IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013 5 Smart Grid Last-Mile Communications Model and Its Application to the Study of Leased Broadband Wired-Access Felipe Gómez-Cuba, Student Member, IEEE, Rafael Asorey-Cacheda, and Francisco J. González-Castaño Abstract—This paper addresses the modeling of specificSmart Grid (SG) communication requirements from a data networking research perspective, as a general approach to the study of dif- ferent access technologies suitable for the last mile (LM). SGLM networks serve customers’ Energy Services Interfaces. From functional descriptions of SG, a traffic model is developed. It is then applied to the study of an access architecture based on leased lines from local broadband access providers. This permits con- sideration of the potential starvation of domestic traffic, which is avoided by applying well-known traffic management techniques. From previous results obtained for a purpose-built WiMAX Fig. 1. A wired ISP access network carries SGLM data. SGLM network, the intuition that a leased broadband access yields lower latencies is verified. In general, the proposed traffic Even though communications architectures connecting model simplifies the design of benchmarks for the comparison of candidate access technologies. households to the SG are usually called Advanced Metering Interfaces (AMI), we prefer to refer to them as Last-Mile net- Index Terms—Communication systems traffic control, diff- works to emphasize that meter reading is simply one possible serv networks, quality of service, smart grids, subscriber loops, WiMAX. application. For instance, AMIs and Distribution Automation Systems (DAS) may coexist in these architectures [5]. In a recent work [6] we assessed the viability of WiMAX for I. INTRODUCTION ad-hoc infrastructures for SGLM communications, showing that traffic priorities play a key role in performance. In this paper HE SMART GRID (SG) has raised many expectations as we generalize our traffic model to any access technology and a result of the upcoming renovation of the electric grid, T perform a deeper analysis of the traffic-management plane of which will require state-of-the-art communications, computing, SGLM communications. We also discuss some relevant candi- management, and control technologies. Utilities expect im- date access technologies for SGLM communications. provements in automation, integration of future energy sources We validate our model by analyzing a leased broadband and rapid-response automation mechanisms, while customers scenario, where domestic users possess some form of broad- demand rich domestic applications for home management, sat- band wired Internet access, the Internet provider is willing to isfaction of their ecological concerns, and energy cost savings reach a data carrying agreement with the electric company, and [1], [2]. the required bandwidth of SGLM traffic is comparably small Regarding the Smart Grid Last-Mile (SGLM), there is an (meaning that the existing network can handle the increase open discussion on the most suitable communication technolo- without extending its capacity) (Fig. 1). gies [3], [4]. This paper proposes a generalized SGLM traffic The rest of this paper is organized as follows: Section II model designed to simplify the comparison of competing solu- describes SGLM architectural constraints and provides a list tions, from a data networking research perspective. of access technologies to be considered. In Section III we review functional characteristics of SG communications and Manuscript received March 29, 2012; revised September 04, 2012; accepted develop an SGLM traffic model that allows the comparison October 02, 2012. Date of publication February 13, 2013; date of current version February 27, 2013. This work was supported by projects CALM of access technologies. In Section IV we discuss our earlier (TEC2012-21405-c02-01), MICINN, Spain; and MEFISTO (10TIC006CT), results for a WiMAX SGLM network. Based on the proposed Xunta de Galicia, Spain. Paper no. TSG-00142-2012. model, Section V describes the setup and simulation results in F. Gómez-Cuba is with GRADIANT. EdificioCITEXVI,local14,Campus Universitario de Vigo, 36310 Vigo, Spain (e-mail: [email protected]; . a new scenario: a leased broadband SGLM network. Finally, R. Asorey-Cacheda is with AtlantTIC, Universidade de Vigo, ETSE Teleco- Section VI concludes the paper. municación, Campus, 36310 Vigo, Spain (e-mail: [email protected]). F. J. González-Castaño is with GRADIANT. Edificio CITEXVI, local 14, II. ACCESS TECHNOLOGIES FOR SMART GRID LAST-MILE Campus Universitario de Vigo, 36310 Vigo, Spain and also with AtlantTIC, Universidade de Vigo, ETSE Telecomunicación, Campus, 36310 Vigo, Spain COMMUNICATIONS (e-mail: [email protected]; [email protected]). In this section we will briefly describe the variety of access Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. technologies that are available for SGLM communications. Digital Object Identifier 10.1109/TSG.2012.2223765 More detailed comparisons can be found in the literature [3], 1949-3053/$31.00 © 2013 