Network Model

The OSI Reference Model:

OSI Reference Model is consists of seven layers design for the help and reference for the study purposes. This is best to teach in universities for learning the network model. It is basically step by step procedure of all protocols implementation on different layers according to their responsibilities. Each layer in OSI model defines its own specific functions. This is not implemented.  In the real environment TCP/IP model is implemented which consists of 5 layers. All 7 layers functionality of OSI model is combined in the TCP/IP.  DoD is model developed by Department of Defense. It is consists of 4 layers. 7 Layers of OSI model is combined here in 4 layers. Below figure shows OSI layers protocols, Application and concepts used in each layer.

  OSI Reference Model

Application Layer	HTTP, FTP, SMTP, Telnet, SNMP
Presentation Layer	ASCII, EBCDIC, JPEG, GIF, TIFF, MPEG, WAV, MIDI.
Session Layer	SQL, NFS, RPC, X Window, NetBIOS, Winsock, BSD socket
Transport Layer	TCP, UDP, SPX TCP, UDP, SPX
Network Layer	IP, IPX, AppleTalk
Data Link Layer	LAN( Ethernet), WAN(HDLC, PPP, Frame Relay, ATM)
Physical Layer	Ethernet, RS-232, V.35

TCP/IP Model

Application Layer	Same as (Application + Presentation + Session) Layers of OSI
Transport Layer	Same as OSI Transport Layer
Network Layer	Same as OSI Network Layer
Data Link Layer	Same as OSI Data Link Layer
Physical Layer	Same as OSI Physical Layer

DoD Model

Process /Application	Same as (Application + Presentation + Session) Layers of OSI
Host-to-Host	Same as OSI Transport Layer
Internet	Same as OSI Network Layer
Network Access	Same as (Datalink + Physical) Layer of OSI

The upper 3 layers define the communication between applications running at different end systems and the communication between an application and its users. The lower 4 layers define how data is transmitted between end systems. Below the full functionality of each layer is defined.

•      Application Layer provides interface between applications and the presentation layer. Remember that applications such as web browsers are not reside in this layer. Actually they use this interface for communication with remote applications at the other end. Protocols run in this layer are HTTP, FTP, SMTP, Telnet, SNMP.
•       Presentation Layer provides data formats, presents data, and handles compression and encryption. For example FTP defines how FTP transfer data between 2 end systems. The receiving end will reassemble data according to the format used by sender and pass them back to the application layer. Presentation layer provides these formats ASCII, EBCDIC, JPEG, GIF, TIFF, MPEG, WAV, MIDI.
•       Session layer Defines how to setup / establish, control / manage, and end / terminate the presentation layer sessions between 2 end systems. For each session a port numbers used to keep different application data separated from each other. These are different application used in session layer SQL, NFS, RPC, X Window, NetBIOS, Winsock, BSD socket.
•    Transport layer Provides reliable and unreliable application data delivery services, as well as segmentation and reassembly of applications data. Important concepts about this layer are connection-oriented, connectionless, error recovery, acknowledgment, flow control, and windowing. Protocols used are TCP, UDP, SPX.
•      Network layer Defines end-to-end packet delivery and tracking of end system locations with logical addressing IP addresses. Determines the best path to transfer data within an internetwork through the routes learning via routing protocols. Allows communication between end systems from different networks. There are 2 types of packets data packets and routing update packets.  Protocols used are IP, IPX, AppleTalk.
•      Data Link layer defines how to transmit data over a network media (how to place network layer. packets onto the network media – cable or wireless) with physical addressing. Allows communication between end systems within the same network. Protocols used are LAN (Ethernet), WAN( HDLC, PPP, Frame Relay, ATM).
•      Physical layer defines specifications for communication between end systems and the physical media, it tells how to place data link layer frames onto the media. Defines connector shapes, number of pins, pin usages or assignments, electrical current levels, and signal encoding schemes. For example Ethernet, RS-232, V.35.

Network Devices

There are 4 main Network Devices; each operates on different layers of OSI model. Below lists defines some comparison points between these devices:

•      Routers: are Network layer (L3) devices. Their main functions are locating specific networks, Where is it? Which is the shortest path or best way to reach there? They create separate broadcast domains.
•       Switches & Bridges: are Data Link layer (L2) devices. Switches are more intelligent than Bridges as it has more specific MAC table of network devices address. Their main role is locating specific hosts within the same network. Devices connected to a switch do not receive data that is meant only for devices connected to other ports. They create separate collision domains for devices connected to them (segmentation) but the devices are still reside in the same broadcast domain. VLAN technology is then used to separate broadcast domains. Multiple sub networks are created with concept of VLAN.
•      Hubs & Repeaters: are Physical layer (L1) devices. Hubs are not smart devices. They send all the bits received from one port to all other ports; hence all devices connected via a hub receive everything the other devices send. All devices connected to a hub reside in the same collision and broadcast domains. A collision domain is an area of an Ethernet network where collisions can occur. If an end system can prevent another from using the network when it is using the network, these systems are considered reside in the same collision domain.