Computer Communication Architecture OSI MODEl TCp/ip model Prepared by: Er. Bednidhi Rijal

OSI (Open System Interconnect) Model Introduced in 1978 and revised in 1984 Formulates the communication process into structured layers There are seven layers in the model, hence the name the 7-Layer model The model acts as a frame of reference in the design of communications and networking products Prepared by: Er. Bednidhi Rijal

All People Seem To Need Data Processing 7 Layers 7. Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer All People Seem To Need Data Processing Prepared by: Er. Bednidhi Rijal

Contd.. Application Layer (Layer-7) The top layer of the OSI model Provides a set of interfaces for sending and receiving applications to gain access to and use network services, such as: networked file transfer, message handling and database query processing Some examples of network applications are DNS, Mail, Telnet, FTP, NFS PDU (protocol data unit) – User Data Prepared by: Er. Bednidhi Rijal

Contd.. Presentation Layer (Layer-6) Manages data-format information for networked communications (the network’s translator) For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand This layer is also responsible for certain protocol conversions, data encryption/decryption, or data compression/decompression PDU – formatted data Prepared by: Er. Bednidhi Rijal

Contd.. Session Layer (Layer-5) Enables two networked resources to hold ongoing communications (called a session) across a network Responsible for initiating, maintaining and terminating sessions Responsible for security and access control to session information (via session participant identification) Responsible for synchronization services, and for checkpoint services PDU – formatted data Prepared by: Er. Bednidhi Rijal

Contd.. Transport Layer (Layer-4) Offers end-to-end communication between end devices (ie. from one process to another) through a network. Manages the flow of data between parties by segmenting long data streams into smaller data chunks (based on allowed “packet” size for a given transmission medium) Reassembles chunks into their original sequence at the receiving end Provides acknowledgements of successful transmissions and requests resends for packets which arrive with errors The most common transport layer protocols are the connection-oriented Transmission Control Protocol (TCP) and the connectionless User Datagram Protocol (UDP). PDU – segments Prepared by: Er. Bednidhi Rijal

Contd.. Network Layer (Layer-3) The network layer is responsible for the delivery of individual packets from the source host to the destination host. Handles addressing messages for delivery Responsible for deciding how to route transmissions between computers This layer also handles the decisions needed to get data from one point to the next point along a network path This layer also handles packet switching and network congestion control PDU – Packets Prepared by: Er. Bednidhi Rijal

Contd.. Data Link Layer (Layer-2) The data link layer is responsible for moving frames from one hop (node) to the next. Handles special data frames (packets) between the Network layer and the Physical layer At the receiving end, this layer packages raw data from the physical layer into data frames for delivery to the Network layer At the sending end this layer handles conversion of data into raw formats that can be handled by the Physical Layer PDU – Frame In LAN, data link layer is divided in the 2 layers: Logical Link Control Sub-layer (LLC) Medium Access Control Sub-layer (MAC) Prepared by: Er. Bednidhi Rijal

Contd.. Physical Layer (Layer-1) Converts bits into electronic signals for outgoing messages Converts electronic signals into bits for incoming messages This layer manages the interface between the computer and the network medium (coax, twisted pair, etc.) This layer tells the driver software for the MAU (media attachment unit, ex. network interface cards (NICs, modems, etc.)) what needs to be sent across the medium The bottom layer of the OSI model PDU – Bits or Bytes Prepared by: Er. Bednidhi Rijal

Summary Prepared by: Er. Bednidhi Rijal

Advantage of layered Approach Independent of the implementation Reduces complexity Standardizes interface Facilitates modular engineering Ensures interoperable technology Simplifies teaching and learning Prepared by: Er. Bednidhi Rijal

TCP/IP Reference model Also called DOD (Department of defense) model Prepared by: Er. Bednidhi Rijal

The Application Layer (Layer 4) On top of the transport layer is the application layer. It contains all the higher-level protocols. Example: Virtual terminal (TELNET) protocol – allows a user on one machine to log onto a distant machine and work there. The file transfer protocol - provides a way to move data efficiently from one machine to another. Electronic mail (SMTP) – to transfer mail Domain Name System (DNS) for mapping hostnames onto their network addresses. HTTP for fetching pages on wave. Prepared by: Er. Bednidhi Rijal

The Transport Layer (Layer 3) The layer above the internet layer in the TCP/IP model is called the transport layer. It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. Defined two end-to-end transport protocols: TCP (Transmission Control Protocol) - a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. UDP (User Datagram Protocol) – an unreliable, connectionless protocol for applications that do not want TCP's sequencing or flow control and wish to provide their own. Prepared by: Er. Bednidhi Rijal

The Internet Layer (Layer 2) Its job is to permit hosts to inject packets into any network and have them travel independently to the destination (potentially on a different network). They may even arrive in a different order than they were sent. The internet layer defines an official packet format and protocol called IP (Internet Protocol). The job of the internet layer is to deliver IP packets where they are supposed to go. Similar to the OSI network layer. Prepared by: Er. Bednidhi Rijal

The Host-to-Network Layer (Layer 1) The host has to connect to the network using some protocol so it can send IP packets to it. Prepared by: Er. Bednidhi Rijal

A Comparison of the OSI and TCP/IP Reference Models The OSI and TCP/IP reference models have much in common. Both are based on the concept of a stack of independent protocols. Both have layers. Both have comparable transport and network layers. Despite these fundamental similarities, the two models also have many differences. Three concepts are central to the OSI model: 1. Services. 2. Interfaces. 3. Protocols. The biggest contribution of the OSI model is that it makes the distinction between these three concepts explicit. The service definition tells what the layer does, not how entities above it access it or how the layer works. It defines the layer’s semantics. Prepared by: Er. Bednidhi Rijal

Contd.. A layer’s interface tells the processes above it how to access it. It specifies what the parameters are and what results to expect. It, too, says nothing about how the layer works inside. It can use any protocols it wants to, as long as it gets the job done. These ideas fit very nicely with modern ideas about object-oriented programming. The TCP/IP model did not originally clearly distinguish between services, interfaces, and protocols. The only real services offered by the internet layer are SEND IP PACKET and RECEIVE IP PACKET. As a consequence, the protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively easily as the technology changes. Prepared by: Er. Bednidhi Rijal

Contd.. The OSI reference model was devised before the corresponding protocols were invented. This means OSI model was not biased toward one particular set of protocols. With TCP/IP, protocols came first and the model was really just a description of the existing protocols. Difference between the two models is the number of layers: the OSI model has seven layers and the TCP/IP model has four. Both have (inter)network, transport, and application layers, but the other layers are different. The OSI model supports both connectionless and connection-oriented communication in the network layer, but only connection-oriented communication in the transport layer (the transport service is visible to the users). The TCP/IP model supports only one mode in the network layer (connectionless) but both in the transport layer, giving the users a choice. Prepared by: Er. Bednidhi Rijal

A Critique of the OSI Model and Protocols 1. Bad timing. 2. Bad technology. 3. Bad implementations. 4. Bad politics. Prepared by: Er. Bednidhi Rijal

A Critique of the TCP/IP Reference Model The model does not clearly distinguish the concepts of services, interfaces, and protocols. This model is not at all general and is poorly suited to describing any protocol stack other than TCP/IP. TCP/IP model does not distinguish between the physical and data link layers. Host to network layer is not really a layer at all in the normal sense of the term as used in the context of layered protocols. It is an interface (between the network and data link layers) Prepared by: Er. Bednidhi Rijal