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The OSI 7-layer model explained First introduced in 1978, the OSI (Open Systems Interconnection) 7-layer model was developed by the ISO (International Standards Organization) in the days when all communications protocols were proprietary and inter-manufacturer communication almost impossible. The industry has come a long way since then – but the OSI model is still the underlying set of ‘rules’ that makes it all possible. Frederick Chivers, Krone’s technical applications specialist, reminds us how the model works – especially at the cabling level. The concept behind the Open Systems Interconnection model was to enable any device or system operating with any protocol to communicate with another device or system using its own protocol. This removed the restrictions on users of being forced to operate with a specific set of proprietary hardware or software. ISO did not stop after formulating their layered model, a complete suit of network protocols were developed. We would all be using these now if it hadn’t been for the success of TCP/IP and Ethernet. These were available a year or so before ISO had completed their protocol Standards and gained an unassailable market dominance. As TCP/IP over Ethernet does not conform to the ISO model why is the model so widely used? The answer is that it allows comparison of any network technology from any industry. Data, telephony, satellite communication and mobile phone systems can all be made comprehensible by relating their components to the model. When network techies talk about such things as Layer 2 vs Layer 3 switching they don’t have to explain the nuances of different systems inner workings to understand exactly what is going on. So even though ISO was too late with the protocols the model itself is immensely important. The OSI model defines seven distinct ‘layers’. Each layer has a set of specifications and functions that it performs. What makes the open systems approach work is the ability for communications at a given layer to be able to interface correctly, come-what-may, with both the higher and lower layers.
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As you can see from the diagram, every data communication has to go through the model twice – from application layer to physical layer and then back to another application layer. Most explanations start with a very lightweight treatment of the physical layer and work through to the application layer. But since the physical layer is where we, in the cabling business, all earn our living we’ll go top down:
Layer 7 – Applications Not, as you might expect, application programs like word processors or spreadsheets, but the protocols they can use to communicate with remote systems. You may have heard of some such as HTTP, FTP, and most famously WWW, there are many more. The application layer handles functions that the programs need like; • Resource sharing and device redirection • Remote file access • Inter process communication • Network management
Layer 6 – Presentation This layer converts the data format of the sending application into that of the receiving application. . Common functions here are; • Data compression • Encryption • Conversion of bit order; CR to CR/LF; integer to floating point etc. • Character code translation e.g. ASCII to EBCDIC At the presentation layer we’ve escaped from the reality of physical devices – everything is understood by the systems in terms of ‘virtual’ or ‘logical’ devices. This is what gives the ISO model its ability to seamlessly interconnect so many different hardware and software combinations.
Layer 5 – Session Establishes and terminates communication sessions between hosts and handles naming translations. A good analogy for the session layer is the boss’s secretary. The secretary responsible for co-ordinating meetings, phone conversations, appointments, and preventing the bosses (higher
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layers) from disturbance when they are busy. The session layer does the same job between processes running on two different machines by establishing, maintaining and then terminating a ‘session’ to transfer messages. Processes at this layer include; • Connection and disconnection of any node from the network • Authentication of user access • Permitting multiple applications to share a virtual circuit • Fault recovery if a break in service occurs The session layer registers whether devices must send and receive data alternatively or concurrently, the protocols to be used for the session, communications modes, error checking and recovery.
Layer 4 – Transport The transport layer is responsible for the interfacing between the application software and the available hardware – protecting the upper layers from errors, data losses and out of sequence data. The transport layer provides: • Message segmentation – splitting outgoing messages into smaller units (frames) that the network layer can handle. • Re-assembly of the frames of an incoming data stream into the larger message size (in the correct order of course). • Message acknowledgement • Message traffic control – telling the far end to wait when buffers are full. • Multiplexing of several sessions onto one logical or virtual link – and keeping track of which frames belong to which session! TCP, and UDP are examples of Layer 4 protocols. The transport layer and the layers above are called end-to-end layers. They are oblivious of the details of the underling communication facility. It is also the last layer that deals with messages. The next three layers are very different.
Layer 3 – Network This layer interfaces the higher end-to-end layers with the lower physical dependant layers. Whereas the higher layers are only required on end point equipment, the network layer and lower layers are also implemented on intermediate network equipment such as routers (Layers 1, 2 and 3), switches (Layer 1 and 2) and hubs (Layer 1). There is no knowledge of messages at this layer, only the message fragments known as packets. The Network layer addresses and routes packets.
