Gateways, Linking Devices, Switches, Device Servers, Media Converters, Modems, Wireless…Confusing? Well, you are not alone. Communication between two different networks is challenging.
OSI Model
First, let’s start with taking a look on the basic model for all data communication, the OSI model.
The most basic communication is just using sub-sets of the OSI model. Layer 1 represent the lowest level, i.e. the physical communication line and its electrical signals. Layer 7 is the user level where the data model makes it possible for two devices to exchange and understand exchanged data. A common example is that even if we can hear someone speaking, we must understand his language to be able to understand the meaning of the “data”. Devices use a common Layer 7 which speaks the same language and can freely exchange data that both devices understand.
A basic RS-232 interface just defines Layer 1, making sure that two RS-232 devices can be electrically interconnected without electrical conflicts. To be able to communicate and interpret data between two devices, additional protocol software needs to be added in both devices, defining the communication rules and data formats.
The popular CAN protocol, used in most automotive applications nowadays, just communicates on Layer 2. Inside an automobile, the vendor uses CAN as communication link, but the vendor adds their own Layer 7 protocol on top of CAN to make this understandable for all nodes communicating inside the car. Any other CAN node outside the car is not able to participate in the communication if the same Layer 7 protocol isn’t implemented.
In the industrial marketplace devices from several different vendors need to be interconnected and therefore there are several open Layer 7 protocols based on CAN like DeviceNet and CANopen. Two DeviceNet nodes from two different vendors can communicate and exchange data because they use the same Layer 7 protocol.
Ethernet and the TCP/IP protocol covers the Layer 1-4 of the OSI model, which gives more functions and features than the CAN technology. Two Ethernet nodes can be interconnected in order for them to communicate, but TCP/IP only provides a transport mechanism of the data and performing service tasks like establishing connections, handling re-sends if any of the communication bits are corrupted etc.
However, two nodes using Ethernet and TCP/IP can only talk to each other, but they do not understand each other because they do not use the same “language”, i.e. the Layer 7 is not defined by TCP/IP protocol. On top of TCP/IP there must be applications like SNMP, HTTP, FTP etc to be able to exchange and process any data from the network.
All these IT protocols are standardized layer 7 protocols. In the Industrial area there are additional layer 7 protocols that handle real time data exchange. Two examples are EtherNet/IP (IP = Industrial Protocol) and Modbus-TCP that both are used on top of the TCP/IP protocol.
OK, with this information in the back of our minds, let’s take a look into how to convert from one network to another network.
Media Converters & Repeaters
Media converters normally operate in the Layer 1 and Layer 2 area of the OSI model. What they do is that they take the electrical signals from one physical media and converts them to another physical media. One example is a RS-232 RS-422 converter that re-formats the electrical signals between these two networks. Another example is an Ethernet 100Mbit CAT5 Cable 100Mbit Fiber converter.
Media converters are very useful in Industrial applications when going from one physical media to another, but they just re-format the signals, they do not provide any additional value to the application.
As signals travel along a network cable, they degrade and become distorted in a process that is called attenuation. If a cable is long enough, the attenuation will finally make a signal unrecognizable. A Repeater enables signals to travel further. It works at the OSI’s Physical layer to regenerate the network signal and then it resends the signal on other segments.
Repeaters are basically used to allow longer distances between the nodes on the network. The repeater takes a weak electrical signal from one segment, regenerates it, and passes it to the next segment. Repeaters do not translate or filter anything. For a repeater to work, both segments that the repeater joins must have the same access method.
For example, a repeater cannot connect a segment using CSMA/CD (Ethernet) to a segment using Token Passing.
Industrial Ethernet Switches
Switches can be viewed as intelligent repeaters. They amplify the signals of the Physical network, but they also provide intelligent analyzes on the bit packages received. An Ethernet Switch with store-and forward technology looks on the entire data frame it receives on any of its ports. It starts with verifying that no communication bits are corrupted, by checking the CRC checksum. If it is not OK, the data frame will be discarded. If CRC check is successful the switch will look at the destination address and forward the data frame to the port where this destination address is located.
Device Servers
Device Servers are also refererred to as Bridges. Bridges operate in the Layer 1-4 of the OSI model. They establish connections, re-transmissions, error handling etc. Data is exchanged easily between bridges, but still the data is just sent as a chunk of anonymous data. Layer 4 has no mechanism of processing the data itself.
For Ethernet it is very popular to use Device Servers to bridge between a serial network, such as RS-232, up to Ethernet. The Device Server has a complete TCP/IP protocol stack included and when it’s connected to a serial port, the data from that port is encapsulated in a TCP/IP frame and can be transported over Ethernet. On the destination side of Ethernet the serial data is just extracted out from the TCP/IP frame. This may sound like a Media Converter but there is complicated timing and protocol processing being performed by the Device Server.
If the serial device is communicating with a PC-based system, the PC normally runs application software that handles the extraction of the serial data from the TCP/IP frame.
