Presentation on theme: "Lecture 2 The OSI model Chapter 2, specifically pages Dave Novak"— Presentation transcript:
1 Lecture 2 The OSI model Chapter 2, specifically pages 42-58 Dave Novak School of Business Administration, University of VermontSources: 1) Network+ Guide to Networks, Dean 20132) Comer, Computer Networks and Internets, 2004
2 Lecture Outline Examine the seven layers of the OSI model in detail What the OSI model is and how it is used in networkingUnderstand how protocols at different layers interactOSI’s relationship to networking protocolsDifference between MAC and IP addressDefine encapsulation
3 OSI model and networking OSI model provides a universal framework for network communicationPredates popularity of TCP/IPDefines relationships between various protocols, the specific services provided by protocols, and the layers of the model where the protocols operate
4 The OSI model and networking Networked computers use many different protocols simultaneouslyProtocols are responsible for providing different types of network services and functions
5 The OSI model and networking Different layers of OSI are responsible for doing different things by providing different types of services and functionsThe idea behind “layering” is to separate functionality and services by individual layer where there is no redundancy in the services / functions between layersEach layer of the OSI has a specific set of functions and services that are handled at that layer
6 The OSI model and networking The OSI model is a standardized framework for sub-dividing communications system functionality and services into separate layers
7 OSI modelThe collection of networking protocols that operate at the various OSI layers are referred to as a protocol stackProtocols running on a networked computer work together to provide all services required by a particular applicationServices provided by the protocols are not redundant – if a protocol at one layer provides a particular service, the protocols at the other layers do not provide the same serviceProtocols at different layers provide services to each other – allowing interaction between adjoining layers
8 OSI model Promotes open system communication OSI is a theoretical representation or framework for network servicesDoes NOT prescribe hardware or softwareDoes NOT describe how software programs on different computers interact, or how they interact with humans
9 A Protocol Stack The collection of protocols that operate at the various layers of the OSI model are referred toas a protocol stackThe protocols in the protocol stack worktogether to provide all services required by anapplicationProtocols at the different layers perform specificfunctions that are NOT duplicated by otherprotocols at other layers
10 Protocol Interaction Services performed at a particular layer of the OSI model at the sendingcomputer are also performed (or undone)at the corresponding layer of thereceiving computerThe Session Layer (5) at the sendingcomputer does not communicatedirectly with the Session Layer at theReceiving computer messages arepassed down from the Session Layer (5)to the Transport Layer (4) and so on atthe sending computerMessages are passed up from theTransport Layer (4) to the Session Layer (5)at the receiving computer
11 The OSI Reference Model 7654321The Application Layer (7) is thetop most layer – it is NOT the samething as an applicationMS Word is an example of an applicationMS Word is NOT a protocol that operatesat the Application Layer (7) of the OSIFTP is an example of an Application Layer(7) protocolThe Physical Layer (1) is the bottommost layer of the OSI model – itaddresses the transmission of bits overa particular medium
14 OSI modelRefer to the additional reading – Webopedia definition of OSIThe OSI model defines a framework for implementing networking services via specific PROTOCOLS (depending on the protocol stack being used) in seven layers
15 Physical Layer (1)Concerned with transmitting signals (representing raw bits) over a communication channelTransmitting signals via energy of some form or anotherData are not organized into frames or packets at Layer 1
16 Physical Layer (1)Addresses the nature of the medium and types of signals usedCable type: coaxial, twisted pair, fiberSignal type: Light pulses, electrical voltage, radio wavesWhy would one be concerned about the medium that is used? What difference does it make?
17 Physical Layer (1)Hardware dealing with transmission of signals is defined at layer 1 of the OSICables, hubs, repeatersDo not “understand” packets or frames only signalsThe NIC provides a physical connection to the network and bridges layers 1 and 2
18 Physical Layer (1)Networking technologies may use a variety of physical layer optionsEthernet (the most popular wired networking technology) supports a number of different physical layer optionsDesignationCable TypeTopologySpeedMax Segment Length10Base5RG-8 coaxialBus10 Mbps500 meters10Base2RG-58 coaxial185 meters10BaseTCAT 3 UTPStar100 meters100BaseFX62.5 / 125 multimode fiber100 Mbps412 meters100BaseTXCAT 5 UTP
19 Physical Layer (1)Different types of media may be used to implement a given LAN technologyFor example, depending on the standard being followed, an Ethernet LAN may require coaxial, fiber, or twisted pair wiringEach standard has different topology and medium requirements
20 Physical Layer (1) Max length of cable Type of connectors What happens if you exceed max cable length standards?Type of connectorsBit rate (data transmission rate)Monitor data error rates
21 Data Link Layer (2)Converts signals and streams of bits into frames and vice versaCreates and recognizes frame boundariesWhat is a frame and why is it important?
22 Data Link Layer (2)The frame format is different for various networking technologiesEthernetToken RingATM
24 Data Link Layer (2)Access control technique of various technologies defined at Layer 2The way in which networked devices “gain access to the medium”, communicate with other devices, and transfer data differs based on the technology being usedFor example, the process two devices use to communicate over wireless is different from Ethernet
25 Data Link Layer (2)Error detection in the bit to frame conversion processIdentifies and corrects frame errorsErrors related to LAN communicationBetween 2 hostsThe physical address or MAC address is contained in the frame header
26 Data Link Layer (2)Provides conduit or link between the hardware and software on the computer and the physical network mediumThis is done via the NIC
27 Data Link Layer (2) Sublayers LLC – Interface to layer 3. Controls frame synchronization, flow control,and error checkingMAC – Interface to layer 1. Controls how PC accesses and transmits dataspecifies the Media Access Technique used
28 Data Link Layer (2)In terms of network design – Data Link Layer (2) is single most important layer in determining what hardware is usedLAN technology and topology requires certain physical layer options and vice versaImplies access technique used
29 Data Link Layer (2)Data link layer protocols designed to work locally (LAN or subnet centric)Other higher-layer protocols (at layers 3 and 4) are required for error detection and flow control in communicating remotely and over larger distancesHardware that recognizes frames operates at layer 2 of the OSISwitch, bridge
30 Network Layer (3) Packages data/frames into IP datagrams Higher level, routable network addresses (like the IP address) are recognized and managedHardware that recognizes network addresses (like IP addresses) works at Layer 3Routers
31 Network Layer (3)Responsible for end-to-end communication
32 Network Layer (3) Responsible for end-to-end communication How is end-to-end functionality different from functionality provided by Data Link Layer (2) protocols?
