Computer Networks
Department of BCA
Loyola College of Arts & Science
Mettala
UNIT 1
Contents -
Representation and its flow,
of data network
protocols and standards
Data Representation
Data is collection of raw facts which is processed to deduce information. There may be different forms in which data may be represented. Some of the forms of data used in communication are as follows;
Data can be represented by using different forms as shown in figure
Text
Text includes combinations of alphabet in small case as well as upper case. It is stored as pattern of bits.
In data communication , text is represented as a bit pattern Unicode : 32 bits
ascii_ - first 127 characters in Unicode.
4
Numbers
Numbers include combination of digits from 0 to 9. it is stored as a pattern of bits . prevalent encoding system : asci, Unicode.
Images
An image is worth a thousand words‖ is a very famous saying. In computers images are digitally stored.
A Pixel is the smallest element of an image. To put it in simple terms, a picture or image is a matrix of pixel elements.
The pixels are represented in the form of bits. Depending upon the type of image (black n white or color ) each pixel would require different number of bits to represent the value of a pixel.
The size of an image depends upon the number of pixels (also called resolution)
and the bit pattern used to indicate the value of each pixel.
Audio
Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from text, numbers , or images.
It is continuous, not discrete. Even when we use a microphone to change voice or music to an electric signal, we create a continuous signal
Video
video refers to broadcasting of data in form of picture or movie.
Data flow
Two devices communicate with each other by sending and receiving data. The data can flow between the two devices in the following ways. 1: simplex
2: half duplex
3: full duplex
Diagram pf data flow methods
Simplex
In simplex ,communication is unidirectional only one of the device sends the data and the other one only receives the data. Example in the below diagram : a CPU send data while a monitor only receives data.
Half duplex
In half duplex both the stations can transmit as well as receives but not at the same time.
When one device is sending other can only receives and vice versa(as shown below in figure)
Example: walkie - talkie
Full duplex
In full duplex mode, both stations can transmit and receives at the same time.
Example: mobile.
Protocols
A protocol is basically a synonym for the rule. In computer networks, basically,
Communications occurs between entities in different systems. An entity is anything that is capable od sending or receiving information . any two entities cannot simply send bitstreams to each other and expect to be understood.
A protocol is a set of rules that mainly govern data communications. The protocol mainly defines what is communicated, how it is communicated, and when it is communicated.
Key elements of a protocol
The key elements of a protocol are as given below:
syntax This term mainly refers to the structure or format of the data which simply means the order in which data is presented. For example, A simple protocol might expect the first 8 bits of data to be the address of the sender, then the second 8 bits to be the address of the receiver, and then the rest of the stream to be the message itself.
Key elements of a protocol
Semantics This term mainly refers to the meaning of each section of bits. How does a particular pattern to be interpreted, and On the basis of interpretation what action is to be taken? For example, does an address identify the route to be taken or the final destination of the message?
Timing This term mainly refers to two characteristics: At what time the data should be sent and how fast data can be sent. For example, if a sender produces data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and there will be some data loss.
Standard
Standards are essential in creating and maintaining an open and competitive market for equipment manufacturers and in guaranteeing national and international interoperability of data and telecommunications technology and processes. Standards provide guidelines to manufacturers, vendors, government agencies, and other service providers to ensure the kind of interconnectivity necessary in today's marketplace and in international communications. Data communication standards fall into two categories: de facto (meaning "by fact" or "by convention") and de jure (meaning "by law" or "by regulation").
Standards are of two types :
De Facto Standard. De Jure Standard.
De facto standard
De Facto Standard : The meaning of the work ” De Facto ” is ” By Fact ” or “By Convention”.
These are the standard s that have not been approved by any Organization , but have been adopted as Standards because of it’s widespread use. Also , sometimes these standards are often established by Manufacturers.
For example : Apple and Google are two companies which established their own rules on their products which are different . Also they use some same standard rules for manufacturing for their products.
OSI & TCP/IP�Models
OSI Model
What is the OSI Model?
OSI stands for Open System Interconnection is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.
OSI model was developed by the International Organization for Standardization (ISO) in 1984.
it is now considered as an architectural model for the inter-computer communications.
OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.
OSI consists of seven layers, and each layer performs a particular network function.
Different layers of OSI model-
There are the seven OSI layers :
Physical layer:
The lowest layer of the OSI reference model is the physical layer.
It is responsible for the actual physical connection between the devices.
The physical layer contains information in the form of bits.
It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together.
The functions of the physical layer are :
Bit synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at bit level.
Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per
second.
Physical topologies: Physical layer specifies the way in which the different, devices/nodes are arranged in a network i.e. bus, star or mesh topology.
Transmission mode: Physical layer also defines the way in which the data flows between the two
connected devices. The various transmission modes possible are: Simplex, half-duplex and full-duplex.
* Hub, Repeater, Modem, Cables are Physical Layer devices.
Data Link Layer (DLL) :
The data link layer is responsible for the node to node delivery of the message.
The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of DLL to transmit it to the Host using its MAC address.
Data Link Layer is divided into two sub layers : Logical Link Control (LLC)
Media Access Control (MAC)
The packet received from Network layer is further divided into frames depending on the frame size of
NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header.
The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address
The functions of the data Link layer are -
Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
Physical addressing: After creating frames, Data link layer adds physical addresses (MAC address) of sender and/or receiver in the header of each frame.
Error control: Data link layer provides the mechanism of error control in which it detects and retransmits
damaged or lost frames.
Flow Control: The data rate must be constant on both sides else the data may get corrupted thus , flow control coordinates that amount of data that can be sent before receiving acknowledgement.
