KEY TERMS in Computer Networks

Following are some important terms, which are frequently used in context of Computer Networks.

Terms	Definition
1. ISO	The OSI model is a product of the Open Systems Interconnection project at the International Organization for Standardization. ISO is a voluntary organization.
2. OSI Model	Open System Interconnection is a model consisting of seven logical layers.
3. TCP/IP Model	Transmission Control Protocol and Internet Protocol Model is based on four layer model which is based on Protocols.
4. UTP	Unshielded Twisted Pair cable is a Wired/Guided media which consists of two conductors usually copper, each with its own colour plastic insulator
5. STP	Shielded Twisted Pair cable is a Wired/Guided media has a metal foil or braided-mesh covering which encases each pair of insulated conductors. Shielding also eliminates crosstalk
6. PPP	Point-to-Point connection is a protocol which is used as a communication link between two devices.
7. LAN	Local Area Network is designed for small areas such as an office, group of building or a factory.
8. WAN	Wide Area Network is used for the network that covers large distance such as cover states of a country
9. MAN	Metropolitan Area Network uses the similar technology as LAN. It is designed to extend over the entire city.
10. Crosstalk	Undesired effect of one circuit on another circuit. It can occur when one line picks up some signals travelling down another line. Example: telephone conversation when one can hear background conversations. It can be eliminated by shielding each pair of twisted pair cable.
11. PSTN	Public Switched Telephone Network consists of telephone lines, cellular networks, satellites for communication, fiber optic cables etc. It is the combination of world’s (national, local and regional) circuit switched telephone network.
12. File Transfer, Access and Management (FTAM)	Standard mechanism to access files and manages it. Users can access files in a remote computer and manage it.
13. Analog Transmission	The signal is continuously variable in amplitude and frequency. Power requirement is high when compared with Digital Transmission.
14. Digital Transmission	It is a sequence of voltage pulses. It is basically a series of discrete pulses. Security is better than Analog Transmission.

Comparison of OSI Reference Model and TCP/IP Reference Model

Following are some major differences between OSI Reference Model and TCP/IP Reference Model, with diagrammatic comparison below.

OSI(Open System Interconnection)	TCP/IP(Transmission Control Protocol / Internet Protocol)
1. OSI is a generic, protocol independent standard, acting as a communication gateway between the network and end user.	1. TCP/IP model is based on standard protocols around which the Internet has developed. It is a communication protocol, which allows connection of hosts over a network.
2. In OSI model the transport layer guarantees the delivery of packets.	2. In TCP/IP model the transport layer does not guarantees delivery of packets. Still the TCP/IP model is more reliable.
3. Follows vertical approach.	3. Follows horizontal approach.
4. OSI model has a separate Presentation layer and Session layer.	4. TCP/IP does not have a separate Presentation layer or Session layer.
5. OSI is a reference model around which the networks are built. Generally it is used as a guidance tool.	5. TCP/IP model is, in a way implementation of the OSI model.
6. Network layer of OSI model provides both connection oriented and connectionless service.	6. The Network layer in TCP/IP model provides connectionless service.
7. OSI model has a problem of fitting the protocols into the model.	7. TCP/IP model does not fit any protocol
8. Protocols are hidden in OSI model and are easily replaced as the technology changes.	8. In TCP/IP replacing protocol is not easy.
9. OSI model defines services, interfaces and protocols very clearly and makes clear distinction between them. It is protocol independent.	9. In TCP/IP, services, interfaces and protocols are not clearly separated. It is also protocol dependent.
10. It has 7 layers	10. It has 4 layers

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Diagrammatic Comparison between OSI Reference Model and TCP/IP Reference Model

The TCP/IP Reference Model

TCP/IP means Transmission Control Protocol and Internet Protocol. It is the network model used in the current Internet architecture as well. Protocols are set of rules which govern every possible communication over a network. These protocols describe the movement of data between the source and destination or the internet. These protocols offer simple naming and addressing schemes.

 

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Overview of TCP/IP reference model

TCP/IP that is Transmission Control Protocol and Internet Protocol was developed by Department of Defence’s Project Research Agency (ARPA, later DARPA) as a part of a research project of network interconnection to connect remote machines.

