The communication over the electrical power supply networks â Powerline Communications (PLC) - is specified in a European standard CENELEC EN 50065.
POWERLINE COMMUNICATIONS FOR ACCESS NETWORKS - Performance Study of the MAC Layer Halid Hrasnica and Ralf Lehnert Chair for Telecommunications, Dresden University of Technology 01062 Dresden, Germany Email: {hrasnica | lehnert}@ifn.et.tu-dresden.de Phone: +49 351 463-{3474 | 3945}; Fax: +49 351 463-7163
Abstract: The importance of the access networks increases after the deregulation of the telecommunication market. Today’s access networks are still the property of former monopolistic companies. New network operators try to realize their own access to the subscribers. There are two ways for the expansion of the access networks: building of new networks, which is expensive and needs a long time, and usage of the existing infrastructure (xDSL, CATV), which are also very often a property of former monopolists. Because of that, the usage of electrical power distribution networks for the access area in telecommunications, called Powerline Communications (PLC), becomes more and more attractive. In the paper we present possibilities for the application of PLC in the communications access networks and the position of current technology and regulatory developments. Furthermore, we consider the MAC layer of PLC networks and discuss some possible variants for suitable MAC protocols for PLC. We analyze the PLC network structures as well as the behavior of the telecommunications services which will be used in PLC systems. A very important part for the development of the MAC protocols is a consideration of the disturbance in the PLC transmission channel. We describe a general simulation model for the performance analysis of PLC networks which also includes the disturbance modeling. Key words: Powerline Communications, access networks, MAC protocol, modeling, performance evaluation
1
sufficient only for some metering functions (load management for an electrical network, remote meter reading, etc.), data transmission with very low bit rates and realization of few number of transmission channels for voice connections. Our point of interest is usage of the PLC networks with much higher transmission rates (from 1 Mbps and more).
INTRODUCTION
The use of the electrical power supply networks for communication proposes is known since the beginning of 20th century. The first carrier frequency systems have been operated in high-voltage electrical networks which were able to span distances over 500 km using 10W signal transmission power [1]. Such systems have been used for internal communication of electrical utilities and realization of remote measuring and control tasks. Also the communication over medium- and low-voltage electrical networks has been realized for the same usage.
In the last two years we find several pilot products for the realization of communication access networks using PLC (NorWeb UK/Canada, ONELINE Germany, ...) using the low-voltage electrical power supply networks, at higher transmission rates. There are also already available products using the medium-voltage networks as well as the solutions for the application of PLC within buildings/houses (In-Door). An advantage of the usage of PLC in any of this three areas is, that the electrical power grids can be used also for communication and accordingly, build up of new telecommunication networks could be reduced.
The communication over the electrical power supply networks – Powerline Communications (PLC) - is specified in a European standard CENELEC EN 50065. The standard provides a frequency spectrum from 9 to 140 kHz (up to 500 kHz in USA and Japan) for the powerline communications. This makes possible data rates up to several thousand bits per second which are
1
Our topic of interest is application of the PLC systems in the telecommunication access area. The access networks are very important for network providers because of the following reasons: •
About 50% of investment is needed for the access area.
•
After the deregulation of the telecommunication market in a large number of countries the access networks are still the property of former monopolistic companies. Because of that, new network providers try to find a solution to realize their own access network to the subscribers.
•
network which can not be realized in frequency spectrum specified by CENELEC. Therefore, the PLC systems need a frequency spectrum up to 20, 30 MHz and even higher. A PLC system (working at higher frequencies) acts as an antenna producing electro-magnetic radiation. This causes disturbances for other communication services, especially various radio systems, working in the concerned frequency spectrum. Because of that, some limits for the radiation of the PLC systems and accordingly signal power in the PLC has to be defined to ensure a mutual existence of the PLC and other communication systems. Regulation bodies and other interested organizations (ETSI, PLCforum, ...) are working on the specification of the limiting values but there is still no standardization for the PLC besides the CENELEC standard. However, it is expected that the limits will be hard which means, that the signal power used in the PLC systems has to be very low.
A rapid development of new telecommunication services increases the demand for more transmission capacity in backbone and WAN networks as well as in the access area.
There are two possibilities for the expansion of the access networks: building of new networks and usage of existing infrastructure. Building of new access networks can be realized with the following techniques: •
Wireless access networks (DECT, WLAN, ...)
