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Chapter 2

Chapter 2. The OSI Model. By Dr. Sukchatri P RASOMSUK School of ICT, University of Phayao. Objectives. On completion of this chapter, you will be able to perform the following tasks: Describe how data traffic is exchanged between source and destination devices

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Chapter 2

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  1. Chapter 2 The OSI Model By Dr.Sukchatri PRASOMSUK School of ICT, University of Phayao

  2. Objectives • On completion of this chapter, you will be able to perform the following tasks: • Describe how data traffic is exchanged between source and destination devices • Identify the roles and functions of a hub, switch, and router, and where they best fit in the network

  3. Contents • 2.1 Introduction & Definition • 2.2 The Model : Layered Architecture • 2.3 Functions of the Layers • 2.4 TCP/IP Protocol Suite

  4. 2.1 Introduction & Defining Components of the Network Mobile Users Home Office Internet Branch Office Main Office

  5. Defining Components of the Network Branch Office Floor 2 ISDN Server Farm Remote Floor 1 Telecommuter Campus

  6. Network Structure Defined by Hierarchy Core Layer Distribution Layer Access Layer

  7. Access Layer Characteristics Access Layer End station entry point to the network

  8. Distribution Layer Characteristics • Access Layer Aggregation Point • Routes traffic • Broadcast/Multicast Domains • Media Translation • Security • Possible point for remote access Distribution Layer

  9. Core Layer Characteristics Core Layer • Fast transport to enterprise services • No packet manipulation

  10. 2.2 The OSI Model

  11. 2.3 Functions of the Layers • Layer 7 : Application Layer • Layer 6 : Presentation Layer • Layer 5 : Session Layer • Layer 4 : Transport Layer • Layer 3 : Network Layer • Layer 2 : Data Link Layer • Layer 1 : Physical Layer

  12. OSI Model Overview Application Application (Upper) Layers Presentation Session

  13. Application Application (Upper) Layers Presentation Session OSI Model Overview Transport Layer Network Layer Data Link Data Flow Layers Physical

  14. Role of Application Layers EXAMPLES Telnet HTTP User Interface Application

  15. Role of Application Layers EXAMPLES Telnet HTTP User Interface Application ASCII EBCDIC JPEG • How data is presented • Special processing such as encryption Presentation

  16. Role of Application Layers EXAMPLES Telnet HTTP User Interface Application ASCII EBCDIC JPEG • How data is presented • Special processing such as encryption Presentation Keeping different applications’ data separate Operating System/ Application Access Scheduling Session

  17. Telnet HTTP ASCII EBCDIC JPEG Operating System/ Application Access Scheduling Role of Application Layers EXAMPLES User Interface Application • How data is presented • Special processing such as encryption Presentation Keeping different applications’ data separate Session Transport Network Data Link Physical

  18. Role of Data Flow Layers EXAMPLES • Move bits between devices • Specifies voltage, wire speed and pin-out cables EIA/TIA-232V.35 Physical

  19. Role of Data Flow Layers EXAMPLES • Combines bits into bytes and bytes into frames • Access to media using MAC address • Error detection not correction 802.3 / 802.2 HDLC Data Link • Move bits between devices • Specifies voltage, wire speed and pin-out cables EIA/TIA-232V.35 Physical

  20. Role of Data Flow Layers EXAMPLES Provide logical addressing which routers use for path determination IP IPX Network • Combines bits into bytes and bytes into frames • Access to media using MAC address • Error detection not correction 802.3 / 802.2 HDLC Data Link • Move bits between devices • Specifies voltage, wire speed and pin-out cables EIA/TIA-232V.35 Physical

  21. Role of Data Flow Layers EXAMPLES TCP UDP SPX • Reliable or unreliable delivery • Error correction before retransmit Transport Provide logical addressing which routers use for path determination IP IPX Network • Combines bits into bytes and bytes into frames • Access to media using MAC address • Error detection not correction 802.3 / 802.2 HDLC Data Link • Move bits between devices • Specifies voltage, wire speed and pin-out cables EIA/TIA-232V.35 Physical

