Issuu on Google+

THE OSI & TCP/IP REFERENCE LAYERS


A BRIEF HISTORY •

When computers were first linked together into networks, moving information between different types of computers was a very difficult task.

In the early 1980s, the International Standards Organization (ISO) recognized the need for a standard network model. This would help vendors to create interpretable network devices. The Open Systems Interconnection (OSI) reference model, released in 1984, addressed this need.

The OSI model describes how information makes its way from application programs through a network medium to another application program in another computer. It divides this one big problem into seven smaller problems.

Each of these seven problems is reasonably self-contained and therefore more easily solved without excessive reliance on external information. Each problem is addressed by one of the seven layers of the OSI model


DEFINITIONS There are various definitions to the OSI reference layers. Below are a few:  Open Systems Interconnection ( OSI ) is a standard reference model for communication between two end users in a network. The model is used in developing products and understanding networks ( http://whatis.techtarget.com/definition/0,,sid9_gci523729,00.html)  The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. ( http://www.webopedia.com/quick_ref/OSI_Layers.asp)  The OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI reference model breaks this approach into layers


SIMPLY PUT All definitions provided above can be surmised as the OSI reference model being: “A MODEL THAT HELPS TO BREAK DOWN NETWORK FUNCTIONS VIS-ÀVIS STANDARDISATION , SCALABILITY AND INTEROPERABILITY.”


• Standardization: The reference model ensures all equipment manufacturers must conform to a certain standard thereby ensuring a minimum level of quality assurance. • Scalability: Various aspects/parts of a computer network can be reviewed, developed and troubleshot without affecting other parts of the network. Each part can therefore be worked on individually. • Interoperability: Conforming to a reference model ensures different equipment from various vendors can work together successfully on a network. I can therefore connect a Systimax cable to an ethernet port on an HP switch which is in turn connected to a Cisco router successfully.


THE OSI LAYERS


WHAT HAPPENS AT EACH LAYER? It is noteworthy to mention that each layer absolutely relies on the layer beneath it to carry out its function effectively and also communicates with its adjacent layer in a peer system in order for effective communication to take place. Each Layer has protocols that enable its perform its functions.


INDIVIDUAL LAYERS •

The Application Layer (Layer 7) : Primarily an interface between the end user and the application. This layer supports application and end-user processes. Examples of protocols used in this layer include HTTP, FTP, Telnet, SNMP etc

The Presentation Layer (Layer 6) : The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It ensures that information sent by the application layer of one system will be readable by the application layer of another system.

The Session Layer (Layer 5): This layer terminates connections between applications. coordinates, and terminates conversations, between the applications at each end. It deals coordination.

establishes, manages and The session layer sets up, exchanges, and dialogues with session and connection


INDIVIDUAL LAYERSI(Contd)n • The Transport Layer (Layer 4): This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. Protocols used here include TCP and UDP. • The Network Layer (Layer 3): This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding. Here we have routing protocols such as RIP and OSPF as well as routed protocols like IP. Routers are traditionally functional at this layer.


INDIVIDUAL LAYERS (contd) • The Data-link Layer (Layer 2): At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking. Ethernet switches are traditionally functional here.


INDIVIDUAL LAYERS (contd) • The Physical Layer (Layer 1): This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components. There are four general functions which the physical layer is responsible for. These functions are: • Definitions of hardware specifications • Encoding and signaling • Data transmission and reception • Topology and physical network design


THE TCP/IP Protocol Suite • The TCP/IP model, also referred to as the DOD (Department of Defence) model was created by the Department of Defense (DoD) to ensure and preserve data integrity as well as maintain communications in the event of catastrophic war. So it follows that if designed and implemented correctly, a TCP/IP network can be a truly dependable and resilient one.


TCP/IP LAYERS • The DoD model is basically a condensed version of the OSI model— it’s composed of four, instead of seven, layers


The OSI & TCP/IP models compared


ENCAPSULATION & DECAPSULATION • In peer communication, data from the application layer is passed down to the transport layer where it is encapsulated with transport layer information. This information includes source and destination port numbers amongst other control information. The product, referred to as a segment is passed down to the network/internetwork layer where it is further encapsulated with network layer information which include the source and destination IP addresses. The encapsulated segment, now referred to as a packet is passed down to the datalink /network interface layer where it is framed with layer 2 control information and subsequently passed on the physical layer where it is changed into formats transmissible over the relevant medium. At the receiving end, a reverse of the aforementioned process takes place till it reaches the application layer.


PUTTING IT ALL TOGETHER • We shall give an example of how the reference layers are applicable in a communication process. Given a scenario of a user on PC0 who attempts to reach a remote web server. The URL to the web server is www.test.com. We shall follow the path of the packet as it leaves PC0 for the very 1st time till its able to communicate with the web server


Packet at PC0


INSIDE THE PACKET


The Packet at Switch 0


INSIDE THE PACKET


PACKET AT ROUTER 0


INSIDE THE PACKET


PACKET AT ROUTER 1


INSIDE THE PACKET


PACKET AT REMOTE SWITCH


INSIDE THE PACKET


PACKET AT DNS SERVER


INSIDE THE PACKET


INSIDE THE PACKET


DNS PACKET RESOLVED • At this point, the DNS Server resolves the query by translating the domain name www.test.com to 192.168.3.2. The DNS query process highlighted above is then reversed until the information reaches PC0.


WEB PAGE REQUEST • PC0 can then request for the web page from the web server in steps exactly similar to those used to resolve the domain name from the DNS server with differences lying now in the protocol (now http) the destination IP and Port numbers.


• The packet undergoes encapsulation and decapsulation processes until it reaches the web server, which then serves the requested page to the PC.


CONCLUSION • We have been able to highlight the OSI and TCP / IP reference models • We have seen the role played by each of the layers and the protocols that permit them to play these roles • We have examined the applicability of these layers to the scenario of a web page request.


THANK YOU!!!

• ANY QUESTIONS?


OSI LAYERS