HomePlug AV

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HomePlug AV is a network via a power line ( carrier frequency system ) with a data transfer rate of up to 200 Mbit / s. Home networks via power lines offer an alternative solution to other network technologies . In addition to the previous HomePlug 1.0 specification with 14 Mbit / s data rate or 85 Mbit / s, the HomePlug AV standard can enable transmission rates of up to 200 Mbit / s. HomePlug AV was introduced by the HomePlug Alliance in August 2005. The specification also allows HDTV content to be broadcast. HomePlug AV devices do not have to be backwards compatible with HomePlug 1.0, but must not interfere with HomePlug 1.0 connections on the same cable.

Applications

The abbreviation AV in the name of the specification stands for audio and video . Its objective is to enable time-critical and data-rich applications beyond the scope of HomePlug 1.0, for example IP telephony , video-on-demand and IPTV in HDTV quality.

Installation and security

HomePlug AV can be put into operation using plug-and-play technologies and auto-configuration. As long as the preset password is not changed, the integrated 128bit AES encryption is ineffective.

Properties of HomePlug AV

  • Transmission medium: 50/60 Hz AC lines, coaxial cables or twisted pair cables with appropriate adapters (optionally also baluns )
  • Link data rate: 200 Mbit / s
  • TCP data rate: up to about 68 Mbit / s
  • UDP data rate: up to about 90 Mbit / s
  • Area coverage: up to 300 meters (entire household)
  • Transmission method: CSMA / CA . TDMA planned but not yet implemented
  • Ethernet compatibility: Yes (CSMA)
  • Coexistence with home automation : Yes ( X10 , LonWorks , CEBus )
  • Frequency range PHY: 2–28 MHz (1.8–30 MHz possible on coaxial cable)
  • Number of carrier frequencies: 917 (1155 without notch filter )
  • Modulation methods: OFDM , 1024/256/64/16 / 8- QAM , QPSK , BPSK

The HomePlug AV architecture

In order to be able to meet the QoS requirements of entertainment electronics (but also the requirements for VoIP ), the HomePlug AV system works with both connection-oriented and connectionless communication channels.

Connection-oriented communication channels - such as ISDN lines - represent logical point-to-point connections in which defined periods of time are available for data transmission. As soon as an application has reserved such a transmission path between sender and receiver, no other application can access it for the duration of the transmission. The implementation requires a precise and network-wide time control of all participating communication partners, which can only be implemented through a central control mechanism. All data packets arrive in the order in which they were sent.

Connectionless communication channels are, for example, 10BASE2 and WLAN . If several applications send data packets at the same time, these are unusable and must be sent again. To prevent collisions from occurring again, the affected transmitters initially allow a short period of time to elapse, the length of which varies randomly. A central control can be omitted, which makes the system less complex and therefore more robust against disturbances. The order in which packages arrive at the recipient cannot be foreseen. The recipient must put the packages in the correct order.

In order to be able to implement these two modes of operation in a single network, a central coordinator (CCo) first ensures that all stations (STA) in the network are synchronized to exactly the same clock. The CCo then assigns time slices, i.e. distributes the total transmission rate - the gross transport capacity (bits per second) of the transmission channel - into several areas and dynamically assigns them to the other stations in the network.

CCo - the coordinating center

Since HomePlug AV networks should function without a superordinate entity, every station must be able to save such information. There is a Central Coordinator (CCo) in every HomePlug AV adapter. As soon as a HomePlug AV station is connected to the house power grid and supplied with energy, it waits on the medium (power cable) for control information, so-called beacons, from other HomePlug AV stations. If it does not receive anything, it automatically becomes a CCo and communicates this to its surroundings by starting to send out beacons itself.

If a second HomePlug AV station is plugged in, it receives the signals from other beacons and subdivides itself into the existing CCo by adding information about its own characteristics ( MAC address , network key, etc.) to the CCo in the list of recognized stations . Discovered Station List (DSL)).

A CCo controls at least one logical AV Logical Network (AVLN). An AVLN consists of several AV stations (STAs) that all use the same network membership key (NMK). The NMK gives the stations exclusive access to an AVLN so that they can communicate securely and undisturbed. The AV stations in turn send beacons at periodic intervals specified by the CCo. These Discover Beacons contain information about the AV station and the AVLN to which it belongs. Each station that hears a discover beacon enters this information in its own list of discovered stations (DSL). Information from stations from another AVLN is also entered in a second list, the Discovered Network List (DNL). The CCo then queries the stations at periodic intervals, has their DSLs and DNLs transmitted by them and creates a topology map from this.

The central coordinator uses the topology map created from the collected DSL and DNL information to decide whether there is a station somewhere in the network that is better suited as a CCo than himself. The following priority list serves as a decision-making aid:

  1. Selection by the user
  2. Skills of the CCo
  3. Number of discovered STAs in the Discovered Station List
  4. Number of discovered STAs in the Discovered Network List

save energy

If all stations are inactive, the CCo puts the entire AVLN into a power-saving mode . In this state, only a small CSMA area remains active so that the stations can initiate a connection establishment - as well as a small PCF area in the real-time time slices to be able to receive detection and proxy beacons. The stations only have to switch their receivers on during these time periods in order to remain part of an AVLN. The transmitter and receiver can remain switched off for the remainder of the beacon period. This makes it easier for manufacturers to certify their products for the Energy Star program .

