ABSTRACT
Data centers (DCs) is becoming increasingly an integral part of the computing infrastructures of most enterprises. Therefore, the concept of DC networks (DCNs) is receiving an increased attention in the network research community. Most DCNs deployed today can be classified as wired DCNs as copper and optical fiber cables are used for intra-and inter-rack connections in the network. Despite recent advances, wired DCNs face two inevitable problems; cabling complexity and hotspots. To address these problems, recent research works suggest the incorporation of wireless communication technology into DCNs. Wireless links can be used to either augment conventional wired DCNs, or to realize a pure wireless DCN.
As the design spectrum of DCs broadens, so does the need for a clear classification to differentiate various design options. In this paper, we analyze the free space optical (FSO) communication and the 60 GHz radio frequency (RF), the two key candidate technologies for implementing wireless links in DCNs. We present a generic classification scheme that can be used to classify current and future DCNs based on the communication technology used in the network. The proposed classification is then used to review and summarize major research in this area. We also discuss open questions and future research directions in the area of wireless DCs.
POTENTIAL WIRELESS TECHNOLOGIES IN DCNs
Fig.2. Electromagnetic Spectrum
Wireless communication is one of the active areas of research in the communication field today. In wireless communication, information is transferred from the transmitter to the receiver without the need for a confined medium (e.g.,cable). Figure 2 depicts part of the electromagnetic (EM) spectrum. The wave- length of a signal decreases as the frequency increases and different frequencies across the EM spectrum have different propagation properties. According to Friis law, the effective area of an antenna decreases as frequency squared.
PROPOSED CLASSIFICATION OF DCN ARCHITECTURES
Fig.3. Classification of conventional wired DCNs
DCN architectures are broadly classified into switch-centric and server-centric architectures. In switch-centric DCNs, servers operate only as computing nodes and switches are used for data routing. In server-centric DCNs, servers perform both, computation and data routing.
Wired DCNs are commonly classified based on switching schemes into three classes (see Figure 3); namely, electrical (circuit or packet switching), optical (packet, circuit, or burst switching), and hybrid.
Fig.4. Proposed data center network (DCN) classification
In Figure 4, for the sake of brevity, we only show Hybrid wired augmented with RF and Hybrid wired augmented with FSO DCNs. It might be noted that, using the proposed classification, an electrical/optical DCN in conventional classification can be classified as a pure electrical/optical DCN, respectively. On the other hand, a hybrid DCN in conventional DCN classification falls under the hybrid wired DCN class.
SUMMARY OF TECHNIQUES FOR ADOPTING 60GHZ IN DCNs
Fig.5. Intra and inter-rack communications in 60GHz wireless DCs as envisioned by Ramachandran et al
In 2008, Ramachandran et al. nurtured the idea of using 60 GHz technology in DCNs. The authors identify the requirements of a DCN and the problems encountered due to wires. They discuss the suitability and the challenges of the use of 60 GHz inside DCNs. Ramachandran et al. envision three complementary deployment scenarios for both intra and inter- rack communications (see Figure 5). An array of antennas is used in order to create directional beam with small beam width. For intra-rack communication, Ramachandran et al. suggest using a reflector to create indirect LOS links, whereas for inter- rack communication, LOS, indirect LOS, or multi-hop links can be used.
Fig.7. Rack and server design in Cayley DCN
The proposed design by Shin et al. features novel cylindrical rack design [see Figure 7]. A rack consists of S stories and each story holds C prism-shaped containers in which servers are stored. Racks are arranged in a semi-regular mesh topology resulting in a densely connected subgraph that is a member of Cayley Graphs (CG). Two wireless transceivers are mounted on both ends of each server node. One is used for intra-rack communication, and the other is used for inter-rack communication.
APPROACHES FOR DEPLOYING FSO IN DCNs
Fig.10. Design proposed by Riza et al
In, Riza and Marraccini discuss different applications in which power smart FSO links can be utilized. One of the applications is inter-rack communications in wireless DCNs. A transceiver is mounted to a pedestal platform that sits on top of each rack. The pedestal allows for vertical and rotational motion such that LOS links between different racks can be established [see Figure 10]. Power smart FSO link can adapt to the varying link length as a rack establishes the links with different racks in the DCN.