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The addressing only specifies the destination, for example in TCP/IP networks this is where the IP address is applied and read. The network layer provides: • Routing of frames among networks • Traffic control • Frame fragmentation • Logical to physical address mapping (names to numbers) • Usage accounting for statistics and billing Commonly known protocols at the network layer are ISDN, ATM and the most prevalent – the IP part of TCP/IP.
Layer 2 – Data Link Control The job of the data link control layer is twofold. To ‘shield’ the upper layers from any concerns about the physical transmission channel and to provide error-free transfer of data frames from one node to the next node over the physical layer. The data link layer receives raw bits (‘I’s and ‘0’s) from the physical layer and assembles them into the logical groups or frames that the upper layers require. To do this, the data link control layer has to: • Establish and terminate the logical link between nodes. • Control frame traffic • Transmit and receive frames sequentially • Acknowledge frames • Detect and recover from errors in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt. • Create and recognise frame boundaries The data link layer is actually split into two sub layers – the Media Access Control sub layer covering address management (those well known MAC addresses) and the Logical Link Control sub layer, which manages flow and error control, automatic requests for retransmission (ARQ) methods and handshake processes.
Layer 1 –Physical For many years denigrated by the people fascinated with the higher layer stuff with flashing lights, the need for higher and higher bandwidth has finally brought the physical layer onto the radar of most IT and network people. And not before time.
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As we in the industry have known for a long time, the quality of the physical infrastructure can make a great deal of difference to the overall endto-end performance of a data communication system. The physical layer is concerned with the transmission of a raw bit stream over a physical medium. The physical layer is not just the cable or fibre, it can and does include electronics such as the line interfaces of devices like NICs (network interface cards). But, at the physical layer, the system knows nothing of data structure – just ‘1’s and ‘0’s. However, the job of getting a ‘1’ to the other end still as a ‘1’ and not corrupted to a ‘0’ is actually a considerable feat in the harsh real-world of attenuation, crosstalk, induced noise and impedance mismatches. The physical layer describes the electrical, optical, mechanical and functional interfaces to the physical transmission medium and carries the signals for all the higher layers. It has four facets: • Data Encoding – modifying simple binary ‘1’s and ‘0’s into electrical or optical states that are best carried by, and detected at the far end of, the physical medium. For example, what physical state represents a ‘1’, how the receive station knows when a ‘bit-time” starts and how the receiving station delineates a frame. • Transmission techniques – for example whether the encoded bits will be transmitted by baseband (digital) or broadband (analogue) signals. • Physical medium transmission – transmits bits as electrical or optical signals and determines factors such as how many volts or dB (or what change in volts as dB) represents a given signal state on the specific physical medium (STP, UTP, co-ax, fibre, radio, etc.). • Physical medium attachment. Basic, but absolutely essential, factors like will an external transceiver be used, what connectors are used, how many pins do the connectors have and what is each pin used for? Of course the physical layer is where we find our Cat 5e and Cat 6 structured copper and fibre cabling systems. It is in the transmission of highspeed data over distance (even if that is only 100m) that the communication message actually suffers its greatest potential problems. At high frequencies – and we’re talking VHF radio frequencies for Cat 5e and Cat 6 – twisted pairs and cables act as radio antennae broadcasting their signals to adjacent pairs and cables. These pairs and cables pick up interference and, because the attenuation over distance is quite severe, the layer of the signal we’re trying to recover is not large compared to the crosstalk and interference signals. What’s even worse is that poorly impedance-matched components cause reflections and losses that further distort the signals meaning that many ‘1’s can get to the other end of the physical layer as ‘0’s and vice versa.
OSI versus good installation As you’ll realise, the OSI model isn’t too bothered about ‘1’s arriving as ‘0’s. After all, layer 2 has the job of recognising errors and requesting faulty frames to be retransmitted. On lightly loaded networks this isn’t a problem but as Krone labs have demonstrated recently, the amount of “sorry didn’t quite hear that, can you send it again” traffic that begins to be carried as networks are subjected to higher loads and faster speeds can easily outweigh the wanted signal.
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The effect that bit errors have on network performance is dependant on a number of factors such as how busy the effected channel is, the number of frames that need to be retransmitted (“sliding windows”), and whether forward error correction is implemented. So, although the exact percentage degradation is unpredictable it is dispropotionately high e.g. one error in 25,000 bits can easily reduce throughput to less than 2% of the maximum data rate. The worst culprit, our labs have found, is poor impedance matching, due to cables and connectors that are not closely matched and less than perfect terminations – where any excess un-twisting of paired wires can badly affect real data throughput. The more attention we pay to facts like impedance matching and high quality installation practices, the better will be the overall end-to-end, application-to-application, communication performance. And the happier our customers.
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