Power line communications (PLC) systems have been used for many years by various companies in the Spanish manufacturing sector. However, technological advances achieved over the last five years have made it possible to use this technology in the home. In other words, an effective data transmission rate (measured in Mbps) high enough to introduce this technology into the leisure and entertainment telecommunications sector has been achieved.
PLC devices are available on the market at very competitive prices, allowing all types of computer equipment to be interconnected using the existing low-voltage (220V) wiring system found in most homes. Transmission rates of up to 150 Mbps are achievable, and it is possible to establish connections with very stringent quality of service requirements (for example, for HDDVD, High Definition Video Distribution). The natural competitor to PLC technology is 802.11g wireless networks, although these only have a capacity of 54 Mbps and in many cases the coverage does not reach all corners of the home, which can leave some rooms without access to the Intranet and the Internet.
The HomePlug AV specification, developed
by the HomePlug PowerLine Alliance [1], is a non-profit association of leading industries in power line communications (PLC). Founded in March 2000, its first industry standard was released in June 2001.
HomePlug AV (HPAV, where AV stands for Audio/Video) is the standard currently used to specify and design electronic devices for this type of communication. It represents a natural evolution of the earlier Turbo HomePlug specification, whose primary objective was to provide broadband internet access. However, in addition to ensuring high-speed internet access, the HPAV standard is designed to also enable the distribution of high-definition audio and/or video content in the home.
Although the formal specification is relatively recent (mid-2005), several manufacturers are already distributing HPAV equipment at reasonably affordable prices. This underscores the high level of interest that the ICT production sectors are showing in this market niche. Indeed, its main advantage is that electrical outlets are available in every corner of a home, and these outlets must be used since most communication equipment requires power.
As will be seen in later sections, the technology required to meet HPAV specifications is very complex. This is because the communication channel, being designed solely for the transmission of electrical power, is very hostile to signal transmission. In fact, its electrical characteristics are of very low performance (drastic attenuation with a strong frequency dependence, presence of noise signals of various types—white, impulsive, and interfering—impedance mismatch depending on the use of the outlets, etc.) [2]. Furthermore, there is the fact—perhaps more serious—that all of these characteristics vary significantly over time [3][4]. Clearly, the HPAV standard has had to wait for communications technologies to reach a certain level of maturity. This delay could explain the relatively limited use of this technology in Spanish homes.
The HPAV specification also aims to equip its systems with the necessary automatic management mechanisms, making the equipment Plug & Play for the user and quick and easy for the service provider to configure. Furthermore, since the communication channel is shared, the HPAV standard also specifies the use of encryption techniques with the necessary automation for key generation and renegotiation processes. The latest specification includes the possibility of transmitting not only over power lines but also over coaxial cable. This appears to be the only solution (see the conclusions section) currently available to achieve very high transmission rates.
Finally, it's worth mentioning that, in addition to the HomePlug Alliance, other regulatory bodies exist within the PLC sector (which is understandable given the technology's long history; for example, equipment for voice transmission over train overhead lines was already being manufactured in the 1980s). Some of the most prominent are the Universal Powerline Association, the Consumer Electronics Powerline Communications Alliance, and the Open PLC European Research Alliance.
System Architecture
Figure 1 shows the architecture diagram of an HPAV system. As can be seen, the architecture defines the data and control planes. In the first case, the classic layered division of communication systems based on local area network (LAN) standards is observed, which greatly facilitates the integration of the device with the computer terminals currently on the market. The
control plane is unique in that it is monolithic; that is, connection management is performed by understanding the physical, access, and convergence layers as a whole, through the use of crosslayer techniques. This allows for a much more efficient and flexible design of the connection management unit when adapting it to future innovations.
The HPAV architecture requires that, among all the modem-PLCs that make up the same network, one of them must act as the central station. This station will be the only one that has a Central Coordinator (CCo) in charge of reserving the bandwidth needed for connections with QoS requirements (both synchronous and asynchronous but with priorities).
Data Plane: Physical Layer.
Signal transmission over the physical medium operates in the frequency band between 2 and 28 MHz. The modulation mechanisms are such that, with this narrow bandwidth, they are capable of transmitting 200 Mbps (channel speed). The use of Turbo Convolutional Codes for automatic error detection and recovery reduces the actual bit rate of transmitted information by approximately 150 Mbps.
Symbols are transmitted using OFDM (Orthogonal Frequency Division Multiplexing) [5] with a total of 917 available orthogonal carriers. This type of multi-carrier modulation has two major advantages: first, by using so many carriers, the symbol period is long enough to make the transmission immune to the effects of fading and impulse noise, both very common phenomena in PLC environments. Furthermore, each of these 917 tones can be independently modulated using anything from a simple BSK to 1024-QAM. Each carrier selects its modulation based on its own signal-to-noise ratio, allowing the system to easily adapt to sudden changes in the communication channel's transfer function. Logically, the control plane (and more specifically the unit called the Connection Manager) is responsible for constantly measuring the channel characteristics to appropriately adapt the generated signal.
