Computer Networks. Communica)on Reference Models. Week 02. College of Information Science and Engineering Ritsumeikan University

Transcription

1 Computer Networks Communica)on Reference Models Week 02 College of Information Science and Engineering Ritsumeikan University

2 Today s Agenda l Quiz (from last week s lecture and the Homework) l Review Introduction to Computer Networks l The OSI Model l Comparing TCP/IP to OSI l Homework reading 2

3 Objective of Communication Models l The models give network designers and administrators a general overview of the steps and modules of the communication process l Modularity allows the different layers to be considered independently l The models provide the theoretical framework for standards and protocols

4 The Purpose of the OSI Model l OSI stands for Open Systems Interconnection l considered the fundamental framework for the way all devices on a network should communicate (Ciccarelli and Faulkner,) l Gives engineers an overview of how information passes from one part of a network to the next

5 Seven Layers of OSI 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical

6 Seven Layers of OSI User A 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical User B Application Presentation Session Transport Network Data Link Physical

7 A Mnemonic for the OSI Layers APSTNDP: All People Seem To Need Data Processing 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical

8 Seven Layers of OSI (APSTNDP) Layer Format Purpose 7. Application Data Network process to application 6. Presentation Data Data representation & encryption 5. Session Data Interhost communication 4. Transport Segments End-to-end connections, reliability 3. Network Packets Logical addressing 2. Data Link Frames Physical addressing 1. Physical Bits Media, signal, & binary addressing

9 7. The Application Layer l Name does not mean application programs but often means network APIs (application programming interfaces) l This is the layer closest to the user and it formats the data for a particular function l Printers, applications (programs), and database services also operate on the applications layer

10 Application Layer Examples l HTTP, Hypertext Transfer Protocol, for requesting information such as a web page l SMTP, Simple Mail Transfer Protocol, for mail transfer agents to relay messages between machines l DNS, Domain Name Systems, which connect the names of web servers to their Internet Protocol addresses

11 6. The Presentation Layer l This layer performs data presentation, data compression and data encryption l Data presentation makes sure the data is in a format compatible with the receiving device l Data compression simplifies the data to reduce the amount of data to send l Encryption makes the transfer secure by scrambling the message l The presentation layer at the receiver performs: decryption, expansion (decompression), presentation

12 Presentation Layer Examples l MIME, Multipurpose Internet Mail Extensions, to support non-ascii character sets, attachments, etc, in messages l PGP, Pretty Good Privacy, which encrypts and decrypts data using public and private keys l MPEG, JPEG, ASCII, Unicode, and all other data format conversions

13 5. The Session Layer l Tasks include: establishing a connection, maintaining the session, ending the connection, dialog control, dialog separation l Connection is established with packets -- syn (synchronization) and ack (acknowledgement) l Dialog control is about controlling who sends and how much is sent, for smooth data transfer l Dialog separation puts markers between data packets so lost packets can be resent

14 Session Layer Examples l TCP, Transmission Control Protocol, which establishes connections through sockets, which are numbered abstractions for session-based communications l RTP, Real-time Transfer Protocol, which establishes sessions for streaming media

15 4. The Transport Layer l The transport layer makes sure packets are sent and, sometimes, makes sure they are received l Connection-oriented transmissions require an ack message to be sent back so missing or corrupted packets can be resent, which is slow but reliable l Connectionless transmissions assume packets that are sent are received, which is fast but unreliable because packets can be lost and are not retransmitted

16 Transport Layer Examples l TCP, which assembles packets and segments into a coherent and error-free stream of bits and bytes l UDP, Universal Datagram Protocol, another protocol for sending packets (datagrams) without any need for setting up a session or check transmission (such as when speed is more important)

17 3. The Network Layer l The network layer adds logical addresses known as network addresses to the packets l Logical addresses do not depend on actual physical machines or locations l This layer first ads the network address to the packet l Then it maps the address to the physical address l It determines the best path for the packet l It makes sure the format is correct for the destination network

18 Network Layer Examples l IP, Internet Protocol, which resolves addresses by ARP (Address Resolution Protocol) l Various routing protocols, as described in later lessons

19 2. The Data Link Layer l This layer has two sublayers: the Logical Link Control (LLC) sublayer and the Media Access Control (MAC) sublayer l The LLC sublayer multiplexes network protocols and can provide flow control and error notification l The MAC sublayer manages connections to the physical medium and encapsulated packets within frames

20 Data Link Layer l MAC addresses (6 bytes, assigned by the manufacturer) identify physical devices on a LAN l LLC provides MAC addresses to the network layer for routing l MAC allows only one connected device to transmit at a time (CSMA/CA), or provides recovery when two transmissions overlap and conflict (CSMA/CD)

21 Data Link Layer Examples l IEEE (wired Ethernet) local area networks l ATM (Asynchronous Transfer Mode) highperformance networks (e.g., Internet backbone), and older digital telephone and cable TV networks l SLIP (Serial Line Internet Protocol) modem-to-modem protocol for carrying Internet Protocol over telephone lines

22 1. The Physical Layer l The physical medium connecting two computers together l Controlled by NICs (Network Interface Cards) l NIC converts the data bits into a signal to be transferred across the physical medium: copper wire, optical fibre, radio, etc.

23 Physical Layer Examples l RS-232, which is a common physical interface between machines and peripheral devices l T1, 10BASE-T, 10BASE-TX, and other wired media specifications for Ethernet l POTS, the Plain Old Telephone System, which (as the user-end part of the PSTN) uses twisted pair l Various radio (e.g., ), optical, and electrical media described in later classes

24 OSI in Devices End User 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical Router, etc Application Presentation Session Transport Network Data Link Physical

25 OSI Model Compared to TCP/ IP OSI Layers l Application l Presentation l Session l Transport l Network l Data Link l Physical TCP/IP Layers l Application or Process Layer l Host-to-Host or Transport Layer l Internet Layer l Network Interface Layer l Physical Layer

26 Homework l Read the lecture notes and the slides l Understand and know the layers of the OSI model l Understand and know all the technical terms introduced

27 Quiz (just for fun) 1. What do the following acronyms stand for? a) PSTN b) NIC c) OSI d) MAC 2. What are the names of the seven layers of the OSI model? 3. Write the name of (or acronym for) one actual example of each layer in the model.