03. osi reference-layer
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03. osi reference-layer
- 2. What is Protocol ? A protocol is a set of rules that make communication on a network more efficient. Operational sequence to carry our a specific task. Example : DNC, DHCP, HTTP.
- 3. Overview During the past two decades there has been a tremendous increase in the numbers and sizes of networks. Many of the networks, however, were built using different implementations of hardware and software. As a result, many of the networks were incompatible and it became difficult for networks using different specifications to communicate with each other.
- 4. International Organization for Standards (ISO) • To address this problem, the International Organization for Standardization (ISO) researched many network schemes. The ISO recognized that there was a need to create a network model that would help network builders implement networks that could communicate and work together (interoperability) and therefore, released the OSI reference model in 1984.
- 6. Layered Communication Location A I like Message rabbits L: Dutch Information Ik hou for the Remote van Translator konijnen Fax #:--- Information L: Dutch for the Ik hou Remote van Secretary konijnen Source: Tanenbaum, 1996
- 7. Layered Communication Location A Location B I like J’aime Message rabbits les lapins L: Dutch Information L: Dutch Ik hou for the Ik hou Remote van van Translator konijnen konijnen Fax #:--- Fax #:--- Information L: Dutch L: Dutch for the Ik hou Ik hou Remote van Secretary van konijnen konijnen
- 8. Layered Communication Location A Location B Layers I like J’aime rabbits Message 3 les lapins L: Dutch Information L: Dutch Ik hou van for the remote 2 Ik hou van translator konijnen konijnen Fax #:--- Fax #:--- Information L: Dutch L: Dutch for the Ik hou Ik hou remote van secretary 1 van konijnen konijnen
- 9. Why a Layered Network Model? 7 Application • Reduces complexity (one big problem to seven smaller 6 Presentation ones) 5 Session • Standardizes interfaces 4 Transport • Facilitates modular engineering 3 Network • Assures interoperable 2 Data Link technology • Accelerates evolution 1 Physical • Simplifies teaching and learning
- 10. Devices Function at Layers 7 Application 6 Presentation NIC Card 5 Session 4 Transport 3 Network 2 Data Link 1 Physical Hub
- 11. Host Layers } 7 Application 6 Presentation Host layers: Provide accurate data delivery between 5 Session computers 4 Transport 3 Network 2 Data Link 1 Physical
- 12. Media Layers } 7 Application 6 Presentation Host layers: Provide 5 Session accurate data delivery between computers 4 Transport } 3 Network 2 Data Link Media layers: Control physical delivery of messages 1 Physical over the network
- 13. OSI Layers } 7 Application 6 Presentation Software layers 5 Session 4 Transport Heart of OSI } 3 Network 2 Data Link Hardware layers 1 Physical
- 14. Layer Functions 7 Application Provides network services to application processes (such as electronic mail, file transfer, and terminal emulation) Closest to User & Data Integrity.
- 15. Layer Functions 7 Application Network services to applications 6 Presentation Data representation • Ensures data is readable by receiving system • Translates between multiple data formats using a common data format. • Encryption & Decryption • Compression & Decompress.
- 16. Layer Functions 7 Application Network services to applications 6 Presentation Data representation 5 Session Inter-host communication • Establishes, manages, and terminates sessions between applications. • Synchronizes Dialogue.
