OSI Model

1. OSI Model Honolulu Community College Cisco Academy Training Center Semester 1 Version 2.1.1

2. Overview • Past 2 decades has seen a tremendous growth in networking. • Manufacturers developed their own equipment, leading to problem of compatibility between equipment of different vendors. • Proprietary systems like DECnet, SNA and TCP/IP would not communicate with each other. • To address this problem, ISO (International Organization for Standardization) conducted a study, and in 1984 released the OSI model.

3. What is the OSI Model? • The OSI reference model is the primary model for network communications. • OSI model represents the communication process, how data is moved from one workstation to another on the network. • A layered model: • Breaks the communication process down into 7 tasks (represented by the 7 layers). • Network functions are specified for each layer. • Provides a standard to insure compatibility and interoperability.

4. Why a Layered Model? • Reduces complexity. • Standardizes interfaces. • Facilitates modular engineering. • Ensures interoperable technology. • Accelerates evolution. • Simplifies learning and teaching.

5. Network Communications • Originates at a source, and travels to a destination. • Information that travels on the network is the data, packet, or data packet. • Data packet includes the source information along with other elements needed to make communication possible and reliable. • Source & destination addresses. • Information for error detection and error correction. • Sequence numbers. • Information identifying the application.

6. Media • Media is the material through which data packets travels. • Telephone wires • Category 5 UTP (unshielded twisted pair) • STP (shielded twisted pair) • Coaxial cables (used for cable TV) • Thinnet - 10base2 • Thicknet - 10base5 • Optical fibers (thin glass fibers that carry light) • Multi-mode (62.5/125 microns) • Single-mode

7. Protocol • A set of rules, or an agreement, that determines the format and transmission of data. • Protocol is like a language. • If devices do not use the same protocol, they will not understand each other.

8. OSI Model • Provides standards. • Represents the communication process in a network. • Breaks communications process into 7 separate, independent tasks. • Each layer represents one task.

9. Application Layer (7) • Provides network services to applications. • Email, FTP - file transfer protocol, Terminal emulation, Telnet Presentation Layer (6) • Data Representation • Ensures data is readable. • Data format and structure. • TIFF, GIF, JPEG, Quicktime, MIDI,etc. • Negotiated data transfer syntax. • Coding. • Compression.

10. Session Layer (5) • Interhost communications. • Establishes, manages and terminates sessions between applications. • Provides support for layers above it.

11. Transport Layer (4)

12. Network Layer (3)

13. Data Link Layer (2)

14. Physical Layer (1) • Foundation layer of the OSI model. • Defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. • Voltage levels and timing of voltage changes. • Physical data rates. • Maximum transmission distances. • Physical connectors.

15. OSI Layers - key words • Application - browsers. • Presentation - common data format. • Session - dialogues and conversations. • Transport - quality of service, and reliability. • Network - path selection, routing, and addressing. • Data link -frames and media access control. • Physical - signals and media.

16. Encapsulation • If one computer wants to send data to another computer, the data must first be packaged in a process called encapsulation. • Encapsulation is packaging the data, as you would do to a letter when sending it through the mail. • Encapsulation wraps data with the additional protocol information before transmission. • Additional information is in the form of headers, and/or trailers.

17. Data Encapsulation • Each layer ‘encapsulates’ the data from the upper layer. • Data is wrapped with the appropriate headers and trailers. • This happens at each layer as data flows down the OSI model.

18. Data Encapsulation Example • Consider sending email. • In the OSI model, the email program user interface is the application layer. • The host layers (upper 4 layers) provide the data, which is then physically delivered by the media layers (lower 3 layers) over the network.

19. Details: Email Example(host layers) • Application Layer: the email program user interface, used to create the email message. • Presentation Layer: the alphanumeric characters are coded in ASCII or some other code. • Session Layer: the characters are formed into an email message for transmission. • Transport Layer: segments the data for transport through the network (TCP header).

20. Email Example (media layers) • Network Layer: segmented data is put into packets (datagrams) with a network header. • header has source and destination logical addresses (IP). • Data Link Layer: packet is put into a frame. • Frame header has source and destination physical addresses (MAC), frame trailer is also added. • Frame is encoded for transmission. • Physical Layer: signals transmitted over the networking media.

22. Peer to Peer Communication • Each layer uses its own layer’s protocol to communicate with peer layer in the other system. • Uses PDU - protocol data unit • Peer-layer communications uses services of the layers below it. • Each layer takes the upper layer’s data and puts it in its own PDU. • This is data encapsulation.

24. TCP/IP Reference Model • The historical and technical open standard of the Internet is TCP/IP. • TCP/IP reference model & TCP/IP protocol stack make data communication possible. • Made up of only 4 layers. • Application layer - handles high-level protocols, issues of representation, coding, and dialog control. • Combines application, presentation & session OSI layer functions.

25. TCP/IP Reference Model (cont.) • Transport layer - deals with the quality-of-service issues of reliability, flow control, and error correction. • Similar to OSI transport layer. • Internet layer - sends data packets through internetwork & have them arrive at the destination independent of the path and networks they took. • Similar to OSI network layer.

26. TCP/IP Reference Model (cont.) • Network layer - also called the host-to-network layer. • Concerned with all of the issues that an IP packet requires to actually make a physical link. • Includes the LAN and WAN technology details. • Combines Data link and Physical OSI functions.

27. TCP/IP Protocol Graph • Illustrates some of the common protocols that are specified by the TCP/IP reference model. Application Transport Internet Network

28. TCP/IP Protocol Graph (cont.) • Transport layer uses two protocols - TCP & UDP. • TCP - connection oriented, reliable. • UDP - connection-less, unreliable. • Only one network protocol - IP (internet protocol). • IP serves as a universal protocol, Application Transport Internet Network

30. Summary • The OSI reference model provides a standard which ensures greater compatibility and interoperability between various types of network technologies. • Encapsulation - process in which data is wrapped with a particular protocol header/trailer before transmission. • Peer-to-Peer Communications - each layer uses it own protocol data units (PDUs) to communicate with its peer layer. The End