Orthogonal frequency-division multiplexing (OFDM) is a modulation technology that has been widely adopted in many new and emerging broadband wireless and wireline communication systems. Due to its capability to transmit a high-speed data stream using multiple spectral-overlapped lower-speed subcarriers, OFDM technology offers superior advantages of high spectrum efficiency, robustness against inter-carrier and inter-symbol interference, adaptability to server channel conditions, etc. In recent years, there have been intensive studies on optical OFDM (O-OFDM) transmission technologies, and it is considered a promising technology for future ultra-high-speed optical transmission. Based on O-OFDM technology, a novel elastic optical network architecture with immense flexibility and scalability in spectrum allocation and data rate accommodation could be built to support diverse services and the rapid growth of Internet traffic in the future. In this paper, we present a comprehensive survey on OFDM-based elastic optical network technologies, including basic principles of OFDM, O-OFDM technologies, the architectures of OFDM-based elastic core optical networks, and related key enabling technologies. The main advantages and issues of OFDM-based elastic core optical networks that are under research are also discussed.

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A Survey on OFDM-Based Elastic Core Optical Networking

As 100-Gb/s coherent systems based on polarization- division multiplexed quadrature phase shift keying (PDM-QPSK), with aggregate wavelength-division multiplexed (WDM) capacities close to 10 Tb/s, are getting widely deployed, the use of high-spectral-efficiency quadrature amplitude modulation (QAM) to increase both per-channel interface rates and aggregate WDM capacities is the next evolutionary step. In this paper we review high-spectral-efficiency optical modulation formats for use in digital coherent systems. We look at fundamental as well as at technological scaling trends and highlight important trade-offs pertaining to the design and performance of coherent higher-order QAM transponders.

High-Spectral-Efficiency Optical Modulation Formats

A novel paradigm that changes the scene for the modern communication and computation systems is the Edge Computing. It is not a coincidence that terms like Mobile Cloud Computing, Cloudlets, Fog Computing, and Mobile-Edge Computing are gaining popularity both in academia and industry. In this paper, we embrace all these terms under the umbrella concept of “Edge Computing” to name the trend where computational infrastructures hence the services themselves are getting closer to the end user. However, we observe that bringing computational infrastructures to the proximity of the user does not magically solve all technical challenges. Moreover, it creates complexities of its own when not carefully handled. In this paper, these challenges are discussed in depth and categorically analyzed. As a solution direction, we propose that another major trend in networking, namely software-defined networking (SDN), should be taken into account. SDN, which is not proposed specifically for Edge Computing, can in fact serve as an enabler to lower the complexity barriers involved and let the real potential of Edge Computing be achieved. To fully demonstrate our ideas, initially, we put forward a clear collaboration model for the SDN-Edge Computing interaction through practical architectures and show that SDN related mechanisms can feasibly operate within the Edge Computing infrastructures. Then, we provide a detailed survey of the approaches that comprise the Edge Computing domain. A comparative discussion elaborates on where these technologies meet as well as how they differ. Later, we discuss the capabilities of SDN and align them with the technical shortcomings of Edge Computing implementations. We thoroughly investigate the possible modes of operation and interaction between the aforementioned technologies in all directions and technically deduce a set of “Benefit Areas” which is discussed in detail. Lastly, as SDN is an evolving technology, we give the future directions for enhancing the SDN development so that it can take this collaboration to a further level.

How Can Edge Computing Benefit From Software-Defined Networking: A Survey, Use Cases, and Future Directions

With the widespread adoption of the Internet of Things (IoT), the number of connected devices is growing at an exponential rate, which is contributing to ever-increasing, massive data volumes. Real-time analytics on the massive IoT data, referred to as the “real-time IoT analytics” in this paper, is becoming the mainstream with an aim to provide an immediate or non-immediate actionable insights and business intelligence. However, the analytics network of the existing IoT systems does not adequately consider the requirements of the real-time IoT analytics. In fact, most researchers overlooked an appropriate design of the IoT analytics network while focusing much on the sensing and delivery networks of the IoT system. Since much of the IoT analytics network has often been taken as granted, the survey, in this paper, we aim to review the state-of-the-art of the analytics network methodologies, which are suitable for real-time IoT analytics. In this vein, we first describe the basics of the real-time IoT analytics, use cases, and software platforms, and then explain the shortcomings of the network methodologies to support them. To address those shortcomings, we then discuss the relevant network methodologies which may support the real-time IoT analytics. Also, we present a number of prospective research problems and future research directions focusing on the network methodologies for the real-time IoT analytics.

