A Wireless Data center is a type of data center that uses wireless communication technology instead of cables to store, process and retrieve data for enterprises. The development of Wireless Data centers arose as a solution to growing cabling complexity and hotspots. The wireless technology was introduced by Shin et al., who replaced all cables with 60 GHz wireless connections at the Cayley data center.
Motivation.
Most DCs deployed today can be classified as wired DCs because they use copper and optical fiber cables to handle intra- and inter-rack connections in the network. This approach has two problems, cable complexity and hotspots. Hotspots, also known as hot servers, are servers that generate high traffic compared to others in the network and they might become bottlenecks of the system. To address these problems several researchers propose the use of wireless communication into data center networks, to either augment existing wired data centers, or to realize a pure wireless data center
Although cable complexity at first seems like an esthetical problem, it can affect a DC in different ways. First, a significant manual effort is necessary to install and manage these cables. Apart from that, cables can additionally affect data center cooling. Finally, cables take up space, which could be used to add more servers. The use of wireless technologies could reduce the cable complexity and avoid the problems cited before, moreover, it would allow for automatic configurable link establishment between nodes with minimum effort.
Wireless links can be rearranged dynamically which makes it possible to perform adaptive topology adjustment. This means that the network can be rearranged to fulfil the real-time traffic demands of hotspots, thus solving the hot servers problem. Additionally, wireless connections do not rely on switches and therefore are free of problems such as single-point of failure and limited bisection bandwidth.
Requirements.
The Data Center Network (DCN) is the infrastructure responsible to provide intra and inter-DC networking services, therefore is essential to design efficient high-speed/high bandwidth DCN to satisfy the high computing and communication demands from the DC. Another basic requirements, such as scalability and fault tolerance should also be addressed. In 2008, Ramachandran et al. talks specifically about the requirements that a wireless DCN should met, they are as follows:
Technologies.
There are two candidate technologies to enable wireless DCNs, the most cited one is 60GHz radio frequency (RF), the other option is free space optical (FSO).
Challenges.
Since DCNs needs high bandwidth in order to exchange big amount of data, new communication technologies must be capable of achieving high link and network capacities and also it should meet other requirements of a data center network. Hence, In order to implement wireless communication technologies in data centers, there are several challenges that may face any wireless technology to be deployed in DCNs like:
Proposed designs.
Cayley DCN.
Architecture.
In the concept of wireless data centers, efficient use of open spaces is significant in order to optimize resource multiplexing. Since the maximum number of live connections proportional to the volume of data center per single antenna beam. Cayley DCN proposes to use cylindrical racks that are contains levels called stories and each row contains pie-shaped containers. (Fig. 1). These containers stores servers in it and servers faces both inner rack space and inter-rack space. Each server has two transceivers positioned at opposite ends of it. One for intra-rack communication and another one for inter-rack communication. Network Interface Cards (NIC) that are used in traditional data centers are replaced by a custom-built Y-switch with transceivers to connect the server's system bus. In this way, classic network switching fabric that exists in traditional databases are eliminated, which means, there is no need classic networking switches in the novel topology since every server is able to route data. This communication is maintained by geographical routing protocol which tries to find the shortest path between source and destination using coordinates of racks, the ordinal number of stories and the index number of the server in the stories. A server uses three routing tables to determine next hop which are inter-rack routing table, inter-story routing table, intra-story routing table.
Cost and Performance.
The technical studies have been done to evaluate network performance, failure resilience and cost. During the tests, since the wireless transceivers and Y-switches are not yet available in the market, estimations have done based on the expected price range of 60 GHz transceivers and simulations are developed for performance evaluations of Cayley data center and Conventional data centers (CDC) with different designs.
Based on conducted experiences with Cayley data center and CDC that are configured with different oversubscription rates, Cayley data centers have better performance at maximum aggregate bandwidth compared to CDC. Cayley data centers are taking advantage of less switching design and doubles CDC's bandwidth during packet delivery outside of a rack.
Cayley data center has a drawback in packet delivery latency and scalability because it uses multi-hop routing and uses intermediate nodes to convey packets to other nodes. Therefore, as the traffic load increases, the maximum latency quickly increases as well. Hence, Cayley data centers are not able to reach the same scalability level of CDC's, since CDC has stable wired links and network hops with smaller numbers
Cayley data centers are more resilient to failures compared to conventional data centers since Cayley topology has dense connectivity and have minimized the number of switches that are a critical point for failures for data centers. Experiences shows that server nodes will be fully connected until 20% of nodes, 59% stories, and 14% racks fail. However, more than 99% of connections are preserved to %55 nodes and stories, 45% of racks are failed
Cost analysis of Cayley data centers has been done based on assumptions since 60 GHz transceivers are not commercially in the market yet. However, it is expected that transceivers’ price will not be expensive if the silicon chips are used. In the case, the cost of a 60-GHz transceiver less than $90, Cayley data centers will be much cheaper. A 60 GHz transceiver consumes maximum 0.3 watts, while the top of rack switches (TOR), aggregation switches (AS), and core switches (CS) consume in the range of 170-620 watts. Therefore, total power switch packet consumption of CDC is at least 12 times more than Cayley data centers. Moreover, due to the absence of cabling, maintenance costs will be significantly lower
References.
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