Data Bandwidth

In computer networks, bandwidth is the amount of data that can be carried from one point to another in a given time period; (generally a second). Network bandwidth is usually expressed in bits per second (bps); kilobits per second (kb/s), megabits per second (Mb/s), or gigabits per second (Gb/s). It sometimes thought of as the speed that bits travel, however, this is not accurate.

For example, in both 100Mb/s and 1000Mb/s Ethernet, the bits are sent at the speed of electricity. The difference is the number of bits that are transmitted per second.

• A combination of factors determines the practical bandwidth of a network.


• The properties of the physical media.


• 
  

  The technologies chosen for signaling and detecting network signals.

  
  


• 
  

  Physical media properties, current technologies, and the laws of physics play a role in determining the available bandwidth.

  
  

The table shows the commonly used units of measure for data bandwidth

Unit	Abbrivation	Decimal Value	Binary value	Decimal Size
Bit	b	0 or 1	0 or 1	1/8 of a byte
Byte	B	8 bits	8 bits	1 byte
Kilobyte	KB	1,0001 bytes	10241 bytes	1,000 bytes
Megabyte	MB	1,0002	10242	1,000,000 bytes
Gigabyte	GB	1,0003	10243	1,000,000,000 bytes
Terabyte	TB	1,0004	10244	1000,000,000,000 bytes
Petabyte	PB	1,0005	10245	1000,000,000,000,000 bytes

Throughput

The measurement of a bits transfer across the media over a given period is called throughput. It is a measure of how many units of information a system can process in a given amount of time. Due to some factors, it generally does not match the specified bandwidth in physical layerimplementations. Many factors manipulate it, including following:

• 
  

  The type of traffic

  
  


• 
  

  The amount of traffic

  
  


• 
  

  The latency created by the number of network devices between source and destination

  
  


• 
  

  Error rate

  
  

Latency is the amount of time, to include delays; for data to travel from one given point to another.

The networks with multiple segments, throughput can’t be faster than the slowest link in the path from source to destination. Even if all or most of the segments have high bandwidth; it will only take one segment in the path with low throughput to create a tailback to the throughput of the entire network.

The average transfer speed over a medium is often described as throughput. This measurement includes all the protocol overhead information; such as packet headers and other data that is included in the transfer process. It also includes packets that are retransmitted because of network conflicts or errors.

There is another measurement to evaluate the transfer of usable data that is known as goodput. Goodput is the measure of usable data transferred over a given period of time. Goodput is throughput minus traffic overhead for establishing sessions, acknowledgments, and encapsulation. It only measures the original data.

Physical Layer Functions

The physical layer standards deal with three basic functions which are described below:

Physical Components

The physical layer takes frames from the data link layer then converts these frames into electrical, electromagnetic; optical signals through different line coding techniques.Transmits these signals through wired/wireless telecommunication links (cables/antennas) to next hop.

Components of this layer are the electronic hardware devices; media, and other connectors such as NIC, cable, and connector that transmit and carry the signals. Hardware components such as NICs, interfaces and connectors, cable materials, and cable designs are all specified in standards associated with the physical layer.

Encoding

Encoding is a technique of converting a stream of data bits into a predefined "code”. Codes are groupings of bits used to provide a predictable pattern that can be recognized by both the sender and the receiver. In the case of networking, encoding is a pattern of voltage or current used to represent bits; the 0s and 1s.

Signaling

The physical layer must generate the electrical, optical, or wireless signals that represent the "1" and "0" on the media. The method of representing the bits is called the signaling method. The physical layer standards must define what type of signal represents a "1" and what type of signal represents a "0". This can be as simple as a change in the level of an electrical signal or optical pulse. For example, a long pulse might represent a 1 whereas a short pulse represents a 0. Modulation techniques are a common method to send data. Modulation is the process by which the characteristic of one wave modifies another wave.

The Physical Layer

The Physical layer is the lowest layer of OSI Model. It provides the resources to transport the bits; that make up a data link layer frame across the network media. This layer accepts a complete frame from the data link layer and encodes it as a series of signals that are transmitted onto the local media.(see figure 3.1) The encoded bits that comprise a frame are received by either an end device or an intermediate device.

This layer also deals with the physical connection to the network and with transmission and reception of signals. This layer defines electrical and physical details represented as 0 or a 1.It's also decided when the data can be transmitted or not and how the data would be synchronized.

