OSI Reference Model. Computer Networks lab ECOM Prepared By : Eng. Motaz Murtaja Eng. Ola Abd Elatief

Transcription

1 Islamic University of Gaza Faculty of Engineering Computer Engineering Department Computer Networks lab ECOM 4121 OSI Reference Model Prepared By : Eng. Motaz Murtaja Eng. Ola Abd Elatief May /2010

2 OSI Reference Model Late in 1970s the International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) Reference Model to describe how information is transferred from one machine to another, from the point when a user enters information using a keyboard and mouse to when that information is converted to electrical or light signals transferred along a piece of wire or radio waves transferred through the air. Advantages: Dividing the complex network operations into manageable layers. The possibility of changing one layer without having to change all other layers. Simplifies teaching and learning.

3 The Upper Layers The top three layers define how the applications within the end stations will communicate with each other and with users. None of the upper layers knows anything about networking or network addresses. The Lower Layers The four bottom layers that define how data is transferred through a physical wire or through switches and routers. These bottom layers also determine how to rebuild a data stream from a transmitting host to a destination host s application.(how data is transmitted endto-end).

4 Application Layer It provides the interface that a person uses to interact with the application. This interface can be command-line-based or graphics-based. Cisco IOS routers and switches have a command-line interface (CLI), whereas a web browser uses a graphical interface. Protocols: File Transfer Protocol (FTP) File Transfer Protocol, or FTP, is a protocol used to upload files from a workstation to a FTP server or download files from a FTP server to a workstation. It is the way that files get transferred from one device to another in order for the files to be available on the Internet. When ftp appears in a URL (Uniform Resource Locator) it means that the user is connecting to a file server and not a Web server and that some form of file transfer is going to take place. Most FTP servers require the user to log on to the server in order to transfer files. Hyper Text Transfer Protocol (HTTP) In contrast, Hyper Text Transfer Protocol, or HTTP, is a protocol used to transfer files from a Web server onto a browser in order to view a Web page that is on the Internet. Unlike FTP, where entire files are transferred from one device to another and copied into memory, HTTP only transfers the contents of a web page into a browser for viewing. FTP is a two-way system as files are transferred back and forth between server and workstation. HTTP is a one-way system as files are transported only from the server onto the workstation's browser. When http appears in a URL it means that the user is connecting to a Web server and not a file server. The files are transferred but not downloaded, therefore not copied into the memory of the receiving device. Telephone etwork (Telnet) terminal emulation program for TCP/IP networks. The Telnet program runs on your computer and connects your PC to a server on the network. You can then enter commands through the Telnet program and they will be executed as if you were entering them directly on the server console. Electronic Mail ( ) Transmission of messages over communications networks can use Simple Mail Transfer Protocol (SMTP) or the X.400 standard to deliver messages between different applications.

5 Presentation Layer Defining data encryption, compression, and format example, text is represented in two different forms: ASCII and EBCDIC. ASCII (the American Standard Code for Information Interchange, used by most devices today) uses seven bits to represent characters. EBCDIC (Extended Binary-Coded Decimal Interchange Code, developed by IBM) is still used in mainframe environments to represent characters. Text can also be shaped by different elements, such as font, underline, italic, and bold. There are different standards for representing graphical information BMP, GIF, JPEG, TIFF, and others. This variety of standards is also true of audio (WAV and MIDI) and video (WMV, AVI, and MPEG). Session Layer Protocols: Start, control and ends conversations (communication sessions). Keeps different application separate. etwork File System ( FS) Developed by Sun Microsystems and used with TCP/IP and Unix workstations to allow transparent access to remote resources. Structured Query Language (SQL) Developed by IBM to provide users with a simpler way to define their information requirements on both local and remote systems. Remote Procedure Call (RPC) A broad client/server redirection tool used for disparate service environments. Its procedures are created on clients and performed on servers. X Window Widely used by intelligent terminals for communicating with remote Unix computers, allowing them to operate as though they were locally attached monitors. AppleTalk Session Protocol (ASP) Another client/server mechanism, which both establishes and maintains sessions between AppleTalk client and server machines.

6 Transport Layer Establishing a connection-oriented (reliable) and connectionless (unreliable) communications. Segments and reassembles data into a data stream, sequencing and combination. Provide end-to-end data transport services and can establish a logical connection virtual circuits between the sending host and destination host on an internetwork. Protocols: Transmission Control Protocol (TCP) is a connection-oriented protocol. Sequenced Packet Exchange (SPX) is a connection-oriented protocol. User Datagram Protocol (UDP) is a connection-less protocol. Ex: Connection-Oriented Communication Establishing a connection-oriented session Transmitting segments with flow control

7 Windowing: Data throughput would be low if transmitting machine had to wait for an acknowledgment after sending each segment. Controls how much information is transferred from one end to the other. Acknowledgments: Data won t duplicate or lost, positive acknowledgment with retransmission.

