Chapter 2 (Handout 1 only sections 2.1, 2.2 and 2.3) Dr. Clincy Professor of CS Dr. Clincy Lecture 1 OSI Open Systems Interconnection Developed by ISO (International Organization for Standardization)
Contains seven layers Dr. Clincy Intro and Lecture Application Presentation Session Transport Network Data Link Physical 2
OSI Reference Model ? Bottom 3 layers (Data Comm Layers) Responsible for getting data or info to destination Routing and switching occurs Define the electrical and physical standards Performs bit ordering, transmission of the bits, and error detecting and correcting Top 4 layers Creation and interpretation of the data Protocols for data formatting, message syntax, dialogue management, message sequences and info presentation are used Dr. Clincy Intro and Lecture 3
OSI Physical Layer (Old CS3501/CS3503 covered this) Responsible for transmission of bits Always implemented through hardware Encompasses mechanical, electrical, and functional interfaces Encoding and Decoding issues: how 0s and 1s are converted to signals Signal translation (ie. electrical to optical) Signal Multiplexing and Demultiplexing Signal Modulation and Demodulation Transport medium: Coaxial, Twisted Pair, Optical, etc.. Transmission Rate/Data Rate how fast to send bits Transmission mode: transmission direction (simplex, duplex) Dr.Physical Topology: network layout Clincy Intro and Lecture 4 OSI Data Link Layer (partially Old CS3501/CS3503 partially covered and CS4622 partially covers)
Responsible for error-free, reliable transmission of data Framing, Flow control, Error control (detection/correction), Access Methods Makes use of physical address because with in the same network Network Layer Actually sends the packets (groups of frames) from node to node using a routing algorithm Data Link Layer Takes raw data (bits) and transform them into frames, error control, etc. Physical Layer Transmit and receive the raw data (bits) Dr. Clincy Intro and Lecture
5 OSI Network Layer (CS4622 covers) Responsible for routing of messages through networks Concerned with type of switching used (circuit v. packet) Handles routing among different networks NOTE: with in the same network, only the DATA LINK layer is needed amongst multiple networks, the NETWORK LAYER is needed No need for routing with in the same network (LAN) Routing across internetworks Makes use of logical address vs physical address because not with in the same network Dr. Clincy Intro and Lecture 6
OSI Network Layer (CS4622 covers) High Level View of Network Layers Functions: 1. Transmitting data packets through a network in a timely manner 2. There are more than one route between the source and destination, the network layer chooses the best route (next hop) based on some criteria. Examples - A Node Cost Link Cost Distance Spare Cap. Low Util. Z 3. Makes sure the network does not become congested when link or node failures occur. Passes data between two networks (differing networks)
Dr. Clincy Intro and Lecture 7 Recap - OSI Already covered bottom 3 layers (Data Comm) Dr. Clincy Application Presentation Session Transport Network Data Link Physical
Lecture 8 OSI Upper Layers Application Presentation Session Transport Peer-to-Peer Processes .. End-to-End nodes only Dr. Clincy Lecture 9
OSI Transport Layer Isolates messages from lower and upper layers Breaks down message size (segmentation) (down) and performs re-assembly (up) Monitors quality of communications channel (oversee all hops) Selects most efficient communication service necessary for a given transmission (could change over hops) Flow and Error control for Source and Sink Dr. Clincy Lecture 10 OSI Session Layer Establishes logical connections between systems (up/down) Manages log-ons, password exchange, log-offs (up/down) Terminates connection at end of session (up/down) Dr. Clincy
Lecture 11 OSI Presentation Layer Provides format and code conversion services Examples File conversion from ASCII to EBDIC Invoking character sequences to generate bold, italics, etc on a printer The source and sink could operate using different encoding schemes the presentation layer makes the translations Security Compression Dr. Clincy Lecture 12 OSI Application Layer
Provides access to network for end-user (end-user being a human being or software application) Users capabilities are determined by what items are available on this layer (ie. remote log-in, file transfer, email service, directory service, etc.) Dr. Clincy Lecture 13 Recap: What happens at the Intermediate Nodes ? Rx Tx 7 Intermediate Nodes 3 1 1
A Dr. Clincy B C Lecture Q T Z14 COMPLEXITY TO CONSIDER Any particular node in an internetwork can be functioning as follows simultaneously: Tx to other internetwork nodes Rx from other internetwork nodes Intermediate node to some other internetwork nodes Dr. Clincy
Lecture 15 OSI in Action: Outgoing File Transfer The File Transfer Program issues a command to the Application Layer Application passes it to Presentation, which may reformat, encrypt, encode, compress, passes to Session (adds overhead) Session requests a connection, passes to Transport (adds overhead) Transport breaks file into chunks, adds error-checking and flowcontrol info, process-to-process, passes to Network (adds overhead)
Dr. Clincy Lecture Network selects the datas route (internetworking), passes to Data Link (adds overhead) Data Link adds error-control and flow-control info, passes to Physical (adds overhead) Physical translates bits to signal and transmits the signal, which includes information added by each layer 16
OSI in Action: Incoming File Transfer Physical receives signal and translates to bits, passes to Data Link Data Link checks for errors and performs flow control on bits, formulates bits into some formation (frames), passes to Network Network verifies routing (if intermediate node, determines next hop), passes to Transport Transport checks for errors and performs flow control on the chunks, reassembles the chunks, passes to Session
Session determines if transfer is complete, may end session, passes to Presentation Dr. Clincy Lecture Presentation may reformat, perform conversions, decode, decrypt, decompress, pass to Application layer Application presents results to user (e.g. updates FTP program display) 17 US Postal System Analogy Illustrate how the US Postal System is very similar to how networking works
Will help students better understand (versus memorize) data comm and networking Upper Layers creating and interpreting the signal, data or info Lower Layers getting the signal from one place to the next
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