## 3 Proposed Design
### Proposed Solution  
Following the discussion in Section 3.2, it is observed that wireless biomedical sensors generate a large number of small packets in a limited time period which poses challenges for the MAC layer, especially on channel access process. The main issue is the involvement of overheads for a single tiny frame transmission. These overheads include the waiting time until a successful channel access and the underutilization of the channel after successful channel access. In the case of the IEEE 802.15.4 slotted mode, the waiting time consists of the random backoff exponent generated time, CCA, CW, acknowledgement waiting time, SIFS time and turnaround time. The underutilization of the channel refers that a single frame in the case of patient monitoring applications consists of 2 bytes data with minimum 9 bytes MAC header and 6 bytes PHY header and transmits 17 bytes on a channel which provides the maximum capacity of 133 bytes including MAC and PHY headers. To address these issues, we propose to use a frame aggregation mechanism, which sends multiple MAC frames under a single PHY header within a single successful channel access. The concept of frame aggregation is successfully used in IEEE 802.11n [43,44] and makes a huge difference in throughput and delay performance by efficiently utilization in channel access mechanism. This helps to reduce the overheads, i.e., the waiting time before a successful channel access and underutilization of the channel capacity. Figure 5 presents the frame aggregation mechanism in detail. As in a single channel, access multiple frames are transmitted which means we send multiple frames in the cost of a single frame overhead with lesser PHY header overhead. Moreover, aggregating the frame under a single PHY header almost fully utilizes the channel. In the following section, we present the proposed design and its evaluation and validation. Figure 5 presents the design of the frame aggregation mechanism at the MAC layer. Initially, the data is received in the form of MSDUs. After applying the MAC headers, it becomes MPDU and ready for the physical layer. The combination of multiple MPDUs creates aggregated MPDU (A-MPDU). A-MPDU is generated before passing to the PHY layer for final transmission. The MAC does not wait for a certain number of MPDUs to create A-MPDU, so if a node gets a channel access the MAC takes available MPDUs to make A-MPDU for transmission. The destination of the all MPDUs must be same. The maximum size for an A-MPDU frame must not exceed than 127 bytes. Each encapsulated MPDU  
Figure 4: ECG frame structure. Figure 5: Frame aggregation mechanism. consists of a delimiter at the start and padding bits at the end. The purpose of the delimiter is to define a separation pattern of individual MPDU in A-MPDU. The delimiter represents the information including MPDU length, reserved bits (first four bits for future use), cyclic redundancy checks (CRC) and unique pattern. MPDU Length field represents the length of the MPDU. CRC design uses reserved and MPDU length fields for its calculation. The unique pattern is helpful to find the next delimiter. Figure 6 presents the delimiter structure. In this aggregation mechanism, selective retransmission is possible due to the presence of an individual frame check sequence (FCS) for each MPDU. All the MPDUs in an A-MPDU have the same traffic identifier (TID) to effectively work with block acknowledgement (BA) mechanism. In the de-aggregation process, the receiving node first validates the CRC integrity, in the case of successful CRC check, the A-MPDU is de-aggregated and data is passed to the application layer. The block acknowledgement mechanism is widely used with IEEE 802.11n to support the frame aggregation mechanism. The IEEE 802.15.4 already gives the option to use a block acknowledgement [37]. The A-MPDU are capable of enhancing channel access performance significantly; however, in the wireless environment, the chances of frame error get high by increasing the frame size.