In this fourth article in our technical discussion on the WirelessHART protocol, we look at the advantage of time division multiple access (TDMA) found in WirelessHART over carrier sense multiple access (CSMA). Both methods of allowing multiple nodes to communicate on a single channel are implemented at the MAC sub-layer of the OSI model data link layer.
Being polite vs. taking turns
CSMA can be compared to a diner conversation. Each participate waits for a lull in the conversation before speaking. If two or more diners begin speaking at the same time they will stop and wait a random time before beginning again. Whoever starts speaking first after this pause gains control of the conversation.
In a CSMA implementation such as Zigbee, each node listens to the channel to see if it is active before sending. If it starts sending and detects a collision, it sends a collision signal and waits a random amount of time before trying to send again. If the channel is busy, it continues to wait for an opening.
If CSMA is like a dinner conversation, TDMA is more like a committee meeting. The chairman allots a certain amount of time in a given order for each speaker on the agenda. The speaker cannot be interrupted without first getting the attention of the chair.
In WirelessHART, the network manager serves as the chair, allotting a certain time slot for each node to transmit data. During the allotted time slot, the node has exclusive access to the channel, ensuring no collisions take place. To summarize, in CSMA, nodes are being polite while in TDMA they take turns.
When it comes to good manners in communication protocols, TDMA has the advantage. In CSMA collisions will occur, effectively lowering bandwidth and causing potentially creating latency issues. With TDMA, everyone gets a turn without interruption. This decreases latency issues and improves bandwidth utilization.
Being polite may work at a quiet diner party with a handful of guests, but imagine it working at a crowded town council meeting. In a diner party, manners are sufficient to allow everyone a turn to speak. In a crowded meeting, it’s going to take an agenda to give everyone the chance to speak.
Similarly, in a small network Zigbee’s CSMA may be adequate. But on a network crowded with nodes, WirelessHART’S TDMA is the best way to guarantee delivery of data within a time limit.