Quality of Service

The service relies on periodic time slice management ( Time Division Multiple Access - TDMA), which organizes logical connections in time slices of sufficient duration to meet the respective QoS requirements. The network-wide time control (engl. By periodic beacon Beacons ) possible to the Central Coordinator emits. A beacon period extends over two full 50/60 Hz alternating current periods and lasts 40 or 33⅓ milliseconds.

Encryption

When transmitting data over the power grid, there are no physical limitations such as those given by cables in an Ethernet . That is why (eavesdropping) security must be ensured by other, logical means. Access control measures must ensure that only approved devices can log into a HomePlug AV network (AVLN).

The entire data transport and almost all control traffic in an AVLN are protected by 128-bit AES encryption. Only a few control commands that cannot be encrypted are excluded. The Network Encryption Key (NEK) serves as the key, which is only applied to individual network segments when MPDUs (MAC Protocol Data Unit) are generated. The key assignment can be done in the following ways: The preset key is used, which was programmed into all stations at the factory to give users a plug and play experience during the initial installation. However, this does not provide security because all HomePlug AV adapters have this key. The user defines a network password (NPW) and enters it directly into a new station. A hash algorithm generates the NEK - a 128-bit AES key.

HomePlug AV stations can be members of several logical networks in a physical network. The ability to manage multiple security keys enables HomePlug AV Stations to join multiple AVLNs.

AVLN 1 can, for example, connect an LCD television to the satellite receiver, while an AVLN 2 establishes the connection to the six speakers of the 5.1 surround system, which via AVLN 3 also from an (AV) network player or an IPTV -SetTopBox can be controlled. The PC in the study uses an AVLN 4 to communicate with the notebook in the living room - possibly connected via WLAN - and the storage server in the basement, which via AVLN 3 also serves as a data source for the LCD television.

HomePlug AV can also coordinate several neighboring networks (NN). As soon as the central coordinator (CCo) discovers a neighboring network, it negotiates a schedule with its CCo for the transmission of data in its network so that there is no overlap (interference).

Disturbance of other transmission methods

Powerline modems according to the HomePlug AV specification send the data in the frequency range of 2–28 MHz or 2–68 MHz via unshielded power lines. The powerline signals emitted due to the lack of shielding of conventional electrical installations usually cause massive reception interference in the shortwave range within a radius of 100–500 m around the building installation used.

Products that follow the HomePlug AV specifications use notch filters to minimize interference to avoid interference from other transmission technologies that use the same frequency range (shortwave radio, CB radio, amateur radio, various other radio services). Unfortunately, the lowering of the signal level through intermodulation products is only approx. 20 dB in practice, so that interference with other radio services occurs even in shortwave frequency ranges that are not used by the powerline modem. Corresponding short-wave interference from HomePlug AV-specified products cannot therefore be ruled out.

These disruptions can lead to the operator being requested by the Federal Network Agency to decommission his HomePlug adapter. On certain alternative media, such as coaxial cables, these notch filters can in practice be eliminated if the media itself ensures adequate shielding.

Applied notch filters, f = frequency (in MHz):

  • 00,000 ≤f ≤ 01.71 0- AM broadcast band and below
  • 01.71 0<f < 01.8 00- Between AM and 160 meter tape
  • 01.8 00≤ f ≤ 02.0 00- 160 meter amateur tape
  • 03.5 00≤ f ≤ 04.0 00- 80 meter amateur tape
  • 05.33 0≤ f ≤ 05.407 - 5 MHz amateur band
  • 07.0 00≤ f ≤ 07.3 00- 40 meter amateur tape
  • 10.10 0≤ f ≤ 10.15 0- 30 meter amateur tape
  • 14.0 00≤ f ≤ 14.35 0- 20 meter amateur tape
  • 18.068 ≤ f ≤ 18.168 - 17 meter amateur tape
  • 21.0 00≤ f ≤ 21.45 0- 15 meter amateur tape
  • 24.89 0≤ f ≤ 24.99 0- 12-meter amateur tape
  • 00,000 ≤f ≥ 28.0 00- 10 meter amateur tape

Summary

HomePlug AV enables audio and video as well as Ethernet packets to be distributed over the power lines of a household. The maximum data transfer rate of 200 Mbit / s is sufficient for example for two HDTV signals (66–84 Mbit / s), home theater audio or CD audio in two rooms (9–11 Mbit / s) and six VoIP calls (1 Mbit / s) to be distributed in the home network; and has enough reserves to transmit IP data at 10 Mbit / s. However, like WLAN, HomePlug AV cannot guarantee that the data transfer rates determined under optimal conditions will be achieved in practical use.

Web links

Individual evidence

  1. ↑ HomePlug AV Specification, Version 1.1. May 21, 2007, archived from the original ; accessed on March 31, 2019 (English).