Manchester Code

Maintaining Synchronization in a Data Stream

A simple transmission model uses a high voltage for '1' and a low voltage for '0'. This method, known as NRZ (Non-Return-to-Zero), works with varied data. A long sequence of identical bits, such as 11111111 or 00000000, keeps the signal at a constant level for many bit periods.

This waveform makes reception more difficult. The receiver needs a clock to sample the signal at the right moments and read each bit. Without transitions, its clock can shift relative to the sender's clock, causing samples to arrive too early or too late. The receiver can then lose bit boundaries and misinterpret samples in the run. Manchester encoding places timing information directly in the data signal. The receiver must maintain this timing throughout the complete frame.

Manchester Encoding and the Mid-Bit Transition

Manchester is a self-synchronizingline code. Every bit period has a transition in the middle, regardless of whether a '0' or a '1' is sent. The transition gives the receiver timing information for maintaining synchronization. A transition at a bit boundary can also occur, depending on adjacent symbols; the guaranteed mid-bit transition is the timing reference.

Encoding rules

Two conventions are used. For Ethernet, IEEE 802.3 defines the following level assignment:

• Logic '0': A high-to-low voltage transition occurs in the middle of the bit period. The signal is high for the first half and low for the second.
• Logic '1': A low-to-high voltage transition occurs in the middle of the bit period. The signal is low for the first half and high for the second.

Signal Properties and Trade-offs

The design of Manchester encoding determines its properties and creates a trade-off between synchronization and occupied bandwidth.

Main advantages

• Clock recovery: A transition in every bit gives clock-recovery circuits, such as a Phase-Locked Loop, regular edges to track the data timing. In a channel that meets the receiver requirements, this supports synchronization.
• No DC component: Every bit period contains equal-magnitude positive and negative levels for equal times, so the average DC voltage of an ideal waveform is zero. This supports transmission through AC-coupled network components, such as transformers, which cannot pass a sustained DC component.

Effect on Bandwidth

In the polar model described here, each Manchester symbol consists of two intervals with opposite polarities, each lasting $T_b/2$. For independent, equally likely bits, the first positive null of the power spectral density occurs at $2R_b$, where $R_b=1/T_b$ is the bit rate.

For rectangular pulses, the first null of the polar NRZ spectrum occurs at $R_b$. Manchester encoding therefore has twice the one-sided first-null bandwidth of polar NRZ. The bandwidth required by a physical channel also depends on pulse shaping, the channel response, and the permitted distortion.

Applications

Its self-clocking property led to the use of Manchester encoding in early Local Area Network (LAN) standards.

• 10BASE-T Ethernet: The 10 Mb/s standard for twisted-pair copper uses Manchester encoding.
• Token Ring: The LAN technology specified by IEEE 802.5 uses Differential Manchester, where the mid-bit transition provides clocking.
• RFID: Manchester code appears in selected passive Radio-Frequency Identification (RFID) protocols, where transitions support reception without a separate clock line.

Faster Ethernet uses other schemes. 100BASE-TX combines 4B/5B coding with MLT-3, while 1000BASE-X uses 8B/10B. Manchester remains a simple example of a self-clocking code in computer networking.