Chapter 9HomePlug 1.0.1 Coexistence

9.1Overview

HomePlug AV STAs are likely to operate in environments containing legacy HomePlug 1.0.1 STAs. HomePlug 1.0.1 STAs transmit in the same frequency band as HomePlug AV STAs, resulting in interference. Hence, while true interoperability is not required, a means for coexistence is essential for both HomePlug AV and HomePlug 1.0.1 STAs to properly operate under such heterogeneous network conditions. HomePlug 1.0.1 STAs do not understand Beacon-based medium access and the interpretation of contention-free and CSMA Regions. To prevent HomePlug 1.0.1 stations from interfering with HomePlug AV transmissions, and to provide predictable behavior for HomePlug AV in the presence of HomePlug 1.0.1 STAs, all HomePlug AV stations shall implement the HomePlug AV Hybrid mode, as defined in Sections 9.3 through 9.7.

HomePlug AV Hybrid Mode not only defers HomePlug 1.0.1 stations from accessing the medium during the periodic contention-free intervals, it also enables fair sharing of the medium between HomePlug AV and HomePlug 1.0.1 CSMA/CA traffic. This section provides details on the HomePlug AV and HomePlug 1.0.1 coexistence. Section 9.2 provides a brief description of HomePlug 1.0.1 behavior relevant to HomePlug AV coexistence.

9.2HomePlug 1.0.1 Behavior

This section describes HomePlug 1.0.1 behavior relevant to HomePlug AV and HomePlug 1.0.1 coexistence.

Note: This section is for informative purpose only and the reader should refer to the HomePlug 1.0.1 specification for detailed information.

9.2.1HomePlug 1.0.1-Prioritized CSMA/CA

HomePlug 1.0.1 uses CSMA/CA access along with priorities. When the medium is perceived to be idle, a station may transmit an MPDU immediately. If the medium is not idle, a station must wait for the current transmission to end, then assert its priority in the Priority Resolution Period (PRP) that follows the transmission.

The PRP consists of two PRSs. Stations normally compete for access to the medium first by asserting their Channel Access Priority (CAP) in the PRP. CA3 is the highest priority and CA0 is the lowest. Each station with CA3 or CA2 traffic asserts a priority signal in the first PRS (PRS0), causing stations with CA1 or CA0 traffic to defer. Stations with CA3 traffic, and those with CA1 traffic that have not deferred, assert a priority signal in the second PRS (PRS1). This signal causes stations with CA2 and CA0 traffic to defer. A station that hears a higher CAP during the PRP ceases transmission and listens to the medium to receive delimiters that will inform it when the next PRP should occur.

A station that does not detect any stations with higher priority traffic in the PRP can contend for the medium in the contention period that starts immediately following the PRP. In this case, it randomly selects a number of contention slots to wait before transmitting. If it detects another transmission before its turn to transmit, it defers. This is inherently a randomized access to the medium, with no guarantee of success.

9.2.2HomePlug 1.0.1 Carrier-Sensing Mechanisms

HomePlug 1.0.1 uses two carrier-sensing mechanisms to determine activity on the medium:

PCS

VCS

The PCS in HomePlug 1.0.1 is limited to Preamble-sequence detection and provided by the PHY. Due to attenuation and noise, PCS based on received signal strength is unreliable. PCS is used in HomePlug 1.0.1 for early detection of the start of a delimiter, so that stations will attempt to receive the delimiter instead of transmitting. Determination of the medium status during the PHY Body of an MPDU is beyond the capability of PCS, and must be performed logically in the MAC using VCS.

VCS uses the information available in the Frame Control fields of the delimiters to predict future states of the medium. The Frame Length field of the SOF delimiter is used to predict when the corresponding End-of-Frame (EOF) delimiter is expected. If the SOF indicates that a response is expected, Frame Length also yields the time when the response delimiter (ACK, NAK, or FAIL) should occur, as this is a fixed time after the EOF. From the SOF until the EOF is expected, the STA attempts to receive a frame from the medium and does not search for another delimiter. When an EOF or a response delimiter is expected, the STA searches for the appropriate delimiter.

Table91: Receiver Actions on Receipt of HomePlug 1.0.1 Delimiters

Delimiter Type Received / Actions at Receiver*
SOF without Response Expected / Attempt to receive PHY body for time given by frame length
Attempt to receive EOF at time computed from frame length
If EOF received, extract CAP
Else ignore
Start PRP at time computed from frame length
SOF with Response Expected / Attempt to receive PHY body for time given by frame length
Attempt to receive EOF at time computed from frame length
If EOF received, extract CAP
Else ignore
Attempt to receive response at time computed from frame length
If response received, extract CAP
Else ignore
Start PRP at time computed from frame length
EOF without Response Expected / Start PRP after CIFS
EOF with Response Expected / Attempt to receive response after RIFS
If response received, extract CAP
Else ignore
Start PRP at CIFS after response should have appeared
Response (ACK/NAK/FAIL) / Start PRP after CIFS

* The transmitter behaves differently; if an expected response is not received, it enters EIFS.

