Kingston University London

Transcription

1 Kingston University London POWERLINE CARRIER COMMUNICATIONS USE CASE BROADBAND OVER POWER LINES (BPL) ANDREAS KREMMYDAS Master of Science in Networking and Data Communications THESIS

2 Kingston University London Powerline carrier communications - use case broadband over power lines (BPL) Dissertation submitted for the Degree of Master of Science in Networking and Data Communications By ANDREAS KREMMYDAS SUPERVISOR STEFANOS ΤSITOMENEAS KINGSTON UNIVERSITY, FACULTY OF SCIENCE, ENGINEERING AND COMPUTING ΤEI OF PIRAEUS, DEPARTMENTS OF ELECTRONICS AND AUTOMA- TION JANUARY 2014

3 Abstract In recent years there has been considerable interest in broadband signal transmission via the power line network. This interest has increased even further due to the possibility of the power line network to offer special telecommunication utilities to the end users. Telecommunications through power grids can be used to bring the digital age to rural sites but also to provide an alternative transmission medium in urban areas. Applications through a power line network such as broadband Internet access, multimedia and smart grid is a new opportunity for telecoms. The power grid network is proposed as an alternative way for broadband data transmission, especially in underdeveloped countries. Broadband over power lines (BPL) is an access technology which uses the medium-voltage (MV) and lowvoltage (LV) lines of the electrical supply network in order to deliver broadband telecommunication services. Nevertheless, power line communication networks can also be narrowband. The electrical network has been used for many years for low frequency communication services, such as monitoring and remote control. Furthermore, telemetry and smart grids can prevent blackouts and reduce significantly the electricity cost for the consumer. However, there are potential obstacles and challenges to take into consideration, as a result of the channel attenuation and stochastic noise waveforms which could lead to a decrease of the network bandwidth capacity. The goals of this study are to present the possibilities of using power line networks, to identify the protocols governing the exchange of information over power lines, and to investigate the operation models of regulating BPL deployment. We consider that BPL technology has transcended the early transitional phase, and the legal or regulatory limitations on the implementation of BPL systems are constantly reduced. I

4 Contents Chapter 1 - Introduction... 1 Chapter 2 - BPL Technology In Building BPL BPL Access Purely wired BPL access networks Hybrid BPL access networks Example of architectures of BPL network GEN GEN BPL access network elements Configuration of the LV distribution network towards BPL technology Optimizing network performance of MV BPL Electromagnetic compatibility (EMC) Chapter 3 - Description of the layers of the BPL network Modulation techniques for BPL systems Signal modulation method based on Orthogonal Frequency Division Multiplexing (OFDM) Spread spectrum modulation Comparing modulation techniques for BPL systems Fault Management Forward error correction (FEC) ARQ mechanisms BPL MAC Layer Multiple access technique Technical sharing of resources Quality control services CAC mechanism Likelihood estimation of the packet size in BPL networks Packet delay IPv6 and BPL BPL Network Security Multicasting Family of protocols Chapter 4 - Power line attributes and channel modeling Losses in BPL network Transmission losses II

5 4.1.2 Electromagnetic Interference Attenuation Delay in signal propagation Noise Problems in signal transmission in BPL network Lack of symmetry Antenna effect The effect of the transformers Modeling of PLC channel Chapter 5 - Applications and processes of BPL technology BPL carrier services Telecommunication BPL services Internet access Telephony Voice over IP (VoIP) Video on Demand (VOD) Telemetry over the electrical networks Telemetry of the induced effects from geomagnetic storms Detecting the fault position with BPL SCADA Systems Smart grid Automatic meter reading (AMR) Smart Home Telemedicine Chapter 6 - Conclusions References III

6 Table of Figures Figure 1: In-building BPL is used to interconnect devices within a house... 3 Figure 2: BPL architecture... 3 Figure 3: HomePlug network... 7 Figure 4: BPL access architecture... 9 Figure 5: Structure of a wired BPL network Figure 6: Hybrid BPL networks Figure 7: Wireless coverage of BPL GEN1 Units Figure 8: General GEN1 infrastructure Figure 9: First GEN1 Architecture Figure 10: Second GEN1 Architecture Figure 11: Third GEN1 Architecture Figure 12: General GEN2 architecture Figure 13: Network elements of BPL Access Figure 14: Physical PLC Access Network Figure 15: Outdoor network (E-D), Indoor network (D-C) Figure 16: First configuration of external network Figure 17: Second configuration of external network Figure 18: Third configuration of external network Figure 19: The ISO/OSI reference model Figure 20: Spectrum of (a) an OFDM subchannel and (b) OFDM signal of 5 carriers at the central frequency of each subchannel, but there is no crosstalk from overlapping subchannels Figure 21: Principle of bandwidth spreading (broadening) in DSSS Figure 22: Types of FEC: Block and convolutional codes Figure 23: OFDM/TDMA scheme Figure 24: FDMA scheme IV

7 Figure 25: Waiting time (gap) between the transmission of data bursts Figure 26: Likelihood estimation of packet size (uplink) Figure 27: Likelihood estimation of packet size (downlink) Figure 28: Packet delay Figure 29: Unicast and multicast transmission Figure 30: Typical structure of a modern BPL system Figure 31: Active and silent periods during conversation V