- 17. Layer Functions 7 Application Network services to applications 6 Presentation Data representation 5 Session Inter-host communication 4 Transport End-to-end connection reliability • Concerned with data transport issues between hosts • Data transport reliability • Establishes, maintains, and terminates virtual circuits • Fault detection and recovery • Flow control
- 18. Layer Functions 7 Application Network services to applications 6 Presentation Data representation 5 Session Inter-host communication 4 Transport End-to-end connection reliability 3 Network Addresses and best path • Provides connectivity and path selection between two end systems • Domain of routing
- 19. Layer Functions 7 Application Network services to applications 6 Presentation Data representation 5 Session Inter-host communication 4 Transport End-to-end connection reliability 3 Network Addresses and best path 2 Data Link Access to media • Physical addressing, network topology, Deliver of Frames. • Error Detection (No correction) FCS
- 20. Layer Functions 7 Application Network services to applications 6 Presentation Data representation 5 Session Inter-host communication 4 Transport End-to-end connection reliability 3 Network Addresses and best path 2 Data Link Access to media 1 Physical Binary transmission • Electrical & Mechanical Procedures. • Wires, Connectors, Voltage Level, Timing of Voltage, • Maximum transmission distance
- 21. Peer-to-Peer Communications Host A Host B 7 Application Application 6 Presentation Presentation 5 Session Session Segments 4 Transport Transport Packets 3 Network Network Frames 2 Data Link Data Link Bits 1 Physical Physical
- 22. Data Encapsulation Host A Host B } { Application Application Presentation Data Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical
- 23. Data Encapsulation Host A Host B } { Application Application Presentation Data Presentation Session Session Transport Transport Network Data Network Header Network Data Link Data Link Physical Physical
- 24. Data Encapsulation Host A Host B } { Application Application Presentation Data Presentation Session Session Transport Transport Network Data Network Header Network Frame Network Data Frame Data Link Data Link Header Header Trailer Physical Physical
- 25. Data Encapsulation Host A Host B } { Application Application Presentation Data Presentation Session Session Transport Transport Network Data Network Header Network Frame Network Data Frame Data Link Data Link Header Header Trailer Physical 0101101010110001 Physical
- 26. Layers 1 & 2: Physical & Data Link Layers
- 27. Physical and Logical Addressing 0000.0c12.3456
- 28. MAC Address 24 bits 24 bits Vendor Code Serial Number 0000.0c12. 3456 ROM RAM • MAC address is burned into ROM on a network interface card
- 29. NIC - Addresses Two Addresses are associated with NIC Physical Address Logical Address MAC IP (Media Access Control) (Internet Protocol) L2 Address L3 Address Permanent Logical (Can be Changed) 48 Bit 32 Bit Hexadecimal Notation Dotted Decimal Notation Ex : 01-5C-D9-6B-03-2E Ex : 192.168.6.1
- 31. Network Layer: Path Determination Which Path? Which Path? • Layer 3 functions to find the best path through the internetwork
- 32. Network Layer Protocol Operations X Y C C A A • Each router provides its services to support upper layer functions
- 33. Network Layer Protocol Operations X Y C C A A B B Host X Host Y Application Application Presentation Presentation Session Router A Router B Router C Session Transport Transport Network Network Network Network Network Data Link Data Link Data Link Data Link Data Link Physical Physical Physical Physical Physical • Each router provides its services to support upper layer functions
- 34. Layers 4–7: Transport, Session, Presentation, and Application Layers
- 35. Transport Layer • Segments upper-layer applications • Establishes an end-to-end connection • Optionally, ensures data reliability
- 36. Transport Layer— Segments Upper-Layer Applications Application Electronic File Terminal Presentation Mail Transfer Session Session Transport Application Application Data Data Port Port Segments
- 37. Transport Layer— Establishes Connection Sender Receiver Synchronize Negotiate Connection Synchronize Acknowledge Connection Established Data Transfer (Send Segments)
- 38. Transport Layer— Sends Segments with Flow Control Transmit Sender Receiver Buffer Full Not Ready Stop Process Segments Go Ready Buffer OK Resume Transmission
- 39. Transport Layer— Reliability with Windowing • Window Size = 1 Send 1 Receive 1 Ack 2 Sender Send 2 Receive 2 Receiver Ack 3 • Window Size = 3 Send 1 Receive 1 Send 2 Receive 2 Sender Send 3 Receive 3 Receiver Ack 4 Send 4
- 40. Transport Layer— An Acknowledgement Technique Sender Receiver 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Send 1 Send 2 Send 3 Ack 4 Send 4 Send 5 Send 6 Ack 5 Send 5 Ack 7
- 41. Transport to Network Layer End-to-End Segments Routed Packets
- 42. Session Layer • Network File System (NFS) • Structured Query Language (SQL) • Remote-Procedure Call (RPC) • X Window System • AppleTalk Session Protocol (ASP) • DEC Session Control Protocol (SCP) Service Request Service Reply • Coordinates applications as they interact on different hosts
- 43. Session Layer In networking a session is a semi-permanent interactive information interchanges, a communication or a meeting between two or more communicating devices, or between a computer and a user (Login Session). A session begins when an application wants to make connection to a remote server, the session layer opens a temporary channel between the two. A session is established at a certain point in time and torn down at a later point in time. There could be more than one session at the same time. The session layer can ‘time stamp’ data. So that it knows where to re-start the transfer.
