Conectronica Revista Maganize industrial de electronica

This article describes the specification HomePlug AV (HPAV) for the data transmission in lines of low tension (220V). Equipment HPAV reaches rates of transmission of up to 150 Mbps, which allows to use services of broadband telecommunications in the home without needing installing a structure of ad hoc wiring.

The systems of data transmission in high tension cables (PLC, Power Line Communications) have been used for already many years by diverse companies of the Spanish productive sector. Nevertheless, the reached technological advances throughout this last lustrum have made possible that this technology can be used in the home. In other words, an effective rate of transmission of information (measured in Mbps) sufficiently high like being able has been reached to introduce this technology within the sector of the telecommunications of the leisure and the entertainment. Devices PLC to very competitive prices exist in the market that allow to interconnect all type of computer science equipment to each other using the system of wiring of low tension (220V) that there is in all the homes. Rates of transmission of up to 150 are obtained Mbps and is possible to establish connections with very severe requirements of quality on watch (for example, for the transmission of HDVD, High Definition Distribution Video). The natural competitor of technology PLC 802.11g is the radio networks, although these only has a capacity of 54 Mbps and in many occasions the cover does not reach to all the corners of the home, which can leave some rooms without access to the Intranet and to Internet. Specification HomePlug AV HomePlug PowerLine Alliance [1] is an association - without spirit of profit of the industries that lead the sector of the transmission of information in lines of low tension (PLC, Power Line Communications). The association was created in March of 2000 and its first industrial standard came to the light in June of 2001. HomePlug AV (HPAV, where abbreviations AV talk about Audio-video) is the standard that at the moment is used in the specification and design of the used electronicses in this type of communications. One is a natural evolution of the previous specification, denominated I disturb HomePlug1, whose main objective was to provide access to broadband Internet. Standard HPAV nevertheless, besides guaranteeing an access to elevated Internet of of great volume, is designed to also allow of video and/or to the distribution contents audio of high resolution in the home.

Finally, to say that to all the framework of information bits a set of bits of control for the correction of errors is added to him “forwards” (FEC, Forward Correction Error). These bits of control calculate using I disturb convolucionales codes and allow to reclaim plots of bits received erroneously with no need to request the broadcasting of the plot. Plane of data: level MAC The level of access to HPAV means can get to establish three types different from transference ways:  Transferences connection-oriented, with requirements of Quality on watch as the guaranteed limitation, bandwidth of jitter and the maximum latency. This service is provided using a system TDMA (Time Division Multiple Access) periodic in time.  Transferences nonconnection-oriented (conectionless), that share a same communication channel (contention) but able to establish priorities to arbitrate the access to means. This way of transference is used by asynchronous services but with requirements of Quality on watch. It is provided by means of a scheme CSMA/CA based on priorities.  Transferences nonconnection-oriented (conectionless), that share a same communication channel (contention). This way of transference is used by the services best-effort by means of a traditional scheme CSMA/CA. In order to provide these ways of transference, HPAV-MAC implements an architecture of centralized management. The modem-PLC in charge to realise this management must activate its CCo (Central Coordinating module, to see figure 1). The CCo is the one in charge to establish and to manage the use of beacon-period, a period of time base2 that is divided as well in three temporary regions (figure 2):

- Beacon region.

- CSMA region.

- Contention-free region.