8 Chapter 1 - Introduction The BPL technology can be described as the simultaneous transmission of power and data without loss to a significant degree. The idea of using power lines to transmit information is quite old and the reason is due to the fact it is a medium with ubiquitous nature. However, it was not considered as a satisfactory means of communication until the last decade due to the low speed and the high cost of development, compared with the services that had already been granted. In recent years, new powerful modeling techniques led the BPL technology to be considered as a realistic and practical way of communication. Due to the existence of the electric network around the globe, PLC has an implementation cost which can be compared with the equivalent cost of wireless technology [36]. So PLC can be a potential competitor of wireless communication in the above-mentioned services. But the deployment of PLC technology is not spreading with the pace of wireless communication, mostly because of challenges in channel modeling. Because of the ubiquitous nature of the LV (low-voltage), MV (mediumvoltage), and HV (high-voltage) power grids, the architecture of these grids is an indisputable candidate for developing an advanced IP-based communication system [37]. The first PLC effort was the transmitting of control messages via Ripple Control method. Low frequencies (100Hz to 900Hz) were used allowing a low data rate and at the same time requiring a very large power, of about 10kW. The transmission method provided in only one direction and used for management of street lighting and the control of the load power. In the middle 80 s, there have been many studies to analyze the characteristics of power transmission lines at higher frequencies. Several measurements over a range of frequencies from 5 khz to 500 khz drew conclusions as to the attenuation produced by the transmission line affecting the signal and the level of signal to noise ratio (SNR) that could have been achieved. The duplex was first implemented in the early 90's. 1

9 From then until today, many companies tried to offer the best possible data transmission over power cords at higher frequencies even with as less depreciation and error rate that was possible. On 15 October 2004 the Federal Communication Commission (FCC) gave approval for the creation and construction of BPL systems by adopting the technical design and rules for the operation of these systems. In the global market there is confusion about the terms of BPL technologies. The following terms characterize the same technology and under this, each provider choosing the marketing term they desire to give to the product commercially: PLC (Power Line Communications) BPL (Broadband over Power Lines) BPLC (Broadband over Power Lines Communications) However, it is common term to refer to BPL access systems while the other two terms refer to in-building networking and in power management systems from electricity companies. During the years and the possible foundation of BPL technology as one of the key solutions to access the global market, one of these terms will dominate obviously over the others. Figure 1 shows a typical in-building BPL system. Figure 2 shows the BPL architecture used by an electricity supplier. The main types of BPL technology are listed below and are analyzed separately in the next chapter: In-building BPL BPL access Control BPL/PLC or DLC (Distribution Line Carrier) 2

10 Figure 1: In-building BPL is used to interconnect devices within a house Figure 2: BPL architecture PLC systems with experimental and commercial operation have resulted international concern about interference to licensed radio services users such as radio amateurs. The European PLC regulation is described in the CENELEC standard [1]. FCC and Japanese standards differ from GENELEC and define a frequency range up to 500 khz. PLC transceivers communicate in the allowed frequency range 3 khz to khz as shown in Table 1. The maximum output level is shown also in Table 1. 3

11 We should note that the frequencies in Table 1 are used for Narrowband PLC networks. However, in order to achieve higher data rates, Broadband PLC transceivers must operate in a frequency range up to 30 MHz [2]. These concerns have been acknowledged and addressed at EU level and FCC with recommendations for provision of techniques to eliminate interference such as spectral blocking. Band Frequency range Maximum Purpose Output Level 3kHz 9kHz 134 dbμv 120 dbμv Electric distribution company use A 9 khz 95 khz 134 dbμv 120 dbμv Electric distribution company use and their licenses B 95 khz 125 khz 116 dbμv Consumer use with no restrictions C 125 khz 140 khz 116 dbμv Consumer use only with media access protocol D 140 khz khz 116 dbμv Consumer use with no restrictions Table 1: CENELEC - EN standard Furthermore, presented by Galli [3], the frequency band that a PLC device can operate defines the following classes: Broadband (BB): Technologies operating in the HF/VHF bands ( MHz) and having a bit-rate in the range of several Mbps to several hundred Mbps on the physical layer. Low and High Data Rate (LDR and HDR) Narrowband (NB): Devices operating in the any of the VLF/LF/MF frequency bands (from 3 khz to 500 khz), which include the CENELEC/FCC bands. More specific: o (LDR): Single carrier technologies for transmission with data rate of some kbps. Typical examples of LDR NB-PLC technologies are LonWorks and CEBus devices. 4

12 o (HDR): multicarrier-based technologies for transmission of data rates between some tens of kbps and up to 500 kbps. Examples of HDR NB-PLC technologies are technologies within the scope of IEEE standards project. 5

