# A Cooperative MAC Protocol for a M2M Heterogeneous Area Network
## 5 M2M Heterogeneous Area Network
### The Blank Burst Algorithm  
It is possible to transmit WLAN packets and 6 LoWPAN packets simultaneously in the same area. However, a WLAN packet transmission can adversely affect 6 LoWPAN packet transmissions owing to inter-network interference. To tackle this issue, we introduce a silence or blanking period that suspends the 6 LoWPAN transmission during a WLAN packet transmission by using a cooperative MAC technique. The Blank Burst (BB) algorithm enforces a silence period on 802.15.4 devices and routers, thus, allowing the 802.11 transmitter to use the shared transmission channel in an interference free mode. In order to inform the end devices of this silence period, the 6 LoWPAN superframe beacon is used where a BB signaling packet is inserted in the beacon field. We refer to this signaling mechanism as the Blank Burst (BB), which is generated and disseminated by the MFDRR. The BB packet transmission timing is shown in Figure 8.  
Figure 7: The MFDRR protocol stack.  
When 6 LoWPAN payloads arrive at the MFDRR's aggregation buffer, these payloads are aggregated into WLAN packets. Once the buffer length reaches a predefined threshold value, the BB control signal is triggered at time \(T1\) as shown in Figure 8. A blank burst request is launched from the WLAN application layer which is relayed to the 6 LoWPAN MAC layer as illustrated in Figure 7. The length of the silence period \(T_{BB}\) is calculated using Equation (4), and included in the BB signal packet. At this point of time, the 6 LoWPAN MAC layer sends a blank burst feedback signal to the WLAN application layer, indicating that the 6 LoWPAN MAC layer is transmitting the BB signal to end users in the next superframe. The BB signal packet is transmitted from the MFDRR which is relayed by the 6 LoWPAN routers to end devices. Upon receiving the BB signaling packet at time \(T2\), end devices move to sleep mode for the \(T_{BB}\) duration. Meanwhile, the 6 LoWPAN packets are aggregated into WLAN packets in the MFDRR which, in turn, are transmitted to the data sink at time \(T2\). The 802.11 interface of the MFDRR uses the CSMA/CA protocol using the distributed coordination function (DCF) to transmit WLAN packets. In a single MFDRR-based network, the 802.11 packets will not experience any collision and, hence, they will be transmitted with a minimum delay. After the completion of this blank burst at time \(T3\), the end devices wake up and resume data packet transmission.  
\[T_{BB}=N\left(T_{DIFS}+T_{backoff\_min}+L_{agg}+T_{SIFS}+T_{ACK}\right) \tag{4}\]  
Equation (4) is used to calculate the blank burst duration where the DCF technique is used to transmit WLAN packets using the CSMA/CA protocol. In Equation (4) \(T_{DIFS}\) denotes the distributed interframe spacing (DIFS); \(T_{backoff\_min}\) denotes the minimum back-off delay; \(L_{agg}\) denotes one WLAN packet transmission time including headers and payload, and \(T_{SIFS}\) denotes short interframe spacing (SIFS). \(T_{ACK}\) represents the acknowledgement packet transmission delay. \(N\) denotes the number of WLAN packets transmitted per BB duration.