# A Design for an Internet Router with a Digital Optical Data Plane
## 7 Related Work  
Two large surveys review high-performance router design [33] and optical routers [27]. High-performance router designs tend to focus on increased electronic parallelism, which does not support all-optical packet processing needed when communication bandwidths and distances necessitate optical transmission modulation formats. Many optical approaches focus on the expectation of reduced energy, although some attempt to refute that claim [27]. Our approach makes no assertions as to the potential for energy savings for the optical approach; we assume that the primary benefit of an optical packet switch is avoiding the complexity and cost inherent in conversion from necessary optical transmission formats to electronic formats for switching. A number of groups have attempted to develop optical packet switches. Most designs fall into two categories: recirculation and burst switching. Recirculation systems handle output port collision by sending colliding packets to a set of fiber delay lines (FDLs) of different delay, in which those packets are returned to the input for a new attempt at switching [34; 35; 36; 37]. Recirculation causes packet reordering, which can severely affect some protocols [38]. Some switches handle only fixed-sized packets that arrive aligned (synchronously) [35], whereas others recirculate variable-length packets into FDLs of different length [37] or have input ports with one FDL staging area for each output port [39]. Our design shifts packets while in transit and each input is assigned its own SDL for each output, so it never recirculates them, thus avoiding packet reordering. Optical Burst Switching (OBS) [6] and Terabit Burst Switching (TBS) [7] schedule groups of adjacent packets with the same destination as if they were brief circuits configured while the packets are in flight. They assume both that these groups can be created at the network edge, typically using large electronic buffers, and that these brief circuits avoid contention while in transit, sometimes by scheduled coordination of burst emission. We consider the delay introduced in gathering sufficient packets for a burst and the global coordination required to avoid burst contention untenable approaches to handling dynamic Internet traffic patterns. Furthermore, a burst is scheduled as a group, reducing the opportunity to schedule each packet individually and thus achieve higher multiplexing efficiency. Some groups have recently revived use of more conventional optical circuits as a complement to packet switching, such as those that would use micro-mirrors, wavelength selection, or electro-optical switches (e.g., Pockels cells). This approach is already widely used commercially and can be very effective when traffic patterns are known sufficiently in advance, both early enough to mask large circuit setup delays to offset the dead time during circuit reconfiguration and involving enough data to warrant a dedicated optical channel. This is rarely the case for Internet traffic in general, but can be used for provisioning links between Internet routers in the Internet core (where traffic is  
Figure 17: Forward shift **(top** solid curve) achieves throughput efficiency approaching that of electronic routers and can be over 93% efficient (95% as efficient as electronic VOQ show as dashed curves, **right** curve) with as few as 6000 bytes of SDL buffering. Forward shift is much more effective than backward shift for most nontrivial buffer sizes (over 1070 bytes). large enough and where its backbone patterns are predictable in the aggregate), especially over long timescales. Known traffic patterns are also a hallmark of many data center applications, in which hybrid use of optical circuits can complement packet switching [8]. Our approach avoids being limited to such environments. Some groups have explored so-called'subwavelength' fast optical switching to handle packets. These often focus on the switching mechanism itself, especially using wavelength conversion mechanisms.