Special Report: Next Generation Internet Applications
These delay lines consist of different length fibers, tuned to deliver the optical signal after specific delay intervals. Several methods of optical buffering have been proposed; however, none have been widely implemented at this time. Node architectures can be categorized as single or multi-stage, feed-forward or feedback buffers.[13] Figure 11 depicts the single-stage feed-forward 4-input/4-output optical buffer structure. Single-stage feed-forward optical buffers resolve contention by taking all of the optical signal inputs, converting them so that they reside on individual wavelengths (@ and A), multiplexing them together (B), and then distributing them to a set of delay lines, each having a unique delay time (C). The individual output port selects a specific delay line to resolve contention. The aggregation is then demultiplexed (D and E). Finally, the individual output port transmits the chosen wavelengths (? and ?). Single-stage feed-forward optical buffers provide a costly way to implement contention resolution. The large number of components necessary for implementation is unattractive to large-scale network designers. Also there is no defined way for higher priority packets to supersede lower priority packets. Selected wavelengths multiplexed for output. Figure 11: Single-Stage Feed-Forward 4X4 Optical Buffer Structure Single-stage feedback optical buffers try to reduce the component burden associated with single-stage feed-forward optical buffers. Single-stage feedback optical buffers do not require a wavelength conversion or multiplexing operation prior to delay line allocation. Single-stage feedback buffering differs from single-stage feed-forward buffering in that the single set of delay lines in the buffer are shared by all wavelengths. Wavelengths are directed to a delay line appropriate to resolve the contention, thereby providing the benefit of packet prioritization. If a packet is superceded by one of higher priority, the packet is sent through another delay line and the process continues. Single-stage feedback optical buffers allow for scalability. Unfortunately, some lower priority packets could experience substantial power loss while having to make several delay line iterations. This power loss creates the necessity for optical amplification which can lead to signal degradation.
Multi-stage feed-forward optical buffers are currently being proposed to resolve multiple contentions and should accommodate more efficient, faster buffering. Cascading multistage feed-forward buffers could accommodate large switching fabrics by providing the large-scale buffering necessary for operation. Research is in progress to develop practical methods for this form of buffering.
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