Unit 5

COMPUTER NETWORKS AND DATA TRANSMISSION

Definition of terms

Data communications refers to the transmission of this digital data between two or more computers and a computer network or data network is a telecommunications network that allows computers to exchange data. The physical connection between networked computing devices is established using either cable media or wireless media. The best-known computer network is the Internet.

i) Network
A collection of independent entities that are arranged in such a manner to exchange data, information or resources e.g television network, telephone networks.

ii) Computer networks
A collection of computers linked together using transmission media for the purpose of communication and resource sharing.

iii) Transmission Media
----is a physical and non-physical link between two or more computers and in which a signal can be made to flow from source to destination.

iv) Resource Sharing
-----is the sharing of the resources that are attached to the network for access by users eg file, printers , data, application programs etc.

v) Data Communication
A process of transmitting data signal from one point to another through the network.

vi) Telecommunication
The communication i.e. transferring of data and information over significant distances is known as telecommunication.

Advantages of computer network are:

• Data and software of computer can be shared with other computer on the network.
• Only the authorized user of a network can use the facilities of the network.
• Computers on the network can communicate with each other.

The disadvantages of computer network are:

• Data and information may be stolen by computer hackers if the security of network is not reliable.
• If any computer in a network gets affected by computer virus, there is high chance of spreading computer viruses on the other computer.
• Computers on the network have to depend on the server computer for resources.
• This sharing of information may leak the privacy of other clients.

Explain how computer network reduce expenses in an office.

- Computer Networks can allow businesses to reduce expenses and improve efficiency by sharing data and common equipment, such as printers, among many different computers. At the same time, the network may be connected through cables, telephone lines, infrared beams etc, which is cheaper and helps to reduce the expenses.

TERMS USED IN DATA COMMUNICATION

i)         Data signal
Is a voltage signal level in the circuit which represents the flow of data. This can be either Analog or Digital in nature

ii)        Signal modulation and Demodulation
Is a process of converting data signal to and from a form that is suitable for transmission over a
transmission medium.
MODEM - Converts digital signal by superimposing it on an analog carrier signal which is transmitted over analog telephone line. A process known as Modulation

A modem at the receiving end converts the analog signal into digital form a process called Demodulation.

iii)        Multiplexing
Is a process of sending multiple data signals over the same medium

e.g a wire conductor can carry several data signal either simultaneously or at different times.

                iv)       Demultiplexing
                           A process of separating the multiplexed signals at the receiving end.

v)        bandwidth
- Is a maximum amount of data that the transmission medium can carry at any one time. e.g a cable having a bandwidth of 100 mbps.

vi)             Baseband
- A digital signal that is generated and applied to transmission medium directly without modulation.
- It utilizes the full capacity of transmission medium hence at any time, only one signal can be sent unless they are multiples.

 Attenuation/ Signal loss
- Is the decrease in magnitude and energy as a signal progressively moves along a transmission medium. The signal is not boosted; it will totally be lost along the way and may never reach the destination.
- It corrected by placing a signal amplifier (repeater station) along the medium at appropriate distances in order to receive the weak signal, clean it, amplify it then restarts it.

MODES OF DATA COMMUNICATION
There are three modes of data communication
i) Simplex
ii) Half Duplex
iii) Full Duplex

1. Simplex
Communication in only one direction e.g radio broadcast. The listener cannot communicate back through receiver.


2. Half Duplex
- Communication in both direction but one direction at a time e.g sender sends information then the receiver can reply e.g radio call

3.Full Duplex
- Communication occurs in both directions simultaneously e.g a computer sending and receiving data on a network.

TYPES OF COMPUTER NETWORKS
Types of computer networks are classified according to size. There are three common networks
1. Local Area Network.

2. Metropolitan Area Network.


3. Wide Area Network.


TYPES OF NETWORK TOPOLOGIES

- Refers to the way which computers and other devices have been arranged or how data is passed from one computer to another in a network

- Topology is viewed in two ways
1. Logical Topology/ Signal Topology
- Deals with the way data pass from one device to the next in the network e. g Ethernet and Token Ring


2. Physical Topology
- Refers to the physical layout or arrangement of components on the network e.g Star, Bus, Ring, Mesh, Tree/Hierarchical Topologies

1. Bus Topology
alternatively referred to as a line topology, a bus topology is a network setup in which each computer and network device are connected to a single cable or backbone. The following sections contain both the advantages and disadvantages of using a bus topology with your devices.

Advantages of bus topology

• It works well when you have a small network.
• Easiest network topology for connecting computers or peripherals in a linear fashion.
• Requires less cable length than a star topology.

Disadvantages of bus topology

• Difficult to identify the problems if the whole network goes down.
• It can be hard to troubleshoot individual device issues.
• Not great for large networks.
• Terminators are required for both ends of the main cable.
• Additional devices slow the network down.
• If a main cable is damaged, the network fails.

2. Ring Topology
A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring. Data travels from node to node, with each node along the way handling every packet.