Fig.11. FireFly by Hamedazimi et al
In FileFly, FSO transceivers are placed on ToRs. In order to perform link steering, the authors propose the use of switch- able mirrors (SMs) or Galvo Mirrors (GMs). In the case of SMs, every FSO transceiver is equipped with several SMs (see Figure 11). SMs are preconfigured and aligned to a receiving FSO on a different rack. According to the states of SMs (i.e., glass/mirror), a link is directed to devices on other racks through the reflection off a mirror mounted to the ceiling. Links are established by switching relevant SMs to mirror/transparent states.
WIRELESS DCNs :CHALLENGES AND LESSONS
Fig.16. Design proposed by Arnon (a) Side view. (b) Top view
In the case of inter-rack communication, racks are arranged in circular cells such that neighboring racks can communicate using LOS OWC links. Moreover, ToRs within a cell can communicate with Aggregate (or core) switches located at a higher layer as shown in Figure 16. Aggregate (or core) switches can communicate with each other at a higher layer on top of the layer of ToRs. However, a complete topology of a DC using the proposed design has not been addressed, and thus, it is not clear how racks, aggregate, and core switches, are connected on a large scale.
Fig.17. Design proposed by Joseph et al. (a) Intra-rack. (b) Inter-rack top (top) and side (bottom) views
They envision intra-rack communication to be performed using a ToR optical switch employing a multiple lens array. Servers in the rack send information to the ToR Switch as shown in Figure 17-(a). The optical switch then directs the information back to the servers using data shower beams. The switch can be placed at the top, bottom, or middle of the rack cabinet.
For the inter-rack communications, optical switches or transceivers are mounted to a polygonal structure. For example, Figure 17-(b) depicts six switches (transceivers) mounted to a hexagonal structure. Similar to the work by Marraccini and Riza the structure is mounted to a pedestal system that allows rotational and vertical height adjustments. This arrangement can be very useful for cellular FSO DCNs.
FUTURE RESEARCH DIRECTIONS IN WIRELESS DCNs
The incorporation of wireless communication technologies in DCNs is still in its infancy, thus, it still needs great investigation and development in order to become an efficient practical reality. Some interesting design considerations and open questions involve.
- Hybrid versus Pure DCNs
- Goodness Metrics
- Network Architecture
- Cost Tradeoffs
- Visible Light Communication
- Hybrid Wireless DCNs
SUMMARY
DCs have become a critical part of today’s computing and enterprise infrastructures. Currently deployed wired DCs suffer from increasing cabling complexity and hotspots problems. This has motivated the researchers to investigate the possibility of incorporating wireless technologies into DCs. Existing surveys and classifications on DCs chiefly focus on wired DCs. In this paper, we present a detailed survey on wireless DCs.
We start by comparing the two potential candidate technologies for wireless communication in DCs, namely; 60 GHz and FSO. Comparison shows that both technologies are unlicensed and have link length suitable for the confined environment of DCs. Moreover, 60 GHz and FSO technologies depend on LOS links, but 60 GHz technology has lower practical bandwidth and can be affected by interference. On the other hand, FSO links require careful alignment to maintain the LOS.
We propose a classification that can be used to classify any DC, including existing wired and emerging wireless DCs. Our classification is based on the communication technologies used to realize the DCN. According to the proposed classification, wired DCs can be classified as pure electrical/optical wired DC, or hybrid wired DC. On the other hand, wireless technology can be used either to augment wired DCs resulting in hybrid DCs, or to realize pure RF/FSO DC. We discuss different wireless-based DC designs and collate the major work in the field to jump-start researchers to tap into the growing research on wireless DCs.
Source: University of Nebraska
Authors: Abdelbaset S.Hamza | Jitender Deogun | Dennis R.Alexander
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