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Transcript of OSI Model
Standardization), and OSI stands for Open System
Interconnection. It is the key part of networking because
its common framework for developers and students of
networking to work with and learn from. It is a important networking model and teaching tool. It is made up of 7 layers- Application, Presentation, Session, Transport, Network, Data Link, Physical. Application Provides interfaces for applications to access network
services, for example: File sharing, message handling,
database access, and error recovery. The PDU is referred
to as data. Protocols used as this layer are: HTTP, FTP,
SMB/CIFS, TFTP, and SMTP. Problems that can occur
at this layer would be: missing/misconfigured client or server software, incompatible commands used to communicate between a client and server. Handles data formatting and translation, for outgoing
messages it converts data into a specified format by the Application Layer, or for incoming messages , it reverses. It also handles protocol conversion, data encryption/decryption, data compression/decompression, data representation incompatibilities between OSs, and graphic commands. A "redirector" is present at this layer, which requests for service from the computer. Problems that could occur at this layer would be: incompatible or missing translation software due to a problem in the Presentation Layer. Session Permits two computers to hold ongoing communications, "sessions", across a network, so applications on either end of the session can exchange data for as long as the session lasts. Handles communication setup ahead of data transfers when necessary and session teardown when the session ends. Common functions: handle name lookup and user logon/logoff. DNS and other name resolution protocols are used at this layer. Manages the mechanics of ongoing conversations, and a process called "check-pointing" which is a synchronization process between two related streams of data. Transport Network Data Link Physical Overview Presentation Manages data transfer from one application to
another across the network. It breaks long data streams into "segments". Segmenting is needed because networks have maximum frame sizes, MTU. The MTU is 1518 bytes. Also includes flow control and acknowledgments and handles resequencing segments into the original data on receipt to ensure reliabile delivery. Key fields: Source and Destination Port Numbers, Sequence and Acknowledgement Numbers, and Window Size. Problems that can occur at this layer is: segments that are too large between source and destination networks. Handles logical addressing, translates logical network addresses (IP) into phyiscal addresses (MAC), and performs best path selection and routing in an internetwork. It's the "traffic cop" for networking activity because of Access Control. Problems that could occur at this layer is: incorrect IP addresses or subnet masks, incorrect router configuration/operation errors. Works with frames, and is the mediator between the layers. Defines how computers access the network. MAC address is defined at this layer. Turns packets into a frame by adding a header that includes a source and destination MAC addresses. Discards frames containing CRC and other frame errors, but retransmits data. Problems that could occur at this layer is: collision/invalid frames causes by collisions, port network design, line noise, or NIC driver problems. Also, using incompatible network technologies. Converts bit into signals for outgoing messages, and signals into bits for incoming messages. The type of signal generated depends on the medium. Specifies how to encode 1s and 0s, encoding is representing 0s and 1s by a physical signal- electrical voltages or light pulses. Includes all the cables, connectors, hubs, switches etc used on the medium. Problems that could occur at this layer is: incorrect media termination, EMI or noise that scrambles signals, and NICs and hubs that are misconfigured or don't work properly. The OSI model is a helpful way to categorize networking activities. It helps explain how data is formatted and moves up/down the protocol stack, and from computer to computer. There is another model, called TCP/IP which has less layers and was created before OSI. TCP/IP only has four layers: Application, Transport, Internetwork, and Network Access.