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 2/80

3. Contents 2.1 Introduction & Definition 2.2 The Model : Layered Architecture 2.3 Functions of the Layers 2.4 TCP/IP Protocol Suite 3/80

4. 2.1 Introduction & Defining Components of the Network 4/80 Main OfficeBranch Office Home Office Mobile Users Internet

5. Defining Components of the Network 5/80 Floor 2 Floor 1 Server Farm Branch Office Telecommuter ISDN Remote Campus

6. Network Structure Defined by Hierarchy 6/80 Distribution Layer Core Layer Access Layer

7. 7/80 Access Layer Characteristics End station entry point to the network Access Layer

8. 8/80 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 9/80 Fast transport to enterprise services No packet manipulation Core Layer

10. 2.2 The OSI Model 10/80

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 11/80

12. OSI Model Overview 12/80 Application (Upper) Layers Session Presentation Application

13. OSI Model Overview 13/80 Data Flow Layers Transport Layer Data Link Network Layer Physical Application (Upper) Layers Session Presentation Application

14. Role of Application Layers 14/80 Telnet HTTP User Interface EXAMPLES Application

15. Role of Application Layers 15/80 Telnet HTTP ASCII EBCDIC JPEG User Interface How data is presented Special processing such as encryption EXAMPLES Presentation Application

16. Role of Application Layers 16/80 Telnet HTTP ASCII EBCDIC JPEG Keeping different applications’ data separate User Interface How data is presented Special processing such as encryption Operating System/ Application Access Scheduling EXAMPLES Session Presentation Application

17. Role of Application Layers 17/80 Keeping different applications’ data separate User Interface How data is presented Special processing such as encryption Telnet HTTP ASCII EBCDIC JPEG Operating System/ Application Access Scheduling Transport Data Link Network Physical EXAMPLES Session Presentation Application

18. Role of Data Flow Layers 18/80 EIA/TIA-232 V.35 EXAMPLES Physical Move bits between devices Specifies voltage, wire speed and pin-out cables

19. Role of Data Flow Layers 19/80 802.3 / 802.2 HDLC EIA/TIA-232 V.35 EXAMPLES Data Link Physical Combines bits into bytes and bytes into frames Access to media using MAC address Error detection not correction Move bits between devices Specifies voltage, wire speed and pin-out cables

20. Role of Data Flow Layers 20/80 802.3 / 802.2 HDLC EIA/TIA-232 V.35 IP IPX EXAMPLES Network Data Link Physical Combines bits into bytes and bytes into frames Access to media using MAC address Error detection not correction Move bits between devices Specifies voltage, wire speed and pin-out cables Provide logical addressing which routers use for path determination

21. Role of Data Flow Layers 21/80 TCP UDP SPX 802.3 / 802.2 HDLC EIA/TIA-232 V.35 IP IPX EXAMPLES Transport Data Link Physical Reliable or unreliable delivery Error correction before retransmit Combines bits into bytes and bytes into frames Access to media using MAC address Error detection not correction Move bits between devices Specifies voltage, wire speed and pin-out cables Network Provide logical addressing which routers use for path determination

22. Role of Data Flow Layers 22/80 TCP UDP SPX 802.3 / 802.2 HDLC EIA/TIA-232 V.35 IP IPX Presentation Application Session EXAMPLES Reliable or unreliable delivery Error correction before retransmit Combines bits into bytes and bytes into frames Access to media using MAC address Error detection not correction Move bits between devices Specifies voltage, wire speed and pin-out cables Transport Data Link Physical Network Provide logical addressing which routers use for path determination

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. 23/80

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. 24/80

25. 25/80 Physical Layer Functions Defines Media type Connector type Signaling type Ethernet 802.3 V.35 Physical EIA/TIA-232

26. Physical Layer: Ethernet/802.3 26/80 H ub Hosts Host 10Base2—Thick Ethernet 10Base5—Thick Ethernet 10BaseT—Twisted Pair

27. 27/80 Hubs Operate at Physical layer ABCD 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 28/80 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. 29/80

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. 30/80

31. 31/80 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 Data Link Physical EIA/TIA-232 v.35 Ethernet Frame Relay HDLC 802.2 802.3

32. 32/80 Data Source add FCS Length Dest. add Variable2664 0000.0C xx.xxxx Vendor assigned IEEE assigned MAC Layer - 802.3 Data Link Layer Functions Preamble Ethernet II uses “Type” here and does not use 802.2. MAC Address 8# Bytes

33. 33/80 Data Dest SAP Source SAP DataSource addFCSLengthDest add Variable11 802.2 (SAP) MAC Layer - 802.3 Data Link Layer Functions Ctrl 1 or 2 32 Preamble Data Dest SAP AA Source SAP AA Variable11 802.2 (SNAP) Ctrl 03 1or2 OR OUI ID Type