Finally, a set of control bits for forward error correction (FEC) is added to the entire information bit structure. These control bits are calculated using turbo convolutional codes and allow for the recovery of frames of erroneously received bits without requiring frame retransmission.
Data plane: MAC layer
The HPAV media access layer can establish three different types of transfer modes:
Connection-oriented transfers, with Quality of Service requirements such as guaranteed bandwidth, jitter limitation, and maximum latency. This service is provided using a time-periodic TDMA (Time Division Multiple Access) system.
Connectionless transfers, which share the same communication channel (contention) but are capable of establishing priorities to arbitrate access to the medium. This transfer mode is used by asynchronous services with Quality of Service requirements. It is provided using a priority-based CSMA/CA scheme.
Connectionless transfers, which share the same communication channel (contention). This transfer mode is used by best-effort services using a traditional CSMA/CA scheme.
To provide these transfer modes, HPAV-MAC implements a centralized management architecture. The modem-PLC responsible for this management must activate its CCo (Central Coordinator module, see Figure 1). The CCo is responsible for establishing and managing the use of beacon-periods, a base2 time period that is further divided into three time regions (Figure 2):
- Beacon region.
- CSMA region.
- Contention-free region.
The first temporary zone (beacon region) is used by the CCo to notify all modem-PLCs connected to the network about the pre-established connection distribution they will use for the remainder of the period (CSMA and Contention-free regions) for data transmission. This announced distribution is persistent, meaning the CCo commits to not altering this arrangement for a consecutive number of beacon periods. Given the importance of this period for the proper functioning of the HPAV-MAC, the beacon used is a highly robust and reliable electrical signal.
The contention-free region is managed using a TDMA system. The higher layers (TCP or even IP, at least in version 6 of the protocol) of any given modem-PLC communicate with its Connection Manager (CM) module to negotiate the QoS requirements demanded by the application: guaranteed bandwidth, error-free transmission, limited latency, and jitter control. Once the negotiation is complete, the CM communicates with the CCo of the central modem-PLC and informs it of its Quality of Service (QoS) requirements. If the CCo has the capacity to accept the connection, it requests the modem-PLC to estimate the channel capacity by emitting a test tone for each of the OFDM modulation carriers. The result of this estimate is sent to the CCo, which then determines the number of time slots to allocate to the modem-PLC during the contention-free region. Time slots not used for QoS traffic are dynamically used by the other modem-PLCs for the transmission of CSMA/CA frames in non-persistent mode. If these frames are not used, connections with QoS requirements can, with the CCo's permission, use this bandwidth to increase their capacity.
Finally, the CSMA region provides the time interval during which modem-PLCs can persistently contend for channel ownership using a priority-based CSMA/CA scheme.
Control Plane: Central Coordinator (CCo)
Each CCo is responsible for controlling an AV Logical Network (AVLN), which consists of several modem-PLCs that share a Network Membership Key (NMK). This key allows each AVLN member to communicate with the other members privately and securely.
The CCo is responsible for the following tasks:
Dynamic bandwidth management.
Periodically broadcast a beacon indicating the AVLN for which it is responsible.
Learn the topology of its own AVLN and all others sharing the same communication channel. To do this, simply listen to the AVLN information that all stations broadcast.
Activate power-saving mode in low-load situations.
The first function has already been discussed. It remains to be mentioned that if there is more than one AVLN, the corresponding CCos of each network must coordinate to dynamically share the common communication channel. Generally, the mechanism consists of dividing the beacon period into parts proportional to the number of terminals in each AVLN.
The network map allows you to determine the presence of another modem-PLC in the same AVLN that could assume the role of CCo3 in the future through a
handover protocol. The criteria to follow for the exchange of functions are, in strict order of priority:
1. User selection.
2. CCo capacity.
3. Number of stations discovered on the AVLN map.
4. Number of AVNL stations discovered.
When all stations are inactive, the CCo is also responsible for activating power-saving mode. In this mode, only a small, temporary area of the CSMA region is activated (to allow stations to initiate a transmission) and contention-free (to allow stations to listen to control beacons). During the remainder of the beacon period, transmitters and receivers must be turned off, saving power and preventing unnecessary interference with other signals.
Equipment on the Market:
HPAV technology is mature enough to offer mid-to-high-quality equipment at very competitive prices. Companies like Devolo, Linksys, Atlantis, Aztech, MSI, and Zyxel are perhaps some of the most prominent. Devolo and Linksys have websites in Spanish and allow online purchases of their products. Zyxel has only some of its content translated into Spanish and does not offer online purchases (although its website provides a list of Spanish distributors and their points of sale).
Broadly speaking, there are three types of HPAV devices: PLC modems with an IEEE-802.3 Fast Ethernet interface, those with an IEEE-802.11 wireless interface, and finally, those that connect directly to the computer's PCI bus. Naturally, in all three cases, the device has another interface that connects to the low-voltage power supply via a traditional connector.