13 Chapter 2 - BPL Technology 2.1 In Building BPL Based on the In-Building BPL technology, the interior of the electric LV network of a building space is transformed into a local network. Using internal adapters, In-Building BPL operates numerous electrical outlets available within a building to transfer information between computers and other electronic home appliances, eliminating the need to install additional cables between devices. An implementation and use of the system is the universal user control, the concept of noninvolvement of one company or provider for system monitoring. Broadband In- Building BPL technology uses frequency range of 2 to 28 MHz (HomePlug AV) unlike other similar technologies for in-building communications over the network using the power spectrum below 1MHz (X-10, CEBus, LonWorks, etc.). As far it concerns HomePlug AV, the IEEE 1901standard can extend its range up to 50 MHZ. HomePlug AV2 will use additional frequencies above 30 MHz in order to achieve greater throughput [4]. Compared with other home networking technology, In-Building BPL outperforms in providing adequate transmission rate while numerous electrical outlets can connect a multitude of devices. The negative is the instability of the electric network which can cause undesirable disconnections between devices. Table 2 shows the transmission characteristics of the main home networking technologies. 6

14 Technology Communication Data rate QoS Support 100 Base-T UTP wiring 100 Mbit/s No 1000 Base-T UTP wiring 1 Gbit/s No Bluetooth 4.0 Wireless 24 Mbit/s Yes Wi-Fi n Wireless 600 Mbit/s Yes HomePNA 3.1 Phone line 320 Mbit/s Yes HomePlug AV2 IEEE std Power line 1 Gbit/s 1 Yes Table 2: Home networking standards The global market for In-Building BPL systems is currently dominated by the HomePlug standard. Figure 3 shows a practical implementation of home networking using HomePlug modems. Figure 3: HomePlug network 1 HomePlug Alliance (HPA) [4]. 7

15 2.2 BPL Access Electric generators by transforming mechanical energy to electrical energy produce electricity which is delivered at the final step to the customer. To illustrate, a step-up transformer is used in order to transmit power from a power plant through high-voltage (HV) lines with minimal loss. Moreover, a substation connected, to a HV line, distributes electricity over medium-voltage (MV) lines to a neighborhood step-down transformer where individual customers or businesses are located. Customers are connected to a neighborhood transformer through low-voltage (LV) lines. BPL access architecture includes the route from the substation via MV/LV lines up to customer premises. Regarding the general infrastructure of the grid, there are three voltage levels which correspond to RMS values of voltage in polar form of a three-phase electric power system: High Voltage (HV) of voltage level of 35kV (voltage over 275kV is called ultra-high voltage or UHV), Medium Voltage (MV) of voltage level ranging from 1kV to 35kV, Low Voltage (LV) of voltage level ranging from 230/380 V (100V in USA) to 1kV. By using the above architecture, for many years researchers and power supply companies have made efforts to provide telecommunication services and high signal quality to consumers. The idea of using HV lines as part of the signal path from the provider to the consumer was rejected almost immediately considering the increased noise of the HV line, making it difficult to telecommunications and, secondly, the provision of telecommunications signal to consumers is very demanding because of the distance that usually separates the HV line from urban areas. The most viable solution seemed to inject the signal in MV lines which is close to the nodes of the backbone network (core network) and the passing of the signal through the LV lines to reach the mark in the electrical output of the house and the outward after 8

16 connecting the end user via a BPL modem. This scenario is depicted simplistically in Figure 4. Figure 4: BPL access architecture Besides this solution which is the most traditional and the most prevalent today, in recent years new hybrid technologies have made their appearance that are gaining ground because of their simplicity of implementation and the cost of manufacture. In most external BPL Access networks that are implemented in Europe and in the United States, the frequency band from 2MHz to 30 MHz is used. The upper bound of the common-mode current is proposed to be 30dBµA at the frequency band of 2MHz to 15MHz and 20dBµA at the range from 15MHz to 30MHz. Thus there is the need for further research on methods for the simultaneous development of narrow-band systems (radio and broadcasting stations) and a wide-band system (BPL modems) in the same frequency zone [5]. Lower frequencies have less noise, and it is more suitable for the external network to achieve maximum coverage distance. There should also be adequate separation between the frequencies used for the internal and the external network. The distance within internal network coverage is obviously smaller than the outside distance. So intranets can be used for higher frequencies. In addition, the channel noise produced by electrical devices is much smaller at high frequencies than at low. 9

17 2.2.1 Purely wired BPL access networks According to Figure 5, the telecommunications backbone network is implemented by optical fibers connected to the MV BPL network through the MV nodes. Then the MV network is connected to the LV electrical distribution network, for which the BPL network termination units are called CPE (Customer Premises Equipment) or NTU (Network Termination Unit) and they are located at the end user/consumer. Figure 5: Structure of a wired BPL network Therefore, the topology of the network can be divided into three main sections, each of which has its own architecture: A telecommunications backbone network which is usually an optical ring. The BPL network ring in MV. The BPL network ring in LV, usually implemented with tree or star topology. In its most simple form, this architecture can include telecommunications backbone and a BPL LV network in star or tree topology which, however, prevents 10