- 44. Presentation Layer • Text • Graphics • Data • Visual images ASCII PICT login: EBCDIC TIFF Encrypted JPEG • Sound GIF MIDI • Video MPEG QuickTime • Provides code formatting and conversion for applications
- 45. Presentation Layer ASCII : American Standard Code for Information Interchange. EBCDIC: Extended Binary Coded Decimal Interchange Code. JPEG : Joint Picture Expert Group. TIFF : Tagged Image File Format. GIF : Graphical Image Format. BMP : Bitmap Image. MPEG : Motion Picture Expert Group. AVI : Audio Video Interleave. WAV : Windows Audio Video
- 46. Application Layer COMPUTER APPLICATIONS NETWORK Word Processor APPLICATIONS INTERNETWORK Presentation Graphics Electronic Mail APPLICATIONS File Transfer Electronic Data Interchange Spreadsheet Remote Access World Wide Web Database Client-Server Process E-Mail Gateways Design/Manufacturing Information Location Special-Interest Bulletin Boards Project Planning Network Management Financial Transaction Services Others Others Internet Navigation Utilities Conferencing (Voice, Video, Data) • Internetwork applications Others can extend beyond the enterprise (i.e., to suppliers, etc.)
- 47. Summary • OSI reference model describes building blocks of functions for program-to- program communications between similar or dissimilar hosts • Layers 4–7 (host layers) provide accurate data delivery between computers • Layers 1–3 (media layers) control physical delivery of data over the network
Notas del editor
- This module covers the OSI reference model. It is sometimes also called ISO or 7 layer reference model. The model was developed by the International Standards Organization in the early 1980's. It describes the principles for interconnection of computer systems in an Open System Interconnection environment. We’ll explain what this means.
- The concept of layered communication is essential to ensuring interoperability of all the pieces of a network. To introduce the process of layered communication, let’s take a look at a simple example.
- In this slide, the goal is to get a message from Location A to Location B. The sender doesn’t know what language the receiver speaks – so the sender passes the message on to a translator. The translator, while not concerned with the content of the message, will translate it into a language that may be globally understood by most, if not all translators – thus it doesn’t matter what language the final recipient speaks. In this example, the language is Dutch. The translator also indicates what the language type is, and then passes the message to an administrative assistant. The administrative assistant, while not concerned with the language, or the message, will work to ensure the reliable delivery of the message to the destination. In this example, she will attach the fax number, and then fax the document to the destination – Location B.
- The document is received by an administrative assistant at Location B. The assistant at Location B may even call the assistant at Location A to let her know the fax was properly received. The assistant at Location B will then pass the message to the translator at her office. The translator will see that the message is in Dutch. The translator, knowing that the person to whom the message is addressed only speaks French, will translate the message so the recipient can properly read the message. This completes the process of moving information from one location to another.
- Upon closer study of the process employed to communicate, you will notice that communication took place at different layers. At layer 1, the administrative assistants communicated with each other. At layer 2, the translators communicated with each other. And, at layer 3 the sender was able to communicate with the recipient.
- That’s essentially the same thing that goes in networking with the OSI model. This slide illustrates the model. So, why use a layered network model in the first place? Well, a layered network model does a number of things. It reduces the complexity of the problems from one large one to seven smaller ones. It allows the standardization of interfaces among devices. It also facilitates modular engineering so engineers can work on one layer of the network model without being concerned with what happens at another layer. This modularity both accelerates evolution of technology and finally teaching and learning by dividing the complexity of internetworking into discrete, more easily learned operation subsets. Note that a layered model does not define or constrain an implementation; it provides a framework. Implementations, therefore, do not conform to the OSI reference model, but they do conform to the standards developed from the OSI reference model principles.
- Let’s put this in some context. You are already familiar with different networking devices such as hubs, switches, and routers. Each of these devices operate at a different level of the OSI Model. NIC cards receive information from upper level applications and properly package data for transmission on to the network media. Essentially, NIC cards live at the lower four layers of the OSI Model. Hubs, whether Ethernet, or FDDI, live at the physical layer. They are only concerned with passing bits from one station to other connected stations on the network. They do not filter any traffic. Bridges and switches on the other hand, will filter traffic and build bridging and switching tables in order to keep track of what device is connected to what port. Routers, or the technology of routing, lives at layer 3. These are the layers people are referring to when they speak of “layer 2” or “layer 3” devices. Let’s take a closer look at the model.
- The upper four layers, Application, Presentation, Session, and Transport, are responsible for accurate data delivery between computers. The tasks or functions of these upper four layers must “interoperate” with the upper four layers in the system being communicated with.
- The lower three layers – Network, Data Link and Physical -- are called the media layers. The media layers are responsible for seeing that the information does indeed arrive at the destination for which it was intended.
