Computer Network – Introduction

• Introduction
• Network as an Infrastructure for Data Communication
• Data Communications Components
• Applications of Computer Network
• Network Architecture
• Types of Computer Network
• Network Topologies
• Protocols and Standards
• OSI Reference Model
• TCP/IP Protocol Suite
• Comparison between OSI and TCP/IP Reference model
• Critiques of OSI and TCP/IP Reference model

Introduction

A network is a set of devices (often referred to as nodes) connected by communication links. A link is a communications pathway that transfers data from one device to another. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.
A computer network is a number of computers (also called nodes) connected by some communication lines. Two computers connected to the network can communicate with each other through the other nodes if they are not directly connected.
A computer network is a system in which multiple computers are connected to each other to share information and resources.

Features of Network

• Share Resources from one computer to another
• Create files and store them in one computer, access those files from the other computer(s) connected over the network
• Connect a printer, scanner, or a fax machine to one computer within the network and let other computers of the network use the machines available over network.

Advantages of Network

• Better communication
• Uninterrupted connectivity
• Sharing of Resources
• Bring people together

Types of Network

We can classify the multiple processor system by their physical size. Distance is important as a classification metric because different techniques are used at different scales.

Network as an Infrastructure for Data Communications

• Network infrastructure is the hardware and software resources of an entire network which enable network connectivity, communication, operations and management of an enterprise network.
• It provides the communication path and services between users, processes, applications, services and external networks/the internet.
• Network infrastructure is typically part of the IT infrastructure found in most enterprise IT environments. The entire network infrastructure is interconnected, and can be used for internal communications, external communications or both.
• A typical network infrastructure includes:
  • Networking Hardware
    • Routers
    • Switches
    • LAN Cards
    • Wireless Routers
    • Cables
  • Network Services
    • Satellite
    • Wireless Protocols
    • IP Addressing
  • Networking Software
    • Network Operations and Management
    • Operating System
    • Firewall
    • Network Security Applications
• There are two type of transmission technology that are in use
  • Broadcast links
  • point to point link
• Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.
• 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

1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.
2. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission
   and left uncorrected are unusable.
3. Timeliness: The system must deliver data in a timely manner.
4. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of
   audio or video packets. For example, let us assume that video packets are sent every 30 millisecond. If
   some of the packets arrive with 30-ms delay and others with 40-ms delay, an uneven quality in the video
   is the result.

Data Communication Components

A data communication system has 5 necessary components which are listed as below.

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, fiberoptic cable, and radio waves.
5. Protocol
   A protocol is a set of rules that governs data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.

Data Flow Directions

The direction of flow are of 3 types

• Simplex
  • Unidirectional, One-way
  • A sender can only send but not receive, and a receiver can only receive but not send
  • Radio, Television broadcast are of simplex type
  • Uses the entire capacity of the channel to send the data in one direction
• Half-Duplex
  • Bi-directional, 2-way communication
  • When one is sending, other can only receive or vice-versa
  • Full capacity of channel is taken over by whichever of the xdevice is transm
• Full-Duplex
  • Bi-directional, 2 way communication
  • Able to send and receive data simultaneously
  • Signals going in one direction share the capacity of the link with the signals going in the other directions
  • Sharing can occur in 2 ways
    • Either the link must have 2 physically separate transmission paths
    • Or the capacity of the channel is divided between the signals traveling in both directions
  • Common example: Telephone Network

Applications of Computer Network

Major Applications

• Business Applications
• Home Application
• Mobile Computers, & etc

1. Business Applications

• Resource Sharing
• VPNs (Virtual Private Networks)
• Web application,
• communication medium
• email (electronic mail),
• IP telephony
• Voice over IP (VoIP)
• Desktop sharing
• e-commerce (electronic commerce)

2. Home Applications

Some of the most important uses of the Internet for home users are as follows:
1. Access to remote information
2. Person-to-person communication
3. Interactive entertainment
4. Electronic commerce

3. Mobile Users

Mobile computers, such as notebook computers and Mobile phones, are one of the fastest-growing
segments of the entire computer industry. Although wireless networking and mobile computing are often related, they are not identical, as the below figure shows.