18 the transmission of the signal from the MV network. This implies that there is a backbone network directly accessible for the LV network, which in most cases is practically difficult when resorting to termination of the participation of MV in path signal. The above standard topology uses the OFDM modulation technique. The data from the Internet backbone network are converted to a format signal in OFDM BPL MV devices and then a phase coupling occurs with the MV transmission line. The process is bidirectional, i.e. the signal format is converted respectively to OFDM signal format used in the telecommunications backbone network (PSTN, Internet). The transfer of information from the network towards LV lines providing electrical energy to consumers is through BPL LV devices. These devices, firstly, convert the OFDM signals in a format suitable for BPL LV network and also route the signal to the appropriate LV line. Also, to keep the signal in LV networks for long distances BPL repeaters are used Hybrid BPL access networks Hybrid BPL networks are using wireless technology in order to access the end user, thus avoiding the LV power line network or in order to connect the backbone of the telecommunications network with the BPL LV network, thus avoiding the MV power line network (BPL MMDS). Figure 6 shows these modes of communication networks which are based on hybrid BPL. 11

19 Figure 6: Hybrid BPL networks In the first case, the wireless networking ( a/b/g/n) provides access to the end user using the free zones of 2.4 and 5 GHz. This hybrid network clearly offers significantly improved behavior in terms of interference to amateur radio than a purely wireless network BPL. However, operation on the free frequency band offers no protection from interference from other users in the same zone. This drawback becomes more noticeable in urban areas and can affect the ability to provide services to the end user. In the latter case, access to the end user is from the HV wired network. Wireless networking between the telecommunications backbone and the BPL LV network is implemented using either Wi-Fi technology in the free zones in 2.4 and 5GHz either fixed wireless, microwave links, PMP (Point to Multipoint) to licensed frequencies. This option has the advantage that the communication is protected from interference caused by other systems. 12

20 2.2.3 Example of architectures of BPL network In this section we present the technology and architectures that are used by the company Amperion, a US-based technology provider for BPL supported by Cisco that is involved in the field of access services to consumers. Amperion s architecture which will give us a supervisory look at how to implement the BPL networks, combining purely wired and hybrid BPL networks GEN 1 The first generation (Gen 1) BPL technology is based on the logic of production and installation of three different units called concentrators/importers (injectors), repeaters and exporters (extractors). Importers have the role of the transfer of data from a conventional telecommunications medium such as an optical fiber to the MV electrical supply network. Importers are the units that are always placed at the beginning of each circuit and route the traffic to and from a BPL network. Repeaters are the units used for the regeneration of the signal, when it is necessary (every meters for an over ground network). Repeaters also act as traffic routers for the network path, from the consumer to the gateway, which in this case is the importer. Repeaters perform complete regeneration of the signal entering a very slight delay (microseconds). Finally, the exporters are the units that terminate the circuit and act as bridges of signal transfer from MV to LV. These are the units used in this model, except the network routers and the wireless access points. Thus, as shown in Figure 7, each module can provide services to consumers who are located within the coverage zone in a wireless network. 13

21 Figure 7: Wireless coverage of BPL Gen1 Units Each Gen1 unit can serve up to 40 users simultaneously providing wireless circuits in regulated speeds. The number of users depends on the quality of service (QoS) to be requested. Each telecommunications circuit from importer to exporter by the Gen1 units provides speeds of 20Mbps asynchronous in both directions (Half- Duplex network) in the band from 1 to 30MHz. Finally, Gen1 wireless routers are using protocols IEEE a/b/g/n. The BPL access network equipment consists of injectors (concentrators), repeaters and extractors (exporters). BPL Importers (injectors) are connected with the backbone network via a broadband connection (e.g. fiber optic line E1, DSL line or through IEEE a/b/g/n) and inject the signal into the MV lines. The MV lines may be overhead or underground within a protective tube. The height of overhead lines is about 10 meters above the ground. Three-phase MV cables generally distribute the electricity from the HV substation to the MV. The natural direction of the cables can be horizontal, vertical or triangular. This structure can be changed as needed with one or more branching phases to serve many consumers. Theoretically, the BPL 14

22 signal can be inserted into MV lines between two conductors or between a conductor and earth. BPL Exporters (extractors) provide the interface between the MV and LV lines that transmit the BPL signals to the end consumers. BPL Exporters are usually placed in each LV network providing services to a consumer group. In the long haul distance, depreciation or deformation of the signal due to the transmission through the MV network is such, that BPL providers use repeaters to maintain the required power and fidelity of the signal. Figure 8 shows a basic BPL system that can be deployed in a large area through the existing MV lines providing a cellular network infrastructure. Figure 8: General Gen1 infrastructure By using the first generation units, Gen1 technology resulted to three different BPL network architectures which are described next. The first BPL Gen1 architecture is implemented as shown in Figure 9. Configuration used is OFDM (Orthogonal Frequency Division Multiplexing) for transmitting the signal through the BPL bandwidth using several separate narrowband carriers. Using the BPL importer, information from the core network is suitably formed in OFDM signals, and then is injected through the coupler in one of the phases of the MV line. Due to the duplex nature of the Internet, the importer also converts the modulated OFDM signals of the MV line, in the type of data used in the internet backbone. The bidirectional movement of data is transferred towards and from the 15

