Page 1 - © Richard L. Goldman Networking The OSI Model ©Richard L. Goldman March 29, 2001

Page 2 - © Richard L. Goldman The OSI Model The OSI (Open Systems Interconnection) Model is a networking architecture developed by the ISO (International Organization for Standardization). Note : ISO in not an acronym – iso means equal in Greek The OSI Model is a helpful reference when developing, discussing, or teaching computer-to- computer communications. The OSI Model is only a model. It is not an actual networking architecture or protocol that you would find installed on a computer.

Page 3 - © Richard L. Goldman OSI Model Software Hardware 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical The OSI Model is a seven layer set of specifications designed to allow all computers to communicate with each other. The top six layers contain the software that ensures that data is sent and received in a reliable, error-free manner. The bottom layer consists of the cables, connectors, and equipment that carry the data from one computer to another.

Page 4 - © Richard L. Goldman OSI Model Software Sending Computer Program Hardware 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical When an application in one computer wants to send data to another computer it sends it to the Application layer of the OSI Model. The data is then passed to each succeeding layer until it is finally transmitted on the Physical layer. Each layer performs specific processing and packaging functions required to transmit the data.

Page 5 - © Richard L. Goldman PDU – Packet Data Unit As data is passed from layer to layer it is broken down into smaller pieces called: –Packets, –Data Units, or –Payloads. The official OSI acronym for data placed in packets is a PDU or Packet Data Unit. Note: The PDU changes at each layer of the OSI Model as each layer adds its own formatting or addressing to it.

Page 6 - © Richard L. Goldman OSI Model Software 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link Receiving Computer Program OSI Model Software Hardware 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Sending Computer Program 1 - Physical The Physical layer of the Receiving computer accepts the packaged data. It is then passed back through each layer. The unpacked/reprocessed data is finally delivered to the Receiving Application. Each layer performs a specific re- processing and/or un-packaging function required to receive the transmission.

Page 7 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 8 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file Peer Communication Packet or Data Frame DatagramSegment Message

Page 9 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 10 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 11 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 12 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 13 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 14 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 15 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 16 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 17 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 18 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 19 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 20 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program e

Page 21 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program e

Page 22 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program e

Page 23 - © Richard L. Goldman OSI Model Computer Program OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program file

Page 24 - © Richard L. Goldman OSI Model Software Computer Program OSI Model Hardware 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Software 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program In this manner two computers can communicate with each other over a network.

Page 25 - © Richard L. Goldman Physical Layer The Physical Layer consists of the actual physical components that make up a network: –Cables and connectors –NICs –Hubs –Repeaters The Physical Layer is responsible for transmitting, amplifying, and receiving network traffic signals. The operation of the Physical Layer is governed by the RS- 232-D and 802 protocol standards. OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 26 - © Richard L. Goldman Data Link Layer - Sublayers The Data Link Layer contains two sublayers. LLC – Logical Link Control –Flow Control –Error correction MAC – Media Access Control –Contains Physical Address 12 digit Hex – 1 st 6 Mfg, 2 nd 6 Unique –Provides Access Control Contention Token Passing Polling –Controls Logical Network Topology Ethernet Token Ring Etc. OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 27 - © Richard L. Goldman Data Link Layer - Send The sending Data Link Layer is responsible for creating data frames that are suitable for transmission. To accomplish this the Data Link Layer packages the data into the correct size for transmission and then adds the following components: –Destination Address –Source Address –Control Information –Error Checking Information OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 28 - © Richard L. Goldman Data Link Layer - Receive The receiving Data Link Layer: –Checks the frames it receives for errors and acknowledges receipt of each frame. –The sending Data Link Layer will retransmit a frame if it does not receive an acknowledgement. –Strips away the components added by the transmitting Data Link Layer and passes the PDU to the Network Layer. OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 29 - © Richard L. Goldman Network Layer The Network Layer is responsible for three services: –Routing – Provide a path to the receiving computer through one or more intermediate nodes. –Congestion Control – Route and reroute data to avoid network congestion. –Internetworking – Providing the means to communicate between two different types of networks. OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 30 - © Richard L. Goldman Transport Layer The Transport Layer is responsible for: –Flow Control – Adjusting rate of transmission based on network conditions. –Sequencing – Assigns sequence numbers to PUDs to ensure proper reassembly of the data. –Acknowledges (ACK) and requests retransmission of corrupt or missing data. (TCP & SPX work at this layer.) OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 31 - © Richard L. Goldman Session Layer The Session Layer, negotiates, establishes, and maintains the communications session between two nodes. Acts as the traffic cop by regulating which node can transmit and for how long. Monitor identification of session participants – permits only authorized nodes to communicate. OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 32 - © Richard L. Goldman Presentation Layer The Presentation Layer translates data into a format that the network can understand. Encrypts and decrypts data to provide secure communications. OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 33 - © Richard L. Goldman Application Layer The Application Layer is a set of instructions that act as an interface between a user application running on a computer and the network. It is responsible for network: –File transfers –File management –Message handling ( ) OSI Model 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 - Data Link 1 - Physical

Page 34 - © Richard L. Goldman Layer 1 Devices The Physical Layer Layer 1 Devices (Physical Layer) –Cables and other transmission media –NIC –Transceivers –Repeaters –Hub –MAU

Page 35 - © Richard L. Goldman Layer 1 Devices (Physical Layer) NICs and Cat. 5 Crossover Cable Each computer needs a NIC, Network Interface Card, to physically join a network. This example shows two computers connected with a Cat. 5 crossover cable.