•
Satellite systems
•
New cable/fiber-optic networks
Because of that, the PLC systems will be more sensitive to disturbances from the network environment and the transmission capacity of the PLC systems seems to be limited. The restricted capacity of the PLC networks calls for an adequate organization of MAC (Medium Access Control) layer including a multiple access of numerous stations/subscribers to the PLC transmission medium. The main point of our investigations, which we outline in this work, is development of MAC protocols for the PLC access networks.
First two techniques seem to be expensive and they are also not able to offer higher transmission rates. Building of new cable or optical networks cause higher costs too, and it takes a longer time. Because of that the usage of existing infrastructure seems to be a favorable solution. There are following three possibilities: •
Development of new transmission methods (xDSL)
•
Usage of TV cable networks (CATV)
•
Powerline Communications (PLC)
The paper is organized as follows: First, we consider PLC network and system structures as well as a service specification and their impacts on the investigation of the MAC layer (section 2). In section 3, we define a logical PLC network structure to be used in the investigation of the MAC layer. Further, disturbances and possible error handling methods for the PLC network are analyzed (section 4). In section 5, we consider the MAC layer in PLC networks. At the end, a simulation model for performance analysis of the PLC MAC layer is described (section 6) and in conclusions we outline future investigations on this topic.
A disadvantage of first two solutions is that the existing infrastructure belongs very often to the former monopolistic companies. Because of that the PLC access networks seem to be a reasonable solution (Figure 1).
2 2.1
PLC SYSTEM SPECIFICATION Network Topology
The PLC access networks are connected to the backbone communication networks via a transformer station, as shown in Figure 1, or via any other station in the network. Many utilities supplying electrical power have also their own telecommunication networks which can be used as PLC backbone.
Figure 1: PLC access network
A common feature of the low-voltage supply networks is that they are realized with big differences using various technologies (different types of cable, ...). In our investigations we consider the PLC network topology and
There is a number of subscribers in a low-voltage electrical power supply network which have to share the transmission capacity of a PLC access network. Because of that there is a need for a higher capacity in the PLC
2
its impact to the MAC layer. Topology of a low-voltage power supply network depends on several factors: •
Location of the PLC network: urban or rural residential area, industrial area, business area
•
User/subscriber density: number of users in a PLC network (small – middle - large), user concentration (single houses - small blocks – towers)
•
Network length: short – middle – long
•
Network design - number of network sub sections
applications [3]. The MAC protocol to be implemented in a PLC system has to provide features for realization of different teleservices. We define the following four group of teleservices which have to be considered in the investigations of the MAC layer for PLC networks: •
Connection oriented services, like telephony and other CBR (Constant Bit Rate) services
•
Connection less services without QoS guarantees
•
Specific PLC services
•
Data transmission with QoS guarantees (like VBR – variable bit rate – services)
PLC networks must support the classical telephone service, because of its importance and its big penetration in the communications world. Further requirement will be a CBR service with higher transmission capacity like video. Another important service is data transmission, which allows internet usage. The powerline MAC layer and MAC protocol have to be able to deal with both of the previous mentioned services to ensure an initial position of the PLC systems against other technologies. Also, a possibility for transmission of more sophisticated services, with higher QoS requirements (priorities, delays, ...) should be included into the PLC MAC layer.
Figure 2: PLC network structure Figure 2 shows a possible structure of a PLC Network. As we see, there are generally several network sections from the transformer station to the users. Each section (and also each low-voltage supply network) can have a different structure. There could be various number of connected users in different networks and of course in different network section. The users can be more concentrated or not and they can be distributed in a symmetric or in an asymmetric way. There is also a difference between network lengths and between lengths of the network sections. However, we are able to define some characteristic values describing an average structure of a typical PLC network according to our investigations and information from [2]:
In the consideration of the MAC layer in the PLC networks special emphasis has to be given to the specific PLC services (home automation, energy management, security, ...). Most of this services can be covered by other previous considered kind of services, especially with connection less data transmission. In spite of that, there can be some special requirements of the PLC services which need some QoS guarantees and transmission priorities. This features have to be also included in the functionality of the MAC layer. However, the PLC specific services use low data rates and their realization seems to be not critical.