After a transmission (the end of the EOF if no response is expected, or the end of the response if one is expected), there is a delay of CIFS. Following the CIFS is the PRP as mentioned above, which is followed by the Contention Window (CW). The CW consists of a sequence of Contention Slots (CSs), in which a station that won the PRP can start transmission. The CW extends for the maximum MPDU transmission time or until the first transmission starts, whichever comes first.

If a station becomes confused about the state of the medium (e.g., after a collision or when it receives an errorful SOF), it enters Extended Interframe Space (EIFS). EIFS is a waiting period equivalent to the maximum MPDU duration to allow a station to resynchronize with other stations on the medium. During EIFS, the station searches for a delimiter that will allow it to reestablish VCS and to begin to participate again.

9.2.3HomePlug 1.0.1 Segment Bursting

The HomePlug 1.0.1 station that has broken a longer transmission into multiple MPDUs may employ segment bursting using the CC mechanism. Here, the transmitting station sets the CC bit in the SOF and the EOF, and the responding station sets it in the response delimiter. Stations with traffic at the same or a lower priority level are not allowed to assert priority in the PRP or to transmit on the medium until CC is reset. Segment bursting allows efficient delivery of long transmissions by eliminating waiting time and collisions.

9.2.4Contention-Free Transmissions

Stations with higher priority traffic may interrupt a burst transmission by asserting their priority in the PRP. A station can only do this if it knows the CAP of the transmission, which is available in the EOF and in the response, but not in the SOF. Consequently, stations must assume that the CAP is CA3 (highest priority) until they learn otherwise from an EOF or a response. If a station does not hear either the EOF or a response, it enters EIFS at the time the next contention period should have begun. Since CAP is only known from the EOF or response, the HomePlug 1.0.1 node will assume that CAP is CA3 and will not assert priority in the PRP. This allows the PRP to be used by HomePlug AV transmissions during contention-free access.

9.2.5Link Status

HomePlug 1.0.1 STAs are required to monitor the medium to ascertain whether they are still connected to the network. If the HomePlug 1.0.1 STA does not receive a valid delimiter for five seconds, it starts a polling process, transmitting Channel Estimation Requests until it either obtains a response or gives up. If it decides that it has been disconnected, it may continue to poll the medium to test for reconnection. To keep inactive HomePlug 1.0.1 STAs quiet, valid HomePlug 1.0.1 delimiters must be sent frequently enough to satisfy the link-status function.

9.3HomePlug AV Coexistence Modes

HomePlug 1.0.1 coexistence is an operating mode for HomePlug AV STAs. The CCo of each AVLN determines the mode of operation of STAs associated with it. There are three modes in which an AVLN can operate:

  1. Fully Hybrid Mode: The AVLN operates in Fully Hybrid Mode if interfering HomePlug 1.0.1 stations are detected. In this mode, all CP and CFP transmissions use Hybrid Mode delimiters (refer to Section 3.2.1.1) to coordinate medium access with HomePlug 1.0.1 stations. In a Fully Hybrid mode AVLN that is operating in Uncoordinated mode, the CCo shall also specify Contention Free Period Initiation (CFPI) allocation(s) to defer HomePlug 1.0.1 stations from accessing the medium during Beacon Region and CFP allocations (refer to Section 9.6.1). In a Fully Hybrid mode AVLN that is operating in Coordinated mode, the CCo should also specify Contention Free Period Initiation (CFPI) allocation(s) to defer HomePlug 1.0.1 stations from accessing the medium during Beacon Region and CFP allocations.

Note: A CCo in Coordinated mode mightnot be able to provide CFPI allocation before the Beacon Region if it is Reserved by its neighboring CCo (refer to Section 9.10).

  1. Shared CSMA Hybrid Mode: The AVLN operates in Shared CSMA Hybrid Mode if interfering HomePlug 1.1 stations are detected, and no interfering HomePlug 1.0.1 stations are detected. In Coordinated Mode, the CCo may also specify a Shared CSMA Hybrid Mode if a Coordinating CCo in the INL specified a Shared CSMA or Fully Hybrid Mode (refer to Section 9.10). In this mode, only the Shared CSMA allocations use hybrid delimiters to coordinate medium sharing.
  2. AV-Only Mode: The AVLN operates in AV-Only Mode if no interfering HomePlug 1.0.1 and HomePlug 1.1 stations are detected. In Coordinated Mode, this further requires that none of the Coordinating CCos detects HomePlug 1.0.1 or HomePlug 1.1 stations. In this mode, all CP and CFP transmissions use AV-Only delimiters.