34. 34/80 Each segment has its own collision domain All segments are in the same broadcast domain Data Link Switches and Bridges Operate at Data Link Layer Switches and Bridges Operate at Data Link Layer OR 123124

35. SwitchesSwitches 35/80 Each segment has its own collision domain Broadcasts are forwarded to all segments Memory Switch

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. 36/80

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. 37/80

38. 38/80 Network Layer Functions Defines logical source and destination addresses associated with a specific protocol Defines paths through network Interconnects multiple data links Network IP, IPX Data Link Physical EIA/TIA-232 v.35 Ethernet Frame Relay HDLC 802.2 802.3

39. 39/80 Data Source address Destination address IP Network Layer Functions Header 172.15.1.1 Node Network Logical Address Network Layer End Station Packet

40. Network Layer Functions 40/80 11111111 00000000 10101100 00010000 01111010 11001100 Binary Mask Binary Address 172.16.122.204 255.255.0.0 17216122204 255 AddressMask 25500 NetworkHost

41. 41/80 Routing Table NETINTMetric 1 2 4 S0 E0 1 0 0 1.04.0 1.3 E0 4.3 S0 2.2 E0 2.1 S0 4.1 4.2 1.1 1.2 Routing Table NETINTMetric 1 2 4 E0 S0 0 0 1 Logical addressing allows for hierarchical network Configuration required Uses configured information to identify paths to networks Network Layer Functions

42. Routers: Operate at the Network Layer Broadcast control Multicast control Optimal path determination Traffic management Logical addressing Connects to WAN services 42/80

43. Using Routers to Provide Remote Access 43/80 Internet Telecommuter Branch Office Modem or ISDN TA Mobile User Main Office

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. 44/80

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. 45/80

46. 46/80 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 Network IPXIP Transport SPXTCPUDP

47. 47/80 Encapsulating Data Transport Data Link Physical Network Upper Layer Data TCP Header DataIP Header DataLLC Header 0101110101001000010 DataMAC Header Presentatio n Application Session Segment Packet Bits Frame PDU FCS

48. 48/80 Upper Layer Data De-encapsulating Data LLC Hdr + IP + TCP + Upper Layer Data MAC Header IP + TCP + Upper Layer Data LLC Header TCP+ Upper Layer Data IP Header Upper Layer Data TCP Header 0101110101001000010 Transport Data Link Physical Network Presentation Application Session

49. 49/80 Reliable Transport Layer Functions Synchronize Acknowledge, Synchronize Acknowledge Data Transfer (Send Segments) SenderReceiver Connection Established

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. 50/80

51. Layer 6 : Presentation layer These services allow an application to properly interpret the information being transferred. This includes translation, transformation, formatting and syntax of the information. These functions may be required to adapt the information- handling characteristics of one application process to another. Examples include code translation, structuring of data for display on screen, format control and virtual terminal protocols. The syntactical representation of data has been defined in DIS 8824 and 8825. CCITT has described the presentation protocol for message- handling systems in X.409 and for Telex in X.61. 51/80

52. Layer 7 : Application layer  This layer provides management functions to support distributed applications utilizing the OSI environment.  It is the window through which the applications gain access to the services provided by the communications architecture.  These include identification of the cooperating processes, authentication of the communicant, authority verification, agreement on encryption mechanisms, determination of resource availability and agreement on syntax, e.g. character set, data structure. 52/80

53. Layered protocol Functions: communication, sharing of media, leased line and public-switched connections,orderly interleaving of control and data packets, error-free communication, data integrity, connection-control, point-to- point and multi-point connections 53/80

54. Layered protocol layered protocol configuration: ▫ point-to-point (Non-switched) ▫ Point-to-point (switched) ▫ multipoint (non-switched) ▫ multipoint (switched) ▫ loop system Features of layered protocols: decomposition,service access point, modularity, flexibility, survivability and security 54/80

55. 2.4 TCP/IP Protocol Suite  Transmission Control Protocol/ Internetworking Protocol (TCP/IP)  Used in the Internet, was developed prior to the OSI model.  Consists of 5 layers : - Application Layer - Transport Layer - Network Layer (Internet Layer) - Data Link Layer - Physical Layer 55/80

56. TCP/IP Protocol Stack 56/80 7 6 5 4 3 2 5 4 3 2 Application Presentation Session Transport Network Data Link Physical 1 Application Transport Internet Data Link Physical 1

57. Application Layer Overview 57/80 * Used by the router Application Transport Internet Data Link Physical File Transfer - TFTP * - FTP * - NFS E-Mail - SMTP Remote Login - Telnet * - rlogin * Network Management - SNMP * Name Management - DNS* File Transfer - TFTP * - FTP * - NFS E-Mail - SMTP Remote Login - Telnet * - rlogin * Network Management - SNMP * Name Management - DNS*

