GOOD2USE Knowledge Network PC Powerline Adapter Operating Basics

GOOD2USE Knowledge Network PC Powerline Adapter Operating Basics

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In order to efficiently handle the transmission and receipt of data, there is logical separation between the control and data-handling mechanisms. In enterprise networking, this often referred to as "control plane" and "data plane," respectively. The main control mechanism is a process called the Central Coordinator. It is accompanied by a control process called the Connection Manager.
These two canbe thought of as the chief executive officer, responsible for making decisions, and the chief operations officer, responsible for executing those decisions. Information flow is not just one way, however, as the Connection Manager feeds data back to the Central Coordinator so future decisions can be adjusted as necessary.

The Connection Manager has two sub-functions - MAC and PHY.
First, there is the Physical Layer, which is referenced in the OSI Model as Layer 1 and in Powerline parlance as PHY. The PHY handles management of the channel and information rates, reported in the HomePlug AV spec as 200 Mb/s and 150 Mb/s, respectively. The 50 Mb/s gap results from the Shannon-Hartley theorem. There is a set speed for maximum channel capacity through which information can be transmitted without having to correct for errors. This speed in the HomePlug AV spec is that 200 Mb/s rate. Losing 50 Mb/s is a result of the need to compensate for transmission errors.

Performance at the PHY is achieved via windowed Orthogonal Frequency Division Multiplexing (OFDM) and Turbo Convolutional Code (TCC). IEEE 1901 is the standard that covers power line networking and allows for OFDM or Wavelet modulation for the PHY.

As to the many devices, ODFM splits the available bandwidth into a lot of different channels, multiple channels can be used for different things at 1 time. Device A and B are talking on channel 1 while device A and C are talking on channel 2, etc. OFDM is a channel-management mechanism that can split the available spectrum into sub-spectrum sizes for data transmission. The key benefit in OFDM is multi-path transmission. It's not necessary to wait for a particular lane to clear because there are multiple lanes from over which data can be sent.
OFDM's efficiency is why it is used as the channel-management mechanism in Wi-Fi specifications 802.11g/n/ac instead of Direct-Sequence Spread Spectrum (DSSS), used in 802.11b.

OFDM is also the basis for 4G LTE, WiMAX, and other multiple access systems. .

The figure left (click to enlarge) shows the MAX2992 powerline communication (PLC) baseband modem delivers half-duplex, asynchronous data communication over AC power lines at speeds up to 300kbps (full FCC band data rate). The MAX2992 is a system-on-chip (SoC) that combines the physical (PHY) and media access control (MAC) layers

Following the PHY is the Media Access Control (MAC) layer, referenced in the OSI Model as Layer 2. It's at this layer Quality of Service (QoS) features are handled via Time Division Multiple Access (TDMA) and Collision Sense Multiple Access/Collision Avoidance (CSMA/CA).
This layer is also where the Central Coordinator asserts order throughout the Powerline network through the use of three control regions: Beacon, CSMA and Contention-Free.

First, the Central Coordinator sets a Beacon Period in which it broadcasts a schedule to all Powerline adapters, instructing each node its permitted time frame to send traffic, whether that traffic is Contention-Free or CSMA.
When establishing the Beacon Period, the Central Coordinator syncs it to the AC Line Cycle, which is when the AC current "wave" is pulsed down the wire. Each node then specifies through the Contention-Free region its QoS requirements to meet traffic demand.

If the Central Coordinator can handle the request, it instructs the Powerline adapters to choose the transmission frequency.
This "Tone Map", along with an estimation of channel usage, is sent to the Central Coordinator so it can determine the lifespan of the connections. When persistent bandwidth isn't required, perhaps for interactive types of traffic (think telnet or ssh), allocated time in the Beacon Period may be used by a Powerline adapter to send traffic using CSMA.

Because the QoS mechanisms are reliant on timing, once the Central Coordinator broadcasts the Beacon packet, the MAC synchronizes his swatch upon inspection of the associated timestamp.

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