Introduction
100BASE-TX is one of the most widely adopted Fast Ethernet standards, providing data transfer at 100 Mbps over twisted-pair cabling. It is based on the IEEE 802.3u specification and typically uses Category 5 or higher unshielded twisted pair (UTP) cables. The “TX” designation indicates transmission over two pairs of wires—one for transmitting data and the other for receiving data.
In a 100BASE-TX link, transmitting data from the host system to the physical medium involves a multi-stage process that includes encoding, scrambling, serialization, and conversion into differential signals suitable for twisted-pair transmission. Understanding the transmit data flow helps clarify how Ethernet maintains speed, integrity, and compatibility across devices.
High-Level Overview of the Transmit Path
The transmit data flow in 100BASE-TX can be divided into several logical blocks:
1.Media Independent Interface (MII) Input The host system or MAC (Media Access Control) layer delivers data to the physical layer (PHY) using the MII standard interface. Data is provided as 4-bit nibbles at 25 MHz, representing 100 Mbps throughput.
2.4B/5B Encoding The PHY converts each 4-bit nibble into a 5-bit symbol using a lookup table. This ensures enough transitions for clock recovery and avoids long runs of identical bits.
3.Scrambling The 5-bit symbols are scrambled using a linear feedback shift register (LFSR) to reduce electromagnetic interference (EMI) and ensure a random distribution of ones and zeros.
4.NRZI Encoding Non-Return-to-Zero Inverted (NRZI) encoding is applied to improve signal transition characteristics. A logic “1” is represented by no transition, and a logic “0” is represented by a transition.
5.MLT-3 Encoding Finally, the data is mapped into MLT-3 (Multi-Level Transmit, 3 levels) signaling. This encoding uses three voltage levels (+, 0, -) to represent data transitions, reducing the frequency spectrum of the transmitted signal and making it suitable for twisted-pair copper cabling.
6.Transmission onto the Medium The MLT-3 signal is driven onto the twisted-pair medium via differential line drivers. Termination and impedance matching ensure signal integrity over distances up to 100 meters.
Step-by-Step Transmit Data Flow
1. Media Independent Interface (MII)
•The MII provides a standardized way for the MAC layer to communicate with the PHY.
•For 100BASE-TX, the MII operates at a 25 MHz clock, transferring 4 bits per clock cycle, which equals 100 Mbps throughput.
•The MAC sends data frames (including preamble, start-of-frame delimiter, payload, and CRC) across the MII.
•Control signals on the MII indicate frame boundaries, idle states, and error conditions.
2. 4B/5B Encoding
•Each 4-bit nibble from the MII is converted into a 5-bit codeword.
•This process ensures that the encoded stream has enough transitions for reliable clock recovery and avoids long runs of zeros or ones.
•Example mappings: 0000 (0x0) → 11110 0001 (0x1) → 01001 1111 (0xF) → 11101
•Out of the 32 possible 5-bit codes, only 16 are valid for data, while others are reserved for control symbols (Idle, Start, End, etc.).
•This results in 25% overhead, but it guarantees robust signaling.
3. Scrambling
•After 4B/5B encoding, the data stream undergoes scrambling.
•A scrambling polynomial, often based on a 11-bit linear feedback shift register, ensures pseudo-randomization of the bit sequence.
•Purpose: Minimize electromagnetic interference by avoiding repetitive patterns. Prevent spectral peaks that can interfere with other systems. Maintain DC balance in the signal.
•Importantly, scrambling is self-synchronizing, so the receiver can descramble without requiring a separate synchronization signal.
4. NRZI Encoding
•The scrambled bitstream is then encoded using NRZI (Non-Return-to-Zero Inverted).
•Rule: Logical 0 = Transition Logical 1 = No transition
•NRZI improves transition density, further assisting in clock recovery.
•However, NRZI alone does not guarantee sufficient transition density, which is why 4B/5B encoding is still necessary.
5. MLT-3 Encoding
•The NRZI signal is then mapped into MLT-3 (Multi-Level Transmit – 3 levels) encoding.
•MLT-3 uses three voltage levels: +, 0, and −.
•Rule: A logic 1 causes the signal to move to the next state in the sequence (+ → 0 → − → 0 → + …). A logic 0 means the signal stays in the same state.
•Benefits of MLT-3: Reduces the maximum fundamental frequency to 31.25 MHz (instead of 125 MHz). Makes it easier to transmit over twisted-pair cabling without excessive crosstalk or attenuation.
•Example: Input bitstream: 11001 NRZI output: No transition, No transition, Transition, No transition, Transition MLT-3 signal levels: 0, +1, 0, -1, 0
6. Line Drivers and Transmission
•After MLT-3 encoding, the signal is passed to the line driver stage.
•The line driver provides sufficient current to transmit the differential signals over the twisted-pair cable.
•Impedance matching (typically 100 Ω) is crucial to avoid reflections.
•The result is a balanced, differential signal that travels across the cable to the receiver.
Example of Data Flow in Practice
Let’s walk through a small sequence:
1.Input nibble from MII: 0101 (hex 5)
2.4B/5B encoding: 0101 → 10101
3.Scrambling: Suppose the scrambler outputs 11010
4.NRZI encoding: Start at logic high. 1 → No transition (remain high) 1 → No transition (stay high) 0 → Transition (go low) 1 → No transition (stay low) 0 → Transition (go high)
5.MLT-3 mapping: Sequence generates voltage states such as 0 → +1 → +1 → 0 → -1 → 0.
6.Line driver output: Differential voltages representing MLT-3 signals appear on the cable.
The receiver will invert this process: detect MLT-3 levels, recover the NRZI sequence, descramble, map back from 5B to 4B, and finally deliver the data to the MAC layer.
Advantages of This Data Flow
•Robust clock recovery: 4B/5B and NRZI ensure transition density.
•Noise reduction: Scrambling spreads energy across the spectrum.
•Cable efficiency: MLT-3 lowers bandwidth requirements.
•Compatibility: Works with existing twisted-pair infrastructure.
Challenges in Transmit Path
While effective, the transmit data flow of 100BASE-TX faces several challenges:
•Signal attenuation over 100 meters requires careful cable quality.
•Crosstalk between twisted pairs can distort MLT-3 signals.
•Jitter accumulation can complicate clock recovery at the receiver.
•Encoding overhead (4B/5B adds 25% redundancy) slightly reduces efficiency.
However, these challenges are mitigated through robust PHY design and compliance with Ethernet cabling standards.
Conclusion
The 100BASE-TX transmit data flow is a carefully engineered pipeline that transforms raw nibbles from the MAC layer into robust, noise-resistant, and cable-friendly differential signals. Through 4B/5B encoding, scrambling, NRZI, and MLT-3 encoding, the system ensures reliable 100 Mbps communication over twisted-pair copper.
Each stage plays a vital role:
•MII ensures interoperability between MAC and PHY.
•4B/5B encoding guarantees transition density.
•Scrambling reduces EMI.
•NRZI encoding aids clock recovery.
•MLT-3 signaling makes high-speed data feasible over inexpensive cables.
This transmit data flow highlights the balance between theoretical data representation and practical physical layer transmission, which is the cornerstone of Ethernet technology’s success.
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