The Demand for Real-Time Ethernet

The demand for Ethernet as a real-time control network is increasing (see Figure 3) as manufacturers realise the benefits of employing a single network technology from the boardroom to the plant floor. Decreased product costs coupled with the possibility of overlapping training and maintenance costs for information, field level, control and possibly device networks would greatly reduce the expense to the manufacturers.



Figure 3 — Predicted Sales of Industrial Ethernet nodes


Ethernet offers many benefits at the real-time control level over existing solutions. As a control network, 10 Gbps Ethernet offers bandwidth that is almost 1000x faster than today's comparable fieldbus networks (such as the 12 kbps of PROFIbus) and can also support real-time communication. Distributed applications in control environments require tight synchronisation so that the delivery of control messages can be guaranteed within defined message cycle times. Typical cycle times for control applications are listed in Table 1. Traditional Ethernet and fieldbus systems are not capable of meeting cycle time requirements below a few milliseconds, but the emerging real-time Industrial Ethernet solutions allow cycle times as low as a few microseconds.

Typical Cycle Times for Control Applications
Control Application Typical Cycle Time
Low speed sensors (e.g. temp. pressure) Tens of milliseconds
Drive control systems Milliseconds
Motion control (e.g. robotics) Hundreds of microseconds
Precision motion control Tens of microseconds
High speed devices Microseconds
Electronic ranging (e.g. fault detection) Hundreds of nanoseconds

Table 1

Along with the increased bandwidth and tight synchronisation, real-time Ethernet gives manufacturers the security of using a physical and data-link layer technology that has been standardised by both the IEEE and the ISO. Ethernet can provide reduced complexity with all the attributes required of a field, control or device network - in operations having up to 30 different networks installed at this level [2]. Furthermore, Ethernet devices can also support TCP/IP stacks so that Ethernet can easily gate to the Internet. This feature is attractive to users since it allows remote diagnostics, control, and observation of their plant network from any Internet-connected device around the world with a license-free web browser. Although Ethernet does introduce overhead through its minimum message data size (46 bytes), which is large in comparison to existing control network standards, its increased bandwidth, standardisation and integration with existing plant technology should generate good reasons to consider Ethernet as a control network solution.

Ethernet and CSMA/CD

Ethernet, as defined in IEEE 802.3, is unsuitable for strict real-time industrial applications because its communication is non-deterministic. This is due to the definition of the network's media access control (MAC) protocol, which is based on Carrier Sense Multiple Access/ Collision Detection (CSMA/CD), see Figure 4. The implementation described in the standard uses a truncated binary exponential backoff algorithm.



Figure 4 — IEEE 802.3 CSMA/CD with
Truncated Binary Exponential Algorithm Flow Chart

With CSMA/CD, each node can detect if another node is transmitting on the medium (Carrier Sense). When a node's Carrier Sense is on, it will defer transmission until it determines that the medium is free. If two nodes transmit simultaneously (Multiple Access), the network experiences a collision and all frames are destroyed. Nodes can detect collisions (Collision Detection) by monitoring the collisionDetect signal provided by the physical layer. When a collision occurs, the node transmits a jam sequence.

When a node begins transmission on the medium there is a certain time interval, called the Collision Window, during which a collision can occur. This window is large enough to allow the signal to propagate around the entire network/segment. When this time window is over, all (functioning) nodes should have their Carrier Sense on, and so would not attempt to commence transmission.

When a collision occurs, the truncated binary exponential backoff algorithm is employed at each 'colliding' node. The algorithm works as follows:
Initially: n:=0, k:=0, r:=0.

When a collision occurs, the node enters the algorithm:
It increments n, the Transmit Counter, which counts the number of sequential collisions experienced by a node.
If n > 16, (16 unsuccessful successive transmission attempts), transmission fails and the higher layers should be informed.
If n <= 16, select a number from the set k = min(n, 10) (Truncation).
A random number, r, is selected from the set (0,1,2,4...2 k) (Exponential and Binary).
The node then waits r x slot_time before recommencing a transmission attempt.

One advantage of this backoff algorithm is that it controls the medium load. If the medium is heavily loaded, the likelihood of collisions increases, and the algorithm increases the interval from which the random delay time is chosen. This should lighten the load and reduce further collisions.

Ethernet's CSMA/CD with truncated exponential binary algorithm introduces the possibility of complete transmission failure as well as the possibility of a random transmission time, hence IEEE 802.3's non-determinism and unsuitability for real-time communications — especially on heavily-loaded networks. Re-definition of the media access protocol would solve the problem but would leave the new nodes unable to interoperate with legacy Ethernet nodes.

Ethernet is non-deterministic only if collisions can occur. To implement a completely deterministic Ethernet, it is necessary to avoid all collisions. A collision domain is a CSMA/CD segment where simultaneous transmissions can result in a collision. The probability of collision increases with the number of nodes transmitting on a single collision domain. Completely avoiding collisions in the Ethernet network, therefore, gives a network with a relatively large bandwidth and popularity an opportunity to be developed for the real-time domain. The most common way of implementing a collision-free Ethernet is through the use of hardware.

A solution where two or more nodes compete for medium access to a network segment is called Shared Ethernet.