  22. Role of Data Flow Layers Application Presentation EXAMPLES Session TCP UDP SPX • Reliable or unreliable delivery • Error correction before retransmit Transport Provide logical addressing which routers use for path determination IP IPX Network • Combines bits into bytes and bytes into frames • Access to media using MAC address • Error detection not correction 802.3 / 802.2 HDLC Data Link • Move bits between devices • Specifies voltage, wire speed and pin-out cables EIA/TIA-232V.35 Physical

  23. Layer 1 : Physical layer • Lowest layer of OSI architecture provides services to the link layer, acquiring, maintaining and disconnecting the physical circuits that form the connecting communications path. • Handles the electrical and mechanical interface as well as the procedural requirements of the interconnection medium. • Responsible for bit synchronization and the identification of a single element as a one or a zero. • This layer includes mechanical, electrical, functional and procedural specifications.

  24. Layer 1 : Physical layer • The physical layer is the rough equivalent of the traditional data-terminal-equipment (DTE) to data-communications-equipment (DCE) interface. • Typical protocols at the physical layer include the RS-232, the RS-449 family, CCITT X.25 and X.21 facility interfaces, other CCITT (V) and (X) series recommendations, and the physical aspects of the IEEE 802.X media access protocols for Local Area Networks.

  25. Physical Layer Functions • Defines • Media type • Connector type • Signaling type Ethernet 802.3 V.35 EIA/TIA-232 Physical

  26. Physical Layer: Ethernet/802.3 10Base2—Thick Ethernet10Base5—Thick Ethernet Host Hub 10BaseT—Twisted Pair Hosts

  27. A B C D Hubs Operate at Physical layer Physical • All devices in the same collision domain • All devices in the same broadcast domain • Devices share the same bandwidth

  28. Hubs: One Collision Domain • More end stations means more collisions • CSMA/CD is used

  29. Layer 2 : Data link layer • Link layer services relate to the reliable interchange of data across a point-to-point or multipoint data link that has been established at the physical layer. • Link layer protocols manage establishment, control and termination of logical link connection. They control the flow of user data, supervise recovery from errors and abnormal conditions, and acquire and maintain character and block or frame synchronization. • It attempts to add reliability, flow and error control, and communication management.

  30. Layer 2 : Data link layer • Data link control protocols include character-oriented Binary Synchronous Communication (BSC), ANSI X3.28m, • the more recent bit-oriented ADCCP (Advanced Data Communications Control Procedure) and its international counterpart HDLC, X.25, LAPB, ISDN, LAPD and IEEE 802.X logical link control.

  31. Data Link layer Functions • Defines • Physical source and destination addresses • Higher layer protocol (Service Access Point)associated with frame • Network topology • Frame sequencing • Flow control • Connection-oriented or connectionless 802.2 Data Link Frame Relay HDLC Ethernet 802.3 EIA/TIA-232 v.35 Physical

  32. Data Link Layer Functions MAC Layer - 802.3 # Bytes 8 6 6 2 Variable 4 Preamble FCS Dest. add Source add Length Data Ethernet II uses “Type” here and does not use 802.2. • 0000.0C xx.xxxx Vendor assigned IEEE assigned • MAC Address

  33. Data Link Layer Functions 802.2 (SNAP) 1 1 1or2 3 2 Variable OUI ID Dest SAP AA Source SAP AA Ctrl 03 Type Data OR 802.2 (SAP) 1 1 1 or 2 Variable DestSAP Source SAP Ctrl Data Preamble Dest add Source add Length Data FCS MAC Layer - 802.3

  34. 4 Switches and Bridges Operate at Data Link Layer Data Link OR 1 2 3 1 2 • Each segment has its own collision domain • All segments are in the same broadcast domain

  35. Switches Switch Memory • Each segment has its own collision domain • Broadcasts are forwarded to all segments

  36. Layer 3 : Network layer • Responsible for providing communication between two hosts across a communication network. Services include routing, switching, sequencing of data, flow control and error recovery. • It provides the interface such that higher layers need not know about the underlying topology. • It provides connection management, routing, and error and flow control. • The CCITT X.25 packet layer is the best known network layer protocol for packet- switched networks. X.21 is used for circuit-switched networks.