58. Transport Layer Overview 58/80 Transmission Control Protocol (TCP) User Datagram Protocol (UDP) Transmission Control Protocol (TCP) User Datagram Protocol (UDP) Application Transport Internet Data Link Physical Connection- Oriented Connectionless

59. TCP Segment Format 59/80 Source port (16) Destination port (16) Sequence number (32) Header length (4) Acknowledgement number (32) Reserved (6) Code bits (6) Window (16) Checksum (16)Urgent (16) Options (0 or 32 if any) Data (varies) 20 Bytes Bit 0 Bit 15Bit 16Bit 31

60. 60/80 Application Presentation Session Transport Network Data Link Physical OSI Model PDUFunctional ResponsibilitiesExamples Written Exercise: OSI Model

61. Basics of Subnetting –Subnetworks or subnets –Why Subnets : –A primary reason for using subnets is to reduce the size of a broadcast domain. –Broadcasts are sent to all hosts on a network or subnetwork. –When broadcast traffic begins to consume too much of the available bandwidth, network administrators may choose to reduce the size of the broadcast domain 61/80

62. Basics of Subnetting –Addressing without subnets 62/80

63. Basics of Subnetting - Addressing with subnets 63/80

64. Basics of Subnetting - The 32 bits binary IP Address 64/80

65. Basics of Subnetting - Subnet Mask : To determine the subnet mask for a particular subnetwork IP address follow these steps. (1) Express the subnetwork IP address in binary form. (2) Replace the network and subnet portion of the address with all 1s. (3) Replace the host portion of the address with all 0s. (4) As the last step convert the binary expression back to dotted-decimal notation 65/80

66. Basics of Subnetting –The AND Function 66/80

67. Basics of Subnetting Creating a Subnet : To create subnets, you must extend the routing portion of the address. The Internet knows your network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or 24 routing bits (the network number). 67/80 Address Class Size of Default Host Field Maximum Number of Subnet Bits A 24 22 B 16 14 C 8 6 EX.

68. Basics of Subnetting Determining subnet mask size Subnet Masking : 68/80

69. Basics of Subnetting Class B Subnet Planning Example : Ex 1 :10001100.10110011.11011100.11001000 140.179.220.200 IP Address 11111111.11111111.11100000.00000000 255.255.224.000 Subnet Mask --------------------------------------------------------------------------------------- 10001100.10110011.11000000.00000000 140.179.192.000 Subnet Address 10001100.10110011.11011111.11111111 140.179.223.255 Broadcast addr Ex 2 : 69/80

70. Basics of Subnetting Class C 70/80

71. Basics of Subnetting Private Address Space : There are certain addresses in each class of IP address that are not assigned. These addresses are called private addresses. Private addresses might be used by hosts that use network address translation (NAT), or a proxy server, to connect to a public network; or by hosts that do not connect to the Internet at all. Private Subnets : There are three IP network addresses reserved for private networks. The addresses are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. 71/80

72. Basics of Subnetting Example : Class C network number of 200.133.175.0 You want to utilize this network across multiple small groups within an organization. You can do this by subnetting that network with a subnet address. We will break this network into 14 subnets of 14 nodes each. This will limit us to 196 nodes on the network instead of the 254 we would have without subnetting, but gives us the advantages of traffic isolation and security. To accomplish this, we need to use a subnet mask 4 bits long. Recall that the default Class C subnet mask is 255.255.255.0 (11111111.11111111.11111111.00000000 binary) Extending this by 4 bits yields a mask of 255.255.255.240 (11111111.11111111.11111111.11110000 binary) 72/80

73. Basics of Subnetting This gives us 16 possible network numbers, 2 of which cannot be used: 73/80

74. Basics of Subnetting Allowed Class A Subnet and Host IP addresses 74/80

75. Basics of Subnetting Allowed Class B Subnet and Host IP addresses 75/80

76. Basics of Subnetting Allowed Class C Subnet and Host IP addresses 76/80

77. SummarySummary After completing this chapter, you should be able to perform the following tasks: –Describe how data moves through a network –Identify the roles and functions of routers, switches and hubs, and specify where each device best fits in the network –How to calculate and organize Subnet for each class. 77/80

78. Assignments/ Q&A 1. Written exercise : OSI Model. 2.What are some of the advantages of using the OSI model in a networking environment? 3. Describe the encapsulation process. 4. How many broadcast and collision domains are on a hub? 5. Practice Lab Activity : 10.4.1, 10.6.6, 10.7.5, 10.7.7 78/80