The process that data travel from a source node to a destination node is following:

• The user data is segmented by the transport layer; placed into packets by the network layer, and further encapsulated into frames by the data link layer.Data link layer sent these frames to the physical layer.

• The physical layer encodes the frames and creates the electrical, optical; or radio wave signals that represent the bits(0 and 1) in each frame.

• These signals are then sent to the media, one at a time.

• The destination node physical layer retrieves these individual signals from the media; restores them to their bit representations, and passes the bits up to the data link layer as a complete frame(see figure 3.1).

• Line configuration: - This layer connects devices with the medium;  Point to Point configuration and Multipoint configuration.

• Topologies: - Devices must be connected using the following topologies;  Mesh, Star, Ring and Bus

• Transmission Modes: -  Physical Layer also defines the direction of transmission between devices ( Simplex, Half Duplex, Full Duplex).

Media

Many different types of media can be used for the physical layer. For example, telephone twisted pair, coax cable, shielded copper cable; and fiber optics are the main types used for LANs. Different transmission techniques generally categorized as baseband, or broadband transmission may be applied to each of these media types.

There are three basic types media. The physical layer produces the representation and groupings of bits for each type of media as:

• Copper cable: The signals are patterns of electrical pulses.


• Fiber-optic cable: The signals are patterns of light.


• Wireless: The signals are patterns of microwave transmissions.

To enable physical layer interoperability, all aspects of these functions are governed by standards organizations.

Standards

Upper layer Protocol: - Protocols and operation of the upper OSI layers are totally performed in software. This software is designed by software engineers and computer scientists. IETF (Internet Engineering Task Force ) is an organization which defined the services and protocol for TCP/IP suit.

The physical layer consists of electronic circuitry, media, and connectors. Therefore, it is suitable that the standards governing this hardware are defined by the relevant electrical and communications engineering organizations.

There are many different international and national organizations, regulatory government organizations, and private companies involved in establishing and maintaining physical layer standards. For example, the physical layer hardware, media, encoding, and signaling standards are defined and governed by the following:-

• International Organization for Standardization (ISO)


• International Telecommunication Union (ITU)


• American National Standards Institute (ANSI)


• Institute of Electrical and Electronics Engineers (IEEE)


• Telecommunications Industry Association/Electronic Industries Association (TIA/EIA)


• National telecommunications regulatory authorities including the Federal Communication Commission (FCC) in the USA and the European Telecommunications Standards Institute (ETSI)


• Canadian Standards Association (CSA)


• European Committee for Electrotechnical Standardization(CENELEC)

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Network Interface Card (NIC)

Network Interface Card (NIC) is also referred to as an Ethernet card. It also called network adapter. It is a circuit board that is installed in a computer so that it can be connected to a network.  A network interface card provides the computer with a dedicated, full-time connection to a network. Personal computers and workstations on a local area network typically contain a network interface card specifically designed for the LAN transmission technology. Most new computers have a network interface built-in directly into the motherboard

Network Interface Card enables a computer to connect to a network; such as a home network, or the Internet using an Ethernet cable with an RJ-45 connector and wires

Types of Network Interface

There is a different form of the network card. But the two main ones are wired and wireless. Wireless NICs Uses wireless technologies to connect the network, so they have one or more antennas. Wired NICs just use an RJ45 port since they have an Ethernet cable attached to the end. This makes them much flatter than wireless network cards.

Speed of Network Interface Card

All network interface cards have a different speed rating, such as 10 Mbps, 11 Mbps, 54 Mbps or 100 Mbps, and 1000 Mbps. The speed describes the general performance of the interface card.  It's important that the speed of the NIC does not necessarily determine the speed of the internet connection. This is due to reasons like available bandwidth and the speed you're paying for.For example, if your DSL speed is 16Mbps and your NIC Speed is 100 Mbps, So network interface card will not increase the speed of your internet.  but, if your DSL speed is 16 Mbps and your NIC is 10 Mbps, So your internet speed will also slow down to 10 Mbps.

Network Interface Card Driver

All hardware devices installed in computers need device drivers in order to work with the software on the computer. If installed network card isn't working, it's mean that driver is missing, corrupted or outdated. The updating of driver software required in internet connection in order to download the driver. But problem is that your network card is not working and you cannot access the internet. So you can download a driver on a computer that is connected to the internet from the card manufacturers site and then transfer using a USB drive or CD.

