The km-scale network supporting the signal detection elements must be robust, cost-effective, and transparent, in the sense that data quality and operational flexibility are not compromised. Despite the challenging technical requirements for data quality, a simple all-copper network can suffice.
All timing calibration, control, and communications signals between a given DOM and the surface DAQ are provided by one conventional twisted pair of ∼1 mm diameter copper conductors insulated with PVC. The impedance is in the range of 100 Ω. This pair also supplies power to the DOM. The resulting network is both cost-effective and robust, since the number of connections (i.e., two) needed for all functionality is minimal. The twisted pairs are assembled first in double pairs, as "twisted quads," and enclosed in a common sheath. The required number of twisted quads are then wrapped around a Kevlar strength member and covered with a protective sleeve. Breakouts of individual quads to facilitate connections to the DOMs are periodically introduced at the appropriate positions.
Cable bulk and cost are reduced by planned operation of two DOMs per twisted pair. This means that two neighboring DOMs are ganged together at depth, with a simple network at the junction point to distribute power and transmit impedance-matched signals. DOM operation employs alternate interrogation of each member of the DOM pair to obtain unambiguous and unconflicted data flow. The data rates per DOM are anticipated to be ∼5 kB/s. Even with two DOMs/twisted pair, the bandwidth of the copper links is sufficient to support easily the expected traffic level of 10 kB/s, or equivalently, 80 kbits/s. Fast base-band communication over twisted pair links of 2–3 km can reach 400 kbits/s or significantly higher, depending on protocol. Nevertheless, it should be noted that the margins here are only a factor of ∼4, and any action that would increase the PMT noise rate or data/hit will reduce the margin of safety against network saturation, increasing the motivation to utilize the local coincidence feature.
Data transmission to the surface DAQ occurs by alternate interrogation of the DOMs sharing a twisted pair. Differential send/receive results in very low levels of cross-talk for these digital signals, measured to be less than 1% using the deployed AMANDA string 18 cables. Deployment operations are also simplified substantially, since only a single electrical connector needs to be mated and verified.
The use of an all-copper wire network eliminates the need for the optical fibers that have been used in AMANDA. Optical fibers are obviously capable of transporting analog information at the bandwidth needed, but the copper network can meet this goal with digital transport of information. In the rather unorthodox AMANDA mode of optical fiber use, i.e., being frozen in while supporting large vertical gravitational forces and freeze-in force gradients, failure or degradation of signal was commonly observed, typically in the range of 5–15% of channels. This pattern presents an unacceptable level of loss for an instrument of the scale of IceCube. The actual causes of failure or degradation have remained a subject of speculation, since realistic testing of ideas and possible remedies does not appear feasible. Combined with the goals to save costs, eliminate dozens of plane flights, simplify and shorten deployment procedures, the history of reliability problems generates ample motivation to develop an instrument design based on an all-copper network.
While the all-copper network plausibly meets the goals of robustness and cost-effectiveness, it may seem less apparent that such a network could meet the goal of a distributed time base at the few nanosecond level. As noted earlier, dispersion and attenuation lead to a substantial degradation of pulse rise-time in copper links of this length (∼ 2μs for 2.5 km). Nevertheless, the realization of local ⇒ master clock time transformation with a relative accuracy of 3 ns over a km-scale array is remarkably straightforward, requiring only a tiny fraction of bandwidth and minimal circuitry for this purpose. The exceptional stability of the DOM local oscillators facilitates this process. Preliminary results with the four DOMs in string 18 in AMANDA operated by the Test-Board DAQ give values for time "synchronization" of ∼5 ns rms, in agreement with expectations, and with clear prospects for further improvement by algorithmic modifications.
The measurement and calibration of timing offsets introduced by differences in twisted-pair cable lengths has been performed automatically and remotely via satellite links by system operators in the Northern Hemisphere. No manual interventions by pole personnel are necessary at any time for this purpose. The Reciprocal Active Pulse (RAP) technique exploits the capabilities of the DOM to measure offsets with a statistical precision of ∼1 ns as discussed in section 7.2.7.