•
Number of users in the network: 250 ~ 400
•
Number of network sections: ~ 5
2.3
•
Number of users in a network section: 50 ~ 80
•
Network length: ~ 500 m
There are several multiplex and modulation schemes which are investigated for their application in the PLC transmission systems. Considerations of the suitable transmission schemes for PLC and analysis of the PLC transmission channel can be found in e.g. [4] and [5]. The following two modulation and multiplex methods are discussed for PLC networks:
2.2
PLC Services
The PLC systems have to offer a big palette of telecommunications services with a satisfied quality to be able to compete against other communications technologies applied to the access networks (see introduction). Note that the low-voltage power supply networks are not design for data transfer and that other access technologies work in better transmission conditions. We consider the PLC transmission systems as a bearer service. A bearer service carries teleservices which make possible usage of various communication
Transmission System
•
CDM – Code Division Multiplexing
•
OFDM – Multiplexing
Orthogonal
Frequency
Division
CDM is characterized as insensitive to narrowband disturbances and selective attenuation and also it works with a low signal power [1], which is important because of the radiation problem. However, OFDM is outlined as the best candidate for the application in the PLC
3
OFDM systems can react with a reduction of transmission capacity of the sub-carriers according to the disturbance situation (case B). Because of that, it is possible that the capacity of a transmission channel is also reduced. In this case, the MAC layer has to manage transmission channels with a variable transmission capacity.
transmission systems with higher data rates [4], because its excellent bandwidth efficiency. OFDM provides a data transmission over a number of sub-carriers which makes possible a deviation from critical frequencies used by other communication systems. Therefore, we also consider OFDM for the specification of PLC transmission system in sense of the development and investigation of the MAC layer. As we will show in the next section, there are also some general principles of its consideration which can be applied to CDM based systems and other transmission schemes.
In case C, all available sub-carriers are summarized into a number of channels with a transmission capacity. That means, a number of sub-carriers is grouped according to their available capacity to form a transmission channel with a wished capacity. In this case, the transmission channels do not include always the same sub-carriers.
OFDM transmission systems use a number of sub-carriers distributed in a frequency spectrum (Figure 3). Each subcarrier has a transmission capacity and it is possible to make a groups of the sub-carriers to build up transmission channels with a higher capacity [6].
3 3.1
PLC LOGICAL STRUCTURE Bus Network Structure
Generally, a PLC access network is connected to its backbone network over a base station (Figure 1). That means all communication between the users of a PLC network and the world is carried out over the base station. We assume also that the internal communication between users of a PLC network is done via the base station. There are two transmission directions in a PLC network: •
Downlink/downstream from the base station to the users
•
Uplink/upstream from users to the base station
If we assume that the base station is placed in the transformer station (it is also valid if the base station is placed in any other station in the network) we see the following transmission modes: Figure 3: OFDM channel structure
•
There are three possibilities for the capacity management in the OFDM system to be considered in the development of the MAC layer:
Information sent by the base station in downlink direction is transmitted to all network subsection and is received by all users in the network
•
In uplink direction, information sent by an user is transmitted not only to the base station, but also to all users in the network
•
A) A group of sub-carriers (e.g. SC, Figure 3) with a fixed transmission capacity form the transmission channels (CH)
•
B) A group of sub-carriers with variable transmission capacity form the transmission channels
•
C) The sub-carriers are grouped to build up the transmission channels according to the available transmission capacity of the sub-carriers and to the wished channel capacity
Figure 4: PLC network model That means the PLC transmission medium/cable holds a bus structure in spite of the fact that the low-voltage supply networks have a tree topology. It is also valid if we consider only one network section or any other part of the PLC access network. Because of that it is possible to consider the PLC network structure as a bus system, at least in the investigations of the powerline MAC layer (Figure 4).
In the case A, the MAC layer deals always with the transmission channels having a same transmission capacity (e.g. 64 kbps). The transmission channels include always the same sub-carriers. That means, if one of the sub-carriers is not available (a sub-carrier is disturbed) the transmission channel can not be used nevertheless that other sub-carriers are available.
4
3.2
disturbance is short enough, which means shorter than the duration of a symbol transmitted over physical layer of a PLC system, there is no influence of the disturbance on the transmission. E.g., if the duration of an OFDM symbol is 300 µs and a pulse disturbance shorter than 300 µs does not damage transmitted data.
Channel Structure
As we showed in subsection 2.3, the transmission channels can be used with a constant or variable capacity. If we consider CDM as another candidate for PLC systems we can find a similar channel structure as in OFDM based systems. In this case, the whole available transmission spectrum is divided by orthogonal codes which can be allocated to the particular users or services (CDMA – Code Division Multiple Access). With a possibility for usage of a code an user has a opportunity to send or receive data with a certain transmission speed. That means, a transmission capacity is allocated to the user also like a transmission channel in the OFDM based and we see a channel structure in CDM based systems, too.
