The same RF spectrum of 2 – 30 MHz used in the Manchester, UK trials by NOR.WEB still forms the basis for solutions.
Generally, the spectrum between 2 to 11 MHz is considered for the priority use by access applications, 13 to 20 Mhz for in-house and 20 to 30 MHz reserved for special applications.
Thus, a formula for interworking between access and in-house applications has been established.
Modulation techniques are better understood. Multi-frequency carrier technology, such as OFDM is in use, but some spread spectrum approaches have also been tried successfully. A number of vendors are still secretive about their favoured modulation scheme.
The design of a Powerline interface is achieved in a layered fashion. The physical layer and the Media Access (MAC) layer are implemented in Powerline chip sets. The Powerline channel is a difficult and variable environment, so the MAC layer must implement the dynamic measurements necessary to adapt the transmission requirements of the communications channel to the Powerline channel. The actual communication services are then designed to live on top of the MAC layer. Since the physical and MAC layers are vendor proprietory in most cases, that means troubles for interworking of in-house systems from different vendors.
Several Powerline chipset vendors have established themselves. Powerline system integrators package those chipsets into product. The significant system features are therefore determined by the system integrators. Main.Net is a case in point.
Inter-sytem interworking and interworking of systems by different vendors is being addressed by ETSI and others, but would require to open up the still proprietary specifications for the physical and MAC layers and in consequence, their standardization. This is not likely to occur in the immediate future.
Initially, the applications split into in-house and access systems. Now it appears that a user just has to stay with one vendor to avoid difficulties.
The HomePlug Alliance in the United States announced in July 2001 its HomePlug Specification 1.0 as the standard for in-house solutions, endorsed by about 30 vendor companies.
Europe continues to concentrate on access systems, driven by the PLCforum. There is evidence however, that this split may become less meaningful as time goes on: it has been stated by some access vendors that it was difficult to market their product without a compatible in-house solution. For some vendors today who deploy dynamic power level adjustment, the same technology implementation is used for both access and in-house, thus making such a distinction unnecessary.
Main.net has developed such a powerful Powerline system product (based on Itran chipsets) that they claim their product can be used equally well for access as it can be used for in-house systems. It allows in fact to “re-use” parts of the spectrum by dynamically lowering the transmission power level address dependent such that the same spectrum can be used again in relatively distant parts of the overall system.
With the “frequency band split” convention of using the lower 10 MHz for access (there is generally only one access vendor in a local system) and allocating a comparable bandwidth for in-house systems, the interworking between access and in-house seem all but resolved.
Operating different in-house systems in one local area is a more difficult issue, no good solution has been found yet.
With adaptive modulation level mechanisms, transmission through electric meters and even transformers has been demonstrated.
While HomePlug concentrated on the in-house market, their chip sets are designed to be also usable for access purposes later on. Using sophisticated modulation schemes a throughput of perhaps 15 Mb/s can be readily achieved using the frequency spectrum of 5 – 22 MHz with the modulation power still within the limits of FCC Part 15 rules, covering the majority of typical in-house applications in the US. Using 13 – 22 MHz, only 8 Mb/s may be achievable, however.
A weakness of this technology for access is that the limited information throughput of, currently, about 2 Mb/s is shared by all customers on a substation transformer, which will surely lead to inadequate bandwidth for some customers. Forrester Research has pointed that out as a severe weakness, markedly considering the competition of the emerging DSL and cable services.