The features that stood out during the research, which led to making the TCP/IP reference model were:

• Support for a flexible architecture. Adding more machines to a network was easy.
• The network was robust, and connections remained intact untill the source and destination machines were functioning.

The overall idea was to allow one application on one computer to talk to(send data packets) another application running on different computer.

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Description of different TCP/IP protocols

Layer 1: Host-to-network Layer

1. Lowest layer of the all.
2. Protocol is used to connect to the host, so that the packets can be sent over it.
3. Varies from host to host and network to network.

 

Layer 2: Internet layer

1. Selection of a packet switching network which is based on a connectionless internetwork layer is called a internet layer.
2. It is the layer which holds the whole architecture together.
3. It helps the packet to travel independently to the destination.
4. Order in which packets are received is different from the way they are sent.
5. IP (Internet Protocol) is used in this layer.

 

Layer 3: Transport Layer

1. It decides if data transmission should be on parallel path or single path.
2. Functions such as multiplexing, segmenting or splitting on the data is done by transport layer.
3. The applications can read and write to the transport layer.
4. Transport layer adds header information to the data.
5. Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer.
6. Transport layer also arrange the packets to be sent, in sequence.

 

Layer 4: Application Layer

The TCP/IP specifications described a lot of applications that were at the top of the protocol stack. Some of them were TELNET, FTP, SMTP, DNS etc.

1. TELNET is a two-way communication protocol which allows connecting to a remote machine and run applications on it.
2. FTP(File Transfer Protocol) is a protocol, that allows File transfer amongst computer users connected over a network. It is reliable, simple and efficient.
3. SMTP(Simple Mail Transport Protocol) is a protocol, which is used to transport electronic mail between a source and destination, directed via a route.
4. DNS(Domain Name Server) resolves an IP address into a textual address for Hosts connected over a network.

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Merits of TCP/IP model

1. It operated independently.
2. It is scalable.
3. Client/server architecture.
4. Supports a number of routing protocols.
5. Can be used to establish a connection between two computers.

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Demerits of TCP/IP

1. In this, the transport layer does not guarantee delivery of packets.
2. The model cannot be used in any other application.
3. Replacing protocol is not easy.
4. It has not clearly separated its services, interfaces and protocols.

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Application Layer – OSI Model

It is the top most layer of OSI Model. Manipulation of data (information) in various ways is done in this layer which enables user or software to get access to the network. Some services provided by this layer includes: E-Mail, transferring of files, distributing the results to user, directory services, network resource etc.

 

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FUNCTIONS OF APPLICATION LAYER:

1. Mail Services : This layer provides the basis for E-mail forwarding and storage.
2. Network Virtual Terminal : It allows a user to log on to a remote host. The application creates software emulation of a terminal at the remote host. User’s computer talks to the software terminal which in turn talks to the host and vice versa. Then the remote host believes it is communicating with one of its own terminals and allows user to log on.
3. Directory Services : This layer provides access for global information about various services.
4. File Transfer, Access and Management (FTAM) : It is a standard mechanism to access files and manages it. Users can access files in a remote computer and manage it. They can also retrieve files from a remote computer.

Presentation Layer – OSI Model

The primary goal of this layer is to take care of the syntax and semantics of the information exchanged between two communicating systems. Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data. Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.

 

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FUNCTIONS OF PRESENTATION LAYER:

1. Translation : Before being transmitted, information in the form of characters and numbers should be changed to bit streams. The presentation layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format the computer.
2. Encryption : It carries out encryption at the transmitter and decryption at the receiver.
3. Compression : It carries out data compression to reduce the bandwidth of the data to be transmitted. The primary role of Data compression is to reduce the number of bits to be 0transmitted. It is important in transmitting multimedia such as audio, video, text etc.

Session Layer – OSI Model

Its main aim is to establish, maintain and synchronize the interaction between communicating systems. Session layer manages and synchronize the conversation between two different applications. Transfer of data from one destination to another session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

 

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FUNCTIONS OF SESSION LAYER:

1. Dialog Control : This layer allows two systems to start communication with each other in half-duplex or full-duplex.
2. Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into stream of data. Example: If a system is sending a file of 800 pages, adding checkpoints after every 50 pages is recommended. This ensures that 50 page unit is successfully received and acknowledged. This is beneficial at the time of crash as if a crash happens at page number 110; there is no need to retransmit 1 to100 pages.

