The HomePlug AV MAC is a hybrid of time-division multiple access (TDMA) and carrier-sense multiple access (CSMA). Each network has a designated device, the central coordinator (CCo) that coordinates access to the medium by periodically generating a beacon with information on TDMA and CSMA allocation. TDMA is used by applications requiring QoS, while CSMA is used for elastic applications. Beacon period in HomePlug AV is synchronized to the underlying ac line cycle and is equal to twice the ac line cycle period. Thus, HomePlug AV has a beacon period of 33.33 ms and 40 ms in countries with 60 Hz and 50 Hz AC line cycle frequencies, respectively. Figure 4 shows the beacon period structure in HomePlug AV.

HomePlug AV uses a high level of MAC-PHY cross layer design to cope with the unique characteristics of powerline channels. Important aspects of the HomePlug AV MAC layer design are:

1. efficient two-level MAC framing to overcome impulse noise;
2. ac line cycle-based channel adaptation;
3. dynamic TDMA to handle changing channel conditions;
4. persistent and non-persistent schedules to handle beacon loss; and
5. 128-bit AES based encryption for privacy.

Impulse noise is a common impairment over powerlines. It is handled in HomePlug AV by a combination of aggressive channel adaptation at the PHY level and efficient retransmission at the MAC level. Impulse noise power is typically greater than the signal power level, and is broad spectrum, so it will disrupt one or more PHY symbols per event. Adapting the channel at the PHY layer to overcome this form of impairment would degrade the data rate, and is not feasible. Since there are often several undisrupted symbols between events, adapting to the highest rate that these good symbols will support provides better net throughput. However, this hinges on the ability to retransmit efficiently only the badly damaged portions of each data frame, which is realized by a two-level MAC framing method^[3] with selective acknowledgments.

Noise on powerlines varies with the ac line cycle. The achievable PHY rate at the cleanest portion of the line cycle may be more than 50% higher than that of the noisiest portion. MAC structures used in HomePlug AV facilitates dividing the ac line cycle into several regions for channel adaptation purposes. Synchronization of the beacon with ac line cycle enables static channel conditions for periodic TDMA allocations.

Powerline channel characteristics also can change when electric appliances turn on and off. This may cause an allocation that once was sufficient to fail to satisfy the throughput requirements of a given application. To ensure that QoS guarantees are maintained under such conditions, HomePlug AV TDMA allocations are dynamic. The source of each TDMA stream includes information on the buffer backlog and the physical layer data rates in the frame control of each data frame that it transmits. This information enables the CCo to update allocations as channel conditions or source rates change.

Powerline noise could result in the loss of a beacon. A station losing a beacon will enter listen mode, resulting in reduced throughput. To alleviate this problem, HomePlug recommends beacon persistence^[4]. By doing so, the beacon information is valid over multiple beacon periods. This allows the station to withstand occasional beacon losses.

Powerline signals can propagate from one residence to another. Security mechanisms built on 128-bit AES enable a high level of security. While there may be many stations that can communicate with each other, only stations that share a common network encryption key (NEK) can exchange data packets. To distribute the NEK, which is changed at least hourly, the stations in an AV logical network (AVLN) share a common network membership key (NMK). The NMK is a key distribution key that allows stations to obtain the NEK. Stations obtain the NMK through direct entry of a network password from which the NMK is generated using SHA-256; distribution using the device access key (DAK), unique to each station; or distribution using the less secure unicast key exchange (UKE) protocol^[5].

The maximum PHY data rate that can be achieved by HomePlug AV is 150 Mbps and 189 Mbps with 917 carriers (e.g., in North America) and 1155 carriers, respectively. While these are theoretical maximum values, the actual performance in houses will depend on the size of the house and the types of impairments present. In field tests conducted in five houses (with a total of 240 paths tested), an average PHY data rate of 80 Mbps was obtained. In 80% of the paths tested, the PHY data rate was greater than 60 Mbps.

The efficiency of TDMA-based UDP traffic over HomePlug networks can be as high as 85% (depending on the application data rate). Thus, the average UDP throughput based on field tests is 68 Mbps. Furthermore, the 80% paths minimum UDP throughput is 51 Mbps.

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