Network Architecture

• Computer Network Architecture is defined as the physical and logical design of the software, hardware, protocols, and media of the transmission of data.
• Simply we can say that how computers are organized and how tasks are allocated to the computer.
• Architecture also defines how the computers should get connected to get the maximum advantages of a computer network such as better response time, security, scalability etc.
• Two types of network architecture
  • Client/Server
  • Peer to peer

Client/Server

• In Client Server architecture a central computer acts as a hub and serves all the requests from client computers.
• All the shared data is stored in the server computer which is shared with the client computer when a request is made by the client computer.
• All the communication takes place through the server computer, for example if a client computer wants to share the data with other client computer then it has to send the data to server first and then the server will send the data to other client.
• The central controller is known as a server while all other computers in the network are called clients.

Advantages of Client/Server Architecture

• Data backup is easy and cost effective as there is no need to manage the backup on each computer.
• Performance is better as the response time is greatly improves because the server is more powerful computer than the other computers in the network.
• Security is better as unauthorized access are denied by server computer and all the data goes through the server.
• Scalability is not an issue in this Architecture as large number of computers can be connected with server.

Disadvantages of Client/Server Architecture

• In case of server failure entire network is down.
• Server maintenance cost is high as the server is the main component in this Architecture.
• Cost is high as the server needs more resources to handle that many client requests and to be able to hold large amount of data.

Peer to Peer

• In peer to peer architecture all the computers in a computer network are connected with every computer in the network.
• Every computer in the network uses the same resources as other computers.
• There is no central computer that acts as a server rather all computers acts as a server for the data that is stored in them.

Advantages of a Peer to Peer Architecture

1. Less costly as there is no central server that has to take the backup.
2. In case of a computer failure all other computers in the network are not affected and they will continue
   to work as same as before the failure.
3. Installation of peer to peer architecture is quite easy as each computer manages itself.

Disadvantages of a Peer to Peer Architecture

• Each computer has to take the backup rather than a central computer and the security measures are to be taken by all the computers separately.
• Scalability is an issue in a peer to Peer Architecture as connecting each computer to every computer is a headache on a very large network.

Types of Computer Network

• 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 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.

1. Local Area Network (LAN)
2. Metropolitan Area Network (MAN)
3. Wide area network (WAN)

Local Area Network (LAN)

• 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, 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.

Metropolitan Area Network (MAN)

• MAN network covers larger area by connections LANs to a larger network of computers.
• In MAN various Local area networks are connected with each other through telephone lines.
• The size of the Metropolitan area network is larger than LANs and smaller than WANs (wide area networks), a MANs covers the larger area of a city or town.

Wide Area Network (WAN)

• A Wide Area Network is a network that extends over a large geographical area such as states or countries.
• A Wide Area Network is quite bigger network than the MAN.
• A Wide Area Network is not limited to a single location, but it spans over a large geographical area through a telephone line, fiber optic cable or satellite links.
• The internet is one of the biggest WAN in the world.
• A Wide Area Network is widely used in the field of Business, government, and education.

Advantages of Wide area network (WAN)

• Geographical area: A WAN provides a large geographical area. Suppose if the branch of our office is in a different city then we can connect with them through WAN.
• Centralized data: In case of WAN network, data is centralized. Therefore, we do not need to buy the emails, files or back up servers.
• Get updated files: Software companies work on the live server. Therefore, the programmers get the updated files within seconds.
• Exchange messages: In a WAN network, messages are transmitted fast. The web application like Facebook, Whatsapp, Skype etc.
• Sharing of software and resources: In WAN network, we can share the software and other resources like a hard drive, RAM.
• Global Business: We can do the business over the internet globally.
• High bandwidth: The high bandwidth increases the data transfer rate which in turn increases the productivity

Disadvantages of Wide area network (WAN)

• Security issue: A WAN network has more security issues as compared to LAN and MAN network as all the technologies are combined together that creates the security problem.
• Needs Firewall & antivirus software: The data is transferred on the internet which can be changed or hacked by the hackers, so the firewall needs to be used. Some people can inject the virus in our system so antivirus is needed to protect from such a virus.
• High Setup cost: An installation cost of the WAN network is high as it involves the purchasing of routers, switches.
• Troubleshooting problems: It covers a large area so fixing the problem is difficult.

Network Topologies

Physical Topologies

Bus Topology

• A networking topology that connects networking components along a single cable or that uses a series of cable segments that are connected linearly.
• A network that uses a bus topology is referred to as a “bus network.” Bus networks were the original form of Ethernet networks, using the 10Base5 cabling standard. Bus topology is used for:
  • Small work-group local area networks (LANs) whose computers are connected using a thinnet cable
  • Trunk cables connecting hubs or switches of departmental LANs to form a larger LAN
  • Backboning, by joining switches and routers to form campus-wide networks

Advantages

• Easy to install
• Costs are usually low
• Easy to add systems to network
• Great for small networks

Disadvantages

• out of date technology.
• include difficult reconnection and fault isolation
• Can be difficult to troubleshoot.
• Unmanageable in a large network
• If cable breaks, whole network is down

Ring Topology

• In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it.
• A signal is passed along the ring in one direction, from device to device, until it reaches its destination.
• Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.
• A ring is relatively easy to install and reconfigure.
• Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections. The only constraint are media and traffic considerations (maximum ring length and number of devices).
• In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm.
• The alarm alerts the network operator to the problem and its location.
• However, unidirectional traffic can be a disadvantage.
• In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.
• This weakness can be solved by using a dual ring or a switch capable of closing off the break.
• However, unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.
• This weakness can be solved by using a dual ring or a switch capable of closing off the break.

Advantages

• 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 network server to manage the connectivity between the computers

Disadvantages

• One malfunctioning workstation or bad port in the MAU can create problems for the entire network
• Moves, adds and changes of devices can affect the network
• Network adapter cards and MAU’s a Multistation Access Unit are much more expensive than Ethernet cards and hubs
• Much slower than an Ethernet network under normal load

Mesh Topology

In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated
means that the link carries traffic only between the two devices it connects. To connect n nodes in Mesh
topology, we require n(n-1)/2 duplex mode links.

Advantages:

• The use of dedicated links guarantees that each connection can carry its own data load, thus eliminating
• the traffic problems that can occur when links must be shared by multiple devices.
• Robust, If one link becomes unusable, it does not incapacitate the entire system.
• Advantage of privacy or security.
• point-to-point links make fault identification and fault isolation easy , Traffic can be routed to avoid links with suspected problems.

Disadvantage:

• Required high amount of cabling and the number of I/O ports.
• the sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.
• the hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.

One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office.

Star Topology

In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device .

Advantages:

• Less Expensive than Mesh topology.
• In a star topology, each device needs only one link and one I/O port to connect it to any number of other devices. This factor also makes it easy to install and reconfigure.
• Less Cabling, Addition and Deletion involves only one connection between the devices and the Hub or Switch.
• Easy for Fault identification and fault isolation. If one link fails, only that link is affected. All
• other links remain active.

Disadvantage:

• One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead

An extended star topology links individual stars together by connecting the hubs or switches. A hierarchical topology is similar to an extended star. However, instead of linking the hubs or switches together, the system is linked to a computer that controls the traffic on the topology.

Logical Topologies

The logical topology of a network determines how the hosts communicate across the medium. The two most
common types of logical topologies are broadcast and token passing.

The use of a broadcast topology indicates that each host sends its data to all other hosts on the network
medium. There is no order that the stations must follow to use the network. It is first come, first serve. Ethernet
works this way as will be explained later in the course.

The second logical topology is token passing. In this type of topology, an electronic token is passed sequentially
to each host. When a host receives the token, that host can send data on the network. If the host has no data to
send, it passes the token to the next host and the process repeats itself. Two examples of networks that use token
passing are Token Ring and Fiber Distributed Data Interface (FDDI). A variation of Token Ring and FDDI is
Arcnet. Arcnet is token passing on a bus topology.

Protocols and Standards

Protocols

• A protocol is a set of rules that governs(control) data communications.
• A protocol defines what is communicated, how is communicated, and when it is communicated.
• The key elements of a protocol are :

1. Syntax,
2. Semantics
3. Timing.

Elements of Protocols

Syntax

• Structure or format of the data.
• Indicates how to read the bits – field delineation (border or boundary).
• Syntax should be same in sender and receiver for to communicate.