The price range varies between €50 and €200, depending on whether the device operates at 14 Mbps (HomePlug 1.0), 85 Mbps (Turbo HomePlug), or 200 Mbps (HomePlug AV). For high-definition video distribution applications, bandwidth must be guaranteed, and only equipment certified according to the HPAV standard can provide this (Atlantis and Aztech state on their websites that their equipment supports this configuration). Conversely, if only a group of modem-PLCs is required to share an internet connection, any of the three types will work, since currently the limitation is determined by the bandwidth provided by the telecommunications provider (3 Mbps on average) or by the maximum speed of 54 Mbps achievable with wireless technology.
Alternative Technologies.
In this section, we will briefly review alternative technologies to PLC communications for providing broadband services in homes. All of them use existing home cabling infrastructure, such as telephone or coaxial cable, as their communication channel. Therefore, these technologies (along with PLC) are often referred to as "no-wire," meaning they do not require additional cabling and, unlike "wireless" technology, use copper for transmission. The study will focus on the main features announced by their respective standards bodies.
It is also worth mentioning that, with the exception of PLC systems (which have been on the market for many years), all the technologies discussed in this section are very recent—at least their latest specifications are—and these are the only ones that achieve the necessary quality levels to support broadband services.
The
Multimedia over Coax Alliance (MoCA) is a group of telecommunications companies responsible for establishing standards and certifying products that use those standards for transmitting broadband services over the unused spectrum of coaxial cable lines installed in homes. The first formal specification was approved in February 2006, and the first devices were launched a few months later.
According to studies, MoCA devices are capable of transmitting at channel speeds of 270 Mbps (physical layer), which translates to a bit rate of approximately 135 Mbps (MAC layer), using the unused spectrum of the coaxial cable, ranging from 860 MHz to 950 MHz. This capacity, along with an average latency of around 5 milliseconds, makes this technology the most promising in the sector. Interference in the sidebands of the spectrum (CATV and satellite) is minimal and imperceptible to any of the radio and television devices distributed throughout the building. Furthermore, unlike PLC technology, it is unaffected by impulse noise, primarily produced by the electric motors installed in household appliances. It also does not produce significant interference in the free RF spectrum (a frequent complaint from many RF users regarding PLC systems operating on high-voltage lines) thanks to the coaxial shielding.
This technology does, however, have two major drawbacks. The first is that, although the McOA claims that 90% of US homes have coaxial infrastructure, recent studies show that the actual figure is less than 30%. This percentage is likely much lower in Spain. The second drawback is that there are generally few network connection points: typically, there are only a couple of outlets, one in the dining room and another in the living room. With luck, you might find an additional outlet in one of the bedrooms.
Finally, it's important to note that CATV networks in larger buildings include active devices (amplifiers) that are not designed to be compatible with the MoCA specifications. In this case, the amplifier would have to be replaced, or alternatively, using specific splitters and mixers, both signals would have to be divided and regrouped at the input and output of the amplifier respectively.
HPNA,
like the previous example, is a group of telecommunications companies responsible for establishing specifications and certifying products that use these specifications for transmitting broadband services over the unused spectrum of telephone lines and—since version 3.0—also over coaxial lines installed in homes.
This technology is capable of supporting 240 Mbps. However, field tests conducted in real homes show that the average speed achieved is approximately 100 Mbps. Its transmission bandwidth is well above that used by telephone systems, so it does not cause interference with fax machines or telephones. Furthermore, like PLC systems, 100% of homes and residences in participating countries have telephone infrastructure (even if minimal).
However, it is also true that many homes have only one (or at most two) telephone jacks, which seriously limits the potential market for this technology. Furthermore, many experts assert that this technology has failed to overcome market resistance, leading to a slowdown in sales and, consequently, in the investments required for its development.
Its latest specification (HomePNA 3.1) was approved in January 2007 by the ITU and is known as recommendation G.9954. The most significant innovation is that the standard supports the use of coaxial cable as a transmission medium (already included in version 3.0). The use of coaxial cable has solved one of the most serious problems with telephone lines: the poor condition and faulty connections of the interface (jack) to the user's equipment. Naturally, this solution is only possible when the customer has a coaxial cabling infrastructure.
Wireless 802.11n
is a standard that began in 2004 and is still under development. It is expected to be finally ratified around mid-2008. Using MIMO (Multiple Input Multiple Output) technology, it will be capable of providing a capacity between 4 and 8 times greater than the current 802.11g standard; that is, between 200 and 400 Mbps. We will have to wait to see the first devices to make a sound judgment, but it can already be said that one of the biggest drawbacks of this technology is that performance will degrade significantly as the distance between the device and the access point increases.
Conclusions:
PLC technology (with HomePlug AV as its most important standards body today) is currently a viable solution for deploying broadband services on home intranets. It has been on the market for a long time, and consequently, several manufacturers produce a wide range of mid-to-high-performance equipment at very competitive prices.
Furthermore, there is a growing market trend toward designing equipment capable of transmitting over any type of domestic communication channel. In this regard, both HomePlug AV and HomePNA converge on using coaxial cable as an alternative to their own communication channel.
J. Malgosa Sanahuja, JP Muñoz Gea, P. Manzanares López, JC Sánchez Aarnoutse
{josem.malgosa, juanp.gea, pilar.manzarares, juanc.sanchez}@upct.es
Univ. Politécnica de Cartagena. Telematics Engineering Group
