Superposition Theorm

Superposition Theorem

    Superposition technique is a method that is used in a circuit having multiple independent sources. It is used to determine currents and voltages in a circuit, by considering the contribution of one independent source at a time and then add them up.

    The superposition principle states that the current across (or voltage through) an element in a linear network system is the algebraic sum of the current across (or voltage through) that element due to each independent source acting alone.

    However, to apply the superposition theorem, we should keep two things in mind:

(1) We consider one independent source at a time while all other independent source is turned off.

(a)  Replace other independent voltage sources by 0 V (or short circuits).

(b) Replace other independent current sources by 0 A (or open circuits).

Open Shortest Path First (OSPF)

      Open shortest path first (OSPF) fixes many of issues with RIP and allows routes to be selected dynamically based on the current state of the network. It is complement of RIP. One issue with RIP is limit of its ability to choose the best route and adapt to changing network conditions. Another problem is cost value of 16 for infinity, which makes it possible to use RIP in situation where more than 15 hops might occur between devices.

      This protocol name conveys two important characteristics. The first word refers to the fact that the protocol was developed using open and public RFC process, no license is required to use it. The SPF portion of the name refers to the type of algorithm it uses, which is designed to allow routers to dynamically determine the shortest path between any two networks. 

Overview of OSPF Operation 

    1) The main concept used by OSPF is link state database (LSDB). Each router in the network maintains a data structure called linked database which contain information about the current status of the network. It also describe the link to other network structure in directed graphs, hop count or cost. 

    2) When changes occur in the internetwork, one router update the status of their network to the another routers via link advertisements which are messages to manage the link database.

    3) To determine final router, each router uses its link database to find out shortest path tree. This tree shows the shortest path from one router to another.

    4) As the new information about the router updates, this shortest path tree can be recalculated.

    5) So we can say that OSPF adjust the best route dynamically.

    6) There may be a case, when two paths exist with an equal cost. In this case, traffic or data can be shared between both routes.

OSPF Advantages

1)    Support authentication for security.

2)     Support classful, classless, subnet classful IP addressing.

3)    For large internetwork, OSPF arranged the routes in grouped or hierarchical topology.

4)    It reduced the link state traffic by choosing the shortest path.

5)    Better organization.

OSPF Drawbacks

1)    Complexity

2)    Need more work to properly maintain a internetwork.

Networking: Cisco Packet Tracer Networking: Implement Open shortest path first (OSPF) 

File Transfer Protocol (FTP)

      The purpose of networking applications is to allow different types of information to be sent between networked devices. In the world of computers, information is arranged in the form of files. A file can contain any type of information. The significance of the contents of a file depends entirely on the user of software program that examines it. When those files are created specially for communication, they are called messages. Transferring files and messages between networked devices is the most basic type of network communication. 

    A file is just a collection of information that is treated as a single unit by a computer system. Files are stored in a directory or folders in a file system. In modern computers, files are normally expressed as a sequence of bytes or characters, and each file is read, written, copied or otherwise manipulated as independent object. So, files are the building blocks of information in computers systems. The transfer of information in networking was originally defined in terms of movement of files. Applications of file transfer are electronic mail (Email), network news (Usenet), hypertext (World Wide Web). 

TCP/IP File Transfer Protocol

    File Transfer Protocol perform one main function: allowing files to be copied from one computer to another without much considerations of their contents. Issues arises due to the format differences in the file stored on differing systems. 
1) To ensure that files are sent and received without loss of data, FTP uses the reliable TCP at the transport layer.
2) After a TCP connection is established, an FTP control connection is created by passing the FTP commands over logical connection based on formatting rules defined by Telnet Protocol.
3) Each command sent by the client receives a reply from the server to indicate whether it is succeeded or failed.
4) A data connection is established for each individual data transfer to be performed. FTP supports normal and passive data connections, allowing either the server or client to initiate the data connection. Multiple data types and file types are supported to allow flexibility for various type of transfers.
   Additional support commands are provided to manage the FTP connection as well as to perform support functions such as listing the contents of a directory or deleting the remaining files. 

FTP Connection 
   When a session is set up, a permanent control connection is set up using TCP for passing commands and replies. When files or other data are to be sent, they are passed over separate TCP data connections that are created and then disabled as needed.

                                                              Figure: FTP operational model
The protocol interpreter (PI) manages the control connections, commands and replies. The data transfer process (DTP) manages sending and receiving data between client and server. Note that user interface only interacts with human FTP user. 

FTP Control Connection Establishment, User Authentication 
    Once TCP has been set up, a specific process is follow for user authentication in which we use login process. This process has two purposes: 
1) Allow access to only authorized users. 
2) Server control what types of access each user has 
3) Server can decides the resource for users. 

User name/password login scheme: 
1) First, the user is identified by sending a user name from the user to server using the USER command. Client sent the User's Password using PASS command. 
2) The server checks the User name and password against its user database to verify that the connecting user has valid authority to access the server. 
3) If the information is valid, the server sends back a greeting to the client to indicate that the session is opened. 
   If the user improperly authenticates, the server will request that the user attempt authorization again. After a number of invalid authorization tries, the server may time out and terminate the connection. 
4) After the authentication succeeds, the server then set up the connection to  allow access to certain files or certain type of files, or to read or write file on server.