Page 36 - © Richard L. Goldman Layer 1 Devices (Physical Layer) NICs and Thinnet This example shows two computers connected with a Thinnet, coaxial cable. Terminators are added to complete the circuit.

Page 37 - © Richard L. Goldman Layer 1 Devices (Physical Layer) NICs, Thicknet, and Transceivers This example shows two computers connected with a Thicknet coaxial cable. Vampire Clamp Transceiver Drop Cable AUI connector – Attachment Unit Interface

Page 38 - © Richard L. Goldman OSI Model Software 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program Layer 1 Devices (Physical Layer) OSI Software Layers passing Data through the Hardware OSI Model Software 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical Computer Program Data is passed from the User Application down through each layer of the OSI model until reaches the hardware. It is then transmitted to the other computer. Then back up to the receiving User Application.

Page 39 - © Richard L. Goldman Layer 1 Devices (Physical Layer) Repeater If the distances between the computers exceed the maximum specified length, a repeater may be used to extend the length of the run.

Page 40 - © Richard L. Goldman Layer 1 Devices (Physical Layer) Hub Hubs forward data from one computer to all other computer connected to the hub (broadcast). Hubs function as a multiport repeater by amplifying the signal which can also extends the maximum distance between computes.

Page 41 - © Richard L. Goldman Layer 1 Devices (Physical Layer) MAU – Multistation Access Unit MAU forwards data from one computer to another in a ring until it reaches the computer it is addressed to. MAUs are used to connect computers in a Token Ring network.

Page 42 - © Richard L. Goldman Layer 2 Devices The Data Link Layer Switches are capable of directing network traffic only to the computer it is addressed to. –Also Known As Switching Hubs Layer 1 Switches Bridges are used to pass traffic from one subnet to another while restricting local traffic to its own subnet.

Page 43 - © Richard L. Goldman Layer 2 Devices (Data Link Layer) Switch OSI Model Software 7 - Application 6 - Presentation 5- Session 4 - Transport 3 - Network 2 – Data Link 1 - Physical A switch uses a MAC Address Table to route traffic to its proper destination. The MAC address table is maintained in the MAC sublayer of the Data Link Layer.

Page 44 - © Richard L. Goldman Common Network Devices Layer 5-7 (Application, Presentation, & Session Layers) –Gateway Layer 3 Devices (Network Layer) –Router –Brouter –Layer 3 Switch Layer 2 Devices (Data Link Layer) –Switch (AKA, Switching Hub, or Layer 2 Switch) –Bridge Layer 1 Devices (Physical Layer) –Cables and other network media –NIC –Transceivers –Repeaters –Hub –MAU

Page 45 - © Richard L. Goldman Layer # Layer NameBrief DescriptionDevices 7.ApplicationNetwork services for user applications 1. Gateway 6.PresentationData conversion, encryption, compression 5.SessionEstablish, and maintain communications 4.TransportMaintain packet sequence Request missing packets 3.NetworkLogical address Routes packets 1. Router 2. Brouter 2.Data LinkPhysical address1. Switch 2. Bridge LLC MAC 1.PhysicalElectrical transmission 1. Cabling 2. NIC 3. Hub 4. Repeater

Page 46 - © Richard L. Goldman Layer # Layer NameBrief DescriptionDevices

Page 47 - © Richard L. Goldman Ethernet Topology Standards StandardNameCableSpeedDistanceIEEE Spec 10BaseTEthernetUTP/STP10Mbs100m Base2ThinnetCoax10Mbs185m Base5ThicknetCoax10Mbs500m BaseFEthernet over Fiber Fiber10Mbs500m – 2000m 100BaseTXFast EthernetUTP/STP-Cat 5 (uses 2 pair) 100Mbs (Full Duplex) 100m802.3u 100BaseT4Fast EthernetUTP/STP-Cat 3 (uses 4 pair) 100Mbs (Half Duplex) 100m BaseFXFast Ethernet over Fiber Fiber100Mbs2000m 100BaseVG“Any LAN” (Voice Grade) UTP/STP-Cat 3 UTP/STP-Cat 5100Mbs 100m 213m BaseXGigabit Ethernet UTP/STP-Cat 5 or Fiber 1000Mb/s802.3z