The high-performance computing platforms used for artificial intelligence (AI) and machine learning (ML) in hyperscale data centers need high-speed interconnects like 112 Gbps PAM 4 and faster inside the servers. High-speed interconnects are also required between the servers and storage devices. Twin axial (Twinax) cable assemblies are one way to address those needs.

This article discusses the performance limitations of printed circuit board (PCB) traces and how Twinax cabling can help inside the box. It also reviews how Twinax is structured and how it’s different from single twisted pairs (STP) and coaxial cables. It closes by examining how Twinax can deliver 400 G interconnects and supports AI and ML throughout a hyperscale data center.

PCB limitations

When implementing high-speed interconnects, designers need to balance the requirements for high throughput, signal integrity, solution density, and power consumption. Conventional PCBs have significant performance limitations, and trying to overcome them can be time-consuming, expensive, and ultimately futile.

One way to improve interconnect performance in servers and storage devices is to use high-performance PCB materials. However, those materials are expensive and often only partially solve the problem. Instead, designers can turn to Twinax cables that provide significantly higher performance without a corresponding increase in cost (Figure 1). So, what’s twinax?

Two-core balanced cables

Twinax, sometimes called two-core balanced coaxial cable (coax), consists of two balanced cores. It’s not coax, and even though it has two cores, it’s not STP. A coax consists of a single core surrounded by shielding. STP consists of two wires twisted together and can be shielded or unshielded.

Twinax provides superior speed, bandwidth, and signal integrity compared to coax or STP. A better comparison might be fiber optic cables that are sometimes replaced with Twinax to provide lower cost and lower power consumption solutions. There are four elements in a Twinax cable (Figure 2):

• Two conductors, normally made of stranded copper, are usually packaged in parallel. Some designs claimed to be Twinax have twisted conductors and are not true Twinax cables.
• Insulation: the conductors can be individually insulated, called single extrusion, or insulated in a single extrusion, called co-extrusion. Co-extrusion twinax cables have better skew performance but are more expensive to produce. Single extruded conductors are sometimes contained in a second insulation layer to improve structural integrity.
• Shielding can vary significantly. It can be axially or longitudinally wrapped. A single shield surrounds both conductors. Shields can be made using braided copper or aluminum foil, providing varying degrees of coverage. Shielding improves signal integrity and protection against electromagnetic interference (EMI). Some Twinax cables use both tape and braid shields.
• A jacket made of PVC provides strength and protection from physical damage.

Twinax is compatible

Twinax cables are compatible with numerous datacenter networking interfaces, such as Small Form-factor Pluggable Plus (SFP+), Quad Small Form-factor Pluggable Plus (QSFP+), Quad Small Form-factor Pluggable Plus 28 (QSFP28), and so on.

For example, a 400 G Quad Small Form Pluggable Double Density (QSFP-DD) Twinax cable assembly is available for hot-plug applications with data rates up to 425 Gbps and maximum power consumption of 0.1 W (Figure 3). These Twinax QSFP-DD cable assemblies are suitable for short-range communications and are available in lengths up to 3 meters.

Twinax cable assemblies are also used to implement PCIe and SAS for flyover or mid-board copper (MBCu) connectivity. These cable assemblies provide a three-dimensional solution that bypasses the PCB and is routed over the components from one area to another on the circuit board.

Conventional ribbon cables were used in the past, but they can’t support today’s high-speed interconnects. When data rates hit 28 Gbps and higher, Twinax provides a superior solution compared with ribbon cables (Figure 4).

Summary

Twinax cables can support the high-speed data rates demanded by AI/ML applications in high-performance data centers. They provide a superior alternative to high-performance PCB materials and are compatible with a wide range of interconnect standards found inside servers, storage equipment, and throughout data centers. Over short distances, they can substitute for optical fibers with no loss in performance.

References

How are Twinax and coax cables different?, NNC
Specifying Mid-Board Copper Solutions, TE Connectivity
Twinax Basics, Samtec
What is Twinax Cable?, FS