This is the part of the course that strikes fear into many a network technician, but we’re pretty confident that when you finish this section, you’ll wonder what all the fuss is about! Honest!
The Open Systems Interconnect (OSI) seven-layer model was developed by a standards organization called ISO. Contrary to popular belief, ISO is not an abbre- viation—it’s derived from the Greek word for equal.
ISO wanted a framework into which the major network hardware and soft-ware components and protocols could be placed to give every item a common ref-erence point: a means of relating the components and their functionality to each other and a way of standardizing some of the components and protocols.
Each layer of the model represents a particular aspect of network functional-ity. For example, layer 1—the Physical layer—represents electrical signals, con-nectors, and media types and the way that data is placed on the network media.
For a simple example of how layers work, imagine that you’re designing your own range of NICs, and you’ve gotten to the stage where you’re choosing what connector type to use for the media and card interface.Off you trot to your local electronics store, where a simple, four-pin audio connector catches your eye.You finish your card design and send it to the manufacturer, but when the product hits the stores, it doesn’t sell. That’s because no one else uses that four-pin connector type on their NIC or media, and so no one can hook your cards onto their net- work.In addition, those who have tried to interface to your card were a bit shocked (literally!) to discover that your data signals use a 120-volt reference for binary 1 (in the real world, data signals on a network cable are a fraction of a volt).
What went wrong? You should have used a media connector type and an elec-trical signaling system that conform to the relevant OSI physical layer standards.Also, don’t be surprised if you get sued by that technician with the smoking screw-driver and singed hair!
As well as helping to standardize the design elements of network components,the OSI model helps position and standardize network protocols with reference to one another.As you’ll see, this is important because more than one protocol or action is needed to get your data onto a network (or, indeed, to do the reverse and pick up data from a network). For example, the “protocol” TCP/IP, in fact, refers to two protocols, TCP and IP, and these two protocols don’t work alone.What do they do? Chapter 5 is where you’ll find out.
To summarize, the OSI seven-layer model is a theoretical representation of how a networked device functions and helps us understand the interrelationships among hardware, software, protocols, and applications.Many network technicians refer to network devices by their positions in the model; for example, a repeater (a device mentioned earlier) is a layer 1 (Physical layer) device.
The Layers and What They Represent
Here’s a run through the layers and an overview of their tasks and responsibilities.
Layer 1: Physical Layer
Layer 1 is responsible for defining the network standards relating to electrical sig-nals, connectors, and media types and the way that data is placed on the network media.
Layer 2: Data Link Layer
Layer 2 is responsible for gathering together and completing all of the elements that make up a data packet and putting the whole thing together so that it can be passed to a Physical layer device and on to the network. The Data Link layer assem-bles outgoing packets and generates the CRC. For incoming packets, it checks the data for validity by comparing its locally generated CRC value with that sent in the packet. The Data Link layer also determines whether it is possible or permissible at any instant to try and send data to the network.At any instant, another computer may already be using the network. If you transmit data at the same time, both packets will become corrupted.
Layer 3: Network Layer
Layer 3 understands addressing—how to find the ultimate destination address for a data packet—and routing, to make sure the packet ends up in the right place.
Layer 4: Transport Layer
If the data being sent is bigger than the allowable packet size, the Transport layer breaks the data into smaller,manageable chunks that will fit inside two or more packets. Breaking up data into smaller chunks is also known as fragmentation.The Transport layer is also responsible for confirming whether transmitted packets have reached their destination intact (or at all) and retransmitting them if they haven’t (error correction/management). For incoming packets, the Transport layer reassembles the fragmented data (performs defragmentation), carefully ensuring that received packets are processed in the right order. The Transport layer also manages the flow of data to ensure that packets are sent at a pace that’s suitable for
the receiving device and for general network conditions. Sending data too quickly is like speaking too fast: you may have to keep repeating yourself to get the mes-sage understood, which is actually counterproductive.
Layer 5: Session Layer
Layer 5 sets up, manages, and terminates the data connections (called sessions) between networked devices. These sessions enable networked systems to exchange information.
Layer 6: Presentation Layer
Layer 6 is responsible for managing and translating information by catering to dif-ferences in the ways some computer systems store and manage their data.Presentation layer protocols are also responsible for data encryption.
Layer 7: Application Layer
Layer 7 represents the network-related program code and functions running on a computer system. This program code provides network support for the main applications being run, such as the redirector software discussed earlier, allowing a shared network location to appear on a machine as drive W: and providing ser-
vices such as login authentication. Some application layer functions do exist as user-executable programs. Some file transfer and e-mail applications, for exam-ple, exist entirely on this layer.