23 LV lines in each consumer group by using a BPL exporter. Exporters route data and convert signals from the BPL access point s signals (OFDM) in internal type BPL signals in order to communicate with the BPL modem in user premises and vice versa. The subscriber/consumer has access to the network using a MV BPL modem. The importer and the exporter of the first architecture are using the same frequency F1 used in MV line which is different from the frequency F2 used in LV lines and the subscriber's modem. Due to the channel sharing scheme used by the subscribers, to minimize the time required to access the channel, the CSMA-CD (Carrier Sense Multiple Access with Collision Detection) protocol is used. This type of architecture was designed to be tolerated against crosstalk between semidetached BPL lines, without it being necessary to use isolation filters in distribution lines because all the devices in the MV lines operate in the same frequency band. The signal strength should be such as to avoid intersymbol interference ensuring independent implementation of the system in two or three different neighboring MV lines. Figure 9: First Gen1 Architecture Also in the second Gen1 architecture, OFDM is used as the modulation scheme. The difference from the first architecture lies in the way the signal is received by the subscribers/customers on their premises. Instead of using a device that is connected to the LV line, for example a BPL modem, in the second architecture 16

24 the BPL signal is outputted from the MV line through Wi-Fi devices and transmitted to subscribers' computers and laptops, as shown in Figure 10. Based on this architecture, other technologies may be used instead of Wi-Fi (for example IEEE e, marketed as WiMax) to extend the coverage and application of this architecture at higher levels. In IEEE , the coverage can reach up to 600m, while in the future, with the implementation of WiMax, the coverage may be extended up to a few tens of kilometers. WiMax technology compared to LTE (Long Term Evolution, marketed as 4G LTE) can be deployed to offer multiple solutions as Backhaul, last mile, mobility, and emergency Smart Grid applications, using both licensed and unlicensed frequency bands [6]. Therefore, we must highlight the scalability of this technology, its flexibility and ease on the consumer. The important feature of this second architecture is that BPL does not use the LV power lines, as in the first architecture. Figure 10: Second Gen1 Architecture Moreover, the second Gen1 architecture uses different frequency bands during transmission through the MV lines. This is, first, to separate the BPL traffic from the user to the MV line (upstream) and the traffic from the MV line to the user (downstream) and secondly, in order to minimize crosstalk from neighboring BPL units. If the signal path is the distance of several miles until the subscriber and to recover the depreciation, in this case repeaters are used so the signal can travel from the importer to the exporter without significant loss. BPL repeaters and importers 17

25 transmit and receive signals at different frequencies and use different frequencies from those used by neighboring importers and repeaters. This process allows the efficient operation of the system because it minimizes the possibility of interference from neighboring nodes. The importers of the second architecture can act as an exporter, in case their circuits have the equipment and standards of Wi-Fi. The second Gen1 architecture injects the BPL signal into an MV phase line. The third BPL Gen1 architecture uses the DSSS (Direct Sequence Spread Spectrum) modulation technique for transmitting data on the MV line. All users within each cell share a common frequency band. Due to sharing of the channel by subscribers, to minimize the time required to access the channel the CSMA-CD is used. As in the first BPL architecture, this type of architecture designed to withstand crosstalk between semidetached BPL lines, since all the devices in the MV lines operate in the same frequency band. In a pilot implementation of the third architecture, the technology provider installed in two of the three phases of the MV line two independent BPL channels. Each unit of the third architecture (Figure 11) consists of a concentrator (injector) which provides the interface of a fiber optic connection to the backbone network, and a number of repeaters/regenerators placed along line to keep the signal strength at satisfactory service provision levels. The BPL signal through the MV/LV supplies the user groups that correspond to each exporter. Neighboring systems of the third communication architecture overlap and so the BPL terminals located within the premises of users (modem) and the repeaters, are able to communicate with the injector to provide them with the best quality service without disconnects. 18

26 Figure 11: Third Gen1 Architecture GEN 2 The second generation (Gen 2) introduces a different logic. There is no separation between injectors, repeaters, and extractors, and the unit is uniform and thus enabling the manufacturer and the engineer to design the network easier to implement. This change comes from the fact that the new BPL chip of the units (DS2 Gen2) has the ability to operate in any situation offering the possibility of communication with the physical layer of the selected backbone network (backhaul). DS2 (Design of Systems on Silicon) company was a Spanish powerline chip specialist which it has been acquired by Marvell Technology Group Ltd. on August One of the most important features of the Gen2 unit is the increased speed circuit which reaches 200Mbps on the physical layer. Networking for the consumer is offered via MIMO antennas (Multiple Input Multiple Output) using the IEEE protocol n, which offers increased coverage, speed and security. An additional feature of the enhanced Gen2 units is the ease of installation and maintenance. The small size makes them easy to install and allows passive cooling without the use of fans, which is more economical in consumption. Moreover, it is fully waterproof and immune to hardware failures. The development of the network by using Gen2 units is also quite different of the network with Gen1 units. Each unit uses multicast technology and can communi- 19

