What is protocol? and how it is used in embedded systems?

Question

What is protocol? and how it is used in embedded systems?

Answer 1

An agreed-upon format for transmitting data between two devices. The protocol determines the following:

the type of error checking to be used

data compression method, if any

how the sending device will indicate that it has finished sending a message

how the receiving device will indicate that it has received a message

There are a variety of standard protocols from which programmers can choose. Each has particular advantages and disadvantages; for example, some are simpler than others, some are more reliable, and some are faster.

From a user's point of view, the only interesting aspect about protocols is that your computer or device must support the right ones if you want to communicate with other computers. The protocol can be implemented either in hardware or in software.

Answer 2

A protocol is a system of rules that define how something is to be done. In computer terminology, a protocol is usually an agreed-upon or standardized method for transmitting data and/or establishing communications between different devices. Just as two humans need to have a common language between them before they can begin to share ideas and information, so must computers have a common way of sending information between them. The Internet is often used as an example of a successful protocols-based system in which the implementation of key qualities of protocols, such as error correction and message formatting, are utilized and respected across a wide variety of hardware and software.



                    n practice, we have found that embedded real-time networks require high efficiency, deterministic latency, operational robustness, configuration flexibility, and low cost per node.

Because cost limits the network bandwidth available to many applications, protocol efficiency (message bits delivered compared to raw network bandwidth) is very important. The embedded systems we have studied are characterized by a predominance of short, periodic messages. So, an obvious optimization is to reduce overhead bits used for message packaging and routing (it is not unusual for 8 bits of data to be packed in a message that is 32 or even 64 bits long).

Once message overhead has been reduced as much as possible, media access overhead must be reduced. For the most part, this is accomplished by minimizing the network bandwidth consumed by arbitration (e.g., passing a token or resolving collision conflict). Because worst-case behavior is usually important, efficiency should be evaluated both for light traffic as well as heavy traffic. For example, CSMA/CD (often used in workstation LANs) is highly efficient for light traffic but gives poor performance if heavily loaded, while Token Bus protocols have the reverse properties.

Determinacy, or the ability to calculate worst-case response time is important for meeting the real-time constraints of many embedded control applications. A prioritization capability is usually included in systems to improve determinacy of messages for time-critical tasks such as exception handling and high-speed loop control. Priorities can be either assigned by node number or by message type. Additionally, protocols can support local or global priority mechanisms. In local prioritization, each node gets a turn at the network in sequence and sends its highest priority queued message (thus potentially forcing a very high priority message to wait for other nodes to have their turns first). In global prioritization the highest priority message in the entire system is always transmitted first. This mechanism, which is fundamentally enabled by the media access protocol, is highly desirable for many safety critical applications.

Many applications require robust operation under extreme conditions. We call a protocol robust if it can quickly detect and recover from errors (e.g., duplicate or lost tokens), added nodes, and deleted nodes. In some systems it is also important to quickly recover from a reset or power glitch that forces a restart of the network.

Varied operating environments may dictate use of a media access protocol that is flexible in supporting multiple media as well as mixed topologies. For example, portions of a system may require expensive fiber in noisy environments, while other portions can tolerate low-cost twisted pair wires in benign environments. Further, a bus topology may be optimum for wires, but a ring or star topology maybe needed for fiber.

Finally, a vital consideration is the cost per node. In this article, the order of the media access discussion progresses from very simple to complex, high performance protocols. Simple protocols require less hardware and software resources and are therefore likely to be less expensive. For extremely cost-sensitive high-volume applications, these protocols are good candidates. However, for growth-expected applications, more advanced protocols provide a stronger foundation. In general costs are decreasing over time due to advances in IC manufacturing technology and the increasing availability of off-the-shelf protocols. Consequently, we envision advanced cost-effective protocols used in many embedded applications.