33 Network Layer (3) Example protocol: IP Example services: network addressing, fragmentation/reassembly, routingWhat does routing mean?
35 MAC and IP Address Comparison MAC address (physical address)IP address (network address)
36 Transport Layer (4)Network layer (3) and Transport layer (4) protocols designed to work together as a pairTCP/IP – Internet protocol stackSPX/IPXLayer 4 protocols provide services to complement Layer (3)TCP provides very specific services that IP does not – designed to work together with each protocol providing unique services
37 Transport Layer (4)Provides messaging service for Session layer (5) and hides the underlying network from the upper layersExample protocols: TCP, UDPExample of Transport Layer services: flow control, multiplexing, retransmission, message sequencing
38 Transport Layer (4) Provides end-to-end error control on the network How is this different from Layer 2 error control?How is this different from end-to-end communications provided at Layer 3?
39 EncapsulationEach protocol adds headers to information it receives from the layer above itWhen a datagram is encapsulated, the entire datagram is placed into the payload area of a specific frame formatTransport Layer (4) adds its header to message and passes down to Network Layer (3) Network Layer adds its header in front of Transport Layer header and so on
41 Lower Layers (1 – 4) Layers 1 – 4 are called the LOWER layers Lower layers are concerned with proper transmission of data across the network
42 Upper Layers (5 – 7) Layers 5 – 7 called the UPPER layers Upper layers are concerned with how different application communicate between different hostsDifficult to identify and separate upper layer protocols in some cases – many applications bundle services provided at layers 5, 6, and 7Example: WS FTP not only uses Layer 7 protocol FTP, but manages syntax and compression issues at Layer 6, and session management issues at Layer 5Upper layer protocols/services know nothing about, or understand networking or addressing
43 Session Layer (5)Responsible for establishing, maintaining, and ending communicationThere are no separate Session Layer protocolsSession Layer functions are integrated into other protocols that also include Presentation and Application Layer functionsSet up, manage, and tear down “sessions” or “connections” between Presentation Layer (6) entities
44 Session Layer (5)Coordinates communication and organizes into one of three categories:1) Simplex – only one node can transmit2) Half duplex – both nodes can transmit, but only one at a time. Once one node is finished transmitting data, the other node can transmit3) Full duplex – both nodes can transmit simultaneously without disrupting the other node
45 Session Layer (5)Traffic cop for communications between two nodes on a networkFor Internet applications – mapping between logical ports and sessions
46 Presentation Layer (6)Primary role is to preserve meaning of information transmitted between systemsComputers communicating on a network often use different syntaxEnsures communication between entities is of a form both can understandComputers must negotiate a common syntax so they can choose a transfer syntax that they both have in common and both understandSemantics?
47 Presentation Layer (6)Different applications and programming languages use different data types and different syntaxText-based languages based on sequence while visual languages are based on spatial layout and relationships between symbolsWhich statements in a program are acceptable to the compiler?
48 Presentation Layer (6)Network perspective: applications send messages to each otherApplication perspective: messages contain specific types of dataMany types of data use very standard (universal) formatsMPEG for videoJPEG for still imagesASCII for textNot all data types have universal formatsNot all computers format data types the same way
49 Presentation Layer (6)If needed, systems can select transfer syntax that provides additional services such as:Data compression and decompressionEncryption and decryption
50 Application Layer (7)Entrance point for applications to access the OSI model (structured networking framework) and use network resourcesNOT an application, but access to protocols that provide network servicesCoordinates network servicesIdentify parties and make sure each can be reachedEnsures communication resources exist (For example, is there are modem at the sender’s computer?)
51 Application Layer (7)Most Application Layer protocols provide services (such as mail, print, network management services) that applications (such as WS FTP, Internet Explorer, Outlook) use to access the networkApplication Layer protocols often include Session and Presentation Layer functionsTypical protocol stack consists of 4 separate protocols that run at the application, transport, network, and data-link layersFTP, TCP, IP, specific Ethernet protocols
52 Application Layer (7) FTP, TCP, IP, specific Ethernet protocols Example: WS_FTP (application software) uses FTP (a layer 7 protocol that ALSO performs layer 6 and 5 functions) running over TCP/IP (the layer 4 and 3 protocols – Internet communication) and Ethernet LAN technology (layer 2 protocols specifying frame formats, error detection, addressing on the LAN)The upper layers using FTP are unaware of what type of transport protocol (4), what type of network protocol (3), and what type of LAN technology or data-link protocol (2) is being usedDifferent lower layer protocols CAN be used – the application is not aware of this, and doesn’t need to deal with any lower layer functionality
53 Lecture Summary Details of the OSI model KNOW the layers (A, P, S, T, N, D, P) and be able to describe basic services provided at each layerUsing TCP/IP stack as example, on what layer does IP operate? What about TCP?