Big Data Generation and Acquisition - Big Data: Related Technologies, Challenges and Future Prospects - page 26

aware. Apart from Apple, smart phone operating systems such as Android of
Google and Windows Phone of Microsoft can also collect information in the
similar manner.

In addition to the aforementioned three data acquisition methods of main data
sources, there are many other data collect methods or systems. For example, in
scientific experiments, many special tools can be used to collect experimental
data, such as magnetic spectrometers and radio telescopes. We may classify
data collection methods from different perspectives. From the perspective of data
sources, data collection methods can be classified into two categories: collection
methods recording through data sources and collection methods recording through
other auxiliary tools.

3.2.2 Data Transportation

Upon the completion of raw data collection, data will be transferred to a data
storage infrastructure for processing and analysis. As discussed in Sect. 2.3 ,big
data is mainly stored in a data center. The data layout should be adjusted to improve
computing efficiency or facilitate hardware maintenance. In other words, internal
data transmission may occur in the data center. Therefore, data transmission consists
of two phases: Inter-DCN transmissions and Intra-DCN transmissions.
Inter-DCN transmissions are from data source to data center, which is generally
achieved with the existing physical network infrastructure. Because of the rapid
growth of traffic demands, the physical network infrastructure in most regions
around the world are constituted by high-volume, high-rate, and cost-effective
optic fiber transmission systems. Over the past 20 years, advanced management
equipment and technologies have been developed, such as IP-based wavelength
division multiplexing (WDM) network architecture, to conduct smart control
and management of optical fiber networks [ 20 , 21 ]. WDM is a technology that
multiplexes multiple optical carrier signals with different wave lengths and couples
them to the same optical fiber of the optical link. In such technology, lasers with
different wave lengths carry different signals. By far, the backbone network have
been deployed with WDM optical transmission systems with single channel rate
of 40 Gb/s. At present, 100 Gb/s commercial interface are available and 100 Gb/s
systems (or TB/s systems) will be available in the near future [ 22 ].
However, traditional optical transmission technologies are limited by the band-
width of the electronic bottleneck [ 23 ]. Recently, orthogonal frequency-division
multiplexing (OFDM), initially designed for wireless systems, is regarded as one
of the main candidate technologies for future high-speed optical transmission.
OFDM is a multi-carrier parallel transmission technology. It segments a high-
speed data flow to transform it into low-speed sub-data-flows to be transmitted
over multiple orthogonal sub-carriers [ 24 ]. Compared with fixed channel spacing
of WDM, OFDM allows sub-channel frequency spectrums to overlap with each
other [ 25 ]. Therefore, it is a flexible and efficient optical networking technology.