The first temporary zone (beacon region) is used by the CCo to warn to all the modem-PLC connected to the network on the distribution of pre-established connections that will use the rest of the period (regions CSMA and Contention-free) for the transmission of their data. The announced distribution is persistent in the sense in that the CCo is committed not to alter this organization during a consecutive number of beacon-periods. Given the importance of this period for the good operation of the HPAV-MAC, the beacon that is used it is an electrical signal of a great robustness and reliability. The region contention-free is managed by means of a system TDMA. The levels superiors (TCP or even IP, at least in version 6 of the protocol) of model-PLC anyone have a discussion with their Managing module of the connection (CM) to negotiate the requirements of QoS demanded by the application: guaranteed bandwidth, free transmission of errors, limited latency and control of jitter. Finished the negotiation, the cm communicates with the CCo of the central modem-PLC and it lets know his needs him of Quality on watch. If the CCo has capacity to accept the connection, it asks to him to the modem-PLC that considers the capacity of the channel, emitting a tone of test by each one of the carriers of modulation OFDM. The result of this estimation is sent to the CCo that finally determines the number of temporary grooves that region assigns to him to the modem-PLC during contention-free. The used temporary grooves for the transport of traffic with Quality on watch are not used dynamically by the rest of modem-PLC for the transmission of plots CSMA/CA in not-persistent way. If these last ones are not used, the connections with requirements of Quality on watch can, with the permission of the CCo, to use this bandwidth to increase their capacity. Finally, region CSMA provides the time interval in which the modem-PLC can fight by the possession of the channel by means of a scheme CSMA/CA based on priorities and of persistent form. Plane of control: central coordinator (CCo) Each CCo is responsible to control AV Logical Network (AVLN), which is constituted by several modem-PLC which they share Network Membership Key (NMK), thanks to which each member of the AVLN can communicate with the rest of members of private and safe form. The CCo is responsible for the following tasks:  dynamic Management of the bandwidth.  Emitir periodically in way broadcast a beacon indicating the AVLN for which he is responsible.  Aprender the topology of its own AVLN and the one of all the others that share the same communication channel. For it, it is enough to him with listening to the information of the AVLN that all the stations transmit in way broadcast.  Activar the way of saving of energy in situations of low load. Of the first function already it has been spoken. Lack to mention that in case of having more than a AVLN, corresponding the CCo of each network they must be coordinated to share (dynamically) the common channel of communications. Generally, the mechanism consists of dividing beacon-period in proportional parts to the number of terminals of each AVLN.

The map of the network allows to determine the presence him of another modem-PLC in the same AVLN that could in the future assume the role of CCo3 by means of a protocol of to handover. The criteria to follow for the interchange of functions are, in strict precedence: 1. Selection of the user. 2. Capacity of the CCo. 3. Number of stations open pies in the map of the AVLN. 4. Number of AVNL open pies. When all the stations are inactive, the CCo also is responsible to activate the way power-saving. In this way one only activates a small temporary zone of region CSMA (so that they can initiate a transmission) and contention-free (so that they can listen to the control beacons). During the rest of beacon-period, the transmitters and receivers must go out, saving energy and avoiding possible unnecessary interferences with other signals. Equipment in the market Technology HPAV is sufficiently mature like finding equipment of upper middle quality to very competitive prices. Companies like Devolo, Linksys, Atlantis, Aztech, MSI and Zyxel are perhaps some of most outstanding. Devolo and Linksys have their vestibule in Castilian and allow the purchase online of their products. Zyxel only has part of its contents translated to the Castilian and the purchase is not allowed online (although in his vestibule it facilitates a relation of Spanish distributors and its points of sale). Three classes of devices HPAV exist - grosso way: the modem-PLC with an interface Fast-Ethernet IEEE-802.3, those that a wireless interface IEEE-802.11 has and finally those that go directly connected to bus PCI of the computer. Logically, in the three cases the device has another interface that is the one that is connected to the network of low tension by means of a traditional connector. The range of prices oscillates between the 50 and 200 Euros, following if the 14 device works to Mbps (HomePlug 1,0), 85 Mbps (I disturb HomePlug) or 200 Mbps (HomePlug AV). In applications for the distribution of high resolution video it is necessary to guarantee the bandwidth and that only can provide it the equipment accredited according to the norm HPAV (the equipment of Atlantis and Aztech assures in their vestibule that their equipment admits this configuration). On the contrary, itself they require a modem-PLC group to share an access to Internet, anyone of the three types works, since at present the limitation comes given by the bandwidth provided by the supplier from telecommunications (3 Mbps in average term) or by 54 the maximum volume of Mbps that is reached with the wireless technology. Alternative technologies In this section we will briefly review other alternative technologies to communications PLC for the provision of broadband services in homes. All of them use like communication channel some of infrastructures of already existing wiring in and home: telephone or coaxial cable. For that reason to these technologies (along with the PLC) often is denominated it with the denomination of “no-wire”, meaning that does not need additional wiring and that unlike the technology “wireless”, uses copper support for its transmissions. The study will turn surroundings to the main benefits that they have presented his respective organisms estandarizadores. Also he is interesting to mention that, with the exception of the systems PLC (that have been many years in the market), all the technologies that will be seen in this section they are at least of very recent creation, his last specifications, that on the other hand are the unique ones that reaches quality levels after the support of broadband services.

