Computer networking notes

1. COMPUTER NETWORKS

2. Networks
 A network is a collection of computers, servers, mainframes, network
devices, peripherals, or other devices connected to allow data sharing.
 An example of a network is the Internet, which connects millions of people all
over the world.

3. Uses of networks
• COMMUNICATION. Everything, from the world wide web to online banking to
multiplayer computer games, depends on the ability of computers to
communicate. ...
• ACCESSING RESOURCES. ...
• CENTRALISATION OF DATA. ...
• TRANSFER OF FILES. ...
• INCREASED PRODUCTIVITY. ...
• LEISURE. ...
• WIRED NETWORK. ...
• WIRELESS NETWORKS.

4. Advantages and Disadvantages
• Advantages
• Sharing devices such as printers saves money.
• Site (software ) licences are likely to be cheaper than buying several standalone
licences.
• Files can easily be shared between users.
• Network users can communicate by email and instant messenger .
 Disadvantages
• Purchasing the network cabling and file servers can be expensive.
• Managing a large network is complicated, requires training and a network manager
usually needs to be employed.
• If the file server breaks down the files on the file server become inaccessible

5. Catagories of networks
 Networks differ based on size, connectivity, coverage and design. This
guide explores seven common types of networks, including their benefits
and use cases.
• Personal area network. ...
• Local area network. ...
• Metropolitan area network. ...
• Campus network. ...
• Wide area network. ...
• Content delivery network. ...
• Virtual private network.

6. Communication
 A communication network refers to how information flows within the
organization. Information within an organization generally flows through a
system, rather than being a free flow. Communication networks are regular
patterns of person-to-person relationships through which information flows in an
organization.

7. Advantages and Disadvantages

8. Protocols Architecture
 A network protocol is an established set of rules that determine how data is
transmitted between different devices in the same network.

9. DATATRANSMISSION :
 Data transmission refers to the movement of data in form of bits between two or
more digital devices.
 This transfer of data takes place via some form of transmission media
 FOR EXAMPLE : coaxial cable, fiber optics etc.
 The most widely used codes that have been adopted for this function are the
extended binary coded decimal (EBCDIC) and the american standard code
for information interchange codes (ASCII).
 Both coding schemes cater to all the normal alphabetic, numeric, and punctuation
characters, collectively referred to as printable characters and a range of
additional control characters, known as non-printable characters.

10. Types of DataTransmission :

11.  It is normal practice to transfer data between subunits using a separate wire to
carry each bit of data.
 There are multiple wires connecting each sub-unit and data is exchanged using
a parallel transfer mode.
 This mode of operation results in minimal delays in transferring each word.
 In parallel transmission, all the bits of data are transmitted simultaneously on
separate communication lines.
 In order to transmit n bits, n wires or lines are used. Thus each bit has its own
line.
 All n bits of one group are transmitted with each clock pulse from one device to
another i.e. multiple bits are sent with each clock pulse.
PARALLELTRANSMISSION :

12.  It is speedy way of transmitting data as multiple bits are transmitted
simultaneously with a single clock pulse.

DISADVANTAGE OF PARALLEL TRANSMISSION :
Advantageof parallel transmission :
 It is costly method of data transmission as it requires n lines to transmit n bits at
the same time.

13. SERIAL TRANSMISSION :
 When transferring data between two physically separate devices, especially if the
separation is more than a few kilometers, for reasons of cost, it is more economical
to use a single pair of lines.
 Data is transmitted as a single bit at a time using a fixed time interval for each bit.
This mode of transmission is known as bit-serial transmission.
 In serial transmission, the various bits of data are transmitted serially one after the
other.
 It requires only one communication line rather than n lines to transmit data from
sender to receiver.
 Thus all the bits of data are transmitted on single line in serial fashion.
 In serial transmission, only single bit is sent with each clock pulse.