IEEE 6 IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013 The communications network for the SGLM may be purpose- built or based on connections leased from an Internet Service Provider (ISP), and it may rely on wireless or wired technolo- gies. The former allow faster set-ups and incremental system deployments, whereas the latter are more robust and scale better if the installation serves many users. It is also a matter of dis- cussion whether the NAN will be subdivided in clusters or not; for example, the proposal in [11] has two tiers, with WiFi con- centrators connecting several houses (a Cluster Area Network, CAN) to the DAP, as a single WiMAX NAN. In principle, diverse standards may be used for a pur- pose-built wireless last-mile (LM) network [12]: WiMAX, Fig. 2. The main entities of the SG according to IEEE Std. 2030-2011. IEEE 802.22, WiFi, ZigBee, etc. For a purpose-built wired LM network, there are two possible [4]. For SG communications, the following considerations must types of deployment: be taken into account: • A PLC LM exploiting the last power distribution stage; this • The SG is not a network of light intermediaries and heavy would be cheap, but have a very limited capacity [13]. edges like the Internet. Unlike the IP protocol, which • With independent purpose-built wiring; in this case, instal- covers OSI layer 3, the nodes in the middle of the SG lation costs could be prohibitive, but performance would network perform OSI layer 7 (application) duties. be optimal, specially with optical fiber [14]. • To ensure that SG is viable in areas where broadband ac- A leased wireless access could rely on specific mobile-IP de- cess is not fully available and to prevent monopolies, trans- vices (GSM, UMTS, LTE, etc.) installed in the ESIs [15]. port over user Internet connections should not be the only Finally, in the case of a leased wired access, (over xDSL, alternative. However, since broadband access is available HFC, FTTx, etc.), the operators must provide interfaces to con- in most cases, non-intrusive carrying agreements with In- nect the ESI [5]. ternet providers are of great interest. Table I ranks alternative technologies according to the aspects • An Energy Services Interface (ESI), possibly the Smart of interest for SGLM deployment [16]: Meter (SM) itself, acts as a gateway between utility and • Capacity: The amount of traffic that the link will support. user domains, relaying, filtering or generating cross-do- Lowest capacity systems are suitable for plain remote me- main messages according to a control model. tering but not for advanced information services. • Control is hierarchical, with messages being sent up or • QoS support: Traffic must be differentiated. In particular, down the grid by the control devices in each section. alarm packets should never be dropped and the network According to the IEEE Std 2030-2011 guide for SG interop- must treat real-time traffic adequately. erability [7], the power network is divided into seven domains: • Footprint: The physical impact of the installation for the bulk generation, transmission, distribution, user segment, mar- customers and the grid. kets, management and service provider (Fig. 2). The aforemen- • Complexity: The engineering effort of system deployment. tioned standard characterizes entities and interfaces within those • Coverage: Typical network coverage. Infrastructure costs domains at three different levels: electrical, communications are higher for technologies with smaller cells. and information. The entities described in this paper (ESI, SM, • Trust: Well-established and trustworthy technologies are etc.) are based on this recommendation. preferable to newer and lesser known ones. The ESI acts as a gateway for customers, separating the Home • Reliability: Probability that communication will be suc- Area Network (HAN) from the part of the grid controlled by cessful. the utility. Communications with markets are expected to take • Scalability: Capacity to accommodate more users and ser- place between utilities and producers and will affect customers vices in the future without major changes. indirectly through the ESI, through demand-reduction programs • Cost: Estimated expenditure. and the like [8]. Next we briefly describe the main technologies in Table I: A customer HAN may be composed of appliances from dif- • IEEE 802.16-WiMAX: This standard [17] aims at wireless ferent vendors. Consumer electronics markets are already pro- Metropolitan Access Networks (MAN). In principle, the ducing standards and agreements for interoperability such as the maximum range (5 km) covers a neighborhood, and several ZigBee Smart Energy Profile [9] or the HomePlug technology QoS classes are supported natively. for Power Line Communications (PLC) [10]. • IEEE 802.22-White Spaces: This standard [18] supports By SGLM communications we refer to the flow of data orig- cognitive exploitation of white spaces in the UHF band. It inating at the ESI or relayed from the HAN towards the Distri- is designed for Rural Access Networks (RAN), a low pop- bution Access Point (DAP).