Lecture 2 The OSI model Chapter 2, specifically pages 42-58 Dave Novak School of Business Administration, University of Vermont Sources: 1) Network+ Guide to Networks, Dean 2013 2) Comer, Computer Networks and Internets, 2004 Lecture Outline Examine the seven layers of the OSI model in detail What the OSI model is and how it is used in networking Understand how protocols at different layers interact OSI s relationship to networking protocols Difference between MAC and IP address Define encapsulation OSI model and networking OSI model provides a universal framework for network communication Predates popularity of TCP/IP Defines relationships between various protocols, the specific services provided by protocols, and the layers of the model where the protocols operate The OSI model and networking Networked computers use many different protocols simultaneously Protocols are responsible for providing different types of network services and functions The OSI model and networking Different layers of OSI are responsible for doing different things by providing different types of services and functions The idea behind layering is to separate functionality and services by individual layer where there is no redundancy in the services / functions between layers Each layer of the OSI has a specific set of functions and services that are handled at that layer The OSI model and networking The OSI model is a standardized framework for sub-dividing communications system functionality and services into separate layers 1
OSI model The collection of networking protocols that operate at the various OSI layers are referred to as a protocol stack Protocols running on a networked computer work together to provide all services required by a particular application Services 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 service Protocols at different layers provide services to each other allowing interaction between adjoining layers OSI model Promotes open system communication OSI is a theoretical representation or framework for network services Does NOT prescribe hardware or software Does NOT describe how software programs on different computers interact, or how they interact with humans A Protocol Stack Protocol Interaction The collection of protocols that operate at the various layers of the OSI model are referred to as a protocol stack The protocols in the protocol stack work together to provide all services required by an application Protocols at the different layers perform specific functions that are NOT duplicated by other protocols at other layers Services performed at a particular layer of the OSI model at the sending computer are also performed (or undone) at the corresponding layer of the receiving computer The Session Layer (5) at the sending computer does not communicate directly with the Session Layer at the Receiving computer messages are passed down from the Session Layer (5) to the Transport Layer (4) and so on at the sending computer Messages are passed up from the Transport Layer (4) to the Session Layer (5) at the receiving computer The OSI Reference Model Interaction Between OSI Layers 7 6 5 4 3 2 1 The Application Layer (7) is the top most layer it is NOT the same thing as an application MS Word is an example of an application MS Word is NOT a protocol that operates at the Application Layer (7) of the OSI FTP is an example of an Application Layer (7) protocol The is the bottom most layer of the OSI model it addresses the transmission of bits over a particular medium 2
OSI model OSI model Refer to the additional reading Webopedia definition of OSI http://webopedia.internet.com/quick_ref/os I_Layers.asp The OSI model defines a framework for implementing networking services via specific PROTOCOLS (depending on the protocol stack being used) in seven layers Concerned with transmitting signals (representing raw bits) over a communication channel Transmitting signals via energy of some form or another Data are not organized into frames or packets at Layer 1 Addresses the nature of the medium and types of signals used Cable type: coaxial, twisted pair, fiber Signal type: Light pulses, electrical voltage, radio waves Why would one be concerned about the medium that is used? What difference does it make? Hardware dealing with transmission of signals is defined at layer 1 of the OSI Cables, hubs, repeaters Do not understand packets or frames only signals The NIC provides a physical connection to the network and bridges layers 1 and 2 Networking technologies may use a variety of physical layer options Ethernet (the most popular wired networking technology) supports a number of different physical layer options Designation Cable Type Topology Speed Max Segment Length 10Base5 RG-8 coaxial Bus 10 Mbps 500 meters 10Base2 RG-58 coaxial Bus 10 Mbps 185 meters 10BaseT CAT 3 UTP Star 10 Mbps 100 meters 100BaseFX 62.5 / 125 multimode fiber Star 100 Mbps 412 meters 100BaseTX CAT 5 UTP Star 100 Mbps 100 meters 3
Different types of media may be used to implement a given LAN technology For example, depending on the standard being followed, an Ethernet LAN may require coaxial, fiber, or twisted pair wiring Each standard has different topology and medium requirements Max length of cable What happens if you exceed max cable length standards? Type of connectors Bit rate (data transmission rate) Monitor data error rates Converts signals and streams of bits into frames and vice versa Creates and recognizes frame boundaries What is a frame and why is it important? The frame format is different for various networking technologies Ethernet Token Ring ATM Different Frame Formats Ethernet frame (IEEE 802.3) v2 length >= 1536 B Dest. Source Preamble SFD Length DATA FCS MAC MAC 8 bytes 1 6 6 2 46 1500 bytes 4 Token Ring frame (IEEE 802.5) length > 4500 B Start Access Frame Dest. Source DATA FCS End Frame Del. Control Control MAC MAC Del. Status 1 1 1 6 6 4500 >= 0 4 1 1 Access control technique of various technologies defined at Layer 2 The way in which networked devices gain access to the medium, communicate with other devices, and transfer data differs based on the technology being used For example, the process two devices use to communicate over wireless is different from Ethernet 4