In many transmission systems forward error correction (FEC) mechanisms are applied to cope with the disturbances. In this case, the transmission systems are able to manage a situation when a number of bits is damaged and in spite of that to correct a data contents and to make possible correct data transmission. FEC mechanisms are expected to be implemented also in PLC systems. There are several kinds of the FEC mechanism suitable for various bit error sorts: single, periodic etc. In any case the usage of the FEC mechanism causes an overhead which takes a portion of the network transmission capacity. E.g. about 50 % overhead is used for the FEC in GSM system which improves BER (Bit Error Rate) values from 10-3 (pure wireless transmission channel) to 10-6 [6].
In the systems with CDMA users are able to use a whole frequency and time spectrum for the transmission. We can also imagine a system which use all OFDM sub-carriers for each transmission. To avoid collisions between different users, some kind of access organization has to be done. In this case it could be done occasionally which means that each user receives a part of time to transmit its data. This kind of access organization is TDMA (Time Division Multiple Access). In TDMA system there is number of time slots which repeats during the time (in frames). The time slots offer also a transmission capacity and they can be compared with OFDM or CDMA transmission channels.
In spite of the applied FEC mechanisms and the ability of the system to avoid some kind of disturbances (SNR, symbol duration) it is possible that the transmitted data is damaged. Data transmitted by MAC layer is delivered to the next network sublayer - Logical Link Control (LLC). There it can be recognized if a data segment is error free or not. In case of errors the damaged data has to be retransmitted by an ARQ (Automatic Repeat Request) mechanism. The use of ARQ mechanisms can reduce error probability to a very low value and it is only limited by remaining error probability of CRC (Cyclic Redundancy Check) code used for error recognition. With usage of 240, 216 BCH-CRC (Bose-ChaudhuriHocquenhem-Codes CRC) code (24 bit overhead is added to 216 payload bits) a remaining error probability of 10-8 can be reached, which is considered as sufficient for the data transmission [6].
We can conclude that the PLC transmission systems seems to have a channel structure independent of the used transmission technology. Accordingly, in the development of MAC layer it is possible to deal with logical channels which are managed by a MAC protocol. The logical channels have other meaning for each considered transmission method, but the investigations, done on the logical level, can be applied to any of the methods, of course always considering their particularities.
4
Because of the additional end-to-end transmission delay due by the transmission of the acknowledgements and data retransmissions, ARQ is not suitable for a real-time service like voice. Also, the data retransmissions and transmission of the acknowledgements consume extra network capacity which decrease network utilization. We expect an usage of both FEC and ARQ mechanisms in the PLC transmission systems; selective for certain services (e.g. only FEC for voice connections) and combined (e.g. to increase network utilization in case of data transmission, BER is reduced with usage of FEC and accordingly the probability of data retransmissions initiated by ARQ).
ERROR HANDLING
PLC transmission systems will work with a signal power which has to be below a limit defined by regulation organizations. On the other hand, the signal level has to keep data transmission over PLC network possible. That means there should be a SNR (Signal Noise Ratio) level in the network making the communication possible. So far that SNR is sufficient to avoid the disturbances in the network there is no need for application of other special methods against the disturbances. SNR value has to be able to avoid an influence of the background noise in a PLC network.
ARQ mechanism deals with relative short duration of the disturbances (some ms) which occur on one or several data units. Long term disturbances make one or more transmission channels unavailable for a longer time. In this case, ARQ mechanism would constantly repeat the data making the transmission not efficient. Because of that, the disturbed channels should not be used for any
More difficulties are caused in the PLC transmission systems by impulsive noise which has much higher power than the background noise [7]. In this case SNR is not enough to avoid the disturbance and following data damage (transmission error). But if the duration of a
5
transmission until the disturbance disappears. If a channel affected by a disturbance is used for a transmission, channel re-allocation has to be done to make possible a further transmission of affected connections using other error free transmission channels. A channel reallocation mechanism has to be also included in the features of the PLC MAC layer.
5
The symmetric duplex mode we can find very often in communications networks. However, it is not suitable for access networks with typical internet based data traffic which we expect to have in the PLC access networks. In such cases the traffic load in the downlink transmission direction (Figure 5) seems to be significantly higher than in the uplink direction because of the fact that the user/subscriber transmit mostly smaller files and download larger files form the internet servers, which are usually not placed in the access network.
