2-2008UP-Copyrights reserved1 ITGD4103 Data Communications and Networks OSI Reference Model Lecture-5: OSI Reference Model week 5- q-2/ 2008 Dr. Anwar Mousa University of Palestine International Faculty of Information Technology

2-2008UP-Copyrights reserved2 Contents : 1.OSI Reference Model 2.TCP/IP

2-2008UP-Copyrights reserved3 Layers and Stacks  Many different protocol stacks:  OSI Open Systems Interconnection.  TCP/IP Internet Protocols.  SS7 Signaling System #7  The number of layers, and the name, contents, and function of each layer differ for each protocol stack.

OSI Reference Model

2-2008UP-Copyrights reserved5 Layering  The separation of the communication functions is called layering.  layering is the communication into separate steps that are performed sequentially, defined by specific interfaces for passing the result of each step to the next layer until the overall function, such as the sending or receiving of some amount of information, is completed.  Each layer communicates with the layers above and below through a standard interface.

2-2008UP-Copyrights reserved6 Reasons for layering  Divides interrelated aspects of network operation into less complex elements.  Simplifies the network model.  Enables programmers to specialize in a particular level or layer of the networking model.  Provides design modularity.  Prevent changes in one area from impacting other areas, so each area can evolve more quickly.

2-2008UP-Copyrights reserved7 Layering Responsibility  End- to- end delivery of packets to the network.  Selecting routes for the packets to take.  Implies knowledge of the network topology.  Managing utilization of the links.  Provide flow control (across multiple links).  Spread load among different routes.

2-2008UP-Copyrights reserved8 Layering Principle  Each layer uses its own protocol to communicate with its peer layer in the other system (receiver).  Layer N software on the destination computer must receive exactly the message sent by layer N software on the sending computer.  A layer N+1 entity sees the lower layers N only as a service provider.  Each layer depends on the service function of the layer below.

2-2008UP-Copyrights reserved9 An entity of a particular layer can only communicate with:  Peer layer entity using a common protocol (Peer Protocol)  Adjacent layers to provide services and to receive services.

2-2008UP-Copyrights reserved10 OSI Reference Model  Open Systems Interconnection Reference Model (OSI).  A seven-layer structure designed to describe computer network architectures and the way that data passes through them.  It is a layered model used to describe network protocols and services.

2-2008UP-Copyrights reserved11 Seven Layer Reference Model  Layered model consists of seven layers: 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical

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2-2008UP-Copyrights reserved13 1. The Physical Layer Responsibility:  Transmission of raw bits over a communication channel (how to transfer bits correctly). Issues:  Physical characteristics of interfaces and media.  Representation of bits.  Data rate and transmission mode.

2-2008UP-Copyrights reserved14 The physical layer coordinates all the functions required to transmit a bit stream over the physical medium.

2-2008UP-Copyrights reserved15 2. The Data Link Layer Responsibility:  Transfer data frames correctly and successfully.  Provide an error-free communication link.  Error control and flow control. Issues:  Framing (dividing data into chunks).  Addressing

2-2008UP-Copyrights reserved16 3. The Network Layer Responsibilities:  Move packet inside the network and send it to the destination.  Path selection between end-systems (routing).  Fragmentation & reassembly.  Translation between different network types. Issues:  Forwarding, routing, addressing, switching, and congestion control.

2-2008UP-Copyrights reserved17 4. The Transport Layer Responsibilities:  Delivery of the data between two peer processes.  Connection establishment, management, termination, error control and flow control. Issues:  Headers.  Error detection.  Reliable communication.

2-2008UP-Copyrights reserved18 5. The Session Layer Responsibilities:  Enables two applications on the network to have an ongoing conversation or dialog.  Allows users to establish, manages, and terminate session. Issues:  Control for data exchange  Data synchronization.  Failure recovery.

2-2008UP-Copyrights reserved19 6. The Presentation Layer Responsibilities:  Data translation.  Data formatting.  Data encryption.  Data compression.  Data syntax restructuring.

