TCP/IP PROTOCOL SUITE

The TCP/IP protocol suite was developed prior to the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application.

When TCP/IP is compared to OSI, the host-to-network layer is equivalent to the combination of the physical and data link layers. The internet layer is equivalent to the network layer, and the application layer is roughly doing the job of the session, presentation, and application layers with the transport layer in TCP/IP taking care of part of the duties of the session layer.

TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality.

At the transport layer, TCP/IP defines three protocols: Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). At the network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP).

1. Host-to-Network Layer:

The TCP/IP reference model does not really say much about what happens here, except to point out that the host has to connect to the network using some protocol so it can send IP packets to it. This protocol is not defined and varies from host to host and network to network.

2. Internet Layer:

Its job is to permit hosts to inject packets into any network and have they travel independently to the destination (potentially on a different network). They may even arrive in a different order than they were sent, in which case it is the job of higher layers to rearrange them, if in-order delivery is desired. The internet layer defines an official packet format and protocol called IP (Internet Protocol). The job of the internet layer is to deliver IP packets where they are supposed to go. Packet routing is clearly the major issue here, as is avoiding congestion.

3. The Transport Layer:

The layer above the internet layer in the TCP/IP model is now usually called the transport layer. It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. 

Two end-to-end transport protocols have been defined here. 

The first one, TCP (Transmission Control Protocol), is a reliable connection- oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. It fragments the incoming byte stream into discrete messages and passes each one on to the internet layer. At the destination, the receiving TCP process reassembles the received messages into the output stream. TCP also handles flow control to make sure a fast sender cannot swamp a slow receiver with more messages than it can handle. 

The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable, connectionless protocol for applications that do not want TCP's sequencing or flow control and wish to provide their own. It is also widely used for one-shot, client-server-type request-reply queries and applications in which prompt delivery is more important than accurate delivery, such as transmitting speech or video.

4. The Application Layer:

The TCP/IP model does not have session or presentation layers. On top of the transport layer is the application layer. It contains all the higher-level protocols. The early ones included virtual terminal (TELNET), file transfer (FTP), and electronic mail (SMTP). The virtual terminal protocol allows a user on one machine to log onto a distant machine and work there. The file transfer protocol provides a way to move data efficiently from one machine to another. Electronic mail was originally just a kind of file transfer, but later a specialized protocol (SMTP) was developed for it. Many other protocols have been added to these over the years: the Domain Name System (DNS) for mapping host names onto their network addresses, NNTP, the protocol for moving USENET news articles around, and HTTP, the protocol for fetching pages on the World Wide Web, and many others.

OSI (Open System Interconnect ) Model

In layered architecture of Network Model, one whole network process is divided into small tasks. Each small task is then assigned to a particular layer which works dedicated to process the task only. Every layer does only specific work.

Open System Interconnect is an open standard for all communication systems. OSI model is established by International Standard Organization (ISO). This model has seven layers.

Layer 7 - Application Layer: This layer is responsible for providing interface to the application user. This layer encompasses protocols which directly interact with the user. The application layer is responsible for providing services to the user.

Specific services provided by the application layer include the following:

o Network virtual terminal. A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host.

o File transfer, access, and management. This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.

o Mail services. This application provides the basis for e-mail forwarding and storage.

o Directory services. This application provides distributed database sources and access for global information about various objects and services.

Layer 6 - Presentation Layer: This layer defines how data in the native format of remote host should be presented in the native format of host. The presentation layer is responsible for translation, compression, and encryption.

Specific responsibilities of the presentation layer include the following:

o Translation. The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The information must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format.

o Encryption. To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.

o Compression. Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.

Layer 5 - Session Layer: The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. This layer maintains sessions between remote hosts.   The session layer is responsible for dialog control and synchronization.

Specific responsibilities of the session layer include the following:

o Dialog control. The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either half duplex (one way at a time) or full-duplex (two ways at a time) mode.

o Synchronization. The session layer allows a process to add checkpoints, or synchronization points, to a stream of data.

Layer 4 - Transport Layer: This layer is responsible for end-to-end delivery between hosts. The transport layer is responsible for the delivery of a message from one process to another.

Other responsibilities of the transport layer include the following:

o Service-point addressing. Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called

a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

o Segmentation and reassembly. A message is divided into transmittable segments, with each

segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

o Connection control. The transport layer can be either connectionless or connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.

o Flow control. Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.

o Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission.

