To transfer data effectively, many experiments have shown that creating a uniform chunk of data is better than sending characters singly or in widely varying sized groups. Usually these chunks of data have some information ahead of them (the header) and sometimes an indicator at the end (the trailer). These chunks of data are called packets in most synchronous communications systems.
The amount of data in a packet and the composition of the header can change depending on the communications protocol as well as some system limitations, but the concept of a packet always refers to the entire set (including header and trailer). The term packet is used often in the computer industry, sometimes when it shouldn't be.
You often see the word packet used as a generic reference to any group of data packaged for transmission. As an application's data passes through the layers of the architecture, each adds more information. The term packet is frequently used at each stage. Treat the term packet as a generalization for any data with additional information, instead of the specific result of only one layer's addition of header and trailer. This goes against the efforts of both OSI and the TCP governing bodies, but it helps keep your sanity intact!
A subsystem is the collective of a particular layer across a network. For example, if 10 machines are connected together, each running the seven-layer OSI model, all 10 application layers are the application subsystem, all 10 data link layers are the data link subsystem, and so on. As you might have already deduced, with the OSI Reference Model there are seven subsystems.
It is entirely possible (and even likely) that all the individual components in a subsystem will not be active at one time. Using the 10-machine example again, only three might have the data link layer actually active at any moment in time, but the cumulative of all the machines makes up the subsystem.
A layer can have more than one part to it. For example, the transport layer can have routines that verify checksums as well as routines that handle resending packets that didn't transfer correctly. Not all these routines are active at once, because they might not be required at any moment. The active routines, though, are called entities. The word entity was adopted in order to find a single term that could not be confused with another computer term such as module, process, or task.
The notations N, N+1, N+2, and so on are used to identify a layer and the layers that are related to it. Referring to Figure 1.7, if the transport layer is layer N, the physical layer is N–3 and the presentation layer is N+2. With OSI, N always has a value of 1 through 7 inclusive.
One reason this notation was adopted was to enable writers to refer to other layers without having to write out their names every time. It also makes flow charts and diagrams of interactions a little easier to draw. The terms N+1 and N–1 are commonly used in both OSI and TCP for the layers above and below the current layer, respectively, as you will see.
To make things even more confusing, many OSI standards refer to a layer by the first letter of its name. This can lead to a real mess for the casual reader, because "S-entity," "5-entity," and "layer 5" all refer to the session layer.
Each layer performs N-functions. The functions are the different things the layer does. Therefore, the functions of the transport layer are the different tasks that the layer provides. For most purposes in this book, functions and entities mean the same thing.
This uses the hierarchical layer structure to express the idea that one layer provides a set of facilities to the next higher layer. This is sensible, because the application layer expects the presentation layer to provide a robust, well-defined set of facilities to it. In OSI-speak, the (N+1)-entities assume a defined set of N-facilities from the N-entity.
The entire set of N-facilities provided to the (N+1)-entities is called the N-service. In other words, the service is the entire set of N-functions provided to the next higher layer. Services might seem like functions, but there is a formal difference between the two. The OSI documents go to great lengths to provide detailed descriptions of services, with a "service definition standard" for each layer. This was necessary during the development of the OSI standard so that the different tasks involved in the communications protocol could be assigned to different layers, and so that the functions of each layer are both well-defined and isolated from other layers.
The service definitions are formally developed from the bottom layer (physical) upward to the top layer. The advantage of this approach is that the design of the N+1 layer can be based on the functions performed in the N layer, avoiding two functions that accomplish the same task in two adjacent layers.
An entire set of variations on the service name has been developed to apply these definitions, some of which are in regular use:
An N-service user is a user of a service provided by the N layer to the next higher (N+1) layer.
An N-service provider is the set of N-entities that are involved in providing the N layer service.
An N-service access point (often abbreviated to N-SAP) is where an N-service is provided to an (N+1)-entity by the N-service provider.
N-service data is the packet of data exchanged at an N-SAP.
It is important to remember that all these terms are used in a formal description, because a formal language is usually the only method to adequately describe something as complex as a communications protocol. It is possible, though, to fit these terms together so that they make a little more sense when you encounter them. An example should help.
The session layer has a set of session functions. It provides a set of session facilities to the layer above it, the presentation layer. The session layer is made up of session entities. The presentation layer is a user of the services provided by the session layer (layer 5). A presentation entity is a user of the services provided by the session layer and is called a presentation service user.
The session service provider is the collection of session entities that are actively involved in providing the presentation layer with the session's services. The point at which the session service is provided to the presentation layer is the session service access point, where the session service data is sent. The individual bits of data in the session service data are called session service data units.
Confusing? Believe it or not, after a while you will begin to feel more comfortable with these terms. The important ones to know now are that a layer provides a set of entities through a service access point to the next higher layer, which is called the service user. The data is sent in chunks called service data, made up of service data units.
Communication between two parties (whether over a telephone, between layers of an architecture, or between applications themselves) takes place in three distinct stages: establishment of the connection, data transfer, and connection termination.