MAC LAYER IN PLC SYSTEMS
5.1
Analysis of MAC Protocols
5.1.1
Task of the MAC Layer
MAC protocol have to manage the data transfer over the transmission channels. That means the channel allocation/re-allocation between a number of PLC subscribers and transmission of different kind of services. In subsection 3.2 we saw a similar logical channel structure in various multiple access schemes (FDMA, TDMA, CDMA, TDMA/CDMA, ...).
Figure 5: Transmission directions in PLC access network
A multiple access scheme of a MAC protocol establishes a method of dividing the transmission resources into accessible sections [8]. Transmission channels, defined in our logical structure of the PLC systems (section 3), represent the sections which are accessible for the MAC protocol.
Because of that, the solutions with different transmission capacity in downlink and uplink are more adequate for the access network like PLC. As mentioned above, the different transmission capacities can be set fixed (e.g. 10% uplink, 90% downlink) or the division of the downlink and uplink transmission capacity is managed dynamically depending on the traffic situation in the network. Dynamic duplex mode offers a better network utilization but it is paid with a higher complexity of the system realization.
In the OFDM system presented in subsection 2.3 the transmission channels are distributed in a frequency spectrum, which concerns a kind of FDMA (Frequency Division Multiple Access). Because of the OFDM structure and a number of sub-carriers in each of the transmission channels this multiple access method is called OFDMA (OFDM Access) [9].
5.1.2
Classification of MAC Protocols
A MAC protocol specifies a resource sharing strategy – access of multiple users to the network transmission capacity – applied to a multiple access scheme. Generally, there are three possibilities for the access organization:
A MAC protocol includes also a specification of duplex mode which is applied to a transmission system. Each of the multiple access methods can be combined with a duplex scheme. In telecommunication system there are two duplex schemes (e.g. [6] and [8]):
•
Fixed access
•
FDD – Frequency Division Duplex
•
Dynamic access
•
TDD – Time Division Duplex
•
Reservation based access
A TDMA/FDD solution is applied in GSM mobile networks and TDMA/TDD is used in DECT wireless systems. Any of the duplex solution can be also applied to the PLC systems according to used multiple access scheme and features of the PLC transmission system. There are the following solutions for the realization of the duplex scheme: •
Transmission systems with the fixed access scheme assign to each user a predetermined and fixed capacity (channel or number of channels). That means, a channel is allocated to a user (or several users) independent on its need to transmit data at the moment (e.g. conventional telephony). Fixed access strategies are suitable for continuous traffic, but not for bursty traffic which is typical for data transfer.
The same capacity for both downlink and uplink transmission directions (symmetric)
•
Different, fixed capacity per transmission direction (asymmetric)
•
Dynamic duplex mode
Dynamic access schemes are adequate for data transmission and in some cases it is possible to ensure a satisfactory transmission quality for delay-critical traffic. We give an overview of them in the next part of this subsection (5.1.3).
6
A reservation of the transmission capacity for a particular user is done according to its transmission request/demand which originates from a transmission whish of the user. A transmission request is submitted by user to a central network unit (e.g. base station in PLC network) using either a fixed or a dynamic access schemes. Transmission systems with the reservation access scheme are suitable to carry hybrid traffic (mix of traffic types caused by various services) with variable transmission rate. 5.1.3
token giving a transmission right to another station. Polling method provides centralized organization with a main/central station in the network. The main station sends a polling massage to the network stations which should receive the transmission right. Advantages of a centralized access organization are a relative simple realization of QoS guarantees in the network. Also, technical structure of the network station seems to be simpler than in a decentralized system. On the other hand, a failure of main station causes a break down of the whole network. The centralized systems may produce more transmission overhead (transmission of signaling information), too.
Dynamic Access Schemes
In general, there are two groups of the dynamic access schemes/protocols: •
Contention protocols, with collisions
•
Arbitration protocols, collision-free
The contention access protocols avoid collisions between the transmission of different network users. That means, it could happen that the transmission is unsuccessful, because an other user has transmitted a data at the same time which caused a collision. In case of a collision, a retransmission of collided data is done that causes an additional transmission delay. Therefore, there is no guarantee of QoS for time-critical services and for any kind of reservation of the transmission capacity as well as for realization of the transmission priorities. Further disadvantage is that it is not possible to reach a full network utilization.
ALOHA – pure, slotted, ...
•
CSMA (Carrier 1-to-n-persistent, (CSMA/CD), ...