- The upper four layers, Application, Presentation, Session, and Transport, are responsible for accurate data delivery between computers. The tasks or functions of these upper four layers must “interoperate” with the upper four layers in the system being communicated with.
- If we take a look at the model from the top layer, the Application Layer, down, I think you will begin to get a better idea of what the model does for the industry. The applications that you run on a desktop system, such as Power Point, Excel and Word work above the seven layers of the model. The application layer of the model helps to provide network services to the applications. Some of the application processes or services that it offers are electronic mail, file transfer, and terminal emulation.
- The next layer of the seven layer model is the presentation layer. It is responsible for the overall representation of the data from the application layer to the receiving system. It insures that the data is readable by the receiving system.
- The session layer is concerned with inter-host communication. It establishes, manages and terminates sessions between applications.
- Layer 4, the Transport layer is primarily concerned with end-to-end connection reliability. It is concerned with issues such as data transport information flow and fault detection and the recovery.
- The network layer is layer 3. This is the layer that is associated with addressing and looking for the best path to send information on. It provides connectivity and path selection between two systems. The network layer is essentially the domain of routing. So when we talk about a device having layer 3 capability, we mean that that device is capable of addressing and best path selection.
- The link layer (formally referred to as the data link layer) provides reliable transit of data across a physical link. In so doing, the link layer is concerned with physical (as opposed to network or logical) addressing, network topology, line discipline (how end systems will use the network link), error notification, ordered delivery of frames, and flow control.
- The physical layer is concerned with binary transmission. It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, physical data rates, and physical connectors are defined by physical layer specifications. Now you know the role of all 7 layers of the OSI model.
- Let’s see how these layers work in a Peer to Peer Communications Network. In this exercise we will package information and move it from Host A, across network lines to Host B. Each layer uses its own layer protocol to communicate with its peer layer in the other system. Each layer’s protocol exchanges information, called protocol data units (PDUs), between peer layers. This peer-layer protocol communication is achieved by using the services of the layers below it. The layer below any current or active layer provides its services to the current layer. The transport layer will insure that data is kept segmented or separated from one other data. At the network layer we get packets that begin to be assembled. At the data link layer those packets become frames and then at the physical layer those frames go out on the wires from one host to the other host as bits.
- This whole process of moving data from host A to host B is known as data encapsulation – the data is being wrapped in the appropriate protocol header so it can be properly received. Let’s say we compose an email that we wish to send from system A to system B. The application we are using is Eudora. We write the letter and then hit send. Now, the computer translates the numbers into ASCII and then into binary (1s and 0s). If the email is a long one, then it is broken up and mailed in pieces. This all happens by the time the data reaches the Transport layer.
- At the network layer, a network header is added to the data. This header contains information required to complete the transfer, such as source and destination logical addresses.
- The packet from the network layer is then passed to the data link layer where a frame header and a frame trailer are added thus creating a data link frame.
- Finally, the physical layer provides a service to the data link layer. This service includes encoding the data link frame into a pattern of 1s and 0s for transmission on the medium (usually a wire).
- Now let’s take a look at each of the layers in a bit more detail and with some context. For Layers 1 and 2, we’re going to look at physical device addressing, and the resolution of such addresses when they are unknown.
- Locating computer systems on an internetwork is an essential component of any network system – the key to this is addressing. Every NIC card on the network has its own MAC address. In this example we have a computer with the MAC address 000.0C12.3456. The MAC address is a hexadecimal number so the numbers in this address here don’t go just from zero to nine, but go from zero to nine and then start at "A" and go through "F". So, there are actually sixteen digits represented in this counting system. Every type of device on a network has a MAC address, whether it is a Macintosh computer, a Sun Work Station, a hub or even a router. These are known as physical addresses and they don’t change. Logical addresses exist at Layer 3 of the OSI reference model. Unlike link-layer addresses, which usually exist within a flat address space, network-layer addresses are usually hierarchical. In other words, they are like mail addresses, which describe a person’s location by providing a country, a state, a zip code, a city, a street, and address on the street, and finally, a name. One good example of a flat address space is the U.S. social security numbering system, where each person has a single, unique security number.
