Open System Interconnect model (OSI)
When examining a complex model, it is the situations that are always important to break the condition into small parts that can be understood or that are easily manageable. This is what an organization called International Organization for Standardization did. It broke the communication process into small manageable units or layers, and each unit or layer represents similar functions that provide services to the layers above and request services from layers that are below (ITU-T, 2003). OSI is an acronym for Open System Interconnect model and was developed in the 1980s by the International Organization for Standardization (ISO). The OSI model consists of seven layers namely application, presentation, session, transport network, data link, and finally physical layer. Anything to be communicated like a message starts at the application layer with the top layer and moves down the OSI layers to the bottom layer which is the physical layer. As the message moves through these layers layer-specific information is added and this information is called headers. When the message reaches the receiving end, the headers are stripped or removed as the message travels from the bottom to the top of the layer. So, the sending end encapsulates the message, and the receiving end does de-encapsulation. This is the function of the OSI model (Lorentz, 2005).
Now we are going to discuss each of these layers and state what they do during the process of encapsulation and de-encapsulation (ITU-T, 2003). The topmost layer called the application layer is involved in providing network services to the end-user applications. These services include file access, printing, mail transfer, and word processing. The next layer is called the presentation layer and determines how data is presented to the user. It provides services such as encryption, decryption compression, and decompression of the data. The next layer is the session layer (ITU-T, 2003). It is involved in establishing the connection and ending the connection between the communicating parties. For instance, two communicating mobile phones use a session layer to establish a connection. The fourth layer is called the transport layer and as its name signifies, it is involved in reliable data from the sending device to the receiving device. It is also involved in error detection and correction. The network layer is number three in the OSI reference model. It is involved in determining the most reliable route for the packet to pass through until it reaches its destination. The second layer is called the data link layer and this layer is divided into media access and logical link layer. The main purpose of this layer is to provide a means by which the message being sent can access the media. It also assists to identify the MAC address of the sending device. The first or the bottom-most layer is called the physical layer. Its function is mainly to receive and send raw bits. Bits mean 0s and 1s (Lorentz, 2005). This is because computers only understand binary, but they convert them to a manner in which human beings can understand.
Communication Protocols
Communication protocols can be defined as rules that communicating devices like computers or mobile phones follow to successfully communicate and understand the other partner on the other side (Holzmann, 2001). Since computers have no means of learning these rules, network programmers face the challenge of coming up with these protocols or developing them. For two or more systems to accomplish a given mission, they must exchange a controlled sequence of messages and these messages are the protocols. We cannot fail to mention that computers use controls structures in every system to coordinate the exchange of data between them. Since timing is important in protocol execution the systems maintain timers because it is required that they arrive within certain time intervals (Holzmann, 2001). The main functions of protocols include:
- Data addressing
- Deciding how data is sent
- Compression technique application
- Errors identification
- And deciding how to announce sent and received data
The following are the major communication protocols:
Transmission control protocol/Internet protocol (TCP/IP) represents a set of public standards that specifies how packets of information are exchanged between devices over one or more networks (Holzmann, 2001). TCP/IP consists of four layers and they are the application layer which is the starting point of any communication session. Currently, we have other protocols such as Hypertext Transfer Protocol (HTTP) which governs how files such as sound, text, graphics, and video are exchanged over the Internet or the World Wide Web. Another protocol that operates at this layer is telnet that allows access to a remote host. File transfer protocol allows the transfer of files over the internet. The other layer of TCP/IP is the transport layer. At this layer, we have Transmission Control Protocol (TCP). This is the primary internet protocol for reliable transmission or delivery of data (Holzmann, 2001). It includes services for end-to-end connection, error detection, and recovery. Many applications like email depend on his protocol. Other protocols include routing informationion protocol (RIP), Interior Gateway Protocol (IGP), and internet protocol (IP) which provides services for uniquely identifying a computer on the internet using IP addresses.
Industrial Ethernet
By definition, industrial Ethernet is the usage of Ethernet as the media in the data link layer of the OSI layer model. The data link layer is the second layer of the OSI reference model and it defines how a message being sent accesses the media (Holzmann, 2001). When implementing it, the bus topology is used to define both the logical and physical appearance of the network. Industrial Ethernet is considered the fastest growing network. Ethernet cables come in both category 5 which is known by most people as cat5 and also category 6 mostly known as cat6. The industrial Ethernet has evolved. It used to transmit 10mbs, then due to improvement in technology, it started transmitting in 100mbs and finally, we have gigabit Ethernet that is transmitting at 1000mbs (Lorentz, 2005).
According to Steve Jones, there are several ways that Ethernet affects the overall operation of the network and including increased speed in data transmission. The speed has increased from sub-10kbs with RS232 to 1000 Mbps or one gigabit. It is even expected to go beyond this capacity in the future since there is a need for media that can hold a lot of bandwidth and also transmit that data at high speed because availability is a key factor in the network. The other positive effect on the overall operation of the network is low-cost redundancy (Lorentz, 2005). The cost has been highlighted very well, this is under Jones’ argument that “Ethernet has a characteristic of being a network with active infrastructure. Unlike typical device or control level networks-which generally have a passive infrastructure that limits the number of devices that can be connected and the way they can be connected”. This is made possible due to the availability of the Ethernet switch that creates redundancy into the industrial Ethernet network (ITU-T, 2003). This could not be achieved with the standard field bus networks. Cost-effective networks can be designed to enable effective scalability in the future due to the capability of industrial Ethernet to accommodate a high number of point-to-point workstations or nodes. The following are the advantages of using industrial Ethernet.
- It is possible to use standard devices such as routers, hubs, switches, bridges, access points, and cables.
- One can use industrial Ethernet even when systems are running on different operating systems or different hardware.
- It allows peer-to-peer to co-exist when using TCP protocol.
- You can create several nodes on a link.
- Increased distance.
References
Holzmann, J. (2001) Design and Validation of Computer Protocols. New York Press: Prentice Hall.
ITU-T Recommendation Q.1400. (2003) Architecture framework for the development of signalling and OA&M protocols using OSI concepts, New York, pp 4, 7.
Lorentz, L. (2005) IAONA Handbook Industrial Ethernet, Industrial Automation Open Networking Alliance: Magdeburg.