IEEE 1901

IEEE P1901 ("IEEE P1901 Draft Standard for Broadband over Power Line Networks: Medium Access Control and Physical Layer Specifications") is an IEEE draft standard for broadband over power line networks defining medium access control and physical layer specifications. The P1901 baseline was approved with more than 85% of positive votes during the meeting held in Kyoto, Japan, 16-19 December 2008 (see http://grouper.ieee.org/groups/1901/).

The baseline adopted in December 2008 includes three separate options for PHY and MAC. The third option, which was added in order to get a successful confirmation vote in Kyoto, is compatible with G.hn/G.9960, which is another home networking standard being developed by ITU-T for operation over power lines, phone lines and coaxial cable.

Scope

The project will develop a standard for high speed (>100 Mbit/s at the physical layer) communication devices via alternating current electric power lines: so-called Broadband over Power Line (BPL) devices. The standard will use transmission frequencies below 100 MHz; it will be usable by all classes of BPL devices, including those used for the first-mile/last-mile connection (<1500 m to the premise), to broadband services, as well as BPL devices used in buildings for LANs and other data distribution (<100 m between devices). This standard will focus on the balanced and efficient use of the power line communications channel by all classes of BPL devices, defining detailed mechanisms for coexistence and interoperability between different BPL devices, and ensuring that desired bandwidth and quality of service may be delivered. P1901 will address the necessary security questions to ensure the privacy of communications between users and allow the use of BPL for security sensitive services. This standard is limited to the physical layer and the medium access sub-layer of the data link layer, as defined by the International Organization for Standardization (ISO) Open Systems Interconnection (OSI) Basic Reference Model. The effort will begin with an architecture investigation, and this will form the basis for detailed scope of task groups that will work within P1901 to develop the components of the final standard.

Purpose

New modulation techniques offer the possibility to use the power lines for high speed communications. This new high speed media is open, and locally shared by several BPL devices. Without an independent, openly defined standard, BPL devices serving different applications will conflict with one another and provide unacceptable service to all parties. The standard will provide a minimum implementation subset which allows the fair coexistence of the BPL devices. The full implementation will provide the interoperability among the BPL devices, as well as interoperability with other networking protocols, such as bridging for seamless interconnection via 802.1. It is also the intent of this effort to quickly progress towards a robust standard so powerline applications may begin to impact the marketplace. The standard will also comply with EMC limits set by national regulators, so as to ensure successful coexistence with wireless and telecommunications systems.

"Dual PHY/MAC" Controversy

The proposal selected by the P1901 Working Group in October 2007 has been controversial to some. As with many IEEE standards, compromises are made. IEEE 802.11 (legacy mode) is an example, with three non-interoperable PHYs:

A weakness of this original specification was that it offered so many choices that interoperability was sometimes challenging to realize. It is really more of a "beta-specification" than a rigid specification, initially allowing individual product vendors the flexibility to differentiate their products but with little to no inter-vendor operability.

— Wikipedia, IEEE 802.11 (legacy mode)

The P1901 proposal included two different PHY layers, one based on OFDM modulation and another based on Wavelet modulation. Each PHY is optional, and implementers of the specification would not be forced to include both. This would mean that devices that use the OFDM PHY would not interoperate with devices based on Wavelet PHY. Some members of P1901 maintain that this lack of interoperability defeats the purpose of having a standard.

Dual PHY

P1901 includes two PHY options. The first option ("FFT PHY") is based on FFT OFDM modulation, with a Forward error correction (FEC) scheme based on Convolutional Turbo code (CTC). The second option ("Wavelet PHY") is based on Wavelet OFDM modulation, with a mandatory FEC based on concatenated Reed-Solomon (RS) and Convolutional code, and an option to use Low-Density Parity-Check (LDPC) code.. Although FFT OFDM is the most common choice for powerline communications systems, Wavelet OFDM provides higher transmission efficiency (due to the lack of a guard interval) and deeper notches than traditional FFT OFDM systems.

Dual MAC

Another reason why the current proposal at P1901 has been controversial is the fact that a significant part of the MAC is also duplicated. Instead of specifying a common MAC that can operate over two optional PHYs, the current proposal at P1901 essentially defines two different MAC layers, one designed to operate on top of the FFT PHY and another on top of the Wavelet PHY.

The list of Functional areas of the MAC which are different for each PHY includes the description of Aggregation, Fragmentation and Reassembly Mechanisms, Automatic Repeat request (ARQ) Mechanisms, Selection and management of BM, the way in which Broadcast/Multicast communication is performed, the specification of Buffer Management and Flow Control, protocols for Channel Estimation, Medium Access Control via CSMA/CA, Medium Access Control via TDMA, MAC Frame Formats, Management Messages, specification of MPDU Formats, security protocols such as P1901 Shared Key Mode or RSNA data confidentiality, Synchronization requirements, etc.

Controversy in the media

P1901's lack of interoperability has also been the subject of controversy in the media:

When is a standard not really standard? When it’s the proposed IEEE P1901 standard for powerline networking [...] Unlike the IEEE 802.11n Draft 2.0 standard for wireless networking, the proposed IEEE P1901 standard for powerline networking does not guarantee that products certified as compliant with the IEEE P1901 standard will work together. [...] Powerline networking has a place in the market, but the IEEE P1901 Working Group could learn a few things about gaining consumers’ confidence from their colleagues on the IEEE 802.11n Working Group.

