Media Access Control timeout

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Timeout Media Access Control ( T-MAC , “ Timeout Media Access Control ”) is a network protocol for sensor networks . It changes the sensor network protocol Sensor Media Access Control (S-MAC) in order to be more economical in energy consumption.

Network protocols define down to the smallest detail how the computers in a computer network exchange data with one another. Sensor networks consist of tiny, wirelessly communicating sensor nodes that are deployed in large numbers in an area, network independently and monitor their surroundings in group work with sensors until their energy reserves are exhausted. They thus form a special form of the mobile ad hoc network and place completely different requirements on a network protocol than, for example, the Internet.

T-MAC was presented in 2003 by Tijs van Dam and Koen Langendoen from the Technical University of Delft .

Protocol structure

Sensor nodes can switch to standby mode in which they switch off all components except for the internal clock. A central concern of sensor network protocols is to maximize the duration of these sleep phases in order to save energy. Like its predecessor, the S-MAC, T-MAC relies on a fundamentally regular alternation between waking and sleeping phases. The synchronization scheme adopted by S-MAC means that groups that are as large as possible are formed in the network that follow the same daily rhythm.

Unlike S-MAC to sensor nodes, however, lay in T-MAC bed earlier if it is within a specified time TA no activation event (Engl. Event activation ) as a news announcement or the release of the communication medium for its own transmissions perceive. This timeout gave the protocol its name. With T-MAC, all nodes in a group wake up at the same time, but go to sleep at different times. The length of the time period TA must be tailored to the rest of the circumstances of the protocol to ensure that the nodes do not deactivate before other sensor nodes have been able to establish contact.

Future send request

T-MAC protocol. A wants to send to B, C to D. Immediately after B's CTS, C sends his FRTS. Thick bars mark the waking phases. Sent signals spread locally in all directions, here indicated one level up and down.

S-MAC follows the rendezvous scheme for data transmission . In this case, the transmitter sends a transmission request (RTS, request to send ), which from the receiver with a transmit enable (CTS clear to send is confirmed). The transmitter then transmits its data piece by piece in the form of data packets . Every packet is confirmed by the recipient with an acknowledgment of receipt (ACK, acknowledgment ). If a data packet does not arrive, the ACK is missing and the sender sends the last data packet again. Both the send request and the send confirmation contain the expected duration of the subsequent data transmission so that listening nodes can estimate the likely end of communication.

T-MAC supplements this scheme with a future send request (FRTS, future request to send ) (see figure). If a sensor node receives RTS and CTS from neighboring nodes, it would, as a consequence, remain silent in accordance with the classic scheme so as not to disrupt the communication that was started. With T-MAC, such a node has the option of sending a future send request to its desired conversation partner immediately after the CTS that has been listened to. This FRTS informs the addressee not only about the existing contact request, but also about the duration of the other communication. Both the transmitter and the receiver of the FRTS can then enter a precisely measured sleep phase.

In order to make the FRTS possible in the first place, the start of the data transmission is delayed after the CTS. To do this, the sender first sends a data signal (DS, data send ) that does not contain any user data, but assures the receiver that the actual data transmission will begin soon. Depending on network topology DS and FRST the recipient can collide , d. H. overlap in the communication medium and make each other illegible; however, this is irrelevant since the recipient does not have to rely on the content of the messages. The purpose of the DS packet is to reserve the medium for the period between the receipt of the CTS and the start of the actual data transmission in order to prevent the medium from being taken over by another node.

The two sensor nodes agreed with the help of the future transmission request wake up at an agreed time and in turn start the rendezvous maneuver.

Prioritization through request rejection

T-MAC proposes a second innovation that enables the sensor nodes to take their own priorities into account. The nodes collect messages to be sent in a message outbox until they have the opportunity to send their data. Depending on the amount of data and the size of the available memory, it is possible that this message outlet will fill up. If this is the case, the sensor node will prefer to send messages instead of receiving them. However, the node can only send messages after it has successfully submitted a send request.

According to T-MAC's second proposal, sensor nodes have the option of rejecting a send request sent to them if they prefer to send data themselves instead. To do this, they ignore the received send request and immediately send a send request. The sender of the first send request perceives this second send request and sees from it that the person addressed initially has more important things to do; he therefore refrains from further inquiries.

The developers themselves noticed that caution should be exercised with this second innovation. If there is a high volume of data in the network, it is likely that the message outlets of several nodes will run full at the same time. As a result, too many nodes would reject requests, which would greatly reduce the data throughput or even bring it to a standstill.

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  1. ↑ s. Main source