users/subscribers to a communication network. There is a number of ... We consider the PLC transmission system as a bearer service carrying teleservices ...
Reservation MAC Protocols for Powerline Communications Halid Hrasnica, Abdelfatteh Haidine, Ralf Lehnert Chair for Telecommunications, Dresden University of Technology 01062 Dresden, Germany Email: {hrasnica | haidine | lehnert}@ifn.et.tu-dresden.de Phone: +49 351 463-{3474 | 3474 | 3945}; Fax: +49 351 463-7163
Abstract: In this paper suitable MAC protocols for the implementation in PLC access networks are studied. We propose a logical model for the investigation of the MAC layer according to the applied transmission methods in the PLC systems and to the used telecommunication services. The impulsive noise has a big impact on the error behavior of the transmission, but there are various possibilities to do deal with this problem (FEC, ARQ, ...). In spite of that, the investigation of the MAC layer has to include several disturbance scenarios which are specified in this work. We propose the reservation MAC protocols to be applied to the PLC access networks, because they are suitable to carry hybrid traffic with variable data rates ensuring a high network utilization. Two basic reservation MAC protocols, ALOHA random access and polling based dedicated access, are analyzed. We also discuss several possibilities for the protocol improvement. Key words: powerline communications, MAC, reservation protocol, disturbance scenario, access network
1
Introduction
We investigate the application of the PLC systems in the telecommunication access area because of its importance: high costs (about 50 % of investments is needed for the access networks), deregulation of the telecommunication market which causes a need for realization of the access networks by new network providers [1], [2]. There are two possibilities for the expansion of the access networks: building of new networks (realization of wireless local loop systems, usage of the satellite systems and buildup of new fixed access networks) and usage of existing infrastructure (xDSL techniques, usage of CATV, PLC access networks). Building new access systems causes higher costs and takes a longer time. Because of that the usage of existing infrastructure is a favorable solution. However, the existing communication infrastructure belongs very often to the incumbent network operators and because of that PLC access networks seem to be an promising solution. In the case of a PLC access system a low-voltage power supply network is used for the connection of endusers/subscribers to a communication network. There is a number of subscribers in a low-voltage electrical power supply network who have to share the transmission capacity of a PLC access network. Therefore a high gross data rate on the medium is necessary to ensure a sufficient QoS, making PLC systems competitive to the other access technologies [2]. PLC systems applied to the telecommunication access networks use a frequency spectrum of up to 30 MHz and act as antenna producing electro-magnetic radiation, which causes disturbances to other telecommunication services working in this frequency range. Because of that, PLC networks have to work with a limited signal power which makes PLC systems more sensitive to disturbances from the electrical power supply network and from the PLC network environment. Well-known error handling mechanisms can be applied to the PLC systems to solve the problem of transmission errors caused by the disturbances (e.g. FEC – Forward Error Correction and ARQ – Automatic Repeat reQuest mechanisms). However, the use of this mechanisms consumes a part of the transmission capacity (overhead, retransmission) and therefore decreases the already limited data rate of the PLC systems. Because of the shared transmission medium, PLC networks have to provide a very good network utilization keeping also a sufficient QoS, which can be reached by usage of efficient methods for the network capacity sharing – Media Access Control (MAC) protocols. Various MAC protocols for PLC are investigated in this paper, especially the reservation protocols. The paper is organized as follows: In section 2 we describe briefly a PLC access system considered in this investigation and a logical model for the investigation of the PLC MAC layer. After that (section 3) we present some impacts of noise concerning the PLC MAC layer and possible disturbance scenarios which have to be investigated. In section 4 we
specify the PLC MAC layer and analyze some solutions for MAC protocols and suggest the reservation protocols to be applied to the PLC networks. Afterwards (section 5) we discuss various solutions of reservation protocols for PLC.
2
PLC System Model
2.1 Network Structure PLC access networks are connected to the backbone communication networks via a transformer station, or via any other station in the network. The low-voltage supply systems build various network topologies having a tree structure [1], [2], [3]. There are generally several network sections from a transformer unit to the users where the network sections have also different topologies. Independent of the PLC network topology the communication between the users of a PLC network and a wide area network (WAN) is carried out over a base station, normally placed in the transformer unit (Figure 1).
Figure 1: Logical Bus Structure of the PLC Network We assume that the internal communication between users of a PLC network is done over a base station, too. So, we can recognize two transmission directions within a PLC network: •
Downlink/downstream – for transmission from the base station to the network users
•
Uplink/upstream – providing transmission from each user to the base station
A transmission signal sent by the base station in the downlink direction is transmitted to all network subsections. Therefore, it is received by all users in the network. In the uplink direction, a signal sent by an user is transmitted not only to the base station, but also to all other users in the network. That means, the PLC access network holds a logical bus structure in spite of the fact that the low-voltage supply networks have physically a tree topology [2]. It is also valid, if we consider PLC network section or any other part of the PLC network. Because of that we consider the PLC network structure as a logical bus system (Figure 1).