Layer 3 - Network Layer: This layer is responsible for address assignment and uniquely addressing hosts in a network. The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Other responsibilities of the network layer include the following:

o Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

o Routing. When independent networks or links are connected to create intemetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

Layer 2 - Data Link Layer: The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). This layer is responsible for reading and writing data from and onto the line. Link errors are detected at this layer. The data link layer is responsible for moving frames from one hop (node) to the next.

Other responsibilities of the data link layer include the following:

o Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

o Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.

o Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

o Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and re transmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

o Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

Layer 1 - Physical Layer: The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.

This layer defines the hardware, cabling, wiring, power output, pulse rate etc. The physical layer is responsible for movements of individual bits from one hop (node) to the next.

NETWORK TOPOLOGIES

 A Network Topology is the arrangement with which computer systems or network devices are connected to each other. 

Topologies may define both physical and logical aspect of the network. Both logical and physical topologies could be same or different in a same network.

Point-to-Point

Point-to-point networks contains exactly two hosts such as computer, switches, routers, or servers connected back to back using a single piece of cable. If the hosts are connected point-to-point logically, then may have multiple intermediate devices. But the end hosts are unaware of underlying network and see each other as if they are connected directly.

Bus Topology

All devices share single communication line or cable. Bus topology may have problem while multiple hosts sending data at the same time. 

It is one of the simple forms of networking where a failure of a device does not affect the other devices. But failure of the shared communication line can make all other devices stop functioning.

Both ends of the shared channel have line terminator. The data is sent in only one direction and as soon as it reaches the extreme end, the terminator removes the data from the line.

Star Topology

All hosts in Star topology are connected to a central device, known as hub device, using a point-to-point connection.

If hub fails, connectivity of all hosts to all other hosts fails. Every communication between hosts takes place through only the hub. Star topology is not expensive as to connect one more host, only one cable is required and configuration is simple.

Ring Topology

In ring topology, each host machine connects to exactly two other machines, creating a circular network structure. When one host tries to communicate or send message to a host which is not adjacent to it, the data travels through all intermediate hosts.

Failure of any host results in failure of the whole ring. Thus, every connection in the ring is a point of failure.

Mesh Topology

A host is connected to one or multiple hosts. This topology has hosts in point-to-point connection with every other host or may also have hosts which are in point-to-point connection with few hosts only.

Tree Topology

Also known as Hierarchical Topology, this is the most common form of network topology in use. This topology divides the network into multiple levels/layers of network. 

TYPES OF COMPUTER NETWORKS

Networks are distinguished based on their geographical span. A network can as large as the internet or be as small as distance between your mobile phone and its Bluetooth headphone.

Depending upon the geographical area covered by a network, it is classified as:

– Local Area Network (LAN)

– Metropolitan Area Network (MAN)

– Wide Area Network (WAN)

– Personal Area Network (PAN)

Local Area Network (LAN)

A LAN is a network that is used for communicating among computer devices, usually within an office building or home.

• LAN’s enable the sharing of resources such as files or hardware devices that may be needed by multiple users

• Is limited in size, typically spanning a few hundred meters, and no more than a mile

• Is fast, with speeds from 10 Mbps to 10 Gbps

• Requires little wiring, typically a single cable connecting to each device

• Has lower cost compared to MAN’s or WAN’s.

Metropolitan Area Network (MAN)

A metropolitan area network (MAN) is a large computer network that usually spans a city or a

large campus.

• A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of

buildings to entire cities.

• A MAN might be owned and operated by a single organization, but it usually will be used

by many individuals and organizations.

• A MAN often acts as a high speed network to allow sharing of regional resources.

• A MAN typically covers an area of between 5 and 50 km diameter.

Wide Area Network (WAN)

WAN covers a large geographic area such as country, continent or even whole of the world.

• A WAN is two or more LANs connected together. The LANs can be many miles apart.

• To cover great distances, WANs may transmit data over leased high-speed phone lines or wireless links such as satellites.

Multiple LANs can be connected together using devices such as bridges, routers, or gateways, which enable them to share data.

• The world's most popular WAN is the Internet.

Personal Area Network

A Personal Area Network (PAN) is smallest network which is very personal to a user. This may include Bluetooth enabled devices or infra-red enabled devices. PAN has connectivity range up to 10 meters.

A PAN is a network that is used for communicating among computers and computer devices (including telephones) in close proximity of around a few meters within a room.

Components of Data Communication

 In Data Communications, data generally are defined as information that is stored in digital form. Data communications is the process of transferring digital information between two or more points. 

Data communications can be summarised as the transmission, reception, and processing of digital information. 

For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs). 

The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter.

A data communications system has five components:

1. Message: The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.

2. Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

3. Receiver: The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

4. Transmission medium: The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

5. Protocol: A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices.