Since the users of the Zoomable Brain Database will be distributed across the country, the system stress from flying will be spread over the underlying network. If the network topology appears as a backbone with six Network Access Points (NAP's), like the proposals for the national data highway [15], we can determine the bandwidth required for each link.
We assume:
We can use the analysis from previous sections to determine the system requirements:
Figure 12 depicts the network for the constructed Zoomable Brain Database. Unfortunately, some of the proposals for the national data highway have bandwidths like those in figure 13.
Figure: Bandwidth requirements for the Zoomable
Brain Database system on a topology similar to a possible national
data highway.
Figure: A proposed national data highway network.
In order to allow flying in this environment, user-level requirements
must be relaxed.
We can only hope to fly on such a network by reducing the user-level flying requirements. [1] describes a number of modifications that can be done to the user requirements that will ease the system requirements. The server can reduce the frame resolution in each dimension by one-half (half-flying) to a one-fourth (quarter-flying), reducing the data needed by 1/8th and 1/64th respectively. Likewise, fewer bits of color decrease the data needed for each pixel. The table below summarizes the system benefits of several of these modifications:
Processing and sending 8-bit color reduces CPU and Network requirements by 2/3. Yet many neuroscientists may find scientific analysis difficult under such conditions. In half- or quarter-flying, one frame pixel represents 4 or 16 display pixels, respectively. Thus, network data rates are reduced by 3/4 and 15/16 respectively, but at the expense of image quality. Sending fewer frames per second reduces all system components. However, motion displayed at 3 frames/second has a much rougher flow than does motion displayed at 30 frames/second.
The data reduction from the above modifications can also be combined. If we modify the user-requirements to allow 8 bit color, 3 frames/second and 1/2 flying, the system requirements for the Zoomable Brain Databases would appear as in Figure 14. These reductions in user requirements make it possible to use the proposed infrastructure to achieve some of our goals, but will not be suitable for some neuroscience analysis.
Figure: Bandwidth requirements for the Zoomable
Brain Database system on a topology similar to a proposed national
data highway. User requirements have been reduced to ease system
requirements.
All of our predictions have done the image computation at the server (remote flying) as opposed to image computation done at the client (local flying). But remote flying, while inducing significantly less bandwidth than local flying, increases the amount of server computation. With many database users, the server computation may become prohibitive. As client workstations become increasingly powerful, they may be able to readily perform the needed computations at a sufficient flying rate, reducing server load. In other environments, server load has proven critical for performance [12].
There are many areas for future research. A cost analysis of the tradeoffs between network bandwidths, disk data rates and CPU throughputs can determine what system configuration will be the most economical. Algorithms to manage playout buffers and reduce jitter will be important to preserve flying image quality. A careful study of the effects of flying hardware will determine bottlenecks in current proposed flying systems. The analysis techniques presented in this paper apply to other applications that can be described with analogous user requirements. As in [2], experiments to justify that predicted system requirements accurately meet user requirements are needed to validate the analysis techniques.
Advances in networking, such as those described in this paper, are needed to enable a new era of neuroscience. Researchers will be able to directly access high-quality pristine data from all areas of the brain. Hypotheses will be formulated, evaluated, and scientifically tested through interaction with data collected by scientists on the other side of the world. As we have shown, realizing this dream fully will require gigabits per second of sustained throughput per user, and terabits per second of aggregate bandwidth.