Communication between two OSI applications in the same layer is through queues to the layer beneath them. Each application (more properly called a service user) has two queues, one for each direction to the service provider of the layer beneath (which controls the whole layer). In OSI-speak, the two queues provide for simultaneous (or atomic) interactions between two N-service action points.
Data, called service primitives, is put into and retrieved from the queue by the applications (service users). A service primitive can be a block of data, an indicator that something is required or received, or a status indicator. As with most aspects of OSI, a lexicon has been developed to describe the actions in these queues:
A request primitive is when one service submits a service primitive to the queue (through the N-SAP) requesting permission to communicate with another service in the same layer.
An indication primitive is what the service provider in the layer beneath the sending application sends to the intended receiving application to let it know that communication is desired.
A response primitive is sent by the receiving application to the layer beneath's service provider to acknowledge the granting of communications between the two service users.
The first application sends a request primitive to the service provider of the session layer and waits. The session layer's service provider removes the request primitive from the inbound queue from the first application and sends an indication primitive to the second application's inbound queue.
The second application takes the indication primitive from its queue to the session service provider and decides to accept the request for connection by sending a positive response primitive back through its queue to the session layer. This is received by the session layer service provider, and a confirmation primitive is sent to the first application in the presentation layer. This is a process called confirmed service because the applications wait for confirmation that communications are established and ready.
OSI also provides for unconfirmed service, in which a request primitive is sent to the service provider, sending the indication primitive to the second application. The response and confirmation primitives are not sent. This is a sort of "get ready, because here it comes whether you want it or not" communication, often referred to as send and pray.
When two service users are using confirmed service to communicate, they are considered connected. Two applications are talking to each other, aware of what the other is doing with the service data. OSI refers to the establishment and maintenance of state information between the two, or the fact that each knows when the other is sending or receiving. OSI calls this connection-oriented or connection-mode communications.
Connectionless communication is when service data is sent independently, as with unconfirmed service. The service data is self-contained, possessing everything a receiving service user needs to know. These service data packets are often called datagrams. The application that sends the datagram has no idea who receives the datagram and how it is handled, and the receiving service users have no idea who sent it (other than information that might be contained within the datagram itself). OSI calls this connectionless-mode.
OSI (and TCP/IP) use both connected and connectionless systems between layers of their architecture. Each has its benefits and ideal implementations. All these communications are between applications (service users) in each layer, using the layer beneath to communicate. There are many service users, and this process is going on all the time. It's quite amazing when you think about it.
The amount of data in a packet and the composition of the header can change depending on the communications protocol as well as some system limitations, but the concept of a packet always refers to the entire set (including header and trailer). The term packet is used often in the computer industry, sometimes when it shouldn't be.
You often see the word packet used as a generic reference to any group of data packaged for transmission. As an application's data passes through the layers of the architecture, each adds more information. The term packet is frequently used at each stage. Treat the term packet as a generalization for any data with additional information, instead of the specific result of only one layer's addition of header and trailer. This goes against the efforts of both OSI and the TCP governing bodies, but it helps keep your sanity intact!
Subsystems
A subsystem is the collective of a particular layer across a network. For example, if 10 machines are connected together, each running the seven-layer OSI model, all 10 application layers are the application subsystem, all 10 data link layers are the data link subsystem, and so on. As you might have already deduced, with the OSI Reference Model there are seven subsystems.
It is entirely possible (and even likely) that all the individual components in a subsystem will not be active at one time. Using the 10-machine example again, only three might have the data link layer actually active at any moment in time, but the cumulative of all the machines makes up the subsystem.
Entities
A layer can have more than one part to it. For example, the transport layer can have routines that verify checksums as well as routines that handle resending packets that didn't transfer correctly. Not all these routines are active at once, because they might not be required at any moment. The active routines, though, are called entities. The word entity was adopted in order to find a single term that could not be confused with another computer term such as module, process, or task.
N Notation
The notations N, N+1, N+2, and so on are used to identify a layer and the layers that are related to it. Referring to Figure 1.7, if the transport layer is layer N, the physical layer is N–3 and the presentation layer is N+2. With OSI, N always has a value of 1 through 7 inclusive.
One reason this notation was adopted was to enable writers to refer to other layers without having to write out their names every time. It also makes flow charts and diagrams of interactions a little easier to draw. The terms N+1 and N–1 are commonly used in both OSI and TCP for the layers above and below the current layer, respectively, as you will see.
To make things even more confusing, many OSI standards refer to a layer by the first letter of its name. This can lead to a real mess for the casual reader, because "S-entity," "5-entity," and "layer 5" all refer to the session layer.
N-Functions
Each layer performs N-functions. The functions are the different things the layer does. Therefore, the functions of the transport layer are the different tasks that the layer provides. For most purposes in this book, functions and entities mean the same thing.
N-Facilities
This uses the hierarchical layer structure to express the idea that one layer provides a set of facilities to the next higher layer. This is sensible, because the application layer expects the presentation layer to provide a robust, well-defined set of facilities to it. In OSI-speak, the (N+1)-entities assume a defined set of N-facilities from the N-entity.
