports up to 2km (OM2) distance with 850nm wavelength.
  • Utilizes cost-effective optical components like LEDs.


  • Fast Ethernet over single fiber for long reach applications.
  • Supports up to 10km or 20km distance with 1310nm wavelength.
  • Typically employs FP laser as the transmitter.


  • Fast Ethernet over single fiber for extended reach applications.
  • Supports up to 40km distance with 1310nm wavelength.
  • Generally uses high-output FP laser as the transmitter.


  • Fast Ethernet over single-mode fiber for extensive reach.
  • Supports up to 80km distance with 1550nm wavelength.
  • Utilizes FP or DFB laser as the transmitter.


  • Fast Ethernet over single-mode fiber for ultra-long reach.
  • Supports up to 120km distance with 1550nm wavelength.
  • Employs a DFB laser as the transmitter.

These variations cater to specific reach and performance needs within the 100BASE-FX framework.

Comparison: 100BASE-TX vs 100BASE-FX

Let’s have a quick look about the 100BASE standards comparison.
100BASE Standards Comparison

From the comparison, we can learn the key similarities and differences between 100BASE-TX and 100BASE-FX includes:

Pair of Transmission:

  • 100BASE-TX: Utilizes two UTP Category 5 or STP type 1 wire pairs for signal transmission.
  • 100BASE-FX: Employs two optical fiber pairs for data transmission, ensuring full-duplex communication. One pair transmits from hub to device, while the other handles device-to-hub.

Data Rates:
Both operate at data rates of 125 Mbps, with 100BASE-TX’s fundamental frequency peaking at 31.25 MHz.

Encoding and Decoding:

  • 100BASE-TX: Utilizes the MLT-3 encoding scheme combined with 4B/5B block coding. MLT-3 Encoding involves Multi-Level Transmit with three voltage levels: -1, 0, +1, 0. Sequential voltage transitions enhance DC equalization and spectrum shaping.
  • 100BASE-FX: Adopts the NRZ-I encoding scheme alongside 4B/5B block coding. NRZ-I Encoding is Non-Return-to-Zero binary line code with higher energy pulses compared to return-to-zero (RZ) code.

This concise comparison outlines the distinct attributes of 100BASE-TX and 100BASE-FX, aiding in a better understanding of their functionalities.


Even with the emergence of Gigabit and 10-Gigabit networks, fast Ethernet remains relevant due to its cost-effectiveness and extended reach over multimode cabling compared to higher data rates like 1G and 10G.

Now that you have a grasp of their fundamentals and distinctions, if you’re looking for a 10/100BASE-TX to 100BASE-FX media converter, explore our diverse selection to extend your copper networking over greater distances.

Echo Huang

Echo Huang is an expert wordsmith and marketing professional at Bonelinks with more than 8 years of experience in high technology businesses – fiber optics, IoT, and telecommunication. She is very glad to share industry knowledge and communicate with others.

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