14.  As shown in fig., Suppose an 8-bit data 11001010 is to be sent from source to
destination.
 Then least significant bit (lsb) i,e. 0 will be transmitted first followed by other bits.
 The most significant bit (msb) i.E. 1 will be transmitted in the end via single
communication line.
 The internal circuitry of computer transmits data in parallel fashion. So in order to
change this parallel data into serial data, conversion devices are used.
 These conversion devices convert the parallel data into serial data at the sender side
so that it can be transmitted over single line.
 On receiver side, serial data received is again converted to parallel form so that the
interval circuitry of computer can accept it

15. ADVANTAGEOF SERIALTRANSMISSION :
 Use of single communication line reduces the transmission line cost by the factor
of n as compared to parallel transmission.
 Use of conversion devices at source and destination end may lead to increase in
overall transmission cost.
 This method is slower as compared to parallel transmission as bits are transmitted
serially one after the other.
Disadvantages of Serial transmission :

16. Comparison between Serial and Parallel transmission

17. osi protocol

18. What Is the OSI Model?
 The Open Systems Interconnection (OSI) model describes seven layers that
computer systems use to communicate over a network. It was the first
standard model for network communications, adopted by all major computer
and telecommunication companies in the early 1980s
 The modern Internet is not based on OSI, but on the simpler TCP/IP model.
However, the OSI 7-layer model is still widely used, as it helps visualize and
communicate how networks operate, and helps isolate and troubleshoot
networking problems.
 OSI was introduced in 1983 by representatives of the major computer and
telecom companies and was adopted by ISO as an international standard in
1984.

19. OSI Model Explained: The OSI 7 Layers

20. 7. Application Layer
The application layer is used by end-user software such as web browsers and
email clients. It provides protocols that allow software to send and receive
information and present meaningful data to users. A few examples of
application layer protocols are the Hypertext Transfer Protocol (HTTP), File
Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer
Protocol (SMTP), and Domain Name System (DNS).
6. Presentation Layer
The presentation layer prepares data for the application layer. It defines how
two devices should encode, encrypt, and compress data so it is received
correctly on the other end. The presentation layer takes any data transmitted
by the application layer and prepares it for transmission over the session layer.

21. 5. Session Layer
The session layer creates communication channels, called sessions, between
devices. It is responsible for opening sessions, ensuring they remain open and
functional while data is being transferred, and closing them when communication
ends. The session layer can also set checkpoints during a data transfer—if the
session is interrupted, devices can resume data transfer from the last checkpoint.
4. Transport Layer
The transport layer takes data transferred in the session layer and breaks it into
“segments” on the transmitting end. It is responsible for reassembling the
segments on the receiving end, turning it back into data that can be used by the
session layer. The transport layer carries out flow control, sending data at a rate
that matches the connection speed of the receiving device, and error control,
checking if data was received incorrectly and if not, requesting it again.

22.  3. Network Layer
 The network layer has two main functions. One is breaking up segments into
network packets, and reassembling the packets on the receiving end. The other is
routing packets by discovering the best path across a physical network. The
network layer uses network addresses (typically Internet Protocol addresses) to
route packets to a destination node.
 2. Data Link Layer
 The data link layer establishes and terminates a connection between two
physically-connected nodes on a network. It breaks up packets into frames and
sends them from source to destination. This layer is composed of two parts—
Logical Link Control (LLC), which identifies network protocols, performs error
checking and synchronizes frames, and Media Access Control (MAC) which uses
MAC addresses to connect devices and define permissions to transmit and receive
data.

23. 1. Physical Layer
The physical layer is responsible for the physical cable or wireless connection
between network nodes. It defines the connector, the electrical cable or wireless
technology connecting the devices, and is responsible for transmission of the raw
data, which is simply a series of 0s and 1s, while taking care of bit rate control.
Advantages of OSI Model
 The OSI model helps users and operators of computer networks:
 Determine the required hardware and software to build their network.
 Understand and communicate the process followed by components
communicating across a network.
 Perform troubleshooting, by identifying which network layer is causing an issue
and focusing efforts on that layer.