Error detection in the bit to frame conversion process Identifies and corrects frame errors Errors related to LAN communication Between 2 hosts The physical address or MAC address is contained in the frame header Provides conduit or link between the hardware and software on the computer and the physical network medium This is done via the NIC Sublayers In terms of network design Data Link Layer (2) is single most important layer in determining what hardware is used LAN technology and topology requires certain physical layer options and vice versa Implies access technique used LLC Interface to layer 3. Controls frame synchronization, flow control, and error checking MAC Interface to layer 1. Controls how PC accesses and transmits data specifies the Media Access Technique used 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 distances Hardware that recognizes frames operates at layer 2 of the OSI Switch, bridge Network Layer (3) Packages data/frames into IP datagrams Higher level, routable network addresses (like the IP address) are recognized and managed Hardware that recognizes network addresses (like IP addresses) works at Layer 3 Routers 5
Network Layer (3) Responsible for end-to-end communication 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? Network Layer (3) Frames, Packets, and IP Datagrams Example protocol: IP Example services: network addressing, fragmentation/reassembly, routing What does routing mean? MAC and IP Address Comparison MAC address (physical address) IP address (network address) Transport Layer (4) Network layer (3) and Transport layer (4) protocols designed to work together as a pair TCP/IP Internet protocol stack SPX/IPX Layer 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 6
Transport Layer (4) Provides messaging service for Session layer (5) and hides the underlying network from the upper layers Example protocols: TCP, UDP Example of Transport Layer services: flow control, multiplexing, retransmission, message sequencing 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? Encapsulation Encapsulation Each protocol adds headers to information it receives from the layer above it Upper Layers 4 When a datagram is encapsulated, the entire datagram is placed into the payload area of a specific frame format Transport 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 3 2 Lower Layers (1 4) Layers 1 4 are called the LOWER layers Lower layers are concerned with proper transmission of data across the network Upper Layers (5 7) Layers 5 7 called the UPPER layers Upper layers are concerned with how different application communicate between different hosts Difficult to identify and separate upper layer protocols in some cases many applications bundle services provided at layers 5, 6, and 7 Example: WS FTP not only uses Layer 7 protocol FTP, but manages syntax and compression issues at Layer 6, and session management issues at Layer 5 Upper layer protocols/services know nothing about, or understand networking or addressing 7
Session Layer (5) Responsible for establishing, maintaining, and ending communication There are no separate Session Layer protocols Session Layer functions are integrated into other protocols that also include Presentation and Application Layer functions Set up, manage, and tear down sessions or connections between Presentation Layer (6) entities Session Layer (5) Coordinates communication and organizes into one of three categories: 1) Simplex only one node can transmit 2) Half duplex both nodes can transmit, but only one at a time. Once one node is finished transmitting data, the other node can transmit 3) Full duplex both nodes can transmit simultaneously without disrupting the other node Session Layer (5) Traffic cop for communications between two nodes on a network For Internet applications mapping between logical ports and sessions Presentation Layer (6) Primary role is to preserve meaning of information transmitted between systems Computers communicating on a network often use different syntax Ensures communication between entities is of a form both can understand Computers must negotiate a common syntax so they can choose a transfer syntax that they both have in common and both understand Semantics? Presentation Layer (6) Different applications and programming languages use different data types and different syntax Text-based languages based on sequence while visual languages are based on spatial layout and relationships between symbols Which statements in a program are acceptable to the compiler? Presentation Layer (6) Network perspective: applications send messages to each other Application perspective: messages contain specific types of data Many types of data use very standard (universal) formats MPEG for video JPEG for still images ASCII for text Not all data types have universal formats Not all computers format data types the same way 8
Presentation Layer (6) If needed, systems can select transfer syntax that provides additional services such as: Data compression and decompression Encryption and decryption Application Layer (7) Entrance point for applications to access the OSI model (structured networking framework) and use network resources NOT an application, but access to protocols that provide network services Coordinates network services Identify parties and make sure each can be reached Ensures communication resources exist (For example, is there are modem at the sender s computer?) 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 network Application Layer protocols often include Session and Presentation Layer functions Typical protocol stack consists of 4 separate protocols that run at the application, transport, network, and data-link layers FTP, TCP, IP, specific Ethernet protocols 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 used Different lower layer protocols CAN be used the application is not aware of this, and doesn t need to deal with any lower layer functionality 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 layer Using TCP/IP stack as example, on what layer does IP operate? What about TCP? 9