Semantics

• Interprets the meaning of the bits
• Knows which fields define what action
• Interpretation of the syntax should be same

Timing

• When data should be sent and what
• Speed at which data should be sent or speed at which it is being received

Standards

• Standards provide guidelines to manufacturers, vendors, government agencies, and other service providers to ensure the kind of inter-connectivity necessary in today’s marketplace and in international communications.
• Standards are essential in creating and maintaining an open and competitive market for equipment manufacturers and in guaranteeing.
• Data communication standards fall into two categories:
  • de facto (meaning “by fact” or “by convention”)
    Standards that have not been approved by an organized body but have been adopted as standards through widespread use are de facto standards. De facto standards are often established originally by manufacturers who seek to define the functionality of a new product or technology.
  • de jure (meaning “by law” or “by regulation”)
    Those standards by law or by regulation. These are the standards recognized officially by an Organization.

Standards Organizations
Standards are developed through the cooperation of standards creation committees, forums, and government
regulatory agencies.

Standards Creation Committees

• International Organization for Standardization (ISO): The ISO is a multinational body whose membership isdrawn mainly from the standards creation committees of various governments throughout the world. The ISO is active in developing cooperation in the fields of scientific, technological, and economic activity.
• International Telecommunication Union-Telecommunication Standards Sector (ITU-T): This committee was devoted to the research and establishment of standards for telecommunications in general and for phone and data systems in particular.
• American National Standards Institute (ANSI): Despite its name, the American National Standards Institute is a completely private, nonprofit corporation not affiliated with the U.S. federal government. However, all ANSI activities are undertaken with the welfare of the United States and its citizens occupying primary importance.
• Institute of Electrical and Electronics Engineers (IEEE): It is the largest professional engineering society in the world. International in scope, it aims to advance theory, creativity, and product quality in the fields of electrical engineering, electronics, and radio as well as in all related branches of engineering. As one of its goals, the IEEE oversees the development and adoption of international standards for computing and communications.
• Electronic Industries Association (EIA): Aligned with ANSI, It is a nonprofit organization devoted to the promotion of electronics manufacturing concerns. Its activities include public awareness education and efforts in addition to standards development. In the field of information technology, the EIA has made significant contributions by defining physical connection interfaces and electronic signaling specifications for data communication.

OSI Reference Model

• ISO- International Organizations for Standard
• OSI- Opens System Interconnections
• Stats developing in late 1970s
• Approved by 1984
• The term “Open” in Open System Interconnections denotes “to communicate with any 2 systems”
• There are 7 layers in OSI Reference model
• It is also called OSI layered architecture /OSI Protocol architecture
• The process of breaking up the functions or tasks of networking into layers reduces complexity.
• Each layer provides a service to the layer above it in the protocol specification.
• Each layer communicates with the same layer’s software or hardware on other computers.
• The lower 4 layers are concerned with the flow of data from end to end through the network
• The upper Three layers of the OSI model are orientated more toward services to the applications

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.
• The following figure shows the position of the physical layer with respect to the transmission medium and the data link layer.

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).
• The figure shows the relationship of the data link layer to the network and physical layers.

Network Layer

Other responsibilities of the network layer include the following:

• 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 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.
• Routing. When independent networks or links are connected to create internetworks (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.

Session Layer

• The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes.
• The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.

Presentation Layer

Specific responsibilities of the presentation layer include the following:

• 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.
• 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.
• 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.

Application Layer

• The application layer enables the user, whether human or software, to access the network.
• It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.
• The figure shows the relationship of the application layer to the user and the presentation layer.

Specific services provided by the application layer include the following:

• 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. To do so, the application creates a software emulation of a terminal at the remote host. The user’s computer talks to the software terminal which, in turn, talks to the host, and vice versa. The remote host believes it is communicating with one of its own terminals and allows the user to log on.
• 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.
• Mail services. This application provides the basis for e-mail forwarding and storage.
• Directory services. This application provides distributed database sources and access for global information about various objects and services.

TCP/IP Protocol Suite

• The TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application. 
• The first four layers provide physical standards, network interfaces, internetworking, and transport functions that correspond to the first four layers of the OSI model. 
• The three topmost layers in the OSI model, however, are represented in TCP/IP by a single layer called the application layer. 

