What is a Stack in ata Structure ?

 Introduction to Stacks

Stacks, queues, deques, and lists are data collections with items ordered according to how they are added or removed. Once an item is added, it stays in the same position relative to its neighbours. Because of this characteristic, we call these collections linear data structures.

A stack is an ordered collection of items where the addition of new items and the removal of existing items always takes place at the same end. This end is commonly referred to as the “top”, and the opposite end is known as the “base”.

Items that are closer to the base have been in the stack the longest. The most recently added item is always on the top of the stack and thus will be removed first. 

There are many examples of stacks in everyday situations. Consider a stack of plates on a table, where it’s only possible to add or remove plates to or from the top. 

Or

Imagine a stack of books on a desk. The only book whose cover is visible is the one on top. To access the others, we must first remove the ones sitting on top of them.

Here’s another stack containing a number of primitive data objects:

A stack of primitive objects

Features of Stacks 

One of the most useful features of stacks comes from the observation that the insertion order is the reverse of the removal order.

To make things simpler stack follows LIFO which means item that was Last In would be the item that would be First Out. 

Starting with a clean desk, place books on top of each other one at a time and consider what happens when you begin removing books: the order that they’re removed is exactly the reverse of the order that they were placed. This ability to reverse the order of items is what makes stacks so important.

Below we show the object stack during insertion and removal. Note the objects’ order.

The reversal property of stacks

Considering this reversal property, perhaps you can think of stack examples that occur while using your computer. For example, every web browser has a “Back” button. As you navigate from page to page, the URLs of those pages are placed on a stack. The page you’re currently viewing is on the top, and the first page you looked at is at the base. Clicking on the Back button moves you in reverse order through the stack of pages.

Network Devices

A Network device is a device which acts as a medium between two commune for a transferring or receiving a message. The following are the popular Network devices.

 

1. Repeater – 

• A repeater operates at the physical layer. 
• Its job is to regenerate the signal over the same network before the signal becomes too weak or corrupted so as to extend the length to which the signal can be transmitted over the same network. 
• An important point to be noted about repeaters is that they do not amplify the signal. 
• When the signal becomes weak, they copy the signal bit by bit and regenerate it at the original strength. 
• It is a 2 port device. 

2. Hub –  

• A hub is basically a multiport repeater. 
• A hub connects multiple wires coming from different branches, for example, the connector in star topology which connects different stations. 
• Hubs cannot filter data, so data packets are sent to all connected devices.  In other words, collision domain of all hosts connected through Hub remains one.  
• Also, they do not have the intelligence to find out best path for data packets which leads to inefficiencies and wastage. 

3. Bridge – 

• A bridge operates at data link layer. 
• A bridge is a repeater, with add on the functionality of filtering content by reading the MAC addresses of source and destination. 
• It is also used for interconnecting two LANs working on the same protocol. 
• It has a single input and single output port, thus making it a 2 port device.

Types of Bridges 

• Transparent Bridges:- These are the bridge in which the stations are completely unaware of the bridge’s existence i.e. whether or not a bridge is added or deleted from the network, reconfiguration of the stations is unnecessary. These bridges make use of two processes i.e. bridge forwarding and bridge learning.

• Source Routing Bridges:- In these bridges, routing operation is performed by source station and the frame specifies which route to follow. The host can discover frame by sending a special frame called discovery frame, which spreads through the entire network using all possible paths to destination.

4. Switch – 

• A switch is a multiport bridge with a buffer and a design that can boost its efficiency(a large number of ports imply less traffic) and performance.
• A switch is a data link layer device. 
• The switch can perform error checking before forwarding data, that makes it very efficient as it does not forward packets that have errors and forward good packets selectively to correct port only.  In other words, switch divides collision domain of hosts, but broadcast domain remains same. 

  

5. Routers – 

• A router is a device like a switch that routes data packets based on their IP addresses. 
• Router is mainly a Network Layer device. 
• Routers normally connect LANs and WANs together and have a dynamically updating routing table based on which they make decisions on routing the data packets. 
•  Router divide broadcast domains of hosts connected through it.