2.2 PLC Services PLC systems have to offer a big palette of telecommunications services with a given quality of service (QoS) to be able to compete against other access technologies (section 1). We consider the PLC transmission system as a bearer service carrying teleservices, which make possible usage of various communication applications [4]. The MAC protocol to be implemented in a PLC system has to provide features for realization of different teleservices which can be grouped as follows: •
Connection oriented services, like telephony and other CBR (Constant Bit Rate) services
•
Connectionless services without QoS guarantees (e.g. Internet)
•
Specific PLC services
•
Data transmission with QoS guarantees (like VBR – Variable Bit Rate – services)
PLC networks must support the classical telephone service and data transmission (e.g. internet). The powerline MAC layer has 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 (e.g. VBR, CBR with higher data rates) as well as the features for specific PLC services should be included into the PLC MAC layer [1], [2].
2.3 Transmission System There are several multiplex and modulation schemes which are investigated for their application in the PLC transmission systems (e.g. CDM – Code Division Multiplexing and OFDM – Orthogonal Frequency Division Multiplexing). Related considerations can be found in e.g. [5] and [6].
An OFDM based transmission system can offer a number of transmission channels divided in the frequency spectrum, as described in [2]. In the case of a CDM based system, the whole available transmission spectrum is divided by orthogonal codes. We can also imagine a system which uses all OFDM sub-carriers for each transmission applying a TDMA (Time Division Multiple Access) scheme [2]. In all variants of the transmission technique considered above, we recognize a structure consisting of a number of transmission channels divided in the frequency, time or code spectrum. So, we can conclude that the PLC transmission systems seems to have a logical 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 transmission channels have other meaning for each considered transmission method, but the investigations, done on the logical level, can be applied to any of the transmission schemes.
3
Impact of Noise
3.1 Noise Description The powerline as communications channel has a specific disturbance characteristics. These include the dominant and widely varying noise sources, impedance changes, and multipath effects. Noise sources are electronic, electromechanic, and even induced by the powerlines themselves. Some noise is harmonically related to the 50 or 60Hz power. Light dimmers and related products that use triacs create impulse noise on every cycle or half cycle of the power. Some power supplies, especially poorly designed switching supplies, conduct quite a bit of noise onto the powerline. This noise may have high harmonic content related to the switching frequency of the supply. Some companies, e.g. Intellon [17], has even noted cases where corroded junctions in the building wiring have a semiconductor effect whose nonlinearity induces noise on every power half cycle. Even if all devices were unplugged, there would still be noise present, coupled onto the powerline from outside RF sources. As a consequence of this noise diversity, the powerline channel represents a non-additive white Gaussian noise (NAWGN) environment. According to some measurements reported in [15] and [16], the noise in broadband powerline communication channels is categorized in two main groups: background noise and impulsive noise. While the background noise remains stationary, the impulsive noise has short durations, but a high psd (up to 40 dB above the background noise). For this reason this type is considered as mean cause of bit errors.
3.2 Error Handling We assume that the SNR (Signal Noise Ratio) is high enough to avoid any influence of the background noise in a PLC Network [2], [7]. There are the following possibilities to deal with the impulsive noise/disturbances which makes more difficulties for the PLC networks: •
Sufficient duration of transmitted symbol (e.g. duration of the OFDM symbol) which avoids any influence of a disturbance impulse, if its duration is enough shorter than the symbol duration.
•
Usage of FEC (Forward Error Correction) mechanisms which decrease network data rate because of the overhead. On the other hand, a probability for erroneous data transmission decreases but still remains.
•
ARQ (Automatic Repeat reQuest) mechanisms can deal with relative short disturbances (some ms) applying data retransmissions which cause the degradation of the network data rate, too.
Long–term disturbances make a part of the network capacity unavailable for a longer time. We assume that this part of the transmission capacity is switched-off by a resource allocation management and is not used for any transmission [2], [7]. Accordingly, the long-term disturbances act as background noise and they are not considered in this investigation.
3.3 Disturbance Scenarios Because of their big influence, the impulsive disturbances have to be included in the investigation of the PLC MAC layer and they are considered in our previous simulation studies [2], [9]. The definition of random variables, impulse interarrival and impulse duration, are required. Most of the measurements show that these variables always have exponential distribution, although their mean values depend on the locations, where the measurements were achieved [18].
Noise Scenario
Mean IAT
Mean duration
Hardly disturbed
0.015 s
2.08 ms
Moderately disturbed
0.476 s
0.87 ms
Lightly disturbed
1.903 s
1.82 ms
Table 1: Disturbance Scenarios Therefore, we define three scenarios (Table 1) for the short-term disturbances representing different PLC environments. This disturbance scenarios are proposed to be used for the investigations of suitable MAC protocols for PLC.
