Lecture 2 - The OSI Model.pdf
Transcript
LECTURE 2 THE OSI MODEL OVERVIEW Define basic term: Protocol Background to (the) Layered Architectures OSI Model Layer Functions Protocol Data Units Encapsulation Process BASIC DEFINITIONS Protocol: System of rules allowing two or more nodes to communicate by transmitting and receiv...
LECTURE 2 THE OSI MODEL OVERVIEW Define basic term: Protocol Background to (the) Layered Architectures OSI Model Layer Functions Protocol Data Units Encapsulation Process BASIC DEFINITIONS Protocol: System of rules allowing two or more nodes to communicate by transmitting and receiving data BACKGROUND Computer networks were “closed” by vendor technology e.g. IBM’s SNA, DEC’s DECNet International Organization for Standards (ISO) developed an architectural model (“blueprint”) for “open” communication i.e. compatible applications, protocols, services and devices Allows interoperability through industry standardization and multi-vendor development BACKGROUND (CONT’D) OSI Model Layered architecture Modularizes and simplifies development, design, implementation and support of components Defines Who does What When and How Reference model and not an implementation model THE 7 LAYERS All People Seem To Need Data Processing UPPER LAYERS VS LOWER LAYERS The Upper Layers (5 – 7) are responsible for communication between host applications The Lower Layers (1 – 4) are responsible for actual data transmission between hosts PEER LAYERS Logically, layers communicate with their corresponding layers SERVICE LAYERS Physically, layers communicate with their lower layers ENCAPSULATION Sender’s data wrapped with Protocol Data Units (PDUs) by each lower layer at header and occasionally at trailer Each of sender’s layers communicates with corresponding receiver’s layer LAYER 7: APPLICATION LAYER FUNCTIONS ENCAPSULATION PROCESS Provides Protocol Data Unit: Data a user interface i.e. accepts input and displays output Accepts input from application services (e.g. file, print, message, web, database and application) Passes Data to Layer 6 / Presentation Layer communication protocols for applications e.g. HTTP for Edge/Firefox SMTP/IMAP/POP for Outlook/Thunderbird FTP for File Explorer/Filezilla Uses application processes, interfaces or APIs LAYER 6: PRESENTATION LAYER FUNCTIONS ENCAPSULATION PROCESS Presents data Protocol Data Unit: Data Translates and formats data e.g. Accepts Data from Layer 7 / encoding/decoding Application Layer encrypting/decrypting Encodes, Encrypts and/or Compresses compressing/decompressing Data Passes Data to Layer 5 / Session Layer LAYER 5: SESSION LAYER FUNCTIONS ENCAPSULATION PROCESS Provides dialog control Protocol Data Unit: Data Keeps different applications’ data separate Accepts Data from Layer 6 / Presentation Layer Set up, Manage or tear down session Organizes communication as Passes Data to Layer 4 / Transport Layer Simplex (unidirectional) Half duplex (bidirectional asynchronous) Full duplex (bidirectional synchronous) LAYER 4: TRANSPORT LAYER FUNCTIONS ENCAPSULATION PROCESS Provides end-to-end connection by Protocol Data Unit: Segment establishing logical connection between hosts Accepts Data from Layer 5 / Session Layer (virtual circuit) Adds Transport Header Segments data and delivers through data streams Passes Segment to Layer 3 / Network Layer Provides two types of delivery: Reliable (Connection-oriented) Unreliable (Connection-less) CONNECTION-ORIENTED Sender creates a virtual circuit with receiver using a “three-way handshake” (SYN, SYN/ACK, ACK) Flow control is maintained by: receiver acknowledging delivered segments sender retransmitting unacknowledged segments segments sequenced in proper order by receiver (error correction before retransmit) Windowing is used to specify the number of segments after which acknowledgement is sent LAYER 3: NETWORK LAYER FUNCTIONS ENCAPSULATION PROCESS Provides logical addressing of hosts, which identifies and Protocol Data Unit: Packet locates their network Accepts Segment from Layer 4 / Transport Layer Provides routing, which transports data between networks Adds Network Header Routers use routing tables to perform path determination Passes Packet to Layer 2 / Data Link Layer accept (receive) data destined for their network forward (relay) data destined for known networks drop (reject) data destined for unknown networks LAYER 2: DATA LINK LAYER FUNCTIONS ENCAPSULATION PROCESS Provides hardware addressing of Protocol Data Unit: Frame devices, which identifies and locates Accepts Packet from Layer 3 / Network nodes on network Layer Provides framing and error detection Adds Data Link Header and Trailer (not correction) Passes Frame to Layer 1 / Physical Provides logical topology Layer LAYER 1: PHYSICAL LAYER FUNCTIONS ENCAPSULATION PROCESS Provides physical topology Protocol Data Unit: Bit Moves bits between devices Accepts Frame from Layer 2 / Data Link Layer Represents bits using state transitions of voltage (copper), light (fibre) or Passes Bits to Transmission Media audio (wireless) DECAPSULATION Presentation passes Data to Application Session passes Data Presentation Transport passes Data to Session Network passes Segment to Transport Data Link passes Packet to Network Physical passes Frame to Data Link Transmission Media passes Bits to Physical TO BE CONTINUED… WIRED TRANSMISSION MEDIA
