Internetworking Devices and OSI Model

1.Internetworking Models

When networks first came into being, computers could typically communicate only with computers from the same manufacturer. the Open Systems Interconnection (OSI) reference model was created by the International Organization for Standardization (ISO) to break through this barrier.

The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work in peaceable accord with each other. 

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. 

2.Internetworking Device 

Internet - Means combination of  LAN and WAN

 Networking Device

1. Switches

2. Router 

3. Hubs

4. Repeaters

5. Bridges

Switches - Switches are generally identified as Layer-2 devices, as switches process information stored in the Data-Link header of a frame such as MAC addresses in Ethernet. 

A switch builds a hardware address table, allowing it to make intelligent forwarding decisions based on frame (data-link) headers. A frame can then be forwarded out only the appropriate destination port, instead of all ports.  

Router - Routers are identified as Layer-3 devices, as routers process logical addressing information in the Network header of a packet such as IP addresses..

Hubs - Hubs are Layer-1 devices that physically connect network devices together for communication. Hubs can also be referred to as repeaters.

A hub provides no intelligent forwarding whatsoever, and will always forward every frame out every port, excluding the port originating the frame. As with a bus topology, a host will only process a frame if it matches the destination hardware address in the data-link header. Otherwise, it will discard the frame.  

Switch Forwarding Methods 

1.Store-and-Forward - The Store-and-Forward method copies the entire frame into memory, and

performs a Cycle Redundancy Check (CRC) to completely ensure the integrity of the frame. However, this level of error-checking introduces the highest latency of any of the switching methods.

 

 

2. Cut-Through (Real Time) - The Cut-Through (Real Time) method copies only enough of a frame’s

header to determine its destination address. This is generally the first 6 bytes following the preamble. This method allows frames to be transferred at wire speed, and has the least latency of any of the three methods. No error checking is attempted when using the cut-through method.

3. Fragment-Free (Modified Cut-Through) - The Fragment-Free (Modified Cut-Through) method copies only the first 64 bytes of a frame for error-checking purposes. Most collisions or corruption occur in the first 64 bytes of a frame. Fragment-Free represents a compromise between reliability (store-and-forward) and speed (cut-through).

 

Repeaters - Repeater operates at physical layer of  the OSI model.

Bridges - Bridge work at Layer-2 device as works at Data Link layer of the OSI model and interconnect multiple LANs and manage data flow.

 

3.The OSI Model 

The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO), and formalized in 1984. It provided the first framework governing how information should be sent across a network.

The OSI is a logical model, not a physical one. The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other as well as with users. The bottom four layers define how data is transmitted end to end.

These are the three upper layers and their functions

Application - Provides a user interface

Presentation - Presents Data and Handles such as encryption

Session - Keeps different application data separate

These are the four lower layers and their functions

Transport -  It provides reliable and unreliable delivery

                   Performs error correction before retransmit

Network -   It provides logical addressing, which routers use for path determination

Data Link - Combines packet into bytes and bytes into frame

                    Provides access to media access using MAC address

                    Performs error correction and error detection

Physical  -   Moves bits between device and specifies voltage, wire speed and pinout  of                            cables

-The Application Layer  

The Application layer works as the interface between actual application programs. This means end-user programs like Microsoft Word don’t reside at the Application layer, they interface with the Application layer protocols.  

-The Presentation Layer

The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting. Think of it as the OSI model’s translator, providing coding and conversion services.

-The Session Layer

The Session layer is responsible for setting up, managing, and dismantling sessions between Presentation layer entities and keeping user data separate. Dialog control between devices also occurs at this layer.

Communication between hosts’ various applications at the Session layer, as from a client to a server, is coordinated and organized via three different modes: simplex, half-duplex, and full-duplex. Simplex is simple one-way communication, kind of like saying something and not getting a reply. Half-duplex is actual two-way communication, but it can take place in only one direction at a time, preventing the interruption of the transmitting device. It’s like when pilots and ship captains communicate over their radios, or even a walkie-talkie. But full-duplex is exactly like a real conversation where devices can transmit and receive at the same time, much like two people arguing or interrupting each other during a telephone conversation.

-The Transport Layer  

The Transport layer segments and reassembles data into a single data stream. Services located at this layer take all the various data received from upper-layer applications, then combine it into the same, concise data stream. These protocols provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.

The Transport layer can either be connectionless or connection-oriented, but because Cisco really wants you to understand the connection-oriented function of the Transport layer.

Connection-Oriented Communication

For reliable transport to occur, a device that wants to transmit must first establish a connection-oriented communication session with a remote device—its peer system—known as a call setup or a three-way handshake. Once this process is complete, the data transfer occurs, and when it’s finished, a call termination takes place to tear down the virtual circuit.

-The Network Layer  

The Network layer, or layer 3, manages device addressing, tracks the location of devices on the network, and determines the best way to move data. This means that it’s up to the Network layer to transport traffic between devices that aren’t locally attached. Routers, which are layer 3 devices, are specified at this layer and provide the routing services within an internetwork.  

-The Data Link Layer  

The Data Link layer provides for the physical transmission of data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses and will translate messages from the Network layer into bits for the Physical layer to transmit.  

-The Physical Layer  

The Physical layer does two things: it sends bits and receives bits. Bits come only in values of 1 or 0.