2-2008UP-Copyrights reserved20 7. The Application Layer Responsibilities:  Providing services to applications.  Enable the user to access the network.  Variety of protocols that are commonly used.  Anything not provided by any of the other layers. Issues:  File Transfer.  Appropriate selection of “type of service”.

2-2008UP-Copyrights reserved21 Data transmission using the OSI model:

2-2008UP-Copyrights reserved22 Layering & Headers  Each layer needs to add some control information to the data in order to do its job.  This information is added to the data before being given to the lower layer.  Once the lower layers deliver the data and control information, the peer layer uses the control information.

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2-2008UP-Copyrights reserved25 Important Notes  Data-Link Layer : Communication between machines on the same network.  Network Layer : Communication between machines on possibly different networks.  Transport Layer : Communication between processes (running on machines on possibly different networks).

2-2008UP-Copyrights reserved26 Conclusion

TCP/IP

2-2008UP-Copyrights reserved28 TCP Transmission Control Protocol  TCP is one of the main protocols in TCP/IP networks. Whereas the IP protocol deals only with packets.  TCP enables two hosts to establish a connection and exchange data.  TCP - is responsible for verifying the correct delivery of data from client to server.

2-2008UP-Copyrights reserved29 TCP  TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent.  TCP provides:  Connection-oriented  Reliable  Full-duplex  Byte-Stream

2-2008UP-Copyrights reserved30 Connection-Oriented  Connection oriented means that a virtual connection is established before any data is transferred.  If the connection cannot be established, the user program is notified.  If the connection is ever interrupted, the user program is notified.

2-2008UP-Copyrights reserved31  Reliable means that every transmission of data is acknowledged by the receiver.  If the sender does not receive acknowledgement within a specified amount of time, the sender retransmits the data.  Full Duplex:  TCP provides transfer in both directions.

2-2008UP-Copyrights reserved32  Buffering: TCP is responsible for buffering data and determining when it is time to send a datagram.  Stream means that the connection is treated as a stream of bytes.  The user application does not need to package data in individual datagrams.

2-2008UP-Copyrights reserved33 TCP Segments  The chunk of data that TCP asks IP to deliver is called a TCP segment.  Each segment contains:  Data bytes from the byte stream.  Control information that identifies the data bytes.

2-2008UP-Copyrights reserved34 TCP/IP  The TCP/IP is a protocol suite.  It was initially successful because it delivered a few basic services that everyone needs (file transfer, electronic mail, remote logon) across a very large number of client and server systems.  All TCP/IP protocol suite specifications are in the public domain.  No license fees.  Used to create open system networking environments.

2-2008UP-Copyrights reserved35 TCP/IP  TCP/IP is a way to communicate reliably between hosts. The advantages of TCP/IP:  TCP/IP is bound to no manufacturer.  TCP/IP is usable in LANs and WANs.  TCP/IP makes application independent of the transmission system.

2-2008UP-Copyrights reserved36 TCP/IP Model  TCP/IP Model consists of only five layers.  Physical layer  Network access layer (Data link layer)  Internet layer (Network layer)  Transport layer  Application layer  Session layer and Presentation layer are not actually defined by TCP/IP.

2-2008UP-Copyrights reserved37 Protocol layering and data  Each layer takes data from above.  Adds header information to create new data unit.  Passes new data unit to layer below. Application Transport Network Link Physical Application Transport Network Link Physical Source Destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame

2-2008UP-Copyrights reserved38 5. Application Layer :  Concerned with how data at both ends is handled. 4. Transport Layer :  Manages flow of data. 3. Internet Layer :  Consists of several protocols, primary protocol is IP (providing addressing scheme). 2. Network Access Layer :  Manages transmission of data within the network. 1. Physical Layer :  TCP/IP leaves the physical connection to manage itself.

2-2008UP-Copyrights reserved39 1. Physical Layer  Covers the physical interface between a data transmission device and a transmission medium or network.  Physical layer specifies:  Characteristics of the transmission medium.  The nature of the signals.  The data rate.