•
Sense with
Polling methods
Treatment of different Services
We propose a separated treatment of different telecommunication services transmitted over PLC networks. That makes possible fulfillment of particular QoS guarantees for each of the services. To simplify our further discussion we will consider two types of services; telephony, using 64 kbps transmission channel, and connectionless data transmission, as two borderline types of services.
The collision-free protocols ensure that each network station disposes of an extra part of the transmission capacity (e.g. time periods for transmission). Division of the transmission capacity between network station can be realized according to the following two methods: •
Access Protocol
5.2.2
Collision resolution protocols [10]
Token Passing (Token-Ring, Token-Bus)
5.2.1
For the application of the reservation protocols some extra transmission capacity is needed for the transmission of the signaling information. That means, a number of logical transmission channels (defined in subsection 3.2) is used for signaling. Other channels (rest of the transmission capacity) are available for the transmission of user data.
Multiple Access) – collision detection
•
Organization of the MAC Layer
Our intention is to make possible transmission of different kinds of services as mentioned in subsection 2.2. In this case, the best solution for the access organization is offered by reservation protocols (see subsection 5.1 and [8]). Because of that, we propose such solution for the PLC networks.
We find following types of the contention protocols: •
5.2
With the usage of the reservation access protocols we are able to ensure a collision-free transmission in the PLC system. The Collision-free transmission protocols ensure a good network utilization (subsection 5.1) which is very important factor in the PLC networks because of the expected low transmission capacity (see introduction). Unsuccessful transmission is only possible if a disturbance occur in the network (section 4).
Both of the methods implement a transmission right (by token or polling message) which can be used for a particular data quantity or a time period. This makes possible to realize QoS guarantees in the network, that is not possible with the contention protocols. However, with increasing number of network stations the time between two sending rights for a stations (round-trip time of tokens or polling messages) becomes longer. Both Token Passing and Polling protocols are able to ensure a nearly full network utilization, but only if large number of network stations are active (have data to send).
We assume that the transmission capacity of a logical channel (subsection 3.2) is 64 kbps. That gives possibility to allocate a channel to a voice connection. Free channels, which are not allocated to the voice connections can be used for the connection-less data transmission. We propose priority for the voice connections and all available channels are firstly allocated to the telephony. The data transmission uses remaining number of channels. That means, arrival of a new voice connection
System with Token Passing have a decentralized organization in which the network stations exchange a
7
signaling. Because of that, an applied signaling protocol has to ensure:
causes reduction of the transmission capacity for the data transmission by 64 kbps (one channel). A detailed description of this channel allocation strategy including error handling is given in [11]. 5.2.3
Data Transmission
We propose a full usage of the remaining capacity for the data transmission. That means, an user with the right for the transmission sends data over all available channels for the data transmission. A fair division of the transmission capacity according to the service QoS requirements between network users should be provide by signaling protocol [11].
Efficient transmission of connection requests from the users to the base station in the uplink transmission direction (Figure 5)
•
Optimal utilization of signaling transmission capacity in the downlink direction
There are many protocols which can be applied to the signaling procedure in the PLC networks. Because of the similarity in signaling organization between PLC and wireless networks, caused by usage of similar transmission schemes (modulation, multiplexing) and a similar disturbance scenario, we can use the experience form the implementations and investigations which have be done on this topic (e.g. [6] and [8]).
The data transmission could be also organized in other way: The transmissions from various user are done parallel (in the same time) over different channels. Of course, signaling protocol has to ensure collision-free transmission and acceptable division of the network capacity like in the proposed protocol described above.
Our intention is to use this experience to find a suitable protocol for the PLC systems. In [11] we investigated the usage of an ALOHA random access protocol in the PLC systems. An ALOHA protocol is also applied to the GSM mobile network [6]. In our future work we will extend the investigation to other protocols (see subsection 5.1) which are candidates for PLC, too.
Final decision about the organization of data transmission can be made after the characteristics of the disturbances in the PLC network is known. Knowledge about duration and interarrival times of the disturbances which occur on the transmission channels and about correlation between the disturbance occurrence on the parallel channels is very important for this decision. This consideration has to be done according to the applied multiple access schema (OFDMA, TDMA, ...) in a PLC system.
6 6.1
Because of the expected disturbance influence on the PLC networks we propose a segmentation of the user data into the small data units – PLC segments – with a fixed length, as described in [11]. Accordingly, if a disturbance occurs, only a PLC segment or a number of segments is damaged. The affected PLC segments will be retransmitted by an ARQ mechanism. In this case, smaller data units (PLC segments) ensure also that a fewer network capacity is used for the retransmissions. The length of a PLC segment has to be chosen according to the disturbance characteristic, too.
