One of the newest developments in the QSFP (quad small form factor pluggable) connector design is the ability to reach 50-56 Gbps. Used for high-frequency data transmission in computer centers with four data pairs, it helps to make the connection between server, storage, switch, video and communication systems.
Now comes news from Leoni, a European cable and assembly provider, that it has succeeded in transferring 56 Gbps per channel across passive DACs (direct attach copper cable systems with QSFP, QSFP28, SFP+ and SFP28 connectors) with NRZ modulation. Prototypes with 1- and 2-m transmission lengths showed excellent electrical properties in tests up to 56 GHz.
Leoni’s solution promises major cost-saving potential for network operators and their equipment suppliers: NRZ-modulated 56 Gbps systems allow a passive infrastructure to be retained in computer centers. There is no need for active strengthening of switches and servers with additional electronics, making it possible to save a lot of energy. Furthermore, tests and simulations can be run with existing equipment. The company said that 200 G is only the beginning. Many computer center operators and component manufacturers are going for active components with additional electronics in the connectors when it comes to next generation-wiring up to 400 G across short distances.
With this Leoni solution, it is now possible to keep a passive network structure at 200 G. Leoni’s Business Unit Telecommunication Systems has succeeded in providing a stable-frequency system solution up to 2 m for the QSFP form factor and passive 56 Gbps. With the use of a specially developed high-speed cable and a complex PCB, Leoni can increase the NRZ modulation for a data rate of 56 Gbps, which simultaneously also means providing the bandwidth of the entire system with corresponding SI performance of 56 GHz. With four data pairs (QSFP), 200 G can be securely transferred across these passive cables.
There are only two ways to execute the generational change from 100 G to 200 G or 400 G: either further data pairs must be added or the actual data rate per data pair must be raised. Such raising of the data rate is being normatively discussed right now because of the high frequency. In technical terms, this is only possible to achieve with great difficulty when using existing passive components. A data rate increase by changing the modulation method from NRZ to PAM4 (pulse amplitude modulation) was therefore incorporated in the standard. PAM4 makes it possible, with the same bandwidth, to send twice the data rate in the same time across a connection than with NRZ. However, existing networks must be upgraded to use this active technology.
The switch from passive NRZ to active PAM4 modulation at data rates of up to 56 Gbps per channel does present new challenges in terms of design, measurability, test methods and ways of simulating transmission systems. These are currently being researched and developed.
Leoni provides solutions for both active and passive methods with different data rates and transceiver connectors. The 200 G with NRZ via a QSFP assembly is the latest innovative solution. Yet the focus is already on the next objective: the feasibility study and prototype development for 400 G with NRZ across eight data pairs are currently under way. That equates to a data volume of nearly 12 DVDs per second, which in technical language is known as ‘double density’ (abbreviated DD) and is another milestone in the evolution of high-frequency data transfer in computer centers.