1. Contention-Based MAC Protocols with Reservation Mechanisms

1. Contention-Based MAC Protocols with Reservation Mechanisms

The dynamic reservation approach involves the setting up of some sort of a reservation prior to data transmission. If a node that wants to send data takes the initiative of setting up this reservation, the protocol is considered to be a sender-initiated protocol. Most schemes are sender initiated. In a receiver-initiated protocol, the receiving node polls a potential transmitting node for data. If the sending node indeed has some data for the receiver, it is allowed to transmit after being polled. The MACA by invitation (MACA-BI) and receiver-initiated busy tone multiple access (RI-BTMA) are examples of such schemes.
1.1. Multiple Access Collision Avoidance (MACA)
MACA is a protocol for slotted media access control used in wireless LAN data transmission. MACA is used to avoid data collisions caused by the hidden and exposed terminal problems.
In MACA, a wireless network node announces that it is going to send the data frame, informing the other nodes to remain silent. When a node intends to transmit the data frame, it communicates using a signal known as Request-To-Send (RTS) that includes the length of the data frame to transmit. If the recipient permits the transmission, it responds back to the sender with a signal known as Clear-To-Send (CTS), which includes the length of the data frame that it is about to receive.
In the meantime, the nodes that listen to the RTS signal must remain silent until the data is fully transmitted in order to avoid conflict with CTS. Collisions among RTS packets may still occur in MACA, but they are minimized using a randomized exponential back-off strategy, much like the one that is used in regular Carrier Sense Multiple Access (CSMA).
It is apparent that this RTS-CTS exchange enables nearby nodes to reduce the collisions at the receiver, not the sender. Collisions can still occur between different RTS packets, though. If two RTS packets collide for any reason, each sending node waits for a randomly chosen interval before trying again. This process continues until one of the RTS transmissions elicits the desired CTS from the receiver. MACA is effective because RTS and CTS packets are significantly shorter than the actual data packets, and therefore collisions among them are less expensive compared to collisions among the longer data packets.
Weaknesses of MACA:
When hidden terminals are present and the network traffic is high, the performance of MACA degenerates to that of ALOHA.
MACA does not provide any acknowledgment of data transmissions at the data-link layer. If a transmission fails for any reason, retransmission has to be initiated by the transport layer. This can cause significant delays in the transmission of data.
WLAN data transmission collisions can still happen, and Multiple access with collision avoidance for wireless (MACAW) is a slotted MAC protocol widely used in ad hoc networks. MACAW is brought to extend the functionality of MACA. It demands nodes to send acknowledgments after every successful frame transmission. MACAW is commonly used in ad hoc networks. Moreover, it is the basis of various other MAC protocols found in wireless sensor networks (WSN).
2. Power-Aware or Energy-Efficient MAC Protocols
It is crucial to preserve energy and make use of power efficiently as mobile devices are battery power driven. Power preservation is considered across all the layers of the protocol stack. The following are the guiding principles for power conservation in MAC protocols:
1. Collisions should be avoided since they are the major basis of expensive retransmissions.
2. The transceivers consume most energy in active mode, so they should remain in standby mode (or switched off) whenever possible.
3. The transmitter should be controlled to a lower power mode instead of maximum power because that is enough for the destination node to obtain the transmission.

2.1. Dynamic Power-Saving Mechanism (DPSM)
The concept of sleep and wake states of nodes is used in DPSM for preserving power. It is a type of the IEEE 802.11 scheme where it uses dynamically sized ad hoc traffic indication message (ATIM) windows for achieving longer dozing times of nodes.
The IEEE 802.11 distributed coordination function (DCF) mode is a power saving mechanism where time is divided into beacon intervals used to synchronize nodes. At the commencement of each beacon interval, all nodes must stay wakeful for a fixed period of time known as the ATIM window, which announces the status of packets ready for transmission to the receiver nodes. These announcements are made through ATIM frames and are acknowledged via ATIM-ACK packets during the same beacon interval. Figure 1 shows the method. Performance suffers in terms of throughput and energy consumption if the size of the ATIM window is kept fixed. Each node dynamically and independently chooses the length of the ATIM window in DPSM.

Figure 1 Power-saving mechanism for DCF.

Node A announces a buffered packet for B using an ATIM frame. Node B replies by sending an ATIM-ACK and both A and B stay awake during the entire beacon interval. The actual data transmission from A to B is completed during the beacon interval; since C does not have any packet to send or receive, it dozes after the ATIM window.