Day 1.3 osi reference

Editor's Notes

• #2: 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.
• #5: 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.
• #6: 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.
• #7: 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.
• #8: 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.
• #9: 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.
• #10: 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.
• #11: The session layer is concerned with inter-host communication. It establishes, manages and terminates sessions between applications.
• #12: 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.
• #13: 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.
• #14: 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.
• #15: 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.
• #16: 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.
• #17: 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.
• #18: 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.
• #19: 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.
• #20: 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).
• #21: 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.
• #22: 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.
• #23: Now let’s take a look a layer 3--the domain of routing.
• #24: 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.
• #25: Let’s look at the upper layers of the OSI seven layer model now. Those layers are the transport, session, presentation, and application layers.
• #26: 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.
• #27: 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.
• #28: 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.
• #29: 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.
• #30: 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.
• #31: 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.
• #32: 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.
• #33: 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.
• #34: 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.
• #35: 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.