## 5 Mechanisms Used by Industrial Technologies to Improve Performance Metrics
### Priority Management Schemes  
Timely and reliable data transport is crucial for industrial automation applications. Communication networks are usually designed to meet such criteria by using certain Quality of Service (QoS) mechanisms. QoS mechanisms generally use two techniques to achieve their goals: (i) _traffic classification_ and (ii) _resource reservation_.  
The _traffic classification_ mechanism can be used for channel access and packet delivery along the path between the endpoints, by labeling the packets with a priority value and placing them on the corresponding queue in the path. The _resource reservation_ mechanism is used for allocating and reserving the resources along the path between two end-points for the specific traffic or class of traffic to achieve the desired QoS requirement.  
Fox example, in the wired CAN protocol (a communication system for industrial and automotive applications), a MAC layer technique is used to resolve the contention between several nodes to access the channel. It involves bit-wise priority arbitration for collision resolution that relies on a node's ability to transmit and receive simultaneously. Each packet has a priority value that is used to resolve the contention among different nodes trying to access the channel. The node with a higher priority label in its data packet has a higher chance of accessing the channel. Each contender node transmits its  
Figure 2: The ISA100.11a communication scheduling mechanism that alternates between a TDMA and CSMA-based scheme.  
priority value and receives feedback from the channel simultaneously. A node realizes that it has lost the contention when it detects a higher priority bit on the channel compared to the bit transmitted by the node itself [59, 60].  
This technique cannot be used in wireless sensor networks as they typically have half-duplex transceivers. WIA-PA uses the traffic classification method for addressing different QoS requests. They define four priority levels, based on different classes of data: _command packets_, _process data_, _normal packets_ and _alarm packets_. Low priority packets are declined when the device buffers become full. WirelessHART and ISA100.11a use a combination of traffic classification and resource reservation techniques for providing different QoS requests. When a device wants to establish communication with the central system manager or another device, it sends the contract request (Service request in WirelessHART), including input parameters, such as communication service type (scheduled or unscheduled communication), destination address, traffic classification (best effort queued, real time sequential, real time buffer and network control), requested period, and committed burst for non-periodic communication, to the system manager. The system manager uses its centralized optimization algorithm to determine the required allocation of the network resources (such as graphs and links) and sends a contract response to the source after all necessary network resources have been configured and reserved along the path. However, WirelessHART and ISA100.11a do not specify the specific optimization algorithms that can be used by the system manager to allocate resources.