4
MAC Protocols for PLC
4.1 PLC MAC Layer A multiple access scheme of a MAC protocol establishes a method for dividing the transmission resources into accessible sections [8]. Logical transmission channels, defined earlier (subsection 2.3), represent the sections which are accessible for a MAC protocol. The task of a MAC protocol is to manage the channel allocation/re-allocations between a number of PLC subscribers and transmission of different kind of services [2], [9]. We propose a separated treatment of different telecommunication services transmitted over PLC networks. This allows dedicated QoS guarantees for each of the services. Because of the expected disturbance influence on the PLC networks we propose a segmentation of the user data into the smaller data units – PLC segments – with a fixed length. The data segmentation also allows a simpler realization of the QoS guarantees [2], [9].
4.2 Investigation 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. Fixed access strategies are suitable for continuous traffic, but not for bursty traffic which is typical for data transfer provided in the PLC access networks. 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 [2], [9]. Dynamic protocols with contention can not ensure any guarantees of QoS for time-critical services and also a full network utilization can not be reached. Collision-free dynamic protocols can be realized using Token Passing, Polling or reservation methods. Token passing and polling make possible realization of some QoS guarantees in the network. However, with an increasing number of network stations the time between two sending rights for a stations (round-trip time of tokens or polling messages) becomes longer, making both protocols not suitable for time-critical services [2], [9]. In the case of reservation protocols, a kind of pre-reservation of the transmission capacity for a particular user is done according to its transmission request/demand originating from its transmission request. 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 [8]. Realization of various QoS requirements is also possible and a good network utilization can be reached. Because of that we propose the application of the reservation protocols in the PLC access networks.
4.3 Basic Reservation MAC Protocols For the application of the reservation protocols some extra transmission capacity is needed for the transmission of the signaling information. An efficient transmission of connection requests from the users to the base station in the uplink transmission direction as well as an optimal utilization of transmission capacity in the downlink direction has to be ensured in a signaling channel [2], [9].
Figure 2: ALOHA and Polling Access Methods 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 from the implementations and investigations which have been done on this topic (e.g. [8] and [10]). Either fixed or dynamic MAC protocols can be applied to the signaling channel of the PLC system [2]. In [9] we investigate two opposite protocol solutions for the signaling channel: •
ALOHA - a contention protocol
•
Polling protocol - with dedicated reservation
In the first case, according to the ALOHA protocol (Figure 2), a network station tries to send transmission requests/demands to the base station over a signaling channel and after that waits for an answer from the base station, which includes information about medium access rights for the requested transmission. In the case of collision with a request from an other network station, the both affected stations will try to retransmit their transmission demands after a random time. In the second case, there is a polling procedure realized by the PLC base station which sends so-called polling messages to each network station according to the Round Robin procedure (Figure 2). A station receiving a polling message has the right to send a transmission request and afterwards the base station answers with information about medium access rights. In both ALOHA and polling based protocols the users request a number of PLC segments to be transmitted according to the size of arrived packets (e.g. IP packets).
4.4 Analysis of the Reservation Protocols The results of the investigation done in [9] can be summarized as follows: •
The access times with ALOHA based protocols are significantly shorter than with polling access method in the case that the transmission requests occur relative seldom with accordingly few number of the collisions on the signaling channel. However, if the collision probability increases (e.g. with increasing network load or number of subscribers in the PLC network), the advantage of the ALOHA based protocol disappears.
•
On the other hand, the ALOHA protocol shows a better robustness against disturbances because of its random mechanism and changes its behavior more seldom than the polling based protocol.
To find a suitable solution for the MAC protocol applied to the signaling channel of the PLC network we investigate possibilities for optimization of the both protocol solutions. The ALOHA based protocol can be improved with a degradation of the collision probability in the signaling channel. The disadvantages of the polling based protocol can be improved by inserting a contention component into the protocol making them more robust against disturbances and decreasing the access times.