MoCA

Multimedia to over Coax Alliance (MoCA) is a grouping of companies of the sector of the telecommunications in charge as much to establish standards as to certify the products that use these standards regarding the transmission of broadband services in the phantom without using of the installed coaxial lines in the homes. The first formal specification was approved in February of 2006 and few months later the first equipment was sent to the market. According to the realised studies, the MoCA devices are able to transmit at speeds of channel of 270 Mbps (physical level), that is translated to about 135 Mbps of rate of information of bits (level MAC) using for it the leftover phantom of the coaxial cable, that includes/understands from the 860 MHz to the 950 MHz4. This capacity, along with a latency average surroundings to the 5 milliseconds, does of this most promising technology of the sector. The interference that takes place in the sidebands of the phantom (CATV and Satellite) is minimum and imperceptible in anyone of the devices of radio and television distributed by the building. In addition, unlike technology PLC, it mainly does not affect the produced impulsive noise to him by the installed electrical motors in the electronic equipment of white range. Either producne important interferences in the free phantom of RF (continuous complaint of many users of RF against systems PLC that works in high tension cables) thanks to the screening of coaxial. This technology has, nevertheless, two great disadvantages. First of them it is that although the McOA assures that a 90% of the American homes have infrastructure of coaxial, recent studies demonstrate that in fact 30% are not reached. Probably, in Spain, this percentage will be quite smaller. The second disadvantage is that generally, few points of connection with the network exist: the habitual thing is to locate a pair of takings, one in the dining room and another one in the hall. With luck, it is possible to find an additional taking in some of the rooms. Finally, it is necessary to have present that in networks CATV of buildings of certain spread active devices include (amplifying) that are not designed to be compatible with the MoCA specifications. In this case it would have to replace the amplifier or, by means of spliters and specific mixers, respectively to divide and to regroup both signals to the entrance and exit of the amplifier. HPNA Like in the previous case, Home Phoneline Networking Alliance is a grouping of companies telco ordered so much to establish specifications as to also certify the products that use these specifications regarding the transmission of broadband services in the phantom without using of the telephone lines and - from its version 3.0 - in installed coaxial lines in the homes. This technology is able to support 240 Mbps. Nevertheless, the field tests realised in real homes demonstrate that the reached average rate is of approximately 100 Mbps. Its transmission band is very over the used one by the telephone systems, reason why it produces interferences neither in the fax nor on the telephones. In addition, like the systems PLC, the 100% of the homes and residences of the aligned countries have telephone infrastructure (although she is minimum).

Nevertheless, also it is certain that in many homes there is more no a taking (at the most two) of telephone, which seriously limits the possible market of this technology. In addition many experts assure that he is a technology that has not been able to overcome the reluctance of the market, which has caused a slowing down in the sales and consequently, also in the necessary investments for its development. Its last specification (HomePNA 3,1) was approved in January of 2007 by the ITU and is well-known like the G.9954 recommendation. The greater newness is than the standard admits the use of the coaxial cable like transmission means (already gathered in version 3.0). The use of the coaxial one like transmission means has solved one of the problems more serious than they exist in the telephone lines: the terrible evil been and conexionado of the interface of connection (jack) with the team of the user. Logically, this solution is only possible when the client has infrastructure of coaxial wiring. Wireless 802.11n One is a standard that began in the 2004 and which it follows in phase of development. It is anticipated that finally surroundings in the middle of 2008 will be ratified. Using technology MIMO (Multiple Imput Multiple Output) he will be able to provide a capacity between 4 and 8 times greater than the present standard 802.11g; that is to say, between the 200 and the 400 Mbps. It will be necessary to hope to see the first equipment to make a judgment sensible but already it is possible to be advanced that one of the inconvenient majors of this technology is that the benefits will be degraded strongly as the distance between the device and the joining point increases. Conclusions Technology PLC (of the hand HomePlug AV, its more important estandarizador organism at present) is at the present time a real solution to implant broadband services in the Intranet the homes, since it has been long time in the market and therefore, exist diverse manufacturers who produce a great number of equipment of upper middle benefits to very competitive prices. On the other hand, a tendency in the market is appraised to design equipment that is able to transmit in any type of domestic communication channel. In this sense, as much HomePlug AV as HomePNA converges in using the coaxial cable like an alternative to their own communication channel.