## 4 MAC Layer in WMSN
### Scheduling and Admission Control  
Scheduling, admission control, and buffer management in WMSNs is an open research issue that has attracted the research community in last years but still not really solved. As an starting example, the work in [19] makes some strong assumptions to be considered: on one hand, it uses a coordinated channel scheduling that assumes a _relatively static_ network to overcome the contention overhead incurred by multi-channel MAC protocols. On the other hand, it argues that a _flat_ structure is not suitable for WMSNs. The protocol operation is divided into three sessions (_Request_, _Scheduling_ and _Data Transmission_) and only a heuristic is provided to calculate the scheduling to be used based on priorities. In the cluster-based MAC protocol, COM-MAC [19], a scheduled multi-channel medium access protocol is considered within each cluster where the cluster head coordinates the communication among its members in a contention free manner within both the time and frequency domains. By this \begin{table}
\begin{tabular}{|p{113.8 pt}|p{113.8 pt}|p{113.8 pt}|p{113.8 pt}|p{113.8 pt}|} \hline MAC Protocol & (Single/Multi)-Channel & Contention-(based/free) & Diff. Service & Topology & Cross-Layering \\ \hline \hline T-MAC [21] & single & free (scheduling-based) & no & clustered & no \\ \hline S-MAC [20] & single & free (scheduling-based) & no & clustered & no \\ \hline B-MAC [25] & single & based & yes & flat & yes \\ \hline MMAC [26] & multi & based (IEEE 802.11) & no & flat & no \\ \hline MMSN [27] & multi & based & no & flat & no \\ \hline COM-MAC [19] & multi & free (scheduling-based) & yes & clustered & - \\ \hline Diff. Service Model [28] & - & - & yes & flat & - \\ \hline MAC Protocol in [22] & single & based (CSMA) & yes & flat & no \\ \hline Cross-layer Architecture in [24] & multi (UWB) & free & yes & flat & yes \\ \hline EQ-MAC [29] & single & based (collision-free) & yes & flat (static) & yes \\ \hline Node Admission [30] & single & free (TDMA) & no & flat & yes \\ \hline UWB Technology in [16] & multi & free & yes & flat & yes \\ \hline \end{tabular}
\end{table} 
Table 2: A Comparison between MAC Layer Protocols. Grey rows indicate that the MAC protocol is designed for WSNs but not specifically for WMSNs. way, the nodes, which are assumed to have multiple transceivers being able to operate on a set of available channels simultaneously, avoid collision, idle listening and overhearing problems. The cluster head dynamically allocates time slots and channels for its members according to the current QoS requirements and network traffic status. One work that attacks the scheduling and buffer management problem is [28] based on the fact that different network applications need different QoS requirements such as packet delay, packet loss, bandwidth and availability. And this can be done not by increasing network capacity (as COM-MAC [19]), but by developing a network architecture which is able to guarantee QoS requirements for high priority traffic. It argues that the sensor networks should be willing to spend more resources in disseminating packets that carry more important information by using a differentiated service model for WMSNs. The proposed model can support two major different types of traffic classes: real time class (Expedited Forwarding or EF) and non real time traffic (Assured Forwarding or AF) which is divided into three classes: high priority AF1, medium priority AF2, and low priority AF3. In this model, real time traffic is buffered in a separate queue with low buffer size while non real time traffics are managed by using random early detection (RED) queue management in separate queues also. The delay performance of different traffic classes is evaluated by using two scheduling mechanisms: Priority Queuing (PQ) and Weighted Round Robin (WRR).