Device communication on a remote network

Here we will discuss the role of the network layer and data link layer when devices communicating on a remote network. Here is the example of PC1 which is communicating with a web server on a different network.

Role of the Network Layer Addresses

When the sender of the packet is on a different network from the receiver, the source and destination IP addresses will represent hosts on different networks. This will be indicated by the network portion of the IP address of the destination host.

Source IP address

The IP address of the sending device, the client computer PC1: 192.168.5.100.

Destination IP address

The IP address of the receiving device, the server, Web Server: 172.17.5.254.
Important is that the network of both sending and receiving end is different.

Role of the Data Link Layer Addresses

When the sender and receiver of the IP packet are on different networks, the Ethernet data link frame cannot be sent directly to the destination host because the host is not directly reachable in the network of the sender. In that case, the Ethernet frame must be sent to another device known as the router or default gateway. In the present example, the default gateway is R1. R1 has an Ethernet data link address that is on the same network as PC1. This allows PC1 to reach the router directly.

Source MAC address

Sending device MAC address which is PC1 - 22-22-22-22-22-22

Destination MAC address

When the receiving device, the destination IP address, is on a different network from the sending device, the sending device uses the Ethernet MAC address of the default gateway or router. In this example, the destination MAC address is the MAC address of R1's Ethernet interface, BB-BB-BB-BB-BB-BB. This is the interface that is attached to the same network as PC1.
The Ethernet frame with the encapsulated IP packet can now be transmitted to R1. R1 forwards the packet to the destination, Web Server. This may mean that R1 forwards the packet to another router or directly to Web Server if the destination is on a network connected to R1.
It is important that the IP address of the default gateway be configured on each host on the local network. All packets to a destination on remote networks are sent to the default gateway.

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Devices communication on the same network

To understand how devices communicate within a network. It is important to understand the roles of both the network layer addresses and the data link layer addresses.

Role of the Network Layer Addresses

The network layer addresses, or IP addresses, indicate the original source and final destination. An IP address contains two parts:

• 
  

  Network portion

  
  

The left-most part of the address that indicates which network the IP address is a member. All devices on the same network will have the same network portion of the address.

• 
  

  Host portion

  
  
  The remaining part of the address that identifies a specific device on the network. The host portion is unique for each device on the network.

Note: The subnet mask is used to identify the network portion of an address from the host portion.

In this example, we have a client computer PC1 FTP server on the same IP network.

• 
  

  Source IP address - The IP address of the sending device, the client computer PC1: 192.168.5.100.

  
  


• 
  

  Destination IP address - The IP address of the receiving device, FTP server: 192.168.5.102

  
  

Notice in the figure that the network portion of both the source IP address and destination IP address are on the same network.

Role of the Data Link Layer Addresses

When the sender and receiver of the IP packet are on the same network, the data link frame is sent directly to the receiving device. On an Ethernet network, the data link addresses are known as Ethernet (Media Access Control) addresses. MAC addresses are physically embedded on the Ethernet NIC.MAC Address also called Physical address.

• Source MAC address- This is the data link address, or the Ethernet MAC address, of the device that sends the data link frame with the encapsulated IP packet. The MAC address of the Ethernet NIC of PC1 is AA-AA-AA-AA-AA-AA, written in hexadecimal notation.


• Destination MAC address- When the receiving device is on the same network as the sending device; this is the data link address of the receiving device. In this example, the destination MAC address is the MAC address of the FTP server: CC-CC-CC-CC-CC-CC, written in hexadecimal notation.

The frame with the encapsulated IP packet can now be transmitted from PC1 directly to the FTP server.

 Figure 2.7 Device on the same network

Network Layer and Data link Layer Addresses

The network and data link layers are responsible for delivering the data from the source device to the destination device. As shown in Figure2-4; protocols at both layers contain a source and destination address, but their addresses have different purposes.

Network Layer Addresses

• 
  

  Network layer address (source and destination)

  
  
  Responsible for delivering the IP packet from the original source to the final destination; whichever on the same network or to a remote network.


• 
  

  Data link layer address (source and destination)

  
  
  Responsible for delivering the data link frame from one network interface card (NIC) to another NIC on the same network.

An IP address is the network layer, or Layer 3; logical address used to deliver the IP packet from the original source to the final destination; as shown in Figure 2-5.

The IP packet contains two IP addresses:

• Source IP address- The IP address of the sending device; the original source of the packet.