- For multiple stations to share the same medium and still uniquely identify each other, the MAC sublayer defines a hardware or data link address called the MAC address. The MAC address is unique for each LAN interface. On most LAN-interface cards, the MAC address is burned into ROM—hence the term, burned-in address (BIA). When the network interface card initializes, this address is copied into RAM. The MAC address is a 48-bit address expressed as 12 hexadecimal digits. The first 6 hexadecimal digits of a MAC address contain a manufacturer identification (vendor code) also known as the organizationally unique identifier (OUI). To ensure vendor uniqueness the Institute of Electrical and Electronic Engineers (IEEE) administers OUIs. The last 6 hexadecimal digits are administered by each vendor and often represent the interface serial number.
- For multiple stations to share the same medium and still uniquely identify each other, the MAC sublayer defines a hardware or data link address called the MAC address. The MAC address is unique for each LAN interface. On most LAN-interface cards, the MAC address is burned into ROM—hence the term, burned-in address (BIA). When the network interface card initializes, this address is copied into RAM. The MAC address is a 48-bit address expressed as 12 hexadecimal digits. The first 6 hexadecimal digits of a MAC address contain a manufacturer identification (vendor code) also known as the organizationally unique identifier (OUI). To ensure vendor uniqueness the Institute of Electrical and Electronic Engineers (IEEE) administers OUIs. The last 6 hexadecimal digits are administered by each vendor and often represent the interface serial number.
- Now let’s take a look a layer 3--the domain of routing.
- Which path should traffic take through the cloud of networks? Path determination occurs at Layer 3. The path determination function enables a router to evaluate the available paths to a destination and to establish the preferred handling of a packet. Data can take different paths to get from a source to a destination. At layer 3, routers really help determine which path. The network administrator configures the router enabling it to make an intelligent decision as to where the router should send information through the cloud. The network layer sends packets from source network to destination network. After the router determines which path to use, it can proceed with switching the packet: taking the packet it accepted on one interface and forwarding it to another interface or port that reflects the best path to the packet’s destination.
- Let’s take a look at the flow of packets through a routed network. For examples sake, let’s say it is an Email message from you at Station X to your mother in Michigan who is using System Y. The message will exit Station X and travel through the corporate internal network until it gets to a point where it needs the services of an Internet service provider. The message will bounce through their network and eventually arrive at Mom’s Internet provider in Dearborn. Now, we have simplified this transmission to three routers, when in actuality, it could travel through many different networks before it arrives at its destination. Let’s take a look, from the OSI models reference point, at what is happening to the message as it bounces around the Internet on its way to Mom’s.
- As information travels from Station X it reaches the network level where a network address is added to the packet. At the data link layer, the information is encapsulated in an Ethernet frame. Then it goes to the router – here it is Router A – and the router de-encapsulates and examines the frame to determine what type of network layer data is being carried. The network layer data is sent to the appropriate network layer process, and the frame itself is discarded. The network layer process examines the header to determine the destination network. The packet is again encapsulated in the data-link frame for the selected interface and queued for delivery. This process occurs each time the packet switches through another router. At the router connected to the network containing the destination host – in this case, C -- the packet is again encapsulated in the destination LAN’s data-link frame type for delivery to the protocol stack on the destination host, System Y.
- Let’s look at the upper layers of the OSI seven layer model now. Those layers are the transport, session, presentation, and application layers.
- Transport services allow users to segment and reassemble several upper-layer applications onto the same transport layer data stream. It also establishes the end-to-end connection, from your host to another host. As the transport layer sends its segments, it can also ensure data integrity. Essentially the transport layer opens up the connection from your system through a network and then through a wide area cloud to the receiving system at the other end.
- The transport layer has several functions. First, it segments upper layer application information. You might have more than one application running on your desktop at a time. You might be sending electronic mail open while transferring a file from the Web, and opening a terminal session. The transport layer helps keep straight all of the information coming from these different applications.
- Another function of the transport layer is to establish the connection from your system to another system. When you are browsing the Web and double-click on a link your system tries to establish a connection with that host. Once the connection has been established, there is some negotiation that happens between your system and the system that you are connected to in terms of how data will be transferred. Once the negotiations are completed, data will begin to transfer. As soon as the data transfer is complete, the receiving station will send you the end message and your browser will say done. Essentially, the transport layer is responsible then for connecting and terminating sessions from your host to another host.
- Another important function of the transport layer is to send segments and maintain the sending and receiving of information with flow control. When a connection is established, the host will begin to send frames to the receiver. When frames arrive too quickly for a host to process, it stores them in memory temporarily. If the frames are part of a small burst, this buffering solves the problem. If the traffic continues, the host or gateway eventually exhausts its memory and must discard additional frames that arrive. Instead of losing data, the transport function can issue a not ready indicator to the sender. Acting like a stop sign, this indicator signals the sender to discontinue sending segment traffic to its peer. After the receiver has processed sufficient segments that its buffers can handle additional segments, the receiver sends a ready transport indicator, which is like a go signal. When it receives this indicator, the sender can resume segment transmission.