8 Network Layer Routing through internetwork and network addressing, transporting traffic between devices that are not locally attached, how to fragment segments to smaller packets to accommodate media with smaller maximum transmission size, how routing works and how routes are learned. IP running in CISCO router responsible for examining the destination IP comparing the destination IP with the routing table fragmenting packet and queuing the fragmented packets. Protocols: Internet Protocol (IP) (32 bits). Internetwork Packet exchange (IPX) (80 bits, 4 bytes network portion and 6 bytes node portion MAC address ). Internet Control Message Protocol (ICMP). Address Resolution Protocol (ARP). Reverse Address Resolution Protocol (RARP). Routed protocols Data Packets : Used to transport user data through the internetworking. Protocols transport user data called routed protocols such as IP, IPX.

9 Routing protocols Route Update Packets : Used to update neighbor about networks connected to routers in the internetwork. Protocols that send route updates packets are called routing protocols such as IGRP, EIGRP, RIP, OSPF Routing table used in a router Devices: Router is a device that functions at the Network layer. Data Link Layer ( LLC & MAC): Translate message from network layer into bits for the physical layer to transmit, formatting the message into Data Frame adding header containing the hardware destination and source address.

10 Media Access Control (MAC) Layer Function: Arbitration: which determine when it is appropriate to use the physical medium. For Example: Ethernet uses the Carrier Sense Multiple Access with Collision Detection algorithm for arbitration. The basic algorithm for using Ethernet when there is data to be sent consists of the following steps: 1. Check if a digital signal (Frame) on the wire (Carrier Sense) another NIC might already have a frame on the wire (Multiple Access). 2. If another digital signal (Frame) on the wire Wait, and then listen again. 3. If clear No other host is transmitting send. 4. The transmitting host constantly monitors the wire to make sure no other hosts begin transmitting. 5. If the host detects a signal (Frame) on the wire it sends extended JAM SIGNAL that causes all nodes in the segment to stop sending data (Frames) THINK, Busy Signal (Collision Detection). 6. The nodes respond to that jam signal by waiting a while before attempting to transmit again. Backoff algorithms determine when the colliding stations can retransmit. 7. If collisions keep occurring after 15 tries Time out. Addressing: so that the correct recipient(s) receive and process the data that is sent. Error detection and ordered delivery of frames: which determines whether the data made the trip across the medium successfully. (Parity & CRC). For Example: Ethernet uses Cyclic Redundancy Check (CRC) stored in Frame Check Sequence (FCS). The filed contains a value of 4 Bytes which, when plugged into a mathematical formula along with the frame contents, can determine if the Frame had bit errors.

11 Logical Link Layer (LLC) Layer Function: What is the data? Which determines the type of header that follows the data link header. For Example: One types of Ethernet frames available: IEEE 802.2: SAP (Service Access Point) Field Length in Bytes Description Destination SAP number 1 Identifies the network layer protocol that this is to be sent to Source SAP number 1 Identifies the network layer protocol that originated this data Control field 1-2 Determines the fields that follow this field Data Variable This contains the upper layer network layer packet Protocols: The LLC performs its multiplexing by using Service Access Point (SAP) identifiers. When a network layer protocol is encapsulated in the frames, the protocol of the network data is placed in the SAP field. When the destination receives the frame, it examines the SAP field to determine which upper-layer network layer protocol should process the frame. This allows the destination network device to differentiate between TCP/IP and IPX network layer protocols that are being transmitted across the data link layer connection. LA Protocols: (LLC) (Ethernet) (Token Ring) (wireless) WA Protocols: HDLC PPP Frame Relay ISDN ATM Devices: ICs, Switches and Bridges are examples of devices that function at the Data Link layer.

12 Physical Layer: The physical layer responsible for how binary information is converted to a physical layer signal. For example, if the cable uses copper as a transport medium, the physical layer defines how binary 1 s and 0 s are converted into an electrical signal by using different voltage levels. If the cable uses fiber, the physical layer defines how 1 s and 0 s are represented using an LED or laser with different light frequencies. The physical layer is also defines physical properties for connections and communication, including wires (UTP and fiber) and connectors (RJ-45 and DB-9). The type of interface used on the networking device: The type of interface is commonly called a NIC. A NIC can be a physical card that you put into a computer, like a 10BaseT Ethernet card, or a fixed interface on a switch, like a 100BaseTX port on a Cisco Catalyst 1900 series switch. The type of cable used for connecting devices: Examples of physical layer standards include the following cable types: Category-3, -5, and -5E; and others. The connectors used on each end of the cable: Interface connectors include the following: AUI, BNC, RJ-11, RJ-45, and others. The pin-outs used for each of the connections on the cable: The straight-through cable is used to connect: Host to switch or hub, Router to switch or hub Four wires are used in straight-through cable to connect Ethernet devices. Devices: A hub, repeater and cable connectors are examples of devices that function at the physical layer. A hub and a repeater are examples of devices that function at the physical layer. A hub is really a multiple-port repeater. A repeater receives a digital signal and reamplifies or regenerates that signal, and then forwards the digital signal out all active ports without looking at any data. An active hub does the same thing.