The Open Systems Interconnect (OSI) seven-layer model was developed by a standards organization called ISO. Contrary to popular belief, ISO is not an abbre- viation—it’s derived from the Greek word for equal.
ISO wanted a framework into which the major network hardware and soft-ware components and protocols could be placed to give every item a common ref-erence point: a means of relating the components and their functionality to each other and a way of standardizing some of the components and protocols.
Each layer of the model represents a particular aspect of network functional-ity. For example, layer 1—the Physical layer—represents electrical signals, con-nectors, and media types and the way that data is placed on the network media.
For a simple example of how layers work, imagine that you’re designing your own range of NICs, and you’ve gotten to the stage where you’re choosing what connector type to use for the media and card interface.Off you trot to your local electronics store, where a simple, four-pin audio connector catches your eye.You finish your card design and send it to the manufacturer, but when the product hits the stores, it doesn’t sell. That’s because no one else uses that four-pin connector type on their NIC or media, and so no one can hook your cards onto their net- work.In addition, those who have tried to interface to your card were a bit shocked (literally!) to discover that your data signals use a 120-volt reference for binary 1 (in the real world, data signals on a network cable are a fraction of a volt).
What went wrong? You should have used a media connector type and an elec-trical signaling system that conform to the relevant OSI physical layer standards.Also, don’t be surprised if you get sued by that technician with the smoking screw-driver and singed hair!
As well as helping to standardize the design elements of network components,the OSI model helps position and standardize network protocols with reference to one another.As you’ll see, this is important because more than one protocol or action is needed to get your data onto a network (or, indeed, to do the reverse and pick up data from a network). For example, the “protocol” TCP/IP, in fact, refers to two protocols, TCP and IP, and these two protocols don’t work alone.What do they do? Chapter 5 is where you’ll find out.
To summarize, the OSI seven-layer model is a theoretical representation of how a networked device functions and helps us understand the interrelationships among hardware, software, protocols, and applications.Many network technicians refer to network devices by their positions in the model; for example, a repeater (a device mentioned earlier) is a layer 1 (Physical layer) device.
The Layers and What They Represent
Here’s a run through the layers and an overview of their tasks and responsibilities.
Layer 1: Physical Layer
Layer 1 is responsible for defining the network standards relating to electrical sig-nals, connectors, and media types and the way that data is placed on the network media.
Layer 2: Data Link Layer
Layer 2 is responsible for gathering together and completing all of the elements that make up a data packet and putting the whole thing together so that it can be passed to a Physical layer device and on to the network. The Data Link layer assem-bles outgoing packets and generates the CRC. For incoming packets, it checks the data for validity by comparing its locally generated CRC value with that sent in the packet. The Data Link layer also determines whether it is possible or permissible at any instant to try and send data to the network.At any instant, another computer may already be using the network. If you transmit data at the same time, both packets will become corrupted.
Layer 3 understands addressing—how to find the ultimate destination address for a data packet—and routing, to make sure the packet ends up in the right place.
Layer 4: Transport Layer
If the data being sent is bigger than the allowable packet size, the Transport layer breaks the data into smaller,manageable chunks that will fit inside two or more packets. Breaking up data into smaller chunks is also known as fragmentation.The Transport layer is also responsible for confirming whether transmitted packets have reached their destination intact (or at all) and retransmitting them if they haven’t (error correction/management). For incoming packets, the Transport layer reassembles the fragmented data (performs defragmentation), carefully ensuring that received packets are processed in the right order. The Transport layer also manages the flow of data to ensure that packets are sent at a pace that’s suitable for
the receiving device and for general network conditions. Sending data too quickly is like speaking too fast: you may have to keep repeating yourself to get the mes-sage understood, which is actually counterproductive.
Layer 5: Session Layer
Layer 5 sets up, manages, and terminates the data connections (called sessions) between networked devices. These sessions enable networked systems to exchange information.
Layer 6: Presentation Layer
Layer 6 is responsible for managing and translating information by catering to dif-ferences in the ways some computer systems store and manage their data.Presentation layer protocols are also responsible for data encryption.
Layer 7: Application Layer
Layer 7 represents the network-related program code and functions running on a computer system. This program code provides network support for the main applications being run, such as the redirector software discussed earlier, allowing a shared network location to appear on a machine as drive W: and providing ser-
vices such as login authentication. Some application layer functions do exist as user-executable programs. Some file transfer and e-mail applications, for exam-ple, exist entirely on this layer.