27 cate with more than 2 units simultaneously, thus enabling the design of a network based on two-dimensional architecture. Also, the Gen2 units have the ability to automatically recognize new additions to the network and configure them accordingly, greatly reducing the time of configuration and design of the overall network. Each Gen2 unit has the ability to provide service to 80 concurrent users and their routes through the network using the architecture of faster routing (shortest route), thus giving the end user multiple ways of transmitting information via different routes. The resulting network is full duplex and has dynamic allocation of upload and download traffic in a symmetrical reasonable like Ethernet and not asymmetrical as ADSL s logic. The management of devices can be done through suitable software platform that uses SNMP (Simple Network Management Protocol) offering connectivity with many available platforms. Among other things, the Gen2 units are using a new type of inductive couplers. These new couplers called Shunt Couplers and can be coupling to and from the line communication signals in the frequency band under operation, providing higher transmission speeds and being safer for the network. BPL system manufacturers and service providers look forward to providing a variety of applications to their subscribers. From high quality multi-channel video, audio, Voice over Internet Protocol (VoIP) to real-time applications, which expected to increase demands for additional bandwidth transmission. To support typical transfer rate of around 5Mbps for the average subscriber, BPL systems must operate in MV lines with speeds of 200Mbps or more in the near future. The development of technology has lead BPL to second and third generation, which is based in DS2 technology that allows converting any grid to a data network of 200Mbps data rate or more. 20

28 Figure 12: General Gen2 architecture A simple structure of the future power grid is shown in Figure 12. Via an optical network (Gigabit Ethernet) is provided access to the Internet. Then through the MV line, and through Gen2 units, the signal reaches consumers that can be connected directly to the Internet providing high speed broadband applications BPL access network elements Below we can see the equipment used in many systems and architectures of BPL access. Of course, each company attaches a separate name to each element of the network, but almost all of these function in the manner indicated below. The equipment of a typical BPL network is shown in Figure 13. Network elements are outlined in the following paragraphs separated to MV and LV network elements [7]. 21

29 Figure 13: Network elements of BPL Access MV Node. The MV node is the unit that converts the communication signals from the standard IP format into BPL signal for transmission through the MV power line. For safety reasons, the MV node is not directly connected to the MV line but through a coupling device. The connection to the coupler is usually a coaxial cable. The MV node usually supports a set of functions such as: o Connection of the backbone network to the gateways of the BPL network (Backhaul connection link). o Network connection BPL-LV (with the LV node) o Concentration of BPL signals for transmission through the MV/LV lines. Some architecture segregate MV nodes to components which perform different connection functions.these are: MV Head End. Ensures contact between MV modems at the same MV cluster and the backbone network. MV Modem. The MV modem ensures communication between the LV Head End and the MV Head End using the MV network. 22

30 MV Repeater. Repeater is the device that operates along the MV power lines (overhead and underground) and is used to compensate the attenuation due to transmission in long MV lines or MV lines with high damping. In some systems, MV nodes may perform the function of a repeater. Coupler. The coupler is a device which achieves the coupling of the signal to power line power by using two modes, capacitive and inductive. o Capacitive coupler. Usually preferred for overhead lines. o Inductive coupler. Can be installed to overhead and underground lines without interrupting the power supply and has the ability to withstand high voltages, extreme weather, lightning transients and surges from the power grid. MV Cell. MV cell is called the aggregate of the MV modems that communicate with the same MV Head End. LV Node (Low Voltage Head End - LV HE). The unit that is typically installed at the MV/LV TS (Transformer Station) and performs two functions. Specifically bypasses the transformer and allows communication between the BPL network of the MV line and the BPL network of the LV line while acting as a repeater on the MV line. LV Repeater. The Repeater in LV network intervenes between the LV HE and the CPE (NTU) and usually is placed in outdoor cabinets, on poles, in basements and spaces/cubicles of electricity meters to compensate the attenuation due to transmission in long LV lines or LV lines with high damping. LV Cell. LV cell is called the aggregation of the LV repeaters and NTUs that communicate with the same LV Head End. Network termination equipment (Network Termination Unit - NTU or Customer Premises Equipment - CPE). The CPE or NTU is the termination equipment of the BPL-LV network and is located at the user premises. Essentially it is a LV line interfaces between the BPL network and terminal equipment of a user, such as the computer, a VoIP device etc. 23

31 2.2.5 Configuration of the LV distribution network towards BPL technology In the European networks there are many different variations of the external and the internal network as a result of various standardization efforts [8]. OPERA (Open PLC European research Alliance) is an Open industry standard for Access PLC and has been submitted for IEEE 1901 Access. The Physical PLC Access Network is shown in Figure 14. Head End (HE) is master over its network and connects to backhaul, and the Customer Premises Equipment (CPE) is always slave. Also, the Time Division Repeater (Repeater) can be slave or master [9]. Figure 14: Physical PLC Access Network 24

32 Figure 15: External network (E-D), Internal network (D-C) The start and the end points of the external and internal network are shown above in Figure 15. Figure 16: First configuration of external network In the first configuration (Figure 16), the external network is divided into the central supply lines connecting the street cabinets and into the secondary connection cables that distribute the energy in the various buildings. The street cabinets are switching devices enabling the connection and the support for entry points between LV lines and are mainly used in mesh LV networks. 25