PROTOCOL

• TCP (Transmission Control Protocol)
• UDP (User Datagram Protocol)
• virtual terminal (TELNET)
• file transfer(FTP), and electronic mail (SMTP)
• Domain Name System (DNS),
• HTTP (HyperText Transfer Protocol)
• Stream Control Transmission Protocol (SCTP)
• Address Resolution Protocol (ARP)
• Reverse Address Resolution Protocol (RARP)
• Internet Group Message Protocol (IGMP)
• ICMP (Internet Control Message Protocol)

• TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality; however, the modules are not necessarily interdependent. 
• Whereas the OSI model specifies which functions belong to each of its layers, the layers of the TCP/IP protocol suite contain relatively independent protocols that can be mixed and matched depending on the needs of the system. 
• The term hierarchical means that each upper-level protocol is supported by one or more lower-level protocols. 
• 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); there are also some other protocols that support data movement in this layer. 

Physical and Data Link Layers:At the physical and data link layers, TCP/IP does not define any specific protocol. It supports all the standard and proprietary protocols. A network in a TCP/IP internetwork can be a local-area network or a wide-area network.

Network Layer: At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the Internetworking Protocol. IP, in turn, uses four supporting protocols: ARP, RARP, ICMP, and IGMP. 

1. Internetworking Protocol (IP): The Internetworking Protocol (IP) is the transmission mechanism used by the TCP/IP protocols. It is an unreliable and connectionless protocol–a best-effort delivery service. The term best effort means that IP provides no error checking or tracking. IP assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees.
2. IP transports data in packets called datagrams, each of which is transported separately. Datagrams can travel along different routes and can arrive out of sequence or be duplicated. IP does not keep track of the routes and has no facility for reordering datagrams once they arrive at their destination. 
3. Address Resolution Protocol (ARP): The ARP  is used to associate a logical address with a physical address. On a typical physical network, such as a LAN, each device on a link is identified by a physical or station address, usually imprinted on the network interface card (NIC). ARP is used to find the physical address of the node when its Internet address is known.  
4. Reverse Address Resolution Protocol (RARP): Its allows a host to discover its Internet address when it knows only its physical address. It is used when a computer is connected to a network for the first time or when a diskless computer is booted.  
5. Internet Control Message Protocol: The ICMP is a mechanism used by hosts and gateways to send notification of datagram problems back to the sender. ICMP sends query and error reporting messages.  
6. Internet Group Message Protocol: The IGMP is used to facilitate the simultaneous transmission of a message to a group of recipients.  

Transport Layer: Traditionally the transport layer was represented in TCP/IP by two protocols: TCP and UDP.

• IP is a host-to-host protocol, meaning that it can deliver a packet from one physical device to another. 
• UDP and TCP are transport level protocols responsible for delivery of a message from a process (running program) to another process. 
• A new transport layer protocol, SCTP, has been devised to meet the needs of some newer applications. 
• User Datagram Protocol: The User Datagram Protocol (UDP) is the simpler of the two standard TCP/IP transport protocols. It is a process-to-process protocol that adds only port addresses, checksum error control, and length information to the data from the upper layer. 
• Transmission Control Protocol: The TCP provides full transport-layer services to applications. TCP is a reliable stream transport protocol. The term stream, in this context, means connection-oriented: A connection must be established between both ends of a transmission before either can transmit data. At the sending end of each transmission, TCP divides a stream of data into smaller units called segments. Each segment includes a sequence number for reordering after receipt, together with an acknowledgment number for the segments received. Segments are carried across the internet inside of IP datagram. At the receiving end, TCP collects each datagram as it comes in and reorders the transmission based on sequence numbers. 
• Stream Control Transmission Protocol: The SCTP provides support for newer applications such as voice over the Internet. It is a transport layer protocol that combines the best features of UDP and TCP.

Application Layer: The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model. Many protocols are defined at this layer.

Comparison between OSI and TCP/IP Reference model

 Following are some similarities between OSI Reference Model and TCP/IP Reference Model.

• Both have layered architecture.
• Layers provide similar functionalities.
• Both are protocol stack.
• Both are reference models.

Critiques of OSI and TCP/IP Reference model

A Critique of the OSI Model and Protocols

• Why OSI did not take over the world?
  • Bad timing
  • Bad technology
  • Bad implementations
  • Bad politics
  • Bad timing
  • Bad Implementation
  • Initial version were huge, unwieldy and slow. 
  • Bad Politics
  • TCP/IP part of Unix, OSI – government pushed

A Critique of the TCP/IP Reference Model

• Problems:
  • Service, interface, and protocol not very successful
  • Not a general model
  • Host-to-network “layer” not really a layer
  • No mention of physical and data link layers
  • Minor protocols deeply establish, hard to replace