Regardless of the AVLN’s operating mode, all Beacon MPDUs always use a Hybrid delimiter structure. This choice simplifies the Neighbor Network operations in Coordinated Mode and also enables coexistence with HomePlug 1.1 and Non-HomePlug-based Access Networks (refer to Chapter 10).

In Hybrid Mode, HomePlug AV stations manipulate the fields of HomePlug 1.0.1 Frame Control to keep them synchronized and to control their access to the medium. Since HomePlug 1.0.1 stations use only eight different frame lengths, HomePlug AV MPDU transmissions must be modified to match these lengths. Apart from these, special mechanisms must be used to defer HomePlug 1.0.1 stations from accessing the medium during the Beacon Region and during the CFP allocations.

Coexistence requires HomePlug AV STAs to detect active HomePlug 1.0.1 STAs on the medium, and to change the coexistence modes in response to their presence or absence. HomePlug AV STAs use the presence of delimiters that are only used by HomePlug 1.0.1/1.1 STAs (and never used by HomePlug AV stations in Hybrid Mode) to determine the presence of HomePlug 1.0.1/1.1 STAs. All HomePlug AV STAs shall be capable of detecting HomePlug 1.0.1/1.1 STAs based on HomePlug 1.0.1 delimiters. HomePlug AV STAs that can interoperate with HomePlug 1.0.1/1.1 STAs may also use explicit message exchange to determine the presence of HomePlug 1.0.1/1.1 STAs. All HomePlug AV stations shall be capable of changing their operating mode dynamically based on information provided by the CCo in the Beacon.

9.3.1Detection and Reporting of Active HomePlug 1.0.1 and HomePlug 1.1 STAs

AV STAs shall continuously monitor the medium during the AVLN’s CSMA allocations as well as during their CFP allocations (i.e., CFP allocations for which the STA is either the source or destination of the Global Link) for HomePlug 1.0.1 and HomePlug 1.1 transmissions. The reception of an End of Frame (EOF) delimiter with the 10-bit Reserved Field set to 0x000 shall be considered a valid HomePlug 1.0.1 transmission. HomePlug 1.1 transmissions are identified as described in Section9.8.2.

In HomePlug 1.0.1, the EOF delimiter is transmitted at the end of a HomePlug 1.0.1 Long MPDU. Therefore, when a valid HomePlug 1.0.1 SOF Frame Control is detected along with an invalid AV Frame Control, AV stations shall try to search for an EOF delimiter based on the HomePlug 1.0.1 Frame Length (FL). It is also possible for the EOF delimiter to be detected without a corresponding HomePlug 1.0.1 SOF. Regardless of whether EOF delimiter is detected following a HomePlug 1.0.1 SOF delimiter, it confirms the presence of HomePlug 1.0.1/1.1 station(s).

Note: The Frame Length (FL) field in HomePlug 1.0.1 indicates the duration of the HomePlug 1.0.1 payload. This field is used by AV stations to determine the expected location of the EOF delimiter.

Figure 91 and Figure 92 show the processing for detecting HomePlug 1.0.1/1.1 transmission in AV Only Mode and Hybrid Mode, respectively. For AV Only Mode, Figure 91 assumes that the STA can simultaneously process the received signal for HomePlug 1.0.1 and HomePlug AV Frame Control. Implementations may also use preamble processing to determine whether the preamble is a HomePlug 1.0.1 or an AV preamble and use that information to decide whether a HomePlug 1.0.1 or a HomePlug AV Frame Control needs to be decoded subsequent to the preamble detection.

All AV stations that detected a HomePlug 1.0.1 or HomePlug 1.1 transmission shall report it to the CCo by setting the HomePlug 1.0.1 Detect Flag (HPI0DF) or HomePlug 1.1 Detect Flag (HP11DF), respectively, in the SOF and/or RTS delimiter of all subsequent transmissions they make for a duration of HP1D_ReportDuration. If there are no pending Segments to deliver, the Station may send a stand-alone RTS/CTS delimiter (i.e., RTS/CTS without a Long MPDU to follow).

