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OSI Reference Model Lecture
Layer 1: Physical-> [point-to-point conncections] Concerned with the transmission of a bit stream over a communication channel. Specifications include signal voltages, bit duration, and channel definition. Deals with mechanical and electrical characteristics to activate, maintain, and de-activate the physical connection. Layer 2: Data Link-> [point-to-point conncections] Contains specifications for frames (Ethernet 2, 802.2, 802.3 etc), synchronization, and error control. These specifications change the raw bit stream supplied by layer 1 into data and also addressess reliability and integrity issues. Since Layer 2 handles error control within each packet, the higher layers may assume the each packet is errorFree. [However, a packet could be lost or duplicated. Layer 2 would not know that.] Ethernet frames contain the destination MAC address and source MAC address. To avoid having large collision domains switches should be used over hubs. To avoid large broadcast domains however, layer 3 routers and VLANs must be utilised. Layer 3: Network-> The network layer makes (on the whole destination based) routing decisions across the communication network that may consist of a number of nodes and possible routes and whose structure is known to the network layer. For example, in packet switched networks, it sets up the circuit for the transmission of each packet and then de-allocates and re-allocates the nodes to other services. At this layer, the computer engages in a dialog with the network and requests network services. This layer contains all the specifications for the transmission and switching technologies needed to build circuits through a network of nodes. IP packets are used here which contain destination IP and source IP address. Layer 4: Transport-> This layer provides error detection and control at the message level across a network of nodes. For instance, in packet switched networks, it handles error-free packet sequencing without losses or duplications as well as re-transmissions for bad or lost packets. Levels 3 and 4 are part of network services. IP segments are used here which contain port numbers and allow 3 way handshake virtual circuit to be setup. Layer 5: Session-> The session layer assumes that a reliable virtual point-to-point connection has been made and contains specs for the dialog between the two end systems such as dialog discipline, data grouping, and recovery of an interrupted session. Specs are also included for initiating and concluding a session. Many network specs contain little or no session specs and leave these decisions to the applications. Example: telnet, NFS, SQL and Xwindows. Layer 6: Presentation-> Provides transformation of data to standardize the application interface. Also provides some network services such as encryption, compression, and text re-formatting. eg VPN technologies, jpeg and png graphics. Layer 7: Application-> This layer plays the same role as the 'application interface' in operating systems. Provides network services to users (applications) of the network in a distributed processing environment: eg File Transfer Protocol, email, and web browsers.
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