Full & Half Duplex Ethernet
When Ethernet was standardised in 1985, all communication was half duplex. Half duplex communication restricts a node to either transmit or receive at a time but not perform both actions concurrently.
Nodes sharing a half duplex connection are operating in the same collision domain. This means that these nodes will compete for bus access, and their frames have the potential to collide with other frames on the network. Unless access to the bus is controlled at a higher level and highly synchronised across all the nodes co-existing on the collision domain, collisions can occur and real-time communication is not guaranteed.
Full duplex communication was standardised for Ethernet in the 1998 edition of 802.3 as IEEE 802.3x. With full duplex, a node can transmit and receive simultaneously. A maximum of two nodes can be connected on a single full duplex link. Typically this would be a node-to-switch or switch-to-switch configuration. (The set-up of Figure 5 is for illustration purposes, and unlikely to be implemented in a practical form). Theoretically, employing full duplex links can double the available network bandwidth taking it from 10 Mbps or 100 Mbps to 20 Mbps or 200 Mbps respectively, but in practice it is limited by the internal processing capability of each node. Full duplex communication provides every network node with a unique collision domain. This operation completely avoids collisions and does not even implement the traditional Ethernet CSMA/CD protocol.
Since full duplex links can host a maximum of two nodes per link, such technology is not viable as an industrial real-time solution without the use of fast, intelligent switches capable of connecting links/segments as a network with single collision domains for each node i.e. Switched Ethernet.
Full Duplex, Switched
are data-link layer devices, today's switches are also capable of performing
switching functions based on data from layers 3 and 4. Layer 3 switches
can operate on information provided by IP such as IP version, source/destination
address or type of service. Layer 4 devices can switch depending on the
source/destination port or even information from the higher-level application.
For a real-time Industrial
Ethernet application, an 802.1p/Q implementation has certain advantages:
it introduces standardised prioritisation on Ethernet, allowing control
engineers to implement up to eight different user-defined priority levels
for their traffic. But these standards also have drawbacks including the
extra hardware costs associated with the increased Ethernet frame length
(1522 bytes), which introduces compatibility issues with legacy Ethernet
networks. A real-time implementation using 802.1 p/Q would require full
duplex, switched Ethernet. IEEE 802.1p/Q are acceptable for certain applications
of real-time Ethernet in industry when switch 'through' time is predictable
and an overload situation will not result in hard deadlines being missed.
TCP/UDP/IP for Real-Time Ethernet
With industrial Ethernet, the trend is to define an application layer environment along with the TCP/IP protocol, to realise an industrial automation networking solution. Although some real-time Ethernet solutions e.g. EtherNet/IP perform all their communication, real-time included, through the TCP/UDP/IP stack, most solutions although they provide compatibility with TCP/IP, do not employ this protocol for real-time communication. In a system like EtherNet/IP, TCP is used for initialisation and configuration (explicit) messages while UDP, with its reduced overhead, is used for real-time I/O (implicit messaging).
Typically, for real-time
Industrial Ethernet applications it is sufficient that the solution be
compatible with TCP/IP and the protocol suite is bypassed for all real-time
communication. The ability of a real-time Ethernet solution to intercommunicate
with an office-based system is paramount to achieve the Ethernet-technology
plant of the future.