2-2008UP-Copyrights reserved40 2. Network Access Layer  Equivalent to the OSI’s physical and data link layers.  Concerned with the exchange of data between an end system and the network to which it's attached.  Software used depends on type of network  Circuit switching  Packet switching  LANs

2-2008UP-Copyrights reserved41 3. IP Internet Layer  Uses Internet Protocol (IP)  Provides routing functions to allow data to traverse multiple interconnected networks.  Its function is:  Permit hosts to inject packets into any network and have them travel independently to the destination.  They may even arrive in a different order than they were sent, in which case it is the job of higher layers to rearrange them, if in-order delivery is desired.

2-2008UP-Copyrights reserved42 4. Transport Layer  Defines two standard transport protocols: TCP and UDP  TCP implements a reliable data-stream protocol. (connection oriented)  TCP is responsible for data recovery.  Provides reliability during data exchange.  UDP implements an unreliable data-stream. (connectionless)

2-2008UP-Copyrights reserved43 TCP How it works  The source establishes a connection and sends a limited number of segments and waits for an ACK (acknowledgment) from the destination for some of the segments before sending more.  If no ACK is received within a "timeout" period, then source assumes unacknowledged segments were lost and resends those segments (flow control).  The destination sends a NACK (negative acknowledgement) for any segments with errors and source resends it.

2-2008UP-Copyrights reserved44 Protocol Data Unit (PDU)  A Protocol Data Unit comprises of data and control information which is passed between layers in a protocol stack.  Unreliable, meaning no guarantees that data coming from the upper layer :  Will arrive at their destination.  Will arrive in the order in which the were sent.  Will arrive intact.

2-2008UP-Copyrights reserved45  The User Datagram Protocol (UDP) is composed at the Transport Layer.  Done by taking the data received from the application program at the Application Layer and adding a small header to it.  This datagram is then sent down to the Internet layer to be encapsulated into an IP datagram and sent out in a connectionless mode.  UDP can send its datagrams to multiple destinations.

2-2008UP-Copyrights reserved46 UDP Features  Simple protocol:  No connection establishment delays.  No connection state at sender and receiver.  No congestion control.  Small segment header.

2-2008UP-Copyrights reserved47 UDP Applications  Useful for Applications such as voice and video, where:  Retransmission should be avoided.  The loss of a few packets does not greatly affect performance.  Rate sensitive.

2-2008UP-Copyrights reserved48 5. Application Layer  This layer provides services suitable for the different types of application that might wish to use the network. Common TCP/IP Applications:  Simple mail transfer protocol (SMTP)  File Transfer Protocol (FTP)

2-2008UP-Copyrights reserved49 TCP/IP Protocol Graph

2-2008UP-Copyrights reserved50 The TCP/IP Communication  The flow of data from user to web browser and back.  At each layer, information is either added or removed  Depends on whether data is leaving or arriving at a workstation  Encapsulation - adding information over pre-existing information

2-2008UP-Copyrights reserved51 TCP/ IP Network Communication

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2-2008UP-Copyrights reserved53 OSI versus TCP/IP Model  TCP/IP does not map its layers precisely to OSI model  OSI = 7 layers  TCP/IP = 5 layers  The OSI and TCP/IP reference models have much in common.  Both are based on the concept of a stack of independent protocols.  The functionality of the layers is roughly similar.

2-2008UP-Copyrights reserved54 OSI versus TCP/IP Model  In TCP/IP,  Session Layer’s characteristics(OSI) are provided by the Transport Layer.  The function of presentation layer(OSI) is provided by the Application Layer.

2-2008UP-Copyrights reserved55 OSI vs TCP/IP Session Physical Transport Network Data Link Presentation Application Layer Transport Data Link Internet

2-2008UP-Copyrights reserved56 Conclusion  TCP enables two hosts to exchange data.  TCP provides: Connection-oriented, Reliable, Full-duplex, and Byte-Stream.  TCP/IP Layers: five layers  Physical layer, Network access layer, Internet layer,Transport layer, and Application layer.  TCP and UDP  OSI versus TCP/IP

2-2008UP-Copyrights reserved57 Conclusion

2-2008UP-Copyrights reserved58 Summary