# A Design for an Internet Router with a Digital Optical Data Plane
## 6 Optical Packet Multiplexing
Within each batch, only a subset of packets may succeed in being multiplexed while others may need to be dropped, e.g., if the average output link capacity is exceeded for a long enough period of time. We performed extensive simulations to determine the difference between arbitrary (e.g., random) drop of contending packets vs. intelligent drop, the latter where the smallest number of the shortest packets (i.e., the fewest bytes total) is dropped [24]. Our simulation configuration considered a 32 \(\times\) 32 switch under various input loads from 10% to 100% per port activity and a variety of traffic patterns from Poisson (emulating highly multiplexed traffic, as is typical close to the core) to Pareto (emulating highly bursty traffic, as is typical closer to the edge). The simulator exhaustively considered every possible combination of packet drops, looking arbitrarily far into the incoming packet stream, determining the best choice in every case. We called this the \"precognition switch\" because it performs as if it knows the arriving traffic packet patterns arbitrarily far into the future. Regardless of preference, drops alone were shown to be insufficient, achieving only 65% efficiency, in which even the most intelligent precognition approach was only 3%-5% more efficient than arbitrary random drops (60%). Figure 14: Length of the sequence of mutually overlapping packets (the contention set) for various traffic distributions. Figure 13: Length of the sequence of mutually overlapping packets (the contention set) for various traffic distributions. Efficient multiplexing thus requires time-shifting some of the packets, in order to better utilize the gaps between arrivals to compensate for overlapping arrivals and avoid drops [23]. Electronic packet switches accomplish this time-shift using queues. Queues are first-in, first-out buffering devices that help time-shift and sometimes reorder groups of packets. Electronic queues provide first-in, first-out behavior but are typically implemented less like waiting lines (e.g., ticket queues at the movie theater) and more like parking lots (random access storage). Arriving packets are placed into determined locations based on the packet type and arrival time. They remain parked in this lot until they are scheduled for release, in a process that is more like a valet retrieving the desired car than people completing a ticket purchase (Figure 15, left). Common mechanisms for electronic queuing place these parking lots at the input stage, called Virtual Output Queuing (VOQ), in which packets are parked in separate groups destined for each output, such that packets are released to the output stage with no overlap, so the output passively merges the result. The two common electronic VOQ algorithms are iSLIP [25] and Parallel Iterative Matching (PIM) [26]; these provide the metrics against which we can evaluate optical queuing. Although electronic packets can be parked arbitrarily long in random access memory (RAM), optical packets cannot be'stopped' in this manner. Light can be slowed down, but not stopped. As a result, we consider a different approach to queuing, in which packets merge much as do cars on merging roads. Packets come together where they (or an external party) detect potential contention, and one car 'hits the brakes' so the cars merge passively without slowing down (Figure 15, right). The merging approach, based on gradual and selective slowing of packets, can be accomplished using a switched delay line (SDL) (Figure 16, left inset). An SDL is a sequence of switches that select between a fast and a slow path, which can be implemented in optics using lossless Mach-Zehnder switches that select between waveguides with different refractive indices or of different lengths. Our approach assumes a small number (e.g., 10) of switches in series, with the same delay possible at each switch. This sequence of SDLs is sometimes known as a'staggered delay line' [27], which also behaves similarly to a 'conveyor queue' [28].