24. TCP/IP MODEL

25. the TCP/IP model, it was designed and developed by Department of Defense (DoD) in 1960s and is
based on standard protocols. It stands for Transmission Control Protocol/Internet Protocol. The TCP/IP
model is a concise version of the OSI model. It contains four layers, unlike seven layers in the OSI
model. The layers are:
1. Process/Application Layer
2. Host-to-Host/Transport Layer
3. Internet Layer
4. Network Access/Link Layer
TCP/IP MODEL

26. The first layer is the Process layer on the behalf of the sender and Network Access layer on the behalf of
the receiver. During this article, we will be talking on the behalf of the receiver.

27. 1. Network Access Layer –
 This layer corresponds to the combination of Data Link Layer and Physical Layer of the
OSI model. It looks out for hardware addressing and the protocols present in this layer
allows for the physical transmission of data.
We just talked about ARP being a protocol of Internet layer, but there is a conflict about
declaring it as a protocol of Internet Layer or Network access layer. It is described as
residing in layer 3, being encapsulated by layer 2 protocols.
2. Internet Layer –
This layer parallels the functions of OSI’s Network layer. It defines the protocols which are
responsible for logical transmission of data over the entire network. The main protocols
residing at this layer are :
1. IP – stands for Internet Protocol and it is responsible for delivering packets from the
source host to the destination host by looking at the IP addresses in the packet headers.
IP has 2 versions:
IPv4 and IPv6. IPv4 is the one that most of the websites are using currently. But IPv6 is
growing as the number of IPv4 addresses are limited in number when compared to the
number of users.
2. ICMP – stands for Internet Control Message Protocol. It is encapsulated within IP
datagrams and is responsible for providing hosts with information about network
problems.
3. ARP – stands for Address Resolution Protocol. Its job is to find the hardware address of a
host from a known IP address. ARP has several types: Reverse ARP, Proxy ARP, Gratuitous
ARP and Inverse ARP.

28. 3. Host-to-Host Layer –
 This layer is analogous to the transport layer of the OSI model. It is responsible for
end-to-end communication and error-free delivery of data. It shields the upper-layer
applications from the complexities of data. The two main protocols present in this
layer are :
1. Transmission Control Protocol (TCP) – It is known to provide reliable and error-free
communication between end systems. It performs sequencing and segmentation of
data. It also has acknowledgment feature and controls the flow of the data through
flow control mechanism. It is a very effective protocol but has a lot of overhead due
to such features. Increased overhead leads to increased cost.
2. User Datagram Protocol (UDP) – On the other hand does not provide any such
features. It is the go-to protocol if your application does not require reliable
transport as it is very cost-effective. Unlike TCP, which is connection-oriented
protocol, UDP is connectionless.
4. Application Layer –
 This layer performs the functions of top three layers of the OSI model: Application,
Presentation and Session Layer. It is responsible for node-to-node communication and
controls user-interface specifications. Some of the protocols present in this layer are:
HTTP, HTTPS, FTP, TFTP, Telnet, SSH, SMTP
, SNMP, NTP
, DNS, DHCP, NFS, X Window,
LPD. Have a look at Protocols in Application Layer for some information about these
protocols. Protocols other than those present in the linked article are :

29. 1. HTTP and HTTPS – HTTP stands for Hypertext transfer protocol. It is used by the
World Wide Web to manage communications between web browsers and servers.
HTTPS stands for HTTP-Secure. It is a combination of HTTP with SSL(Secure Socket
Layer). It is efficient in cases where the browser need to fill out forms, sign in,
authenticate and carry out bank transactions.
2. SSH – SSH stands for Secure Shell. It is a terminal emulations software similar to
Telnet. The reason SSH is more preferred is because of its ability to maintain the
encrypted connection. It sets up a secure session over a TCP/IP connection.
3. NTP – NTP stands for Network Time Protocol. It is used to synchronize the clocks on
our computer to one standard time source. It is very useful in situations like bank
transactions. Assume the following situation without the presence of NTP. Suppose
you carry out a transaction, where your computer reads the time at 2:30 PM while
the server records it at 2:28 PM. The server can crash very badly if it’s out of sync.