The Complexity of High-Speed Ethernet Auto-Negotiation (AN) and Link Training (LT)
In this guest blog, Martin Qvist Olsen, VP Marketing & Sales Development for Network Products at Teledyne LeCroy discusses the importance of AN/LT for 100 Gbps per lane Ethernet.
The increasing demand for high-speed Ethernet connectivity, particularly in data centers, has driven the need for interconnect solutions capable of supporting speeds up to 100 Gbps/lane over short distances (2-4 meters). This length corresponds to the typical distance from the bottom of the rack to the top of the rack within data centers. While optical cables have traditionally been used for longer distance, high-speed connections, they are often too power-consuming and expensive for short-distance applications. As a result, electrical cables are still the preferred solution for these scenarios. However, transmitting 100Gbps over even a few meters of electrical cable presents significant signal integrity challenges. Therefore, advanced link equalization and Forward Error Correction (FEC) are critical elements to ensure the appropriate signal quality.
Auto-Negotiation (AN) and Link Training (LT) are two of the essential processes required to establish the characteristics of the link partners including supported link speed(s), FEC enablement, and tuning of transmitter (Tx) equalizers. The purpose of these processes is to ensure the signal integrity of a link is adequate before real payload traffic is transmitted at full line speed.
AN is a well-known process traditionally used in home and office networks with multi-speed 10/100/1000 Mbps connections consumers rely on every day. AN has been included for high-speed Ethernet wherein each link partner can support multiple implementations. For instance, a 100Gbps link can be configured as 100GBASE-CR10 (10 x 10Gbps lanes), 100GBASE-CR4 (4 x 25Gbps lanes), 100GBASE-CR2 (2 x 50Gbps lanes), and 100GBASE-CR1 (1 x 100Gbps lane). The AN process ensures both ends of a link support the same speed, configurations, and FEC implementation.
Whereas AN works at a low link speed rate, LT requires full wire speed to optimize the transmissions. The two link partners communicate via the LT protocol to tune their Tx equalizers to achieve the best possible Bit Error Rate (BER) within the specified time frame. At 100Gbps data rates, FEC will improve BER, reduce lost or retransmitted packets, and generally optimize the link transactions
Despite the IEEE 802.3 standard definitions and application of Auto-Negotiation (AN) and Link Training (LT) exchanges, ensuring initial interoperability between network equipment from different vendors has proven challenging. This highlights why having the right test tools to validate these link transitions is important. The ability to test against every product in the industry isn’t feasible, but testing to the limits of the specification is possible. To validate all capabilities, it is important to have a link partner that can generate all the different coefficients and timeouts along with an Ethernet analyzer with a large buffer to capture and process how the host responds to each of the generated events.
The interoperability challenges in AN/LT stem from several factors. While the standard specifies the AN and LT protocols, it does not define the LT algorithm to search for the best possible signal integrity. The standard leaves room for different starting points and equalizer step sizes, leading to varied implementations by different vendors. In the worst case, the LT algorithm never converges, and the link can never be established.
High-speed Ethernet connections require multiple equalizers to work in harmony. These include transmitter (Tx) equalizers, receiver (Rx) equalizers, and equalizers in active cables (ACCs and AECs). Coordinating the functionality of these equalizers is complex and crucial for maintaining signal integrity and performance.
The complexity of high-speed Ethernet Auto-Negotiation and Link Training underscores the challenges faced by the industry in achieving reliable and interoperable connections. While significant progress has been made, ongoing efforts in standardization, collaboration, and technological innovation are crucial to overcoming these challenges. As the demand for high-speed Ethernet connectivity continues to grow, the industry must continue to test to the limits ensure the seamless and efficient transmission of data within data centers and other critical applications.
The Ethernet Alliance in its mission assists in these endeavors by hosting multi-vendor interoperability demonstrations, both private engineering events and public events like the one organized by the Ethernet Alliance at OFC 2024.
The views and opinions expressed are those of the authors and do not necessarily reflect the official policy or position of the Ethernet Alliance.
