FLEXIBLE TRANSPORT
Implementing end-to-end with FPGAs and transport
Novel management models, such as SDN, require networks to be more dynamic, easier and faster to provision without manual intervention. Xilinx has implemented a flexible optical transport system for this emerging trend using FPGAs and Coherent DSPs, writes Faisal Dada.
difficult to use 100% of the link capacity and also requires that packet flows be managed so that individual links are not adversely affected. When using LAG, links can only be created in multiples of the physical Ethernet rate (N x 100GE), but transport line granularity is best addressed in increments of 25Gbps. Alternatively, the most efficient way to map clients to the transport frame that is transmitted by the modems is to map the entire client signal to the transport frame without alteration. However, mapping physical Ethernet cannot provide the smaller granularity that is required. The Optical Internetworking Forum (OIF) is working on a newer method called FlexE which allows Ethernet rates to be arbitrarily created in fixed increments that are smaller than the physical rate. Utilising such a flexible interface that uses known standard Ethernet
programmable and can change the modulation schemes and FECs used to tradeoff power and reach. We use this programmability in the modems to create the flexible transport system. The table below shows various data transport rates that have been demonstrated or may be possible in the future, based on existing commercially available components, the aforementioned ongoing work and future possibilities. Client interfaces on the other hand have traditionally been fixed Ethernet interfaces utilising standard Ethernet rates and optical components like QSFP28 or CFP-4. In the case of 100 Gigabit Ethernet (100GE) data can be transferred at rates up to 100Gbps. Ethernet link aggregation (LAG) has been used to bond multiple 100GE interfaces together to achieve higher rates. LAG has some limitations where it can be
systems. There are of course the tradeoffs of reach and spectral efficiency with each of the approaches. Alternate techniques like PAM-4 are also being considered for shorter distances required for some data center interconnect applications at lower power and cost structures. To achieve fully flexible transport systems, the end to end connection should be devised such that every part of the connection be able to adjust its throughput based on the networks requirements. In an SDN enabled world, to provide the automation and the on-demand bandwidth, these adjustments may be made by a higher layer management system that has full visibility of the entire network. As it is well understood, modern transport modems (DSPs with their respective ADCs and DACs) are
Faisal Dada Solutions Architect at Xilinx M odern optical transport technologies are employing coherent receivers with transmitters supporting polarisation division multiplexing (PDM) and advanced modulations like QPSK and 16-QAM. Such commercially deployed systems can achieve up to 200Gbps of data transmission per wavelength. Research in the optical transmission domain has shown that techniques like higher symbol rates (i.e. 64Gbaud) and advanced modulations (i.e. 64-QAM) are capable of increasing the data rate associated with coherent optical transport
Data Rate for various Transport Techniques
Modulation Schemes
PDM- BPSK
PDM- QPSK
PDM- 8-QAM
PDM- 16-QAM
PDM- 32-QAM
PDM- 64-QAM
PDM- 128-QAM
25 Gbaud 50Gbps
100Gbps 150Gbps 200Gbps 250Gbps 300Gbps 350Gbps
Data Symbol Rate*
37.5 Gbaud 75Gbps
150Gbps 225Gbps 300Gbps 375Gbps 450Gbps 525Gbps
50 Gbaud 100Gbps 200Gbps 300Gbps 400Gbps 500Gbps 600Gbps 700Gbps
* Actual Symbol rate on the line may be higher to accommodate FEC and Framing Overheads.
ISSUE 6 | Q1 2016 28
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