This PC software will create a virtual serial port on the PC to be used with current applications on the PC. While this method does not add any new features to the original application, the solution is transparent to the attached equipment and requires few, if any, changes to either the hardware or software. In most cases, the end user is not aware that the direct serial connection is being replaced by a virtual one over an Ethernet (IP) network.
In this application the Device Server allows the serial device to be located far away from the PC using the Ethernet network for data transportation of the serial data.
If using a PLC, which is the predominant solution in Industrial Applications, two Device Servers need to be used. One close to the serial device that encapsulates the serial data in the TCP/IP frame and another one close to the serial port of the PLC extracting back the data to serial format from the TCP/IP frame.
Device Servers work’s well in message based networks where the acyclic data packets can be re-transmitted on another type of network, maintaining its original structure and format.
However, most of the industrial networks have also incorporated I/O data traffic that sends cyclic data for fast updates to achieve real time data communication. All these protocols requires Layer 7 processing to bridge data between two different networks.
Another limitation with Device Servers is lack of web-based data handling. Many device servers have a built in static web server for configuration of the Device Server itself, but the fact that a Layer 4 protocol level can not process data values makes it impossible to display data from a application on a built in web server with SSI scripts or Java applets.
See Anybus Serial Server – RS232-Ethernet Virtual COM port
Industrial Gateways (Layer 7 Gateways)
Gateways makes communication possible between different architectures and protocols. They repackage and convert data going from one network to another network so that it can understand the other’s application data.
A gateway repackages information to match the requirements of the destination system. Gateways can change the format of a message so that it will conform to the application program at the receiving end of the transfer. A gateway links two systems that might not use the same Communication protocols and Data formatting structures.
Gateways incorporate the entire suite of the OSI model from Layer 1 up to Layer 7. This allows true data translation and processing on the two different networks and thus a true liking of data between the networks can be achieved. Using a Gateway to bridge between Ethernet and other protocols such as DeviceNet, Profibus or simple serial protocols also allows the Gateway to serve up data from that network directly to a built in web server, enabling true web based control and monitoring.
The area of converting data from a Device with a serial port to Ethernet, or any other network, deserves a closer explanation. The fact that the serial port may not have a Layer 7 protocol at all, makes the data linking difficult. An example can be a device like a barcode reader just transmitting ASCII data embedded in a device specific serial data frame.
Serial to Fieldbus/Ethernet Gateways
Transporting data between RS232/422/485 & ASCII protocols up to an Fieldbus/Ethernet network is made by a “Serial Gateway” often referred to as a “Protocol Converter”. The Anybus Communicator for example, has solved this conversion with using temporary memory storage inside the gateway. The data from the serial /device is mapped into a local memory in the gateway and via a Configurator software the Gateway can define which data bytes from the serial data stream that is Data and what bytes that are just command and control information. The selected data is mapped into the Layer 7 of the selected Network and thus can be received and understood by the other node using the Layer 7 for its data.
A great benefit with this solution is that the Serial Device will look like it had an embedded network card inside and will act in the way a user would expect for a networked enabled device, i.e. the gateway converts selected data bytes from the serial data stream into a format that is understood by a PLC using a Fieldbus System and the device will look like any other device on that Fieldbus network. An example would be an I/O device with digital and anlog process values.
Advanced modes of for example, the Anybus Communicator can also act as an active gateway controlling and communicating with several serial devices (multi-drop function) using a common protocol such as Modbus RTU and linking selected data to a Industrial network such as Profibus or Ethernet. It can also be used as a web enabler to give serial devices a web based management, monitoring and control function.
See Anybus Communicator – Serial to Ethernet/Fieldbus Gateways
Fieldbus/Ethernet to Fieldbus/Ethernet
The growing need for flexibility and diversity within the factory floor is becoming more and more important. Using Industrial Gateways is a way of linking old/new factories and in particular bridging between different fieldbus or Ethernet networks within the same factory or when out in the field. The Anybus X-gateway product range is an industrial Bridge/Gateway between almost any 2 Fieldbus or Ethernet networks. Take a closer look at how the X-gateway can be used to easily exchange I/O data with two networks thus enabling the old factory network and new factory network to communicate with each other.
See Anybus-X – Ethernet/Fieldbus to Ethernet/Fieldbus Bridge/Gateways
Summary
Transporting data between two different networks is not trivial and it is important to analyze the requirement of the application.
– Media converters work well when overcoming electrical difference in two network technologies. Normally no configuration is necessary.
– Device Servers are attractive when using Ethernet based networks and with PC based control.
– Serial Gateways are able to solve all data linking between two networks such as RS232/422/485 to other networks. The drawback is that Gateways are quite complex devices and require set-up during installation to define the data that should be linked between the two networks.
– Bridges/Gateways are able to solve all data linking between two networks. Today this usually means simple exchange of I/O data, but in the future more and more functionality will be encoporated into these products.
– To web-enable data from a serial device with a legacy protocol, you must have a Layer 7 gateway product to interpret the real data and link it to a built in web server.
아이씨엔 매거진 2013년 03월호
