## 2 Mobile M2M Literature Review
### M2M Support in Wireless Networks  
M2M devices using radio technologies will face well-known problems from wireless and cellular networks. Potential issues on the air interface including channel interference, channel quality fluctuation, and noise will be very common due to the multitude of devices and the characteristics of M2M traffic [12; 21], and they can introduce coordination problems in the medium access. According to [PERSON] _et al_. [22], reliability is critical for general acceptance of M2M, since unreliable processing, sensing, or transmission leads to false or lost data, and ultimately to M2M communications' failure from the user's perspective. Although end-to-end service reliability is still a challenge, it is being addressed by standardization efforts.  
As the number of devices competing for the same channel increases, the number of simultaneous accesses will increase, and packet collisions, and signal interference in general, will be more common and result in more packet/data loss. Optimizing the uplink channel access and radio resource allocation is a way to achieve further improvement in performance and resource usage efficiency, avoiding constant transmission deferrals originated from the packet collision avoidance mechanisms and data loss originated from packet collisions, or providing general QoS guarantees. [PERSON] _et al_. [23] introduce contention-based MAC protocols for sensor-to-gateway communications in wireless M2M, and analyse them in terms of delay and energy efficiency. The authors consider an M2M wireless network composed of a large number of devices that periodically wake up their radio interfaces to transmit data to a coordinator, that is, to a gateway. [PERSON] _et al_. [12] propose a joint rate and admission control scheme for QoS provision in M2M communications, using an IEEE 802.11 network, by exploiting heterogeneous networks and accurate predictions of QoS. Wireless networks usually use solely collision avoidance mechanisms, which introduce well-known problems, such as the hidden node problem or the exposed node problem, that other networks do not face, such as cellular networks. Further work needs to be carried using wireless networks, in order to take advantage of the high data rate and low latency common in those networks.  
Techniques that efficiently aggregate the data to be transmitted can be explored to further optimize bandwidth utilization and energy consumption in M2M communications. Two data aggregation schemes based on the Karhunen-Loeve transform for M2M in a wireless network are proposed by [PERSON] _et al_. [24]. Their system includes several sensors, one gateway, and one application server. The sensors transmit the data to the gateway, which transmits all the data to the application server. While gateway-to-application server communications use a reservation-based MAC protocol, the sensor-to-gateway communications use a contention-based MAC scheme and, thus, packet collisions may occur. They determine the optimal duration of the sensor-to-gateway and gateway-to-server transmission phases, in such a way that the best trade-off between the number of packet collisions and compression level from data aggregation is attained.