• Destination IP address- The IP address of the receiving device; the final destination of the packet.

Data Link Layer Addresses

The data link, or Layer 2, physical address(MAC Address) has a different role. The function of the data link address is to deliver the data link frame from one network interface; to another network interface on the same network. This process is illustrated in Figures 2.6

Before an IP packet to be sent over a wired or wireless network; it must be encapsulated in a data link frame so it can be transmitted over the physical medium.

As the IP packet travels from host to router; router to router, and finally router to host; at each point along the way the IP packet is encapsulated in a new data link frame. Each data link frame contains the source data link address of the NIC card sending the frame, and the destination data link address of the NIC card receiving the frame.

The Layer 2, data link protocol is only used to deliver the packet from NIC-to-NIC on the same network. The router removes the Layer 2 information as it is received on one NIC and adds new data link information before forwarding out the exit NIC on its way towards the final destination.

The IP packet is encapsulated in a data link frame that contains data link information, including a:

• 
  

  Source data link layer addresses

  
  
  The physical address of the device’s NIC that is sending the data link frame.


• 
  

  Destination data link layer addresses

  
  
  The physical address of the NIC that is receiving the data link frame. This address is either the next hop router or of the final destination device.

Data Encapsulation , De-encapsulation and Protocol Data Units (PDU),

What is Encapsulation 

Commonly, encapsulation is used for the inclusion of one thing within another thing so that the included thing is not apparent. In networking, encapsulation is the process which includes one data structure within another data structure as a result that the first data structure is hidden for the time being.

As application layer data is passed down the protocol stack on its way to being transmitted across the network media; various protocol information is added at each level and the original data structure is changed. This is known as the encapsulation process.

What is De-encapsulation

Decapsulation is the removal or the making apparent a thing previously encapsulated during the encapsulation process. This process is reversed at the receiving host and is known as de-encapsulation. It is the process used by a receiving device to remove one or more of the protocol headers. The data is de-encapsulated as it moves up the stack toward the end-user application.

Protocol Data Unit (PDU),

The form that a piece of data takes at any layer is called a protocol data unit (PDU). During encapsulation; each succeeding layer encapsulates the PDU; that it receives from the layer above in accordance with the protocol being used. At each stage of the process; a PDU; has a different name to reflect its new functions. The PDUs are named according to the protocols of the TCP/IP suite, as shown below. Figure 2.3 Shows the protocol data unit(PDU) at different layers.

Data - The general term for the PDU used at the application layer and upper layers of the OSI Model.

Segment – This is the transport layer PDU.

Packet – This is the Network layer PDU.

Frame – This is Data Link layer PDU.

Bits - A Physical layer PDU is used when physically transmitting data over the medium.

Figure 2-3 PDU- PDUs at different layers

TCP/IP Communication Process

Figures 2-2 show the complete communication process using an example of a web server transmitting data to a client. This process and these protocols will be covered in more detail in later chapters.

1. Web server is sending data using Hypertext Markup Language (HTML) page to be sent to web clients.

2. The application protocol HTTP header is added to the front of the HTML data. The header contains a variety of information just like IP header, Ethernet header, TCP header as well as the HTTP version the server is using and a status code representing it has information for the web client.

3. The HTTP application layer protocol forwards the HTML-formatted web page data to the transport layer (Layer 4 of the OSI Model). The TCP transport layer protocol is used to handle individual conversations; in this example between the web server and web client.

4. Next, the IP information is added to the front of the  TCP information. IP assigns the appropriate source and destination IP addresses. This information is known as an IP packet.

5. The Ethernet protocol adds information to both ends of the IP packet; known as a data link frame. This frame is delivered to the nearest router along the path towards the web client. This router removes the Ethernet information; analyzes the IP packet; determines the best path for the packet; inserts the packet into a new frame; and sends it to the next neighboring router towards the destination. Each router removes and adds new data link information before forwarding the packet.

6. This data is now transported through the internetwork; which consists of media and intermediary devices.

7. At the receiving end, the client receiving the data link frames that contain the data. Each protocol header is processed and then removed in the opposite order it was added. After removing all headers the user received and see the original data.

OSI Model and TCP/IP Model Comparison

Comparison

The protocols that make up the TCP/IP protocol suite can also be described in terms of the OSI reference model. Network access layer and Application layer of TCP/IP model are further divided In the OSI