In this FAQ, I will explain how common 10G ethernet specifications fit into a typical application. The below application diagram comes from the DS110DF111 datasheet.
Many 10G ethernet specifications are defined by the IEEE 802.3 standard. These specification names are formatted as follows.
- Data rate: 10G = 10 Gbps
- Modulation type: BASE = baseband
- Medium type: K = backplane, S = short optical reach, etc.
- PCS coding: R = 64b/66b encoded
Here are some common 10G IEEE 802.3 specifications which fit into systems implementing TI ethernet retimers and redrivers.
- 10GBASE-KR: This is an electrical specification with a backplane medium that fully defines the features and characteristics of a compliant ethernet PHY. The backplane exists between the switch fabric and the line card in a typical ethernet system. 10G-KR includes support for link training and auto-negotiation protocols.
- 10GBASE-R: This is a more general specification which defines PHY features and characteristics without focusing on a specific medium. 10G-R applies to both backplane applications (MAC <--> PHY) and front port applications (PHY <--> optical module or passive copper). 10G-R does not include support for LT and AN.
- 10GBASE-SR (short reach), 10GBASE-LR (long reach), 10GBASE-ER (extra long reach): These specifications describe the requirements of an optical transceiver and do not provide the electrical requirements of a PHY that can drive the transceiver. Therefore these specifications are not relevant when implementing TI retimers and redrivers in an ethernet system.
Additionally, the following specification is defined by the SFF Committee and applies to front port applications using optical modules.
- SFF-8431: This specification defines the high speed and low speed host and module electrical specifications of SFP+ front port applications. The specification includes host and module eye mask requirements which can be met with the use of a retimer or redriver.
TI offers several 10G ethernet retimers (PN DS1xxDFxxx) and redrivers (PN DS1xxBRxxx, DS1xxMB203, DS1xxKRxxx). These devices are most commonly used in backplane and front port applications to improve signal quality and jitter performance, meeting the requirements outlined by the various 10G ethernet specifications. There are a few factors which should be considered when choosing a device for a specific application.
- Backplane applications: If the use case adheres to the 10GBASE-KR specification and implements link training and/or auto-negotiation protocols, then a redriver with signal path linearity must be used. Retimers do not support LT and AN because the internal clock and data recovery block (CDR) reconstructs signals preventing feedback protocol compatibility. If the use case can adhere to 10GBASE-R specification instead and LT/AN can be disabled, then a retimer can be used in backplane applications for improved signal conditioning capability.
- Front port applications with passive copper: Both retimers and redrivers are suitable for these applications. Retimers offer greater signal conditioning capability because they include a CDR block which reconstructs the signal and effectively resets the jitter budget.
- Front port applications with optical modules: Both retimers and redrivers are suitable for these applications, although retimers’ signal conditioning capability may be more useful for meeting eye mask requirements.