MODELING OF PLC MAC LAYER Performance Analysis
To be able to decide about a suitable MAC protocol for PLC systems, there is a need for comparison of proposed solutions. There are several parameters which can be considered in performance analysis of the MAC protocols:
With the usage of small PLC segments there is a possibility for a finer scaling of the network transmission capacity. That makes possible a simpler realization of the QoS guarantees. However, we must not forget an extra overhead which has to be added to the payload of the PLC segments ([11]). Also with the usage of the re/assembling procedures increases complexity of the user equipment (device) and reaction time of the device (additional delay).
5.3
•
•
Blocking probability of voice connections
•
Transmission delay of data units
•
Loss probability of data units
•
Network utilization, and others
To consider the performance parameters of the investigated protocols we have three possibilities: Analytical, simulation and empirical analysis [12]. The empirical analysis can be done on a working transmission system with the help of measurements. Of course, this method is not suitable for a system in development like PLC. We have already done some investigations using analytical modeling of the PLC networks [11] [13]. In both sdudies we modeled a PLC network on call and burst level. Complexity of the investigated MAC protocol does not allow a conformable implementation in an analytical model. Because of that, we developed a simulation model for the investigations of the MAC layer in the PLC networks.
Signaling Protocols
There is a number of channels in the PLC network (section 5.2) which is used for the organization of the signaling (signaling channels). Because of limited transmission capacity in the PLC networks we propose that only one transmission channel is used for the
8
6.2
network environment. This results with transmission capacity of the PLC networks.
Simulation Model
We implemented the logical structure of the PLC system (described in section 3) and the MAC layer (subsection 5.2) in the simulation model (Figure 6). The YATS simulator [14], developed at our chair, is used for the implementation.
low
In a low-voltage power supply network we find usually about 250 to 400 users which have to share a limited network capacity. Therefore, development of an efficient MAC protocol for the PLC networks, providing good network utilization and usage of various services, is very important part of the investigations. We base our investigations on the PLC systems using OFDM transmission scheme. We defined a logical channel structure of the PLC system to be considered for the specification of the powerline MAC layer. The logical channel structure can be applied also to other transmission techniques (CDM, TDM, ...) which could be used in PLC networks and other communication systems, too. We concluded that the physical three topology of the lowvoltage supply networks can be considered as a logical bus structure in the investigations of the MAC layer.
Base Station
We expect an unpleasant disturbance scenario in the PLC networks and accordingly, an intensive occurrence of transmission errors. The influence of the disturbances can be reduced or avoided with the application of error handling mechanism (FEC, ARQ, channel reallocation) which have to be considered and partly included in the PLC MAC layer.
Figure 6: Simulation Model We have already conclude that there is a big influence of disturbances on the behavior of the PLC transmission systems. Also the results in [13] showed a dependence of the available transmission capacity in a PLC network according to its noise/disturbance environment. Because of that the disturbances are included in our model.
Because of the applied OFDM transmission method we chose the OFDMA multiple access scheme for the PLC system. However, because of the PLC logical structure defined in our investigations we are able to analyze any other access method (CDMA, TDMA, ...), too. According to the expected asymmetric traffic matrix in the PLC access networks we propose a dynamic duplex scheme to be applied to the PLC transmission systems.
There are several kinds of the disturbances caused by various sources [7]. In the simulation model (Figure 6) there is the possibility to model any kind of the disturbances and also to simulate them in parallel. There is possibility to model the disturbances which occur independently on the transmission channels and to model the disturbances which affect more transmission channels, too.
7
a
We propose a collision-free data transmission in the PLC networks to increase utilization of the limited transmission capacity. To the CBR telecommunication services (e.g. telephony) we give a higher priority which ensures capacity reservation for this services. Data packets to be transmitted over PLC are segmented into the small data units with a fixed length. That makes possible an effective realization of QoS guarantees for the particular services and a better opportunity for the error handling in the PLC networks.
CONCLUSIONS
Application of powerline communications (PLC) gives an a possibility for use of the electrical power supply grids for telecommunications. PLC can be applied to high-, middle- and low-voltage power supply networks as well as to the electrical installations within buildings. Importance of the access telecommunication networks increases after the deregulation of the telecommunication market in numerous European countries. Because of that, we investigate the application of the PLC in the telecommunication access area, using the low-voltage power supply networks.