33 Figure 17: Second configuration of external network The second configuration is usually presented as an extension of older areas where underground nodes are used as entries of the central supply lines. This configuration is less resilient from the previous and is common in mixed type building construction. The cables exiting the underground hub arrive directly in the buildings. Figure 18: Third configuration of external network The third configuration of the external network is common in areas with buildings with over 15 houses. Here, the houses surrounding the TS (Transformer Station) are directly connected via electric LV cables. The buildings which the distance between them is several tens of meters are connected via a central supply line that feeds the cabinet. 26

34 2.2.6 Optimizing network performance of MV BPL The MV network is the first part of the distribution network and therefore should have the characteristics of high reliability, low-latency and maximum transmission rate. The topology that can achieve better optimization of the MV network is a point to point topology, which allows the securing of a higher bandwidth and is better suited to MV networks. The choice of circular or mesh point-to-point topologies depend on the topology of the power network and the Transformer Centers (TCs) to be connected [7]. To optimize this point-to-point topology must be taken of the following recommendations. Each adjacent node must use different phases and different frequencies in order to reduce interference. The installation of the connector must be as close as possible to the output of the cable and avoid the traverse of signal from the switches and splitters. In this way the BPL signal is not affected by possibly open switches for any reason. The MV BPL equipment should be as close as possible to the coupler so as to avoid loss of the coaxial cable. The maximum number of MV nodes to reach the backbone network must not exceed the 4 or 5 to achieve low delay and allow the implementation of VoIP services in each TC cell. The maximum distance of a MV node should be checked at every occasion, as the value varies by many factors. The frequency planning must be based on the characteristics of the grid and usually must precede checks with local measurements to find the levels of attenuation and noise. 27

35 2.2.7 Electromagnetic compatibility (EMC) One of the major issues the BPL technology is facing, which is one of the latest and perhaps most difficult steps in the development of the market is that of electromagnetic compatibility. Many surveys have been done and continue to become globally on permissible operating limits of a BPL system. When the signal is transmitted through a power line, a part of it is irradiated in air. The power line could be seen as a large antenna that receives and transmits electromagnetic signals. These disrupt other communications systems, especially radio, using that band of frequencies. So there is the need of the annihilation of crosstalk with signals of other telecommunication systems. The increased use of electric appliances over the electricity network leads to an increase of "contamination" of the frequency domain, which must be kept under control. The principles set out standards of operation aiming to bring precisely the acceptable levels of voltage variation, the emission of harmonics and acceptable flicker emission of devices. It is very important therefore to find ways of limiting the maximum level of emission of energy. In case the wires are in the ground, irradiation is minimal but then there is intense radiation from buildings. While in case the cables are in the buildings are not armored thereby radiate more. Research on the topic of electromagnetic compatibility, has led to some conclusions which are the following: The experimental and commercial operation of BPL telecommunications systems have created international concern about interference to licensed radio services users such as radio amateurs. These concerns have been acknowledged and addressed at EU and FCC level with recommendations and arrangements to provide techniques to eliminate interference, such as spectral blocking. Systems with OFDM modulation already allow selective and programmable spectral blocking and this has been tested at European and international level. 28

36 Standardization of BPL systems is not yet complete. As networks converge, BPL systems are treated as wired communications networks and abide by the same limits to other wired networks such as DSL. Not completed standardization, there are plenty of specifications and reports of field measurements that can form the basis for measurement in installed BPL systems. There are no legal or regulatory restrictions on the installation of BPL systems. Indeed, the European Commission with the recommendation of 06/04/2005 urges Member States to remove any unjustified regulatory obstacles, particularly utility companies for the installation and operation of electronic communications through powerlines and the provision of electronic communication services through these systems. Various techniques and other possible methods have been identified to reduce the risk of interference or facilitate the mitigation of interference problems such as minimizing the power level, avoidance of locally used frequencies, differential signal injection method, using filters and terminations signals. 29

37 Chapter 3 - Description of the layers of the BPL network The exchange of information between remote users although it seems simple enough, it is quite complex. Telecommunication devices can differ, and the information can be exchanged through different networks using different transmission technology. To understand this complex structure, the entire telecommunications function is modeled in separate communication layers. The hierarchical model sets out specific requirements in each layer and the interfaces between them, allowing easier management and modeling of various protocols. Another important benefit of layering is the ability to change the implementation of a service without affecting other elements of a large and complicated system. Today, most communication systems are based on standard ISO/OSI (International Standardization Organization/Open Systems Interconnection). This consists of seven layers, each of which handles specific processes. Each upper layer takes processes closer and closer to telecommunication applications. Below we briefly describe the functions undertaken by each layer of the model [10]. Layer 1 or physical layer. Undertakes the transmission of individual bits within the frame in the telecommunication medium, as well as synchronization, coding, modulation, etc. Layer 2 or data link layer which is divided into two sub-layers: o LLC - Logical Link Control. Specifies the error detection and correction, as well as traffic control. o MAC - Media Access Control. Specifies the access protocols. Layer 3 or network layer. It is responsible for creating and terminating connections as for the process of routing. Layer 4 or transport layer. It specifies the transfer of information from end-toend and the fragmentation of messages, flow control, error management, data security, etc. 30