  37. Layer 3 : Network layer • DoD has developed the IP Internet control protocol. • Other examples of network protocols include the CCITT Q.931 network layer and the ISO 8473 connectionless inter-network protocol.

  38. Network Layer Functions • Defines logical source and destination addresses associated with a specific protocol • Defines paths through network • Interconnects multiple data links IP, IPX Network 802.2 Data Link Frame Relay HDLC Ethernet 802.3 EIA/TIA-232 v.35 Physical

  39. Network Layer Functions Network Layer End Station Packet Sourceaddress Destination address Data IP Header • Logical Address • 172.15.1.1 Node Network

  40. Network Layer Functions Address Mask 172.16.122.204 255.255.0.0 172 16 122 204 10101100 00010000 01111010 11001100 BinaryAddress 255 255 0 0 11111111 11111111 00000000 00000000 BinaryMask Network Host

  41. Network Layer Functions 1.0 4.0 1.1 4.1 2.1 2.2 1.3 4.3 1.2 4.2 E0 S0 S0 E0 Routing Table Routing Table NET INT Metric NET INT Metric 1 E0 0 1 S0 1 2 S0 0 2 S0 0 4 S0 1 4 E0 0 • Logical addressing allows for hierarchical network • Configuration required • Uses configured information to identify paths to networks

  42. Routers: Operate at the Network Layer • Broadcast control • Multicast control • Optimal path determination • Traffic management • Logical addressing • Connects to WAN services

  43. Using Routers to Provide Remote Access Modem or ISDN TA Telecommuter Mobile User Branch Office Main Office Internet

  44. Layer 4 : Transport layer • Highest layer directly associated with the movement of data through the network. • It provides a universal transparent mechanism for use by the higher layers that represent the users of the communications service. • The transport layer is expected to optimize the use of available resources while meeting user requirements. • Responsible for the end-to-end integrity of the edit exchange and must bridge the gap between services provided by the underlying network and those required by the higher layers.

  45. Layer 4 : Transport layer • Classes of transport protocols have been developed that range from extremely simple to very complex. • Simple transport layers can be used when the network provides a high quality, reliable service. • A complex transport protocol is used when the underlying service does not, or is assumed to be unable to, provide the required level of service. • The ISO has promulgated International Standard 8073 as a transport protocol. This standard defines five (5) classes of protocols, ranging from a simple Class (0) to a complex Class (4). • Another transport protocol example is the Transmission Control Protocol (TCP) developed by the DoD and now finding wide application in commercial environments.

  46. Transport Layer Functions • Distinguishes between upper layer applications • Establishes end-to-end connectivity between applications • Defines flow control • Provides reliable or unreliable services for data transfer Transport TCP UDP SPX Network IP IPX

  47. FCS FCS Encapsulating Data Application Presentation PDU Session Upper Layer Data Segment Transport TCP Header Upper Layer Data Network Packet IP Header Data LLC Header Data Data Link Frame MAC Header Data Physical Bits 0101110101001000010

  48. De-encapsulating Data Application Presentation Session Upper Layer Data Transport Upper Layer Data TCP Header Network TCP+ Upper Layer Data IP Header IP + TCP + Upper Layer Data Data Link LLC Header LLC Hdr + IP + TCP + Upper Layer Data MAC Header Physical 0101110101001000010

  49. Reliable Transport Layer Functions Sender Receiver Synchronize Acknowledge, Synchronize Acknowledge Connection Established Data Transfer (Send Segments)

  50. Layer 5 : Session layer • A session binds two application processes into a cooperative relationship for a certain time. • The session layer provides an administrative service that handles the establishment (binding) and release (unbinding) of a connection between two presentation entities. • Sessions are established when an application process requests access to another application process. • Session protocols include ISO 8327, CCITT X.25, ECMA 75 and CCITT T.62 which is intended for use in teletex services.

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