A Survey on Network Methodologies for Real-Time Analytics of Massive IoT Data and Open Research Issues

Software-Defined Networking

Flexibility to support mesh topologies, dynamic capacity allocation, and automated network control and light path setup are key elements in the design of next-generation optical transport networks. To realize these capabilities, reconfigurable optical add/drop multiplexers with dynamic add/drop structures, embedded control planes, and lightpath characterization are required. This article presents the architectures and various ROADM implementations including colorless, directionless, and contentionless add/drop structures. The effect of scaling bit rates beyond 100 Gb/s on ROADM architectures is reviewed including providing variable channel bandwidth depending on bit rate. Automated provisioning and restoration using the GMPLS control plane and optical measurement approaches for lightpaths are also discussed.

Flexible architectures for optical transport nodes and networks

Software defined networks are based on the principle of a centralized control plane separated from the network forwarding or switching plane that it controls. The switching plane can be heterogeneous, composed of network elements from multiple vendors, and it can provide distinct services with different characteristics, configurations, and control at the packet and/or optical layers. Abstracting the control plane from the network elements allows network-platform- specific characteristics and differences that do not affect services to be hidden. In addition, software defined networking (SDN) is based on the principle that applications can request needed resources from the network via interfaces to the control plane. Through these interfaces, applications can dynamically request network resources or network information that may span disparate technologies. For instance, the application layer can dynamically request and obtain network resources at the packet flow, circuit, or even optical level based on application layer requirements. Current SDN implementations focus on Ethernet switching primarily for data center resource optimization. This article reviews the benefits and challenges of extending SDN concepts to various transport network architectures that include optical wavelength and fiber switches, circuit switches, and sub-wavelength optical burst switches. Control plane implementations for optical networks are more complex since they must account for physical constraints including optical signal reachability, bandwidth availability and granularity, light path routing, and light path reconfiguration speed. The longterm goal is to apply SDN concepts across multi-layer multivendor networks in order to support a unified control structure.

Extending software defined network principles to include optical transport

As traffic demands continue to grow, supporting data rates beyond 100 Gb/s will be required to increase optical channel capacity and support higher-rate client interfaces. Advanced modulation formats that adapt to optimize spectral efficiency over a range of channel signal-tonoise ratio conditions are required. Channels can be constructed by varying parameters such as symbol rate, bits per symbol, number of polarizations, and number of optical and electrical subcarriers. Channel capacity can also be increased using advanced techniques such as optical time-division multiplexing, and fibers that support multiple cores and modes. Many channel designs can support higher data rates, but there are trade-offs between complexity, spectral efficiency, and optical reach.

Technical considerations for supporting data rates beyond 100 Gb/s

Data center and cloud architectures continue to evolve to address the needs of large-scale multi-tenant data centers and clouds. These needs are centered around seven dimensions: scalability in computing, storage, and bandwidth, scalability in network services, efficiency in resource utilization, agility in service creation, cost efficiency, service reliability, and security. This article focuses on the first five dimensions as they pertain to networking. Large data centers are targeting support for tens of thousands of servers, exabytes of storage, terabits per second of traffic, and tens of thousands of tenants. In a data center, server and storage resources are interconnected with packet switches and routers that provide for the bandwidth and multi-tenant virtual networking needs. Data centers are interconnected across the wide area network via routing and transport technologies to provide a pool of resources, known as the cloud. High-speed optical interfaces and dense wavelength-division multiplexing optical transport are used to provide for high-capacity transport intra- and inter-datacenter. This article reviews various switching, routing, and optical transport technologies, and their applicability in addressing the networking needs of large-scale multi-tenant data centers.

Technologies and protocols for data center and cloud networking

Network traffic demands are forecast to increase for the foreseeable future, with the challenge being to meet the demand while maintaining or lowering network costs. Simply increasing capacity will not be sufficient; overall bandwidth utilization also needs to improve. A combination of improved transport capacity through increased spectral efficiency and bit rate along with better network utilization by integrating subchannel electrical grooming into the transmission system will be required. Smarter ways to utilize optical capacity are key since transmission costs have been decreasing slower than grooming and switching costs. Integrated transport and switching can improve the efficiency of the client network using techniques such as port virtualization and transit traffic reduction. The baseline for transport networks will be 100 Gb/s PM-QPSK using 50 GHz channel spacing. Moving from a fixed DWDM channel arrangement to support flexible grid and super channels will allow tighter channel (carrier) spacing and should increase capacity by 30 to 50 percent. For shorter distances higher-order modulation such as 16-QAM can double network capacity. To better optimize network efficiency, an architecture that flexibly combines lower rate (sub-100 Gb/s) clients to form channels (carriers) and then superchannels will be required.

Tiejun J. Xia

Papers

Flexible architectures for optical transport nodes and networks

Extending software defined network principles to include optical transport

Technical considerations for supporting data rates beyond 100 Gb/s

Technologies and protocols for data center and cloud networking

High-capacity optical transport networks