Advantages of ring Topology

• Very orderly network where every device has access to the token and the opportunity to transmit
• Performs better than a bus topology under heavy network load
• Does not require a central node to manage the connectivity between the computers
• Due to the point to point line configuration of devices with a device on either side (each device is connected to its immediate neighbor), it is quite easy to install and reconfigure since adding or removing a device requires moving just two connections.
• Point to point line configuration makes it easy to identify and isolate faults.
• Reconfiguration for line faults of bidirectional rings can be very fast, as switching happens at a high level, and thus the traffic does not require individual rerouting.

Disadvantages of Ring Topology

• One malfunctioning workstation can create problems for the entire network. This can be solved by using a dual ring or a switch that closes off the break.
• Moving, adding and changing the devices can affect the network
• Communication delay is directly proportional to number of nodes in the network
• Bandwidth is shared on all links between devices
• More difficult to configure than a Star: node adjunction Ring shutdown and reconfiguration

3. Star Topology

Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central node, typically a switch or hub, which acts as a conduct to transmit messages. In star topology, every node (computer workstation or any other peripheral) is connected to a central node. The switch is the server and the peripherals are the clients.

Advantages

• If one node or its connection breaks it doesn’t affect the other computers and their connections.
• Devices can be added or removed without disturbing the network

Disadvantages

• An expensive network layout to install because of the amount of cables needed.
• The central hub is a single point of failure for the network.

4. Mesh Topology
A mesh network is a network topology in which each node relays data for the network. All mesh nodes cooperate in the distribution of data in the network. It can be applied to both wired and wireless networks.

Advantages of Mesh topology

1) Data can be transmitted from different devices simultaneously. This topology can withstand high traffic.
2) Even if one of the components fails there is always an alternative present. So data transfer doesn’t get affected.
3) Expansion and modification in topology can be done without disrupting other nodes.

Disadvantages of Mesh topology

1) There are high chances of redundancy in many of the network connections.
2) Overall cost of this network is way too high as compared to other network topologies.
3) Set-up and maintenance of this topology is very difficult. Even administration of the network is tough.

Switch Bridge

 Packet forwarding in Switches are performed using ASICs (Application Specific Integrated Circuits). Packet forwarding in Bridges are performed using software. Work at higher speed. Work at lower speed. Switches have more ports. Bridges have less port. Switches can operate on half and full duplex mode. Bridges can operate on half duplex mode only.


Router Gateways

 Coordinates data transfer within internal network. Coordinates data transfer from internal network to internet. All the functionality of gateways is in router. Gateways are integrated into the router. Always have to have hardware to function. Some software’s also can perform as gateways.

TRANSMISSION MEDIA

The transmission media is nothing but the physical media over which communication takes place in computer networks.

Magnetic Media

One of the most convenient ways to transfer data from one computer to another, even before the birth of networking, was to save it on some storage media and transfer physical from one station to another. Though it may seem old-fashion way in today’s world of high speed internet, but when the size of data is huge, the magnetic media comes into play.

For example, a bank has to handle and transfer huge data of its customer, which stores a backup of it at some geographically far-away place for security reasons and to keep it from uncertain calamities. If the bank needs to store its huge backup data then its, transfer through internet is not feasible. The WAN links may not support such high speed. Even if they do, the cost too high to afford.

In these cases, data backup is stored onto magnetic tapes or magnetic discs, and then shifted physically at remote places.

Twisted Pair Cable

A twisted pair cable is made of two plastic insulated copper wires twisted together to form a single media. Out of these two wires, only one carries actual signal and another is used for ground reference. The twists between wires are helpful in reducing noise (electro-magnetic interference) and crosstalk.

There are two types of twisted pair cables:

·      Shielded Twisted Pair (STP) Cable

·      Unshielded Twisted Pair (UTP) Cable

STP cables comes with twisted wire pair covered in metal foil. This makes it more indifferent to noise and crosstalk.

UTP has seven categories, each suitable for specific use. In computer networks, Cat-5, Cat-5e, and Cat-6 cables are mostly used. UTP cables are connected by RJ45 connectors.

Coaxial Cable

Coaxial cable has two wires of copper. The core wire lies in the center and it is made of solid conductor. The core is enclosed in an insulating shield .The second wire is wrapped around over the shield and that too in turn encased by insulator shield. This all is covered by plastic cover.

Because of its structure, the coax cable is capable of carrying high frequency signals than that of twisted pair cable. The wrapped structure provides it a good shield against noise and cross talk. Coaxial cables provide high bandwidth rates of up to 450 mbps.

There are three categories of coax cables namely,

RG-59 (Cable TV),

RG-58 (Thin Ethernet), and

RG-11 (Thick Ethernet).  RG stands for Radio Government.

Cables are connected using BNC connector and BNC-T. BNC terminator is used to terminate the wire at the far ends.

Power Lines

Power Line communication (PLC) is Layer-1 (Physical Layer) technology which uses power cables to transmit data signals. In PLC, modulated data is sent over the cables. The receiver on the other end de-modulates and interprets the data.

Because power lines are widely deployed, PLC can make all powered devices controlled and monitored. PLC works in half-duplex.

There are two types of PLC:

·      Narrow band PLC

·      Broad band PLC

Narrow band PLC provides lower data rates up to 100s of kbps, as they work at lower frequencies (3-5000 kHz).They can be spread over several kilometers.

Broadband PLC provides higher data rates up to 100s of Mbps and works at higher frequencies (1.8 – 250 MHz).They cannot be as much extended as Narrowband PLC.

Fiber Optics

Fiber Optic works on the properties of light. When light ray hits at critical angle it tends to refracts at 90 degree. This property has been used in fiber optic. The core of fiber optic cable is made of high quality glass or plastic. From one end of it light is emitted, it travels through it and at the other end light detector detects light stream and converts it to electric data.

Fiber Optic provides the highest mode of speed. It comes in two modes, one is single mode fiber and second is multimode fiber. Single mode fiber can carry a single ray of light whereas multimode is capable of carrying multiple beams of light.

Fiber Optic also comes in unidirectional and bidirectional capabilities. To connect and access fiber optic special type of connectors are used. These can be Subscriber Channel (SC), Straight Tip (ST), or MT-RJ.

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 intelligence to find out best path for data packets which leads to inefficiencies and wastage.

Types of Hub

·        Active Hub:- These are the hubs which have their own power supply and can clean, boost and relay the signal along with the network. It serves both as a repeater as well as wiring centre. These are used to extend the maximum distance between nodes.

·        Passive Hub :- These are the hubs which collect wiring from nodes and power supply from active hub. These hubs relay signals onto the network without cleaning and boosting them and can’t be used to extend the distance between nodes.

 

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 hot 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

OSI Reference model

Conceived in the 1970s when computer networking was taking off, two separate models were merged in 1983 and published in 1984 to create the OSI model that most people are familiar with today. Most descriptions of the OSI model go from top to bottom, with the numbers going from Layer 7 down to Layer 1. The layers, and what they represent, are as follows:

Layer 7 - Application

To further our bean dip analogy, the Application Layer is the one at the top - it’s what most users see. In the OSI model, this is the layer that is the “closest to the end user”. Applications that work at Layer 7 are the ones that users interact with directly. A web browser (Google Chrome, Firefox, Safari, etc.) or other app - Skype, Outlook, Office - are examples of Layer 7 applications.

 

Layer 6 - Presentation

The Presentation Layer represents the area that is independent of data representation at the application layer - in general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission - this happens at Layer 6.

Layer 5 - Session

When two devices, computers or servers need to “speak” with one another, a session needs to be created, and this is done at the Session Layer. Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

Layer 4 – Transport

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

 Layer 3 - Network

Here at the Network Layer is where you’ll find most of the router functionality that most networking professionals care about and love. In its most basic sense, this layer is responsible for packet forwarding, including routing through different routers. You might know that your Boston computer wants to connect to a server in California, but there are millions of different paths to take. Routers at this layer help do this efficiently.

Layer 2 – Data Link

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Two sub layers exist here as well - the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2.

Layer 1 - Physical

At the bottom of our OSI bean dip we have the Physical Layer, which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in an 802.11 wireless systems), as well as the layout of pins, voltages and other physical requirements. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected. For example power plug, router, switch or computer.

Centralized vs Distributed system

What is a centralized network?

A Centralized network architecture is built around a single server that handles all the major processing. Less powerful workstations connect to the server and submit their requests to the central server rather than performing them directly. This can include applications, data storage, and utilities. 

What are the advantages and disadvantages of centralized networks?

Some key advantages to centralized network architecture are consistency, efficiency, and affordability. 

Network administrators are under pressure to keep machines patched and up-to-date, so having one central server control the whole network means less IT management time and fewer admins. In addition, all the data on a centralized network is required to go through one place, so it’s very easy to track and collect data across the network.

Centralized networks do have their downsides; for example, a single point of failure can be a risk factor for organizations. If the central—or master—server goes down, the individual “client” machines attached to it are unable to process user requests. The impact of this failure will depend on how much the server processes. If the client machines do little more than submit requests, system availability can be totally compromised.

They also offer limited scalability. Because all applications and processing power are housed in a single server, the only way to scale your network is to add more storage, I/O bandwidth, or processing power to the server. This may not turn out to be a cost-effective solution in the long run.

What is a decentralized network?

In computing terms, a decentralized network architecture distributes workloads among several machines, instead of relying on a single central server. This trend has evolved from the rapid advancements of desktop and laptop computers, which now offer performance well beyond the needs of most business applications; meaning the extra compute power can be put to use for distributed processing.  

What are the advantages and disadvantages of decentralized networks? 

A  Decentralized networks offers a wide range of benefits over the more conventional centralized network, including increased system reliability, scale, and privacy.

One of the most important benefits of decentralized network management is the fact that there is no real single point of failure—this is because individual users’ machines are not reliant on a single central server to handle all processes. Decentralized networks are also much easier to scale, as you can simply add more machines to the network to add more compute power. 

In addition to this, a decentralized network architecture allows for greater privacy, as information is not passing through a single point and instead passes through a number of different points. This makes it much more difficult to track across a network.