- In the most basic form of reliable connection-oriented data transfer, a sequence of data segments must be delivered to the recipient in the same sequence that they were transmitted. The protocol here represents TCP. It fails if any data segments are lost, damaged, duplicated, or received in a different order. The basic solution is to have a receiving system acknowledge the receipt of every data segment. If the sender had to wait for an acknowledgment after sending each segment, throughput would be low. Because time is available after the sender finishes transmitting the data segment and before the sender finishes processing any received acknowledgment, the interval is used for transmitting more data. The number of data segments the sender is allowed to have outstanding–without yet receiving an acknowledgment– is known as the window. In this scenario, with a window size of 3, the sender can transmit three data segments before expecting an acknowledgment. Unlike this simplified graphic, there is a high probability that acknowledgments and packets will intermix as they communicate across the network.
- Reliable delivery guarantees that a stream of data sent from one machine will be delivered through a functioning data link to another machine without duplication or data loss. Positive acknowledgment with retransmission is one technique that guarantees reliable delivery of data streams. Positive acknowledgment requires a receiving system or receiver to communicate with the source, sending back an acknowledgment message when it receives data. The sender keeps a record of each packet it sends and waits for an acknowledgment before sending the next packet. In this example, the sender is transmitting packets 1, 2, and 3. The receiver acknowledges receipt of the packets by requesting packet number 4. The sender, upon receiving the acknowledgment sends packets 4, 5, and 6. If packet number 5 does not arrive at the destination, the receiver acknowledges with a request to resend packet number 5. The sender resends packet number 5 and must receive an acknowledgment to continue with the transmission of packet number 7.
- The transport layer assumes it can use the network as a given “cloud” as segments cross from sender source to receiver destination. If we open up the functions inside the “cloud,” we reveal issues like, “Which of several paths is best for a given route?” We see the role that routers perform in this process, and we see the segments of Layer 4 transport further encapsulated into packets.
- The session layer establishes, manages, and terminates sessions among applications. This layer is primarily concerned with coordinating applications as they interact on different hosts. Some popular session layer protocols are listed here, Network File Systems (NFS), Structured Query Language or SQL, X Window Systems; even AppleTalk Session Protocol is part of the session layer.
- The session layer establishes, manages, and terminates sessions among applications. This layer is primarily concerned with coordinating applications as they interact on different hosts. Some popular session layer protocols are listed here, Network File Systems (NFS), Structured Query Language or SQL, X Window Systems; even AppleTalk Session Protocol is part of the session layer.
- The presentation layer is primarily concerned with the format of the data. Data and text can be formatted as ASCII files, as EBCDIC files or can even be Encrypted. Sound may become a Midi file. Video files can be formatted as MPEG video files or QuickTime files. Graphics and visual images can be formatted as PICT, TIFF, JPEG, or even GIF files. So that is really what happens at the presentation layer.
- The presentation layer is primarily concerned with the format of the data. Data and text can be formatted as ASCII files, as EBCDIC files or can even be Encrypted. Sound may become a Midi file. Video files can be formatted as MPEG video files or QuickTime files. Graphics and visual images can be formatted as PICT, TIFF, JPEG, or even GIF files. So that is really what happens at the presentation layer.
- The application layer is the highest level of the seven layer model. Computer applications that you use on your desktop everyday, applications like word processing, presentation graphics, spreadsheets files, and database management, all sit above the application layer. Network applications and internetwork applications allow you, as the user, to move computer application files through the network and through the internetwork.
- To review what we’ve learned – the OSI reference model describes what must transpire for program to program communications to occur between even dissimilar computer systems. Each layer is responsible to provide information and pointers to the next higher layer in the OSI Reference Model. The Application Layer (which is the highest layer in the OSI model) makes available network services to actual software application programs. The presentation layer is responsible for formatting and converting data and ensuring that the data is presentable for one application through the network to another application. The session layer is responsible for coordinating communication interactions between applications. The reliable transport layer is responsible for segmenting and multiplexing information, keeping straight all the various applications you might be using on your desktop, the synchronization of the connection, flow control, error recovery as well as reliability through the process of windowing. The network layer is responsible for addressing and path determination. The link layer provides reliable transit of data across a physical link. And finally the physical layer is concerned with binary transmission.