OSI Reference Model: Layer 5 Hardware

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In the previous few articles I have discussed the first four layers of the Open Systems Interconnect (OSI) Reference Model. In this article I will discuss the fifth. The fifth layer of the OSI Reference Model is called the Session layer. This layer, as you might imagine, is responsible for the management of sessions between two communicating end points. This includes the authentication, set up, termination, and reconnecting if needed.

One of the more interesting aspects of the session layer, or rather the protocols that implement its functionality, is the duplex level. When two end points are communicating with each other they can either communicate in a simplex mode, full-duplex mode, half-duplex mode, or a full duplex emulation mode.


Simplex communication is a one way only type of communication that flows from the designed transmitter to the designed receiver. The radio in your car is an example of this, the station transmits the played-too-often music along with the occasional not-funny joke which is received by the radio in your dashboard; the car radio does not communicate back to the station in any way. Figure 1 shows a diagram of a simplex communication.

Figure 1: A simplex communication diagram (Source: www.allaboutcircuits.com)


Full-duplex means that communication can occur in both directions at the same time. Ethernets are examples of a full-duplex medium; with twisted pair cables one pair of twisted wires can be used for transmitting and another pair used for receiving. This of course refers to newer Ethernets as older ones employing coax cabling are strictly half-duplex (is anyone still using these networks?). Ethernets using fiber optics are also full-duplex. Figure 2 shows a diagram of a full-duplex communication.


Half-duplex communication means that communication between two end points can only occur in one direction at a time. Thinnet and thicknet Ethernets are an example of half-duplex systems as are many walkie-talkie devices. Figure 2 also shows a diagram of a half-duplex communication system.

Figure 2: Half-duplex and full-duplex communication diagram (Source: www.allaboutcircuits.com)

Half-duplex systems may seem somewhat antiquated to many readers of this website as modern computing networks are built for full-duplex communication which typically provides better performance for the users. However, there are many situations where simplex or half-duplex are desirable. Networks designed to feed information from one source to many different end points may not have use for the capability to receive messages from the end points. RSS feeds are an example of a system which sends information out to end users but does not receive information even though it is communicating over a medium which is capable of full-duplex.

Many industrial networks also have no need for full-duplex communication. Think of a widget manufacturer with a widget factory which has a widget conveyor belt. If a big order for widgets comes in the supervisor may want to speed up the conveyor belt. Perhaps the factory has a Programmable Logic Controller (PLC) which the supervisor could use to increase the speed of the conveyor belt, the conveyor belt controller would receive a signal to increase the speed of the conveyor belt then send an acknowledge signal with the current speed of the conveyor belt which is in turn displayed on the PLC. In this example there is absolutely no need for full-duplex communication, and in fact it would only make it more costly and, as always, more components means more things to maintain, so many industrial networks such as this widget factory choose half-duplex communication systems for many of their needs.


For many applications full-duplex is desired even though the medium only supports half-duplex. In these situations there are ways to simulate full-duplex emulation which may be more attractive than upgrading the network to full-duplex.


Time division duplexing (TDD) is very much like time division multiplexing in that it uses the same medium to both send and receive signals which is controlled by a clock. In TDD, both forward and reverse signals use the same medium and are each assigned time slots. One big advantage of this method of full-duplex emulation is that if the amount of data flowing in any one direction is quite variable then the time slot allocation can be optimized for communication in one of the directions to suit the needs of the application, this can happen dynamically. The IEEE 802.16 standard for WiMAX (Worldwide Interoperability for Microwave Access) allows for the use of either TDD or Frequency Division Duplexing. TDD is best suited for asymmetric data communication such as that found on over the Internet, this is because of the dynamic time slot allocation capability.


Frequency division duplexing or FDD is a term used for a communication system which assigns one frequency for uploading and another frequency for downloading. In this type of full-duplex emulation both transmitting and receiving can happen over the same medium but there must be a frequency offset (the bandwidth between the upload and download frequencies) so that the data does not interfere with each other. This frequency offset can be a major disadvantage for some systems. Take WiMAX for example, FDD is supported although this means that the communication between two endpoints takes up more of the frequency spectrum available. On the other hand, TDD can have a greater inherent latency and requires more complex circuitry which may be more power hungry. TDD also requires time offset for the allocated time slots.

FDD systems are typically more advantageous for communication applications which require equal upload and download bandwidths which would thus eliminate the dynamic time slot allocation that TDD provides. Most cellular systems work on FDD.


Echo cancellation is another method of full-duplex emulation. In a communication mode which uses echo cancellation, both end points put data onto the same medium at the same frequency at the same time and each end point receives all data put onto the medium including the data that it sent itself. Each end point must then isolate the data that it sent itself and read all other data. Telephone networks use echo cancellation. This echo cancellation capability can either be implemented in a hardware or a software solution. Although, some forms of echo are desirable. For example, when you speak into a phone your voice is transferred to the ear piece even before it is transmitted to the person you are calling; this is desirable because if you can’t hear yourself speak as you are speaking then you will think the phone isn’t working. Other applications however, such as a dial up modem, are more sensitive to this echo and need to cancel this in order to work properly.

In my next article I will discuss the sixth layer of the OSI Reference Model; the Presentation layer. As always if there are any questions or comments on this article please feel free to send me an email and I will do my best to get back to you promptly.

If you would like to read the other parts in this article series please go to:

If you would like to be notified when Russell Hitchcock releases the next article in this series please sign up to the WindowsNetworking.com Real time article update newsletter.

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