           Figure: FTP Control Connection Establishment, User Authentication 
Note that it is not considered secured today because the user name and password are sent across the control connection in clear text.

Networking: Cisco Packet Tracer Networking: Implement File Transfer Protocol ( FTP)

Comparison of Guided Media

Guided Transmission Media- Twisted Pair

     A twisted-pair cable consists of one or more twisted-pair. Each twisted-pair wire usually is color coded for identification. Its wires are twisted together and separately insulated to each other. They are usually in the form of bundles called cables. They are installed in building during construction.

Transmission Characteristic:

1. Limited distance.

2. Limited Bandwidth of 1 MHz.

3.  Limited data rate of of 100 MHz.

4. Use both analog (amplifier every 5 km) and digital signals (repeater every 2 km).

5.  Susceptible to interference and noise.

Types of Twisted Pair Cables:

a) Unshielded Twisted Pair (UTP): It acts as ordinary telephone wire. It characteristics are: cheapest, easy to install, and suffer from external interference.

b) Shielded Twisted Pair (STP): It is made up of metal braid that reduces interference. More expensive, and harder to handle.

                                               Table: UTP Category

UTP Category	Data Rate	Max. Length	Cable Type	Applications
CAT1	1 Mbps	-	Twisted Pair	Old Telephone Cable
CAT2	4 Mbps	-	Twisted Pair	Token Ring Networks
CAT3	10 Mbps	100 m	Twisted Pair	Token Ring
CAT4	16 Mbps	100 m	Twisted Pair	Token Ring
CAT5	100 Mbps	100 m	Twisted Pair	Token Ring, Ethernet
CAT5e	1 Gbps	100 m	Twisted Pair	Ethernet, Fast Ethernet
CAT6	10 Gbps	100 m	Twisted Pair	Gigabit Ethernet
CAT6a	10 Gbps	100 m	Twisted Pair	Gigabit Ethernet
CAT7	10 Gbps	100 m	Twisted Pair	Gigabit Ethernet

Application of Twisted Pairs:

1. They are used in telephone networks between house and local exchange called Subscriber Loop.

2. Within buildings.

3. For private branch exchange.

4. Local area networks.

Pros and Cons:

1. Easy to work with

2.  Short range

3. Low data rate

4. Cheap

Figure: A twisted-pair cable consists of one or more twisted-pair. Each

twisted-pair wire usually is color coded for identification.

Transmission Media

   Transmission Media is a pathway that carries the data information from transmitter to receiver. There are different types of waves or cables to transmit data. Information is transmitted normally through electrical (Current) or electromagnetic signals (at different frequencies). These signals can be transmitted through different Medias like copper wires, atmosphere, water, and optical fibre. Different media have different properties like bandwidth, ease of installation, cost and delay. Characteristic and quality are determined by signal and medium. For guided transmission, medium is important. For unguided transmission, bandwidth is important. Key concerns in transmission are data rate and distance.

Types of Transmission Media

   Transmission media is divided into two groups:

1.     Guided or Wired or Bounded Transmission Media

2.     Unguided or Wireless or Unbounded Transmission Media

                                     Figure: Transmission medium and physical layer

There are various design factors:
1. Bandwidth: Higher bandwidth gives higher data rate
2. Interference
3. Transmission Impairments
4. Number of Receivers: More receivers introduce more attenuation, need more amplifiers and repeaters.

  Guided or Wired Media are the cables that have physical existence and are limited by environmental geography. Mostly used wired media are co-axial cable, twisted pair cable, and optical cable. Each  media has its own properties like speed of transmission, effect of noise, cost etc.

  Unguided or Wireless Media are the way of transmitting information without using any physical media or cables. It is also known as wireless communication. Example, wireless LAN are used in office and college campuses. This transmission uses radio wave, microwave, infrared waves. 

                           Table: Transmission Characteristics of Guided Media

	Frequency Range	Typical Attenuation	Typical Delay	Repeater Spacing
Twisted Pair	0 to 3.5 KHz	0.2 dB/km	50 µs/km	2 km
Coaxial Cable	0 to 500 MHz	7 dB/km	4 µs/km	1 to 9 km
Optical fiber	186 to 370 Thz	0.2 to 0.5 dB/km	5 µs/km	40 km

Guided Transmission Media

      Twisted Pair

      Coaxial Cable

      Optical Fiber

Wireless Transmission

     Antennas

     Terrestrial Microwave

      Satellite Microwave

      Broadcast Radio

      Infrared

Wireless Propagation

     Ground Wave Propagation

     Sky Wave Propagation

     Line-of-Sight Propagation

Line-of-Sight Transmission

     Free Space Loss

     Atmospheric Absorption

     Multi path

     Refraction

Label: Comparison of Gudied Media