1. Lecture 1 The OSI Model Reference: TCP/IP Protocol Suite, 4 th Edition (chapter 2) 1

2. Outline To discuss the OSI model and its layer architecture and to show the interface between the layers. To briefly discuss the functions of each layer in the OSI model.

3. 2 THE OSI MODEL Introduced in late 1970s, Open Systems Interconnection (OSI) model is an ISO standard covers all aspects of network communications. – OSI is not a protocol, but a model for designing a network architecture and the basis for the protocols creation in OSI stack. OSI model is a layered framework that allows communication between all types of computer systems without requiring changes to the logic of underlying software and hardware. – Consists of seven separated but related layers. – Each defines a part of the process of moving information across a network.

4. 4 Figure 3 The OSI model

5. The OSI model consists of 7 ordered layers. The designers of the OSI model distilled the process of transmitting data to its most fundamental elements. – They identify which networking functions had related uses and collected those functions into discrete groups that become the layers. Each layer defines a family of functions distinct from those of the other layers. By localizing functionality in this fashion, this architecture is both comprehensive and flexible. – Allows complete interoperability between incompatible systems.

6. 6 Figure 4 OSI layers

7. Layer-to-layer communication Interfaces between layers – Define what information and services a layer must provide for the layer above it. – Provide modularity to a network. As long as a layer provides expected services to the layer above it, specific implementation of its functions can be replaced without changes to the surrounding layers. Organization of layers – Network support layers: 1, 2, 3; deals with physical aspect (electrical spec, physical connections and addressing, transport timing), mostly hardware Impl.. – User support layers: 5, 6, 7; allow interoperability among unrelated software systems. (software Impl.) – Transport layer: 4; links two sub-groups.

8. 8 Figure 5 An exchange using the OSI model

9. Layers in OSI Model – physical layer It coordinates functions required to carry a bit stream over a physical medium. – Deals with mechanical and electrical specifications of interface and transmission media. – Defines procedures and functions that physical devices and interfaces have to perform for transmission to occur. It also concerns with – Representation of bits: the type of encoding (bits -> signals, electrical or optical) – Data rate: i.e., transmission rate – number of bits sent per second, or duration of a bit, which is how long it lasts. – Synchronization of bits: sender and receiver clocks must be synchronized.

10. – Line configuration, i.e., connection of devices to media. In a p2p configuration, two devices are connected together through a dedicated link. In a multipoint configuration, a link is shared between several devices. – Physical topology: it defines how devices are connected to make a network. mesh topology (every device connected to every other device), a star topology (devices connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device on a common link) – Transmission mode: the direction of transmission. simplex (one device sends, the other only receives, a one-way communication), half-duplex (two devices send and receive, but not at the same time), or full-duplex (two devices send and receive at the same time).

11. 11 The physical layer is responsible for moving individual bits from one (node) to the next.

12. Layers in OSI Model – Data Link layer It transforms the physical layer, a raw transmission facility to a reliable link. – Makes physical layer appear error-free to upper layer (network layer) It is also responsible for – Framing, it divides the stream of bits received from the network layer into manageable data units called frames. – Physical addressing: if frames are to be distributed to different systems on the network, it adds a header to the frame to define sender and/or receiver of the frame; if the frame is intended for a system outside the sender’s network, the receiver address is the address of the connecting device that connects the network to the next on. – Flow control: it imposes a flow control mechanism to prevent overwhelming the receiver.

13. Data link layer is also responsible for (continued) – Error control, it adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. It is normally achieved through a trailer added to the end of frame. – Access control, when two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

14. Layers in OSI Model – Network layer It is responsible for source-to-destination delivery of a packet, possibly across multiple networks. – If two systems are connected to the same link, there is usually no need for a network layer. Other responsibilities – Logical addressing: if a packet passes the network boundary, another addressing system is needed to help distinguish the source and destination systems. This layer adds a header to a packet coming from the upper layer, including logical addresses of sender and receiver. – Routing: connecting devices (routers or switches) route packets to their final destination when independent networks or links are connected together to create internetworks or a large network.

15. Layers in OSI Model – Transport layer It provides the process-to-process delivery of entire message. – A process is an application program running on the host. It ensures the whole message arrives intact and in order; oversees both error control and flow control at the source-to- destination level. Other responsibilities – Service-point addressing: It gets the entire message to the correct process on the computer. – Segmentation and reassembly: a message is divided into transmitted segments, with each segment containing a sequence number. These numbers enable this layer to reassemble the message correctly upon arriving at the destination, and identify and replace packets that were lost in transmission.

16. Other responsibilities (more) – Connection control: it can be connectionless or connection-oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection-oriented one makes a connection with the transport layer at the destination first before delivering packets. After all data are transferred, the connection is terminated. – Flow control: flow control is performed end to end rather than across a single link. – Error control: error control is performed process-to-process rather than across a single link. The sending transport layer makes sure that entire message arrives at the receiving transport layer without error. Error connection is usually achieved through retransmission.

17. Layers in OSI Model – Session layer It is network dialog controller, establish, maintains and synchronizes the interaction between communicating systems. – Dialog control: it allows two systems to enter into a dialog; allows the communication between two processes to take place in either half- duplex or full-duplex mode. – Synchronization: It allow a process to add checkpoints (synchronization points) into a stream of data. 17

18. Layers in OSI Model – Presentation layer It is concerned with the syntax and semantics of the information exchange between two systems. – Translation: the processes in two systems are usually exchanging information in the form of character strings, numbers, which should be changed to bit streams before being transmitted. It is responsible for interoperability between different encoding methods: sender-dependent format -> common format -> receiver-dependent format – Encryption: it is responsible to assure privacy to carry sensitive information. – Compression: it reduces the number of bits contained in the information, for example, in transmission of multimedia. 18

19. Layers in OSI Model – Application layer It provides user interfaces and support for services such as email, remote file access and transfer, shared database management, and other types of distributed information services. – Network virtual terminal: a software version of a physical terminal and allows a user to log on to a remote host. – File transfer, access and management (FTAM). It allows a user to access files in a remote host, to retrieve files from a remote computer to the local computer, etc. – Email services: email forwarding and storage. – Directory services: provides distributed database sources and access for global information about various objects and services. 19

20. 20 Figure 6 Summary of OSI Layers