| Open System Interconnection (OSI) Model Posted: 09 Jul 2013 09:05 AM PDT Question: What is OSI model? Elaborate your answer.
Answer:
The OSI stand for "Open System Interconnection".
It was first introduced in the late 1970s by the ISO (International Standard Organization).
An OSI is a set of protocol that allow any two different system to communicate regardless of their underlying architecture.
The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and inter-operable.
It consists of seven separate but related layers, each of which defines a part of the process of moving information across network.
These seven layers of OSI model can be divided into two categories:
- Upper Layer (Application Layer):
- This part consist three top layer as: Application layer, Presentation layer, Session layer.
- These layers provide the application services required for the exchange of information.
- Lower Layer ( Data Transport):
- These layer consist remaining four layers as: Physical layer, Data link, Network layer, Transport layer.
- These layers handle the data transport issues.
- In other word these layers provides the end-to-end service necessary for the transfer of data between two system.
 | | Figure: Layers of OSI reference model |
- Physical Layer
- The data units on this layer are called bits.
- This is also concerned with the following:
- Interface between the device and the transmission medium.
- Types of transmission media
- Signals may be electrical or optical
- Data rate
- connection
- Topology
- Transfer rate
- Repeaters & Hub are used in physical lyaer.
- Date Link Layer (DLL)
- The data unit on this layer is called frame (Group of bits )
- This layer divided into two sub layers:
- Media Access Control ( MAC )
- The MAC sub-layer controls how a computer on the network gains access to link resources and grant permission to transmit it.
- Logical Link Control ( LLC )
- The LLC layer controls frame synchronization flow control and error checking.
- In summarize DLL is responsible for hop to hop delivery:
- Framing
- physical Addressing
- Error Control
- "Bridge" uses in DLL.
- Network Layer
- The unit of data at network layer is called packet or Data-gram.
- The network layer establish a route between sending & receiving stations.
- "Router" device used in network layer.
- Network layer is responsible for the source-to-destination delivery:
- Packet
- Logical Addressing
- Routing
- Transport Layer
- In this layer data unit called segment.
- The transport layer is responsible for overall end-to-end validity and integrity of the transmission.
- "Gateway" devices used in transport layer.
- Transport layer responsible for process-to-process delivery as:
- Port Addressing
- Segmentation
- Segmentation and Reassembly
- Connection Control
- Flow Control
- Session Layer
- The session layer decided when to turn communication ON and OFF between two computers.
- The session layer responsible for following:
- Dialog Controller
- Synchronization
- Translation
- Encryption
- Presentation Layer
- In this layer data packet unit called is "Presentation Protocol Data Unit".
- It perform code conversion and data reformatting.
- Compression
- Application Layer
- The application layer is the top layer of OSI model.
- It defines the language and syntax that programs uses to communicate with other programs.
- The application layer enables the user, whether human or software, to access the network and services.
 | | Figure: OSI model & Functionlity |
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| Gateway Posted: 09 Jul 2013 05:22 AM PDT Q. What is Gateway?
Answer:
A gateway is hardware & software devices
Gateway is used to interconnect LAN & WAN.
Gateway makes communication possible between different architecture & environment.
Gateway perform protocol and date onversion.
A gateway operate at the "Transport Layer" and above.
Gateway uses all 7 layers on OSI models.
 | | Figure: Gateway in Networking |
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| Routers Posted: 09 Jul 2013 05:16 AM PDT Question: What is routers?
Answer:
A router is a special purpose computer, that allows to connect to multiple computer networks.
Routers are both hardware & software devices.
Router can connect network segments, and filters and isolate traffic.
Unlike a bridge, a router can connect networks that uses different technology, addressing methods, media types, frame format speeds.
Routers operator at "Network Layer" in OSI model.
 | | Figure: Routers in Networking |
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| Bridge Posted: 09 Jul 2013 05:02 AM PDT A bridge segment or divide a network to isolate traffic related problem.
A bridge sends the data frames only to the concerned segment.
Bridges are used to connect dis-similar network like ethernet system to Token Ring-System. Thus bridge used to join network using CSMA/CD access and token passing access.
Bridge are both hardware and software devices.
Bridge operate "Date Link Layer" in OSI model.
The purpose of bridge as :- Isolate network by MAC addresses.
- Manage network traffic by filtering packet.
- Translate from one protocol to another.
 | | Figure: Bridge in Networking |
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