Clark, Kari Aaron; (2021) Clock Synchronisation Assisted Clock and Data Recovery for Sub-Nanosecond Data Centre Optical Switching. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In current `Cloud' data centres, switching of data between servers is performed using deep hierarchies of interconnected electronic packet switches. Demand for network bandwidth from emerging data centre workloads, combined with the slowing of silicon transistor scaling, is leading to a widening gap between data centre traffic demand and electronically-switched data centre network capacity. All-optical switches could offer a future-proof alternative, with potentially under a third of the power consumption and cost of electronically-switched networks. However, the effective bandwidth of optical switches depends on their overall switching time. This is dominated by the clock and data recovery (CDR) locking time, which takes hundreds of nanoseconds in commercial receivers. Current data centre traffic is dominated by small packets that transmit in tens of nanoseconds, leading to low effective bandwidth, as a high proportion of receiver time is spent performing CDR locking instead of receiving data, removing the benefits of optical switching. High-performance optical switching requires sub-nanosecond CDR locking time to overcome this limitation. This thesis proposes, models, and demonstrates clock synchronisation assisted CDR, which can achieve this. This approach uses clock synchronisation to simplify the complexity of CDR versus previous asynchronous approaches. An analytical model of the technique is first derived that establishes its potential viability. Following this, two approaches to clock synchronisation assisted CDR are investigated: 1. Clock phase caching, which uses clock phase storage and regular updates in a 2km intra-building scale data centre network interconnected by single-mode optical fibre. 2. Single calibration clock synchronisation assisted CDR}, which leverages the 20 times lower thermal sensitivity of hollow core optical fibre versus single-mode fibre to synchronise a 100m cluster scale data centre network, with a single initial phase calibration step. Using a real-time FPGA-based optical switch testbed, sub-nanosecond CDR locking time was demonstrated for both approaches.

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