Beyond gigabit ethernet: physical layer issues in future optical networks

This paper presents a study of the errors observed on an optical Gigabit Ethernetlink in a state of low receiver power. This condition is increasingly likely as networks becomemore complex, with longer ﬁbre lengths, optical switching systems and higher data rates.We discover that some octets and sequences of octets have a far higher probability of beingreceived in error than others. This non-uniformity of error in the physical layer may severelyaﬀect network performance at higher levels, and should be carefully considered as the nextgeneration of optical networks is developed.1 Optical networking motivation.Current work in all areas of networking has led to increasingly complex architectures: our interest isfocused upon the ﬁeld of optical networking, but this is also true in the wireless domain. Our explorationof the robustness of network systems is motivated by the increased demands of these new optical systems.To take advantage of capacity developments oﬀered by optical systems at the short timescales relevant tolocal area networks, packet switching and burst switching techniques have seen signiﬁcant investigation.Our project  is to investigate Optical Packet Switching (OPS) through the construction of a switchedoptical data path based upon semiconductor optical ampliﬁers (SOAs). As part of this work we recognisethat the need for higher data-rates and designs with larger numbers of optical components are forcing ustowards what traditionally have been technical limits.A signiﬁcant amount of work exists that is targeted towards the wide-area telecommunications arena. Incontrast, we are motivated by an OPS for short link lengths — those comparable with current system-areaand local-area networks. The main criteria of our approach are low latency and the avoidance of opticalbuﬀering and all optical signal-processing to ensure lowest cost. Our architecture uses high-speed opticalswitch fabrics for high capacity routing and combines this with wavelength-striping and a separate controlchannel [2,3]. Of particular note is the data path between the sending and receiving end-systems: thispath is clearly non-trivial with a signiﬁcant number of devices such as SOAs and wavelength multiplexand de-multiplex units.There have also been changes in the construction and needs of ﬁbre-based computer networks. In deploy-ments containing longer runs of ﬁbre using large numbers of splitters for measurement and monitoringand active optical devices, the overall system loss may be greater than in today’s point-to-point linksand the receivers may have to cope with much-lower optical powers. Ethernet in the ﬁrst mile [4], alongwith a new generation of switched optical networks, are examples of this trend. In addition, if all othervariables are held constant an increase in bandwidth will require a proportional increase in transmitterpower. Fibre nonlinearities impose limitations on the maximum optical power able to be used in anoptical network. Subsequently, we maintain that a greater understanding of the low-power behaviour ofcoding schemes will provide invaluable insight for future systems.We selected 8B/10B coding as the basis for our work, for which initial results are given in James et al. [5].This codec, originally described by Widmer & Franaszek , is widely used. It converts 8 bits of data fortransmission (ideal for any octet-orientated system) into a 10 bit line code. Although this adds a 25%overhead, 8B/10B has many valuable properties; a transition density of at least 3 per 10 bit code groupand a maximum run length of 5 bits for clock recovery, along with virtually no DC spectral component.In addition to being the standard Physical Coding Sublayer (PCS) for Gigabit Ethernet [7], it is used inFibre Channel, the 800Mbps extensions to the IEEE 1394 / Firewire standard, and will be the basis ofcoding for the electrical signals of PCI Express.2 Bit Error Rate versus Packet Error Experiments.We investigate Gigabit Ethernet on optical ﬁbre (1000BASE-X [7]), under conditions where the receivedpower is suﬃciently low as to induce errors in the Ethernet frames. We assume that while the Functional