 

6. Gateway – 

• A gateway, as the name suggests, is a passage to connect two networks together that may work upon different networking models.
• They basically work as the messenger agents that take data from one system, interpret it, and transfer it to another system. 
• Gateways are also called protocol converters and can operate at any network layer. 
• Gateways are generally more complex than switch or router.

7. Brouter – 

• It is also known as bridging router is a device which combines features of both bridge and router. 
• It can work either at data link layer or at network layer. 
• Working as router, it is capable of routing packets across networks and working as bridge, it is capable of filtering local area network traffic. 

Types of Computer Network

 Computer Network Types

• A computer network is a group of computers linked to each other that enables the computer to communicate with another computer and share their resources, data, and applications.
• A computer network can be categorized by their size. 

A computer network is mainly of four types:

Figure : Types of Computer Network

1. LAN(Local Area Network)
2. PAN(Personal Area Network)
3. MAN(Metropolitan Area Network)
4. WAN(Wide Area Network)

LAN(Local Area Network)

• Local Area Network is a group of computers connected to each other in a small area such as building, office.
• LAN is used for connecting two or more personal computers through a communication medium such as twisted pair, coaxial cable, etc.
• It is less costly as it is built with inexpensive hardware such as hubs, network adapters, and ethernet cables.
• The data is transferred at an extremely faster rate in Local Area Network.
• Local Area Network provides higher security.

Figure : Local Area Network

Personal Area Network (PAN)

• Personal Area Network (PAN) is a the computer network that connects computers/devices within the range of an individual person. 
• As PAN provides a network range within a person’s range typically within a range of 10 meters(33 feet) it is called as Personal Area Network. 
• A Personal Area Network typically involves a computer, phone, tablet, printer, PDA (Personal Digital Assistant) and other and other entertainment devices like speakers, video game consoles etc. 

Figure : Personal Area Network

Metropolitan Area Network (MAN) –

• MAN or Metropolitan Area Network covers a larger area than that of a LAN and smaller area as compared to WAN. 
• It connects two or more computers that are apart but reside in the same or different cities. 
• It covers a large geographical area and may serve as an ISP (Internet Service Provider). 
• MAN is designed for customers who need high-speed connectivity. 
• Speeds of MAN range in terms of Mbps. It’s hard to design and maintain a Metropolitan Area Network.

Figure : Metropolitan  Area Network

WAN(Wide Area Network)

• Wide Area Networks are a form of telecommunication networks that can connect devices from multiple locations and across the globe. 
• WANs are the largest and most expansive forms of computer networks available to date.
• These networks are often established by service providers that then lease their WAN to businesses, schools, governments or the public.
•  These customers can use the network to relay and store data or communicate with other users, no matter their location, as long as they have access to the established WAN. 
• Access can be granted via different links, such as virtual private networks (VPNs) or lines, wireless networks, cellular networks or internet access.

Figure : Wide Area Network

Computer Networking : Basic

 Computer Networking : Basics

What is Data Communication ?

Data Communication is defined as exchange of data between two devices via some form of transmission media such as a cable, wire or it can be air or vacuum also.

For occurrence of data communication, communicating devices must be a part of communication system made up of a combination of hardware or software devices and programs.

Data Communication System Components : 

There are mainly five components of a data communication system:

1. Message

2. Sender

3. Receiver

4. Transmission Medium

5. Set of rules (Protocol) 

All above mentioned elements are described below:

Figure – Components of Data Communication System

1 Message :

     This is most useful asset of a data communication system. The message simply refers to data or piece of information which is to be communicated. A message could be in any form, it may be in form of a text file, an audio file, a video file, etc.

 2  Sender :

  To transfer message from source to destination, someone must be there who will play role of a source. Sender plays part of a source in data communication system. It is simple a device that sends data message. The device could be in form of a computer, mobile, telephone, laptop, video camera, or a workstation, etc.

 3  Receiver :

    It is the destination where finally message sent by source has arrived. It is a device that receives message. Same as sender, receiver can also be in form of a computer, telephone mobile, workstation, etc.

