128b130b

128b/130b is a high-speed line encoding scheme used to transmit data over serial links. In this scheme, each code group carries 128 data bits plus a 2-bit header, producing a 130-bit code group. The header provides framing information to aid synchronization and alignment, while the 128-bit payload is typically scrambled to ensure a balanced distribution of ones and zeros, helping maintain DC balance and reliable clock recovery.

The primary advantage of 128b/130b over older schemes like 8b/10b is efficiency. With only two overhead bits for every 128 data bits, the gross efficiency is about 98.4%, reducing bandwidth loss while preserving the necessary signal properties for reliable transmission. The header also assists in resynchronization after gaps or errors, allowing receivers to regain word alignment quickly.

Implementation and usage: 128b/130b was standardized and widely adopted for PCI Express 3.0, and has continued into later PCIe generations, among other high-speed serial interfaces. Its deployment enables higher data rates without a proportional increase in overhead, making it attractive for modern interconnects that demand both speed and reliability. The encoding typically works in tandem with a scrambler and with downstream error detection and correction at higher protocol layers.

Limitations and considerations: while highly efficient, 128b/130b requires more complex circuitry for encoding and decoding and relies on scrambling to maintain balance. Proper synchronization depends on the header pattern and robust receiver design, and error handling is typically distributed across the transport and application layers.

See also: 8b/10b encoding, 64b/66b encoding, PCI Express.