38 Layer 5 or session layer. It controls the communication between the participant terminals. Layer 6 or presentation layer. It transforms the data into standard format that is expected from the application layer. Layer 7 or application layer. It provides the interface to the end user. Layers 5-7 are closer to applications that the user understands and characterized as Application Network Layers. In contrast, layers 1-4 are responsible for the process of transmission or transport in a network and called Transport Layers [2]. As reported above, the transport layer assumes end-to-end connection, as opposed to layers 1-3 which are borne solely the piece of data transmission over different networks and subnets. Thus, the Transport layer acts as an interface between the Network/Transport Layers (Layers 1-3) and Application Network Layers (Layers 5-7). Figure 19 shows the ISO/OSI model and the transmission of data from a sender to a recipient. We note that the bits transmitted to the physical layer are much more than the number of initial bits from the sender. 31

39 Figure 19: The ISO/OSI reference model 3.1 Modulation techniques for BPL systems Choosing the technical configuration for a telecommunication system is based on the nature and characteristics of the medium on which will become the transmission. The power transmission line operates in a hostile environment for transmitting signals with high noise levels. The modulation techniques chosen should therefore overcome these difficulties. For example must be able to deal with the nonlinear characteristics of the channel. This non-linearity of the transmission channel can make the demodulation quite complicated and expensive, if not impossible, for transmission rates exceeding 10 Mbps for single carrier modulation. Therefore, the configuration should solve this problem without the need for extremely complex balancing (equalization). The impedance mismatch of transmission lines creates sound waves causing dispersion delay and thus causing additional problems. Therefore, the technique should be chosen to be characterized by high flexibility using and discarding specific frequencies that may disturb or used by other services when they can be 32

40 used for signals BPL. Investigations focused on two specific modeling techniques that meet the criteria for high performance in environments inhospitable as the BPL. These are the spread-spectrum modulation and OFDM (Orthogonal Frequency Division Multiplexing) which has been used for the ADSL technology. As a matter of fact, the frequency spectrum that ADSL uses is 1 MHz and the voice data uses only 3 KHz of bandwidth given the maximum frequency of speech signal is 3.4 KHz. Therefore, using a frequency division multiplexing (FDM) scheme we transmit the data using the remainder of the frequencies on the twisted pair cable. Considering the power cords, electricity companies are distributing electricity in the AC frequency of 50/60 Hz throughout the powerline network. Thus, while recognizing that FDM has been a very important factor in ADSL likewise this technique is used in wired BPL access. However, instead of separating the speech signal, the available bandwidth of the power line is shared of the AC frequency of the electrical system and the data transmission signal Signal modulation method based on Orthogonal Frequency Division Multiplexing (OFDM) In Multicarrier Modulation (MCM) the total bandwidth of the channel is divided into a number of subchannels. In each subchannel, a subcarrier is formed at a much lower data rate. Such a modulation scheme can be considered to consist of N independent modulated carriers with different frequencies each. The OFDM modulation is a special version of MCM modulation. Here the carriers are orthogonal to each other, which allows to be selected close enough to one another than in a simple FDM method. Compared with methods such as BPSK and QPSK, the transmitting OFDM symbols have a long duration but narrow bandwidth. Where the symbol duration is less than or equal to the maximum delay dispersion, the received signal contains overlapping versions of the transmitted symbols, a phenomenon called intersymbol interference (ISI). 33

41 Figure 20: Spectrum of (a) an OFDM subchannel and (b) OFDM signal of 5 carriers at the central frequency of each subchannel, but there is no cross talk from overlapping subchannels [39] OFDM systems are usually designed so that each subcarrier is narrow enough to bear flat fading. This also allows the subcarrier to remain orthogonal when the signal is transmitted on a selective fading channel [11]. If the OFDM modulated signal is propagated in such channel, every subcarrier will incur different attenuation. For a transmission system, given bandwidth, the transmission rate of the OFDM symbols is much smaller than that in a single carrier system. For example, for a single carrier system with BPSK modulation, symbol rate coincides with the transmission rate of the bits. The OFDM bandwidth for transmission is divided into Nc carriers, resulting in transmission rate Nc symbols at times less than that for a single carrier. This reduction in the transmission rate makes OFDM naturally more resistant to crosstalk phenomena symbols (intersymbol interference) caused by multipath propagation. The multipath propagation in systems with high bit rates creates crosstalk phenomena, when signals which are transmitted on different paths incur different delay from the main channel thereof overlap at the receiver. In OFDM modulation, such problem occurs when an OFDM symbol overlaps another. As there is no correlation between successive OFDM symbols, the interference between them will result in a disturbed signal. Indeed, the more limited is the used bandwidth, the more intense is the phenomenon of crosstalk and intersymbol interference and this should be addressed. One way to tackle this is to introduce a safety time (Guard Interval) between OFDM symbols. In addition to the symbols transmitted sequentially in different parallel subchannels, the alteration that occurs will not be a large contiguous portion of the signal and thus will allow retrieval on the receiver. The safety time may 34