Transport Layer – OSI Model

The main aim of transport layer is to be delivered the entire message from source to destination. Transport layer ensures whole message arrives intact and in order, ensuring both error control and flow control at the source to destination level. It decides if data transmission should be on parallel path or single path

Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer and ensures that message arrives in order by checking error and flow control.

 

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FUNCTIONS OF TRANSPORT LAYER:

1. Service Point Addressing : Transport Layer header includes service point address which is port address. This layer gets the message to the correct process on the computer unlike Network Layer, which gets each packet to the correct computer.
2. Segmentation and Reassembling : A message is divided into segments; each segment contains sequence number, which enables this layer in reassembling the message. Message is reassembled correctly upon arrival at the destination and replaces packets which were lost in transmission.
3. Connection Control : It includes 2 types :
   • Connectionless Transport Layer : Each segment is considered as an independent packet and delivered to the transport layer at the destination machine.
   • Connection Oriented Transport Layer : Before delivering packets, connection is made with transport layer at the destination machine.
4. Flow Control : In this layer, flow control is performed end to end.
5. Error Control : Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission.

Network Layer – OSI Model

The main aim of this layer is to deliver packets from source to destination across multiple links (networks). If two computers (system) are connected on the same link then there is no need for a network layer. It routes the signal through different channels to the other end and acts as a network controller.

It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.

 

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FUNCTIONS OF NETWORK LAYER:

1. It translates logical network address into physical address. Concerned with circuit, message or packet switching.
2. Routers and gateways operate in the network layer. Mechanism is provided by Network Layer for routing the packets to final destination.
3. Connection services are provided including network layer flow control, network layer error control and packet sequence control.
4. Breaks larger packets into small packets.

DATA LINK Layer – OSI Model

Data link layer is most reliable node to node delivery of data. It forms frames from the packets that are received from network layer and gives it to physical layer. It also synchronizes the information which is to be transmitted over the data. Error controlling is easily done. The encoded data are then passed to physical.

Error detection bits are used by the data link layer. It also corrects the errors. Outgoing messages are assembled into frames. Then the system waits for the acknowledgements to be received after the transmission. It is reliable to send message.

 

FUNCTIONS OF DATA LINK LAYER:

1. Framing: Frames are the streams of bits received from the network layer into manageable data units. This division of stream of bits is done by Data Link Layer.
2. Physical Addressing: The Data Link layer adds a header to the frame in order to define physical address of the sender or receiver of the frame, if the frames are to be distributed to different systems on the network.
3. Flow Control: A flow control mechanism to avoid a fast transmitter from running a slow receiver by buffering the extra bit is provided by flow control. This prevents traffic jam at the receiver side.
4. Error Control: Error control is achieved by adding a trailer at the end of the frame. Duplication of frames are also prevented by using this mechanism. Data Link Layers adds mechanism to prevent duplication of frames.
5. Access Control: Protocols of this layer determine which of the devices has control over the link at any given time, when two or more devices are connected to the same link.

 

PHYSICAL Layer – OSI Model

Physical layer is the lowest layer of all. It is responsible for sending bits from one computer to another. This layer is not concerned with the meaning of the bits and deals with the physical connection to the network and with transmission and reception of signals.

This layer defines electrical and physical details represented as 0 or a 1. How many pins a network will contain, when the data can be transmitted or not and how the data would be synchronized.

 

FUNCTIONS OF PHYSICAL LAYER:

1. Representation of Bits: Data in this layer consists of stream of bits. The bits must be encoded into signals for transmission. It defines the type of encoding i.e. how 0’s and 1’s are changed to signal.
2. Data Rate: This layer defines the rate of transmission which is the number of bits per second.
3. Synchronization: It deals with the synchronization of the transmitter and receiver. The sender and receiver are synchronized at bit level.
4. Interface: The physical layer defines the transmission interface between devices and transmission medium.
5. Line Configuration: This layer connects devices with the medium: Point to Point configuration and Multipoint configuration.
6. Topologies: Devices must be connected using the following topologies: Mesh, Star, Ring and Bus.
7. Transmission Modes: Physical Layer defines the direction of transmission between two devices: Simplex, Half Duplex, Full Duplex.
8. Deals with baseband and broadband transmission.