AV stations shall also maintain statistics on the number of HomePlug 1.0.1/1.1 transmissions that were detected and report them to the CCo using the CC_HP1_DET.CNF message when explicitly requested by the CCo (using CC_HP1_DET.REQ). Stations may also send an unsolicited CC_HP1_DET.CNF message if they detect several HomePlug 1.0.1 transmissions and the network mode is either not changed to Fully Hybrid Mode or if none of the ongoing transmissions reports the detection of HomePlug 1.0.1 stations by setting the HP10DF. Similarly, Stations may send an unsolicited CC_HP1_DET.CNF message if they detect several HomePlug 1.1 transmissions and the network mode is in AV-Only Mode or if none of the ongoing transmissions reports the detection of HomePlug 1.1 stations by setting the HP11DF. HomePlug 1.0.1/1.1 detection statistics shall be reset when CC_HP1_DET.CNF is transmitted in response to a CC_HP1_DET.REQ.

Figure 91: AV Only Mode Processing for Detecting HomePlug 1.0.1 Transmission

Figure 92: Hybrid Mode Processing for Detecting HomePlug 1.0.1 Transmission

9.3.2HomePlug 1.0.1/1.1 Coexistence Mode Changes

The CCo of each AVLN determines the HomePlug 1.0.1 coexistence mode of the AVLN. The Hybrid Mode (HM) Field in the Beacon is used by the CCo to indicate the operation mode of the AVLN.

On start-up, HomePlug AV STAs shall enter Fully Hybrid Mode and listen to the medium for existing activity (in particular, they shall seek the CCo if one is present). If a Beacon from an existing AVLN is detected and the station intends to become part of that AVLN, it shall change its coexistence mode to that of the AVLN. If the STA successfully becomes part of that AVLN, its HomePlug 1.0.1 coexistence operating mode will depend on that AVLN’s operating mode. The STA continues to report HomePlug 1.0.1 and HomePlug 1.1 detection status as described in Section 9.3.1.

If the HomePlug AV station starts a new AVLN in Un-Coordinated Mode, its operating mode shall be based on whether it detected HomePlug 1.0.1 and/or HomePlug 1.1 activity during the startup.

If HomePlug 1.0.1 activity is detected, the AVLN shall start operating in Fully Hybrid Mode.

If HomePlug 1.1 activity is detected and no HomePlug 1.0.1 activity is detected, the AVLN shall start operating in Shared CSMA Hybrid Mode.

If neither HomePlug 1.0.1 activity nor HomePlug 1.1 activity is detected, the AVLN shall start operating in AV-Only Mode.

If the HomePlug AV station intends to start a new AVLN in Coordinated Mode, it initially operates in the same operating mode as that of the coordinating AVLNs. If the AVLN becomes part of the group of Coordinating AVLN, its operating mode is determined by the operating mode of the Coordinating AVLNs, as well as the detection of HomePlug 1.0.1 and/or HomePlug 1.1 activity during startup.

If HomePlug 1.0.1 activity is detected, the AVLN shall start operating in Fully Hybrid Mode.

If HomePlug 1.1 activity is detected and no HomePlug 1.0.1 activity is detected, the AVLN shall start operating in Shared CSMA Hybrid Mode.

If neither HomePlug 1.0.1 nor HomePlug 1.1 activity is detected, and if the Coordinating AVLNs are operating in either Fully Hybrid Mode or Shared CSMA Mode, the AVLN shall start operating in Shared CSMA Hybrid Mode.

If neither HomePlug 1.0.1 activity nor HomePlug 1.1 activity is detected, and Coordinating AVLNs are operating in AV-Only Mode, the new AVLN shall start operating in AV-Only Mode.

The CCo and other stations in the AVLN exchange information to determine the operating mode of the AVLN. When a CCo in AV-Only Mode determines the presence of HomePlug 1.0.1 activity, it shall change the network mode to Fully Hybrid Mode. To allow the AVLN to revert to AV-Only Mode when all HomePlug 1.0.1 nodes become inactive, the CCo maintains a Fully Hybrid Mode Timer that shall be set to FHM_Timeout when it changes to Fully Hybrid Mode. This timer will be continuously reset when new HomePlug 1.0.1 activity is detected/reported while in Fully Hybrid Mode. If the Fully Hybrid Mode Timer expires and no HomePlug 1.1 activity is detected/reported, the CCo shall change to AV-Only Mode.

Similarly, when a CCo in AV-Only mode determines the presence of HomePlug 1.1 activity, it shall change to Shared CSMA Hybrid Mode. To allow the AVLN to revert to AV-Only Mode when all HomePlug 1.1 nodes become inactive, the CCo maintains a Shared CSMA Hybrid Mode Timer that shall be set to SHM_Timeout when it changes to Shared CSMA Hybrid Mode. This timer will be continuously reset when new HomePlug 1.1 activity is detected/reported while in Shared CSMA Hybrid Mode. If the Shared CSMA Hybrid Mode Timer expires and no HomePlug 1.1 activity is detected/reported, the CCo shall change to AV-Only Mode.