Chapter 1: OSI Model Abdullah Konak School of Information Sciences and Technology Penn State Berks Learning Objectives Define the term protocol architecture and explain the need and benefits of a communication architecture. A. Konak IST 220/Ch1: Introduction 2 1
Challenge? Professor A Urdu & English Translator I like rabbits L: Dutch Ik vind konijnen leuk Use Translator Information for the remote translator use Dutch Professor B J aime bien les lapins Chinese & French Translator L: Dutch Ik vind konijnen leuk Secretary Fax:# L: Dutch Ik vind konijnen leuk Information for the remote secretary use fax Secretary Fax:# L: Dutch Ik vind konijnen leuk A. Konak IST 220/Ch1: Introduction 3 What is a Protocol? Shared conventions and rules for communicating information are called protocols Allows entities (i.e. application programs) from different systems to communicate. Includes syntax, semantics, and timing. A protocol architecture consists of a layered set of protocols. A. Konak IST 220/Ch1: Introduction 4 2
Why Use Layered Protocol Architecture? Breaks complex tasks into subtasks Each module handles specific subset of tasks Communication occurs between different modules on the same system between similar modules on different systems Easier application development Network can change without all programs being modified A. Konak IST 220/Ch1: Introduction 5 Standardized Protocol Architectures Vendors like standards because they make their products more marketable Customers like standards because they enable products from different vendors to interoperate Two protocol standards are well-known: TCP/IP: widely implemented OSI: well-known, less used, still useful for modeling/conceptualizing A. Konak IST 220/Ch1: Introduction 6 3
The OSI and TCP/IP (Internet) Protocol Architecture Open Systems Interconnection (OSI) Reference Model: Still an excellent model for conceptualizing and understanding protocol architectures Transmission Control Protocol/Internet Protocol (TCP/IP) A. Konak IST 220/Ch1: Introduction 7 The OSI Layers The lower levels The upper levels Application Presentation Transport Network Data Link Provides user interface Initiates services Transfer data into standard format before transmission Keeps data different applications data separate Control the data exchange End-to-end data error free data transmission Logical addressing for data packets Routing and error handling NIC software function How data encapsulation Error detection and collision prevention Physical Moves bits between devices Specifies voltages, cables, and cables A. Konak IST 220/Ch1: Introduction 8 4
OSI Protocol Architecture Client Application: Web browser Presentation Application Transport Network Server Application: Web Server Data Link A. Konak IST 220/Ch1: Introduction 9 Physical Application Presentation 1 Use A starts a chat program, an application layer entity 2 IST 220 is fun! The user types IST 220 is fun! HOST A 3 The message is coded into an Encoding internationally known code (ASCII) 10100101001010000011111 plaintext Encryption 0101101011010111111000000 ciphertext 4 If necessary, the message is encrypted. Compression 01011010110101111110 5 If necessary, the message is compressed. 01011010110101111110 S1 6 A communication between hosts A and B is established. A session ID is assigned (S1). Transport Network 7 The transport layer protocols breaks the bit stream into smaller segments. Each segment is attached with a sequence number 01011 S1 1 0101101 S1 2 01111110 S1 3 segment segment 8 The network layer protocols make a decision about how to route segments through the network. The address of host B is added to segments, and they are now called packets. 01011 S1 1 B 0101101 S1 2 B 01111110 S1 3 B packet Data Link Physical 9 At the data link layer, each packet is framed according to specifications of the network. Packets are now called frames. 01011 S1 1 B H 0101101 S1 2 B H 01111110 S1 3 BH + A. Konak IST 220/Ch1: Introduction 10-10 At the physical layer, frames are converted into electrical pulses. 5
Application IST 220 is fun! HOST B decoding 8 The message is decoded into text. Presentation Transport Network 10100101001010000011111 decryption 0101101011010111111000000 01011010110101111110 decompression 01011010110101111110 S1 plaintext ciphertext 7 the message is decrypted. 6 the message is decompressed. 01011 S1 1 0101101 S1 2 01111110 S1 3 3 The network layer does not have many functions here. Using the session ID, the session layer 5 sends the bit stream to the correct application 4 Using the sequence numbers to segments are assembled into bit streams at the receiving host. If segments are lost on the way, transport layer protocols acknowledge the sender. 01011 S1 1 B 0101101 S1 2 B 01111110 S1 3 B packet Data Link Physical 2 01011 S1 1 B H 0101101 S1 2 B H 01111110 S1 3 BH + - At the data link layer, each packet is framed according to the specifications of the network. Packets are now called frames. Each frame carries a header with point-to-point physical addresses. A. Konak IST 220/Ch1: Introduction 11 1 At the physical layer reads the electrical pulses. Application Layer The application layer gives services to the user of the network Directory Services FTP Telnet Web browser Directory Services FTP Telnet Web browser Application Layer Application Layer Application Layer Data Application Layer Data To presentation layer From presentation layer A. Konak IST 220/Ch1: Introduction 12 6
Presentation layer The presentation layer is responsible for translation, translating data from the computer's encoding system to some international code. Application Layer Data From Application Layer Application Layer Data To Application Layer Presentation layer Encoded, encrypted, and compressed data H6 Presentation layer Decoded, decrypted, and decompressed data H6 To Layer A. Konak IST 220/Ch1: Introduction 13 From Layer Layer The session layer provides two-way communication between hosts by establishing, synchronizing, and terminating connection. Presentation Layer Data From Presentation Layer Presentation Layer Data To Presentation Layer layer layer SYN SYN SYN H5 SYN SYN SYN H5 To Transport Layer From Transport Layer A. Konak IST 220/Ch1: Introduction 14 7
Transport Layer Prepares data for transmission and provides a reliable mechanism for exchange of data between hosts. From Layer Layer Data To Layer Layer Data Transport layer Transport layer To Network Layer From Network Layer A. Konak IST 220/Ch1: Introduction 15 Network Layer Network layer provides addressing and routing services across different types of networks. Transport Layer Data From Transport Layer Transport Layer Data To Transport Layer Network layer H3 Network layer H3 To Data Link Layer From Data Link Layer A. Konak IST 220/Ch1: Introduction 16 8
Data Link Layer Provides error correction and control services to the higher layers. Framing makes sure that packets can travel different networks. From Network Layer To Network Layer Network Layer Data Network Layer Data Data Link Layer Data Link Layer T2 H2 T2 H2 To Physical Layer From Physical Layer A. Konak IST 220/Ch1: Introduction 17 Physical Layer Transmits bits over a medium; provides electrical and mechanical specifications. Data Link Layer Data From Data Link Layer Data Link Layer Data To Data Link Layer Physical Layer Physical Layer 01010101010101 01010101010101 A. Konak IST 220/Ch1: Introduction 18 9