4  Transmission Medium :

  In entire process of data communication, there must be something which could act as a bridge between sender and receiver, Transmission medium plays that part. It is physical path by which data or message travels from sender to receiver. Transmission medium could be guided (with wires) or unguided (without wires), for example, twisted pair cable, fibre optic cable, radio waves, microwaves, etc.

5.  Set of rules (Protocol) :

      To govern data communications, various sets of rules had been already designed by the designers of the communication systems, which represent a kind of agreement between communicating devices. These are defined as protocol. In simple terms, the protocol is a set of rules that govern data communication. If two different devices are connected but there is no protocol among them, there would not be any kind of communication between those two devices. Thus the protocol is necessary for data communication to take place.

A typical example of a data communication system is sending an e-mail. The user which send email act as sender, message is data which user wants to send, receiver is one whom user wants to send message, there are many protocols involved in this entire process, one of them is Simple Mail Transfer Protocol (SMTP), both sender and receiver must have an internet connection which uses a wireless medium to send and receive email. 

Transmission Modes /Data Flow

Transmission mode means transferring of data between two devices.It is also known as communication mode.Buses and networks are designed to allow communication to occur between individual devices that are interconnected. 

There are three types of transmission mode:-

Simplex Mode

Half-Duplex Mode

Full-Duplex Mode

Figure –Transmission Mode

Simplex Mode

• In Simplex mode, the communication is unidirectional, as on a one-way street.

• Only one of the two devices on a link can transmit, the other can only receive. 

• The simplex mode can use the entire capacity of the channel to send data in one direction.

Example: Keyboard and traditional monitors. The keyboard can only introduce input, the monitor can only give the output.

Figure – Simple Model

Half-Duplex Mode

• In half-duplex mode, each station can both transmit and receive, but not at the same time.

• When one device is sending, the other can only receive, and vice versa. 

• The half-duplex mode is used in cases where there is no need for communication in both direction at the same time. The entire capacity of the channel can be utilised for each direction.

Example: Walkie- talkie in which message is sent one at a time and messages are sent in both the directions.

Figure – Half-duplex Model

Full-Duplex Mode

• In full-duplex mode, both stations can transmit and receive simultaneously. 
• In full_duplex mode, signals going in one direction share the capacity of the link with signals going in other direction, this sharing can occur in two ways:

     ▪ Either the link must contain two physically separate transmission paths, one for sending and other for receiving.

     ▪ Or the capacity is divided between signals travelling in both directions.

• Full-duplex mode is used when communication in both direction is required all the time. The capacity of the channel, however must be divided between the two directions.

Example: Telephone Network in which there is communication between two persons by a telephone line, through which both can talk and listen at the same time.

Figure – Full-duplex Model

Networking Elements – 

The computer network includes the following networking elements:

1 At least two computers

2 Transmission medium either wired or wireless

3 Protocols or rules that govern the communication

4 Network software such as Network Operating System

Network Criteria:

The criteria that have to be met by a computer network are:

1. Performance – 

  It is measured in terms of transit time and response time.

• Transit time is the time for a message to travel from one device to another

• Response time is the elapsed time between an inquiry and a response.

Performance is dependent on the following factors:

• The number of users

• Type of transmission medium

• Capability of connected network

• Efficiency of software

2. Reliability – 

 It is measured in terms of

• Frequency of failure

• Recovery from failures

• Robustness during catastrophe

3. Security – 

It means protecting data from unauthorised access.

Goals of Computer Networks: 

The following are some important goals of computer networks:

1. Resource Sharing –

Many organisation has a substantial number of computers in operations, which are located apart. 

Ex. A group of office workers can share a common printer, fax, modem, scanner etc.

2. High Reliability –

If there are alternate sources of supply, all files could be replicated on two or more, machines. 

If one of them is not available, due to hardware failure, the other copies could be used.

3 Inter-process Communication –

Network users, located geographically apart, may converse in an interactive session through the network.

In order to permit this, the network must provide almost error-free communications.

Flexible access –

Files can be accessed from any computer in the network. The project can be begun on one computer and finished on another.

Other goals include Distribution of processing functions, Centralised management, and allocation of network resources, Compatibility of dissimilar equipment and software, Good network performance, Scalability, Saving money, Access to remote information, Person to person communication etc.,