Access control: When a single communication channel is shared by multiple devices, MAC sub-layer of data link layer helps to determine which device has control over the channel at a given time.
Network Layer:
Network layer works for the transmission of data from one host to the other located in different networks.
It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available.
The sender & receiver’s IP address are placed in the header by the network layer.
The functions of the Network layer are -
Routing: The network layer protocols determine which route is suitable from source to destination. This
function of network layer is known as routing.
Logical Addressing: In order to identify each device on internetwork uniquely, network layer defines an addressing scheme. The sender & receiver’s IP address are placed in the header by network layer. Such an address distinguishes each device uniquely and universally.
**Segment in Network layer is referred as Packet.
** Network layer is implemented by networking devices such as routers
Transport Layer :
Transport layer provides services to application layer and takes services from network layer.
The data in the transport layer is referred to as Segments.
It is responsible for the End to End Delivery of the complete message.
The transport layer also provides the acknowledgement of the successful data transmission and re-transmits the data if an error is found.
Transport layer receives the formatted data from the upper layers, performs Segmentation and also implements Flow & Error control to ensure proper data transmission. It also adds Source and Destination port number in its header and forwards the segmented data to the Network Layer.
Transport Layer reads the port number from its header and forwards the Data which it has received to the respective application. It also performs sequencing and reassembling of the segmented data.
The functions of the transport layer are :
Segmentation and Reassembly: This layer accepts the message from the (session) layer , breaks the message into smaller units . Each of the segment produced has a header associated with it. The transport layer at the destination station reassembles the message.
Service Point Addressing: In order to deliver the message to correct process, transport layer header includes a type of address called service point address or port address. Thus by specifying this address, transport layer makes sure that the message is delivered to the correct process
** Transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls.
Transport Layer is called as Heart of OSI model
The services provided by the transport layer :
Connection Oriented Service: It is a three-phase process which include
In this type of transmission, the receiving device sends an acknowledgement, back to the source after a packet or group of packet is received. This type of transmission is reliable and secure.
Connection less service: It is a one-phase process and includes Data Transfer. In this type of transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication between devices. Connection-oriented service is more reliable than connectionless Service.
* Data in the Transport Layer is called as Segments
Session Layer :
The functions of the Session Layer are:
This layer is responsible for establishment of connection, maintenance of sessions, authentication and also ensures security.
The functions of the session layer are :
Session establishment, maintenance and termination: The layer allows the two processes to establish,
use and terminate a connection.
Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into the data. These synchronization point help to identify the error so that the data is re- synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
Dialog Controller : The session layer allows two systems to start communication with each other in half- duplex or full-duplex.
Presentation Layer:
Presentation layer is also called the Translation layer.
The data from the application layer is extracted here and manipulated as per the required format to transmit over the network.
The functions of the presentation layer are :
Translation : For example, ASCII to EBCDIC.
Encryption/ Decryption : Data encryption translates the data into another form or code. The encrypted data is known as the cipher text and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
Compression: Reduces the number of bits that need to be transmitted on the network.
Application Layer:
At the very top of the OSI Reference Model stack of layers, we find Application layer which is implemented by the network applications.
These applications produce the data, which has to be transferred over the network.
This layer also serves as a window for the application services to access the network and for displaying the received information to the user.
Ex: Application – Browsers, Skype Messenger etc.
**Application Layer is also called as Desktop Layer.
The functions of the Application layer are :
Network Virtual Terminal
FTAM-File transfer access and management Mail Services
Directory Services
OSI model acts as a reference model and is not implemented in the Internet because of its late invention. Current model being used is the TCP/IP model.
TCP/IP Model
TCP/IP model 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.
Layers of TCP/IP Model
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
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 :
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.
ICMP – stands for Internet Control Message Protocol. It is encapsulated within IP datagrams and is responsible for providing hosts with information about network problems.
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
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 :
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.
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
Application Layer:
An application layer is the topmost layer in the TCP/IP mode
It is responsible for handling high-level protocols, issues of representation. This layer allows the user to interact with the application.
When one application layer protocol wants to communicate with another application layer, it forwards its data to the transport layer.
There is an ambiguity occurs in the application layer. Every application cannot be placed inside the
application layer except those who interact with the communication system.
For example: text editor cannot be considered in application layer while web browser using HTTP protocol to interact with the network where HTTP protocol is an application layer protocol.
Following are the main protocols used in the application layer:�HTTP: HTTP stands for Hypertext transfer protocol. This protocol allows us to access the data over the world wide web. It transfers the data in the form of plain text, audio, video. It is known as a Hypertext transfer protocol as it has the efficiency to use in a hypertext environment where there are rapid jumps from one document to another.
SNMP: SNMP stands for Simple Network Management Protocol. It is a framework used for managing the devices on the internet by using the TCP/IP protocol suite.
SMTP: SMTP stands for Simple mail transfer protocol. The TCP/IP protocol that supports the e-mail is known as a Simple mail transfer protocol. This protocol is used to send the data to another e-mail address.
DNS: DNS stands for Domain Name System. An IP address is used to identify the connection of a host to the internet uniquely. But, people prefer to use the names instead of addresses. Therefore, the system that maps the name to the address is known as Domain Name System.
TELNET: It is an abbreviation for Terminal Network. It establishes the connection between the local computer and remote computer in such a way that the local terminal appears to be a terminal at the remote system.
FTP: FTP stands for File Transfer Protocol. FTP is a standard internet protocol used for transmitting the files from one
computer to another computer
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 :
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.
ICMP – stands for Internet Control Message Protocol. It is encapsulated within IP datagrams and is responsible for providing hosts with information about network problems.
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
Conclusion
provides some peace of mind.
References
Thank you