5
Advanced Reservation Protocols
5.1 Improvement of the ALOHA Protocol The ALOHA protocol can be improved due application of CSMA (Carrier Sense Multiple Access) or collision resolution protocols [11]. However, in the considered PLC transmission system the stations are not able to listen the uplink transmission channels [2], so the application of the carrier sense methods is not possible. On the other hand, the implementation of the collision resolution protocols seems to be complex and in spite of that such protocols can reach up to 60 % network utilization. We conclude that the collision probability in the signaling channel can be decreased by reduction of the transmission demands. We analyze the following solutions:
•
Per-burst reservation
•
Piggybacking
After a successful request, made according to the per-burst reservation method, a station receives a transmission right using a certain network capacity as long as there are the user packets to be sent by the station. If the base station recognize that the station does not use the allocated capacity for a while, this network capacity can be allocated to an other station. Per-burst reservation is applied in GPRS (General Packet Radio Service) systems [12]. A disadvantage of the per-burst reservation is a lost network capacity during the phase when a station finished its transmission and the base station is still waiting to recognize it and can not allocate the concerned capacity, which seem to be very valuable in PLC networks, to an other station. The high influence of the disturbances, expected in the PLC systems, causes a need for a frequent reallocation of the transmission resources (e.g. transmission channels). In the case of the per-burst reservation it increases the complexity of the channel reallocation mechanism. Because of that we propose per-packet reservation as described in [2] and [9]. The per-packet reservation causes a higher number of requests causing also a higher collision probability in the signaling channel and accordingly decreases protocol efficiency. This problem can be solved with application of the piggybacking method, which uses segments containing the user data also for the transmission of the requests. E.g. in the last data segment a station can transmit a request, if it has a further user packet to be transmitted. So, the signaling channel is not used for this transmission demand and the collision probability decreases. The piggybacking is also considered for the application in different wireless networks (e.g. [8] and [13]).
5.2 Hybrid Access Protocols A hybrid access method contains both dedicated and random protocol components. So, to the polling based protocol a random component can be added to make it more robust against disturbances and decreasing the access times. One possibility is to allow the random access to the currently free transmission channels. In this case, the station sends the data segments over the free channels and in the same time makes a transmission request [13]. If a collision occurs, a request has to be done in a dedicated time slot and the station has to wait for a transmission permission. This method could be applied if the collisions occur seldom and if the data channels are lightly loaded. Otherwise, we have to take in account all disadvantages of the random access methods in data channels [2]. To avoid it, the free transmission channels should be randomly accessed only for the transmission demands. So, the transmitted data segments can not collide and a station can try to make a transmission request over free data channels without waiting for its dedicated request slot in the signaling channel.
Figure 3: A Hybrid Access Method A further possibility is to divide the signaling channel into a random and a dedicated part (Figure 3). Firstly, the stations try to make the requests in the random part of the signaling. If this is not successful, the dedicated slots are used.
5.3 Adaptive Protocols Disadvantages of random, dedicated and combined protocols can improved with usage of the adaptive protocols [13] which vary their access mechanisms according to the situation in the network. E.g. the stations with a higher data rate get a higher probability to make the requests. This decreases the access times, too. A simple protocol providing such adaptation was proposed for wireless access networks [14]. The stations are divided into two groups: active and inactive stations. The active stations receive the transmission rights according to the dedicated polling access. When an inactive station becomes active, it use a random part of the signaling channel according to the ALOHA method to register itself as an active station. After that, the station receives a dedicated slot for the transmission of the requests. If we can assume that lot of stations are inactive for a longer time, we can conclude that this method decreases the relative long access times in the polling method. Adaptive protocols can vary their parameters depending on current traffic load in the network. For example, the Minimum-Delay Multi-Access Protocol (MDMA) [13] provides usage of the dedicated and random time slots for both data and transmission demands. The stations can make a request or transmit the data in both ways with a defined probability in a random or a dedicated manner. The probabilities for a random or a dedicated access vary according to
the network load. The probabilities can be chosen such, that this protocol become either an ALOHA or a polling method.
Figure 4: An Adaptive Access Method This principle can be applied in the hybrid protocol for the signaling described in subsection 5.2 providing the request slots for both ALOHA and polling access methods. If the hybrid protocol is extended into an adaptive access method, the relation between the number and frequent of random and dedicated slots is calculated according to the network load (Figure 4). We expect that the varying number of random and dedicated slots can improve the performance of the basic hybrid protocol.
6
Conclusions
A PLC transmission system in access networks can be facilitated in a logical model used for the investigations of the PLC MAC layer: a physical tree network topology is modeled in a logical bus structure, the transmission systems offers a number of logical channels. There is a big influence of impulsive noise in the PLC network which can be represented in three disturbance scenarios for the investigations of the MAC layer. We propose reservation MAC protocols because they are suitable to carry hybrid traffic with variable data rates ensuring a high network utilization. The analysis of the basic reservation protocols shows that the ALOHA random protocol can not deal with a large number of transmission demands but is more robust against disturbances than the polling based dedicated access protocol. The ALOHA protocol can be improved with respect to the collision probability by piggybacking. The polling protocol becomes more robust against disturbances by introduction of a contention component. Further improvements can be reached by adaptive protocols which vary their access mechanisms according to the current situation in the network and use both mixed dedicated and random methods, too.
Acknowledgements The authors thank to ONELINE, Barleben, Germany, for supporting our investigations. Part of this work has been done within the EU project PALAS (Powerline as an Alternative Local AcceSs).
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