Services
The entire set of N-facilities provided to the (N+1)-entities is called the N-service. In other words, the service is the entire set of N-functions provided to the next higher layer. Services might seem like functions, but there is a formal difference between the two. The OSI documents go to great lengths to provide detailed descriptions of services, with a "service definition standard" for each layer. This was necessary during the development of the OSI standard so that the different tasks involved in the communications protocol could be assigned to different layers, and so that the functions of each layer are both well-defined and isolated from other layers.
The service definitions are formally developed from the bottom layer (physical) upward to the top layer. The advantage of this approach is that the design of the N+1 layer can be based on the functions performed in the N layer, avoiding two functions that accomplish the same task in two adjacent layers.
An entire set of variations on the service name has been developed to apply these definitions, some of which are in regular use:
An N-service user is a user of a service provided by the N layer to the next higher (N+1) layer.
An N-service provider is the set of N-entities that are involved in providing the N layer service.
An N-service access point (often abbreviated to N-SAP) is where an N-service is provided to an (N+1)-entity by the N-service provider.
N-service data is the packet of data exchanged at an N-SAP.
These terms are shown in Figure 1.8. Another common term is encapsulation, which is the addition of control information to a packet of data. The control data contains addressing details, checksums for error detection, and protocol control functions.N-service data units (N-SDUs) are the individual units of data exchanged at an N-SAP (so that N-service data is made up of N-SDUs).
Making Sense of the Jargon
It is important to remember that all these terms are used in a formal description, because a formal language is usually the only method to adequately describe something as complex as a communications protocol. It is possible, though, to fit these terms together so that they make a little more sense when you encounter them. An example should help.
The session layer has a set of session functions. It provides a set of session facilities to the layer above it, the presentation layer. The session layer is made up of session entities. The presentation layer is a user of the services provided by the session layer (layer 5). A presentation entity is a user of the services provided by the session layer and is called a presentation service user.
The session service provider is the collection of session entities that are actively involved in providing the presentation layer with the session's services. The point at which the session service is provided to the presentation layer is the session service access point, where the session service data is sent. The individual bits of data in the session service data are called session service data units.
Confusing? Believe it or not, after a while you will begin to feel more comfortable with these terms. The important ones to know now are that a layer provides a set of entities through a service access point to the next higher layer, which is called the service user. The data is sent in chunks called service data, made up of service data units.
Queues and Connections
Communication between two parties (whether over a telephone, between layers of an architecture, or between applications themselves) takes place in three distinct stages: establishment of the connection, data transfer, and connection termination.
Communication between two OSI applications in the same layer is through queues to the layer beneath them. Each application (more properly called a service user) has two queues, one for each direction to the service provider of the layer beneath (which controls the whole layer). In OSI-speak, the two queues provide for simultaneous (or atomic) interactions between two N-service action points.
Data, called service primitives, is put into and retrieved from the queue by the applications (service users). A service primitive can be a block of data, an indicator that something is required or received, or a status indicator. As with most aspects of OSI, a lexicon has been developed to describe the actions in these queues:
A request primitive is when one service submits a service primitive to the queue (through the N-SAP) requesting permission to communicate with another service in the same layer.
An indication primitive is what the service provider in the layer beneath the sending application sends to the intended receiving application to let it know that communication is desired.
A response primitive is sent by the receiving application to the layer beneath's service provider to acknowledge the granting of communications between the two service users.
An example might help clarify the process. Assume that two applications in the presentation layer want to communicate with each other. They can't do so directly (according to the OSI model), so they must go through the layer below them.A confirmation primitive is sent from the service provider to the final application to indicate that both applications on the layer above can now communicate.
The first application sends a request primitive to the service provider of the session layer and waits. The session layer's service provider removes the request primitive from the inbound queue from the first application and sends an indication primitive to the second application's inbound queue.
The second application takes the indication primitive from its queue to the session service provider and decides to accept the request for connection by sending a positive response primitive back through its queue to the session layer. This is received by the session layer service provider, and a confirmation primitive is sent to the first application in the presentation layer. This is a process called confirmed service because the applications wait for confirmation that communications are established and ready.
OSI also provides for unconfirmed service, in which a request primitive is sent to the service provider, sending the indication primitive to the second application. The response and confirmation primitives are not sent. This is a sort of "get ready, because here it comes whether you want it or not" communication, often referred to as send and pray.
When two service users are using confirmed service to communicate, they are considered connected. Two applications are talking to each other, aware of what the other is doing with the service data. OSI refers to the establishment and maintenance of state information between the two, or the fact that each knows when the other is sending or receiving. OSI calls this connection-oriented or connection-mode communications.
Connectionless communication is when service data is sent independently, as with unconfirmed service. The service data is self-contained, possessing everything a receiving service user needs to know. These service data packets are often called datagrams. The application that sends the datagram has no idea who receives the datagram and how it is handled, and the receiving service users have no idea who sent it (other than information that might be contained within the datagram itself). OSI calls this connectionless-mode.
OSI (and TCP/IP) use both connected and connectionless systems between layers of their architecture. Each has its benefits and ideal implementations. All these communications are between applications (service users) in each layer, using the layer beneath to communicate. There are many service users, and this process is going on all the time. It's quite amazing when you think about it.
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