The reservation based resource sharing methods gives the best possibilities for the realization of a multi-service transmission system. Because of that, we propose such scheme for the PLC systems. For the investigations of the suitable protocols to be implemented in the PLC signaling scheme, we developed a simulation model which makes possible a comprehensive performance analysis. The simulation model includes implementation of various disturbance scenarios.
However, radiation caused by PLC systems working in a higher frequency spectrum seems to be to high for other telecommunication systems, particularly for various radio services. Because of that, the PLC transmission systems will work with a limited signal power which makes PLC systems more sensitive to disturbances presented in the
9
8
issue: “On Powerline Communications”, 54 (2000) No.1; Germany 2000
FUTURE WORK
The reservation based methods for the network capacity sharing includes a MAC protocol to be implemented for the signaling/reservation. There are many implementations and investigations of reservation schemes done, for the wireless communication systems. We will use this experience and our simulation model to outline one or more suitable protocols which can be applied to the PLC systems. Our first simulation results considering this part of investigations will be published in [15].
[8]
Akyildiz, I. F.; McNair, J.; Martorell, L. C.; Puigjaner, R.; Yesha, Y.: Medium Access Control Protocols for Multimedia Traffic in Wireless Networks; IEEE Network - July/August 1999
[9]
Fazel, K.; Prasad, R.: Multi-Carrier SpreadSpectrum; ETT Vol. 10, No. 4, July-August 1999
[10] Rom, R., Sidi, M.: Multiple Access Protocols – Performance and Analysis; Springer-Verlag New York, USA, 1990 (ISBN 0-387-97253-6)
Because of big importance of the disturbance behavior in the PLC networks, we continue their analysis to specify a adequate disturbance model which can be used in our simulations and further investigations. A relevant disturbance model can be specified after an extensive measurement campaign in various network environments.
[11] Stantcheva, M., Begain, K., Hrasnica, H., Lehnert, R.: Suitable MAC Protocols for an OFDM Based PLC Network – Proceedings of ISPLC2000 - 4th International Symposium on Power-Line Communications and its Applications, 5 - 7 April 2000; Limerick, Ireland [12] Fortier, P., J,: Handbook of LAN Technology; Intertex Publications, McGraw-Hill, Inc. New York, NY, USA, 1992
ACKNOWLEDGEMENTS Part of this work has been done within the EU project PALAS (Powerline as an Alternative Local AcceSs). The authors thank to ONELINE, Barleben, Germany, for supporting our investigations.
[13] Begain, K.; Ermel, M.; Haidine, A.; Hrasnica, H., Stantcheva, M., Lehnert, R.: Modeling of a PLC Access Network; First Polish-German Teletraffic Symposium, Sept. 24-26, 2000, Dresden, Germany
REFERENCES
[14] Baumann, M.: YATS - Yet Another Tiny Simulator, User's and Programmer's Manual, Version 0.3; Dresden University of Technology, Chair for Telecommunications; http://www.ifn.et.tu-dresden.de/TK/
[1]
Dostert, K.: Telecommunications over the Power Distribution Grid – Possibilities and Limitations; IIR-Powerline 6/97, Germany 1997
[2]
Hooijen, O., G. : On the Channel Capacity of the Residential Power Circuit used as a Digital Communications Medium; IEEE Communications Letters, Vol. 2, No. 10, October 1998
[3]
ITU-T Recommendation I.210 (03/93): ISDN Service Capabilities
[4]
Proceedings of ISPLC2000 - 4th International Symposium on Power-Line Communications and its Applications, 5 - 7 April 2000; Limerick, Ireland
[5]
International Journal of Electronics and Communications, Special issue: “On Powerline Communications”, 54 (2000) No.1; Germany 2000
[6]
Walke, B.: Mobilfunknetze und ihre Protokolle – Band 1 (in German); B. G. Teubner Stuttgart; Germany, 1998 (ISBN 3-519-06430-8)
[7]
Zimmermann, M.; Dostert, K.: The Low Voltage Power Distribution Networks as Last Mile Access Network – Signal Propagation and Noise Scenario in the HF-Range; International Journal of Electronics and Communications (AEÜ), Special
[15] Hrasnica, H.; Haidine, A.; Lehnert, R.; Stantcheva, M.: Modeling MAC Layer for Powerline Communications Networks; SPIE's symposium on Voice, Video and Data Communications; conference "Performance and Control of Network Systems IV"; 5-8 November 2000; Boston MA, USA
10
