Problems in Ad Hoc Channel Access Hidden Terminal Problem : Two nodes are said to be hidden from one another (out of signal range) when both attempt to send information to the same receiving node, resulting in a collision of data at the receiver node.

To avoid collision, all of the receiver's neighboring nodes need to be informed that the channel will be occupied. This can be achieved by reserving the channel using control messages, that is, using a handshake protocol. An RTS (Request To Send) message can be used by a node to indicate its wish to transmit data. The receiving node can allow this transmission by sending a grant using the CTS (Clear To Send) message.

Because of the broadcast nature of these messages, all neighbors of the sender and receiver will be informed that the medium will be busy, thus preventing them from transmitting and avoiding collision.

Exposed Node Problem Overhearing a data transmission from neighboring nodes can inhibit one node from transmitting to other nodes. This is known as the exposed node problem. An exposed node is a node in range of the transmitter, but out of range of the receiver. Leads to Wasted bandwidth.

Another solution : A mobile node using an omni-directional antenna can result in several surrounding nodes being "exposed," thus prohibiting them from communicating with other nodes. This lowers network availability and system throughput.

Alternatively, if directional antennas are employed, this problem can be solved. As in diagram node C can continue communicating with the receiving palm pilot device without impacting the communication between nodes A and B. The directivity provides spatial and connectivity isolation not found in omni-directional antenna systems

MAC protocol can be categorized as synchronous or asynchronous in operation, it can also be distinguished by who initiates a communication request

Receiver-Initiated MAC Protocols The receiver (node B) first has to contact the sender (node A), informing the sender that it is ready to receive data. This is a form of polling, as the receiver has no way of knowing for sure if the sender indeed has data to send.

This is also a passive form of initiation since the sender does not have to initiate a request. In addition, there is only one control message used, compared to the RTS-CTS approach

Sender-Initiated MAC Protocols Contrary to receiver-initiated MAC protocols, sender-initiated MAC protocols require the sender to initiate communications by informing the receiver that it has data to send. Examples of these sender-initiated protocols include MACA (Multiple Access with Collision Avoidance)

node A sends an explicit RTS message to node B (the receiver) to express its desire to communicate. Node B can then reply if it is willing to receive data from node A. If positive, it returns a CTS message to node A. Node A then subsequently proceeds to send data

Existing Ad Hoc MAC Protocols

Multiple Access with Collision Avoidance (MACA) MACA aims to create usable, ad hoc, single-frequency networks. MACA was proposed to resolve the hidden terminal and exposed node problems. It also has the ability to perform per-packet transmitter power control, which can increase the carrying capacity of a packet radio.

MACA uses a three-way handshake, RTS-CTS-Data. The sender first sends an RTS to the receiver to reserve the channel. This blocks the sender's neighboring nodes from transmitting. The receiver then sends a CTS to the sender to grant transmission.

This results in blocking the receiver's neighboring nodes from transmitting, thereby avoiding collision. The sender can now proceed with data transmission

MACA has power control features incorporated. The key characteristic of MACA is that it inhibits a transmitter when a CTS packet is overheard so as to temporarily limit power output when a CTS packet is overheard. This allows geographic reuse of channels.

For example, if node A has been sending data packets to node B, after some time, A would know how much power it needs to reach B. If node A overhears node B's response to an RTS (i.e., a CTS) from a downstream node C, A need not remain completely silent during this time. By lowering its transmission power from the level used to reach node B, node A can communicate with other neighboring nodes (without interfering with node B) during that time with a lower power.

In MACA, the slot time is the duration of an RTS packet. If two or more stations transmit an RTS concurrently, resulting in a collision, these stations will wait for a randomly chosen interval and try again.

Compared to CSMA, MACA reduces the chances of data packet collisions Compared to CSMA, MACA reduces the chances of data packet collisions. Since control messages (RTS and CTS) are much smaller in size compared to data packets, the chances of collision are also smaller.

MACA-BI (By Invitation) It uses only a two-way handshake. There is no RTS. Instead, the CTS message is renamed as RTR (Ready To Receive). In MACA-BI, a node cannot transmit data unless it has received an invitation from the receiver.

RTS-CTS handshake is now reduced to a single CTS (CTS-only) handshake after the first hop, and the reduction in the control overhead is hence a function of route length.