— Tom's Hardware Guide: "The Non-standard Powerline Networking Standard",

The IEEE has a long history of "multi-PHY" standards, with 802.11 originally having four PHY which all lacked interoperability. Once the industry converged into a single PHY, 802.11 became one of the most successful standards around. The major difference between 802.11 and P1901 is that 802.11 had a common MAC across all four PHYs, while P1901 does not. P1901's MAC layer is split into multiple different MACs, one for each PHY option:

However, P1901 is arguably a flawed design since it incorporates multiple, incompatible PHY specifications. That would be fine if vendors will be obliged to include all of the standard's PHYs, but they're not, they can pick and choose.

The upshot is that if manufacturer A uses one PHY, and manufacturer B uses another, their products will not work together - despite both being compliant with P1901 and sporting whatever Wi-Fi-style brand products based on the standard end up being sold under.
And it's not like 802.11a, 802.11b and 802.11g where there are clear differences - speed and backwards compatibility - between them.

It gets worse. P1901 also takes in multiple MACs rather than a single specification that can operate with all of the PHYs. This only increases the opportunity for interoperability problems.

— The Register: "G.hn-ing for gigabit",

P1901 timeline

A vote on July 2008 to confirm the "dual PHY" proposal failed to get the required number of backers. A second vote in October 2008 also failed to get the required number of positive votes (75%). The IEEE P1901 process specifies that if the proposal suffers two negative confirmation votes, the process has to be reset, and the three last surviving proposals have to be reconsidered by the group. During the October 2008 P1901, a reset indeed did occur and a new eliminate vote was made which reconfirmed the HomePlug/Panasonic proposal as the candidate P1901 baseline. See http://grouper.ieee.org/groups/1901/. This proposal once again failed to get a successful confirmation vote during the November 2008 meeting in San Francisco.

During the Kyoto meeting (Dec 2008), the P1901 proposal was modified (adding a "G.hn compatible" PHY/MAC) in order to gain broader support, and the group adopted a baseline that included three different PHY and MAC options. The proposal gained broad support (85% for the in-home cluster, 97% for the access cluster and 100% for the coexistence cluster).

Adding the "G.hn compatible" PHY/MAC was a key reason for successfully adopting P1901's baseline, according to an expert from EE Times:

Thanks to growing momentum behind a standard covering all wired home networks, an IEEE group has taken a significant step toward defining a standard for competing powerline technologies. The IEEE's P1901 working group approved last week (Dec. 19) at a meeting in Kyoto a set of baseline technologies powerline nets, breaking a two-year deadlock. The group approved a proposal that defines a set of three physical and media access layer options. The trio includes the G.hn wired home net standard for which the PHY was recently approved by the ITU, a separate standards group.

— EE Times: "Powerline group breaks two-year deadlock",

On February 2009, the tasks of updating the baseline document were allocated to four Technical Subgroups (TSGs). TSG1 is in charge of editorial tasks, TSG2 is in charge of the confirmed FFT and Wavelet PHYs and MACs, TSG3 is in charge of developing the new ITU-T G.hn Compatible PHY/MAC option, and TSG4 is in charge of all coexistence aspects.

IEEE P1901 is still in drafting stage and the text has not been submitted to letter ballot yet. Although IEEE P1901 has not published an official timeline, there is public information available regarding how much time it has historically taken for other IEEE Working Groups (such as 802.11) to approve standards of similar or lower complexity.

According to 802.11, it took 4 years and 5 months for 802.11e (a 211-page long amendment to the IEEE 802.11 standard that defines a set of Quality of Service enhancements for wireless LAN applications) to go from the first letter ballot to final IEEE-SA approval. It took 3 years and 2 months for 802.11i (a 190-page long amendment to the original IEEE 802.11 standard specifying security mechanisms for wireless networks) to go from first letter ballot to final IEEE-SA approval.

Related Standards

ITU-T is developing another home networking standard (G.hn) for operation over power lines, phone lines and coaxial cable. The first document of the G.hn series is Recommendation G.9960, which received consent on Dec 12th 2008. Unlike P1901, G.9960 specifies a single PHY based on FFT OFDM modulation and Low-Density Parity-Check (LDPC) FEC code. Because all G.hn powerline devices share a common PHY, all G.hn powerline devices are interoperable.

IEEE P1675 is another IEEE standard related to Broadband over Power Line. P1675 provides testing and verification standards for the hardware commonly used for Broadband over Power Line (BPL) installations (primarily couplers and enclosures) and provides standard installation methods to ensure compliance with applicable codes and standards.

IEEE P1775 ("Powerline Communication Equipment — Electromagnetic Compatibility Requirements — Testing and Measurement Methods") is an IEEE working group focused on PLC equipment, electromagnetic compatibility requirements, and testing and measurement methods.

See also

• Power line communication

External links

• IEEE P1901 Official site
• IEEE P1901 PAR describes the official scope of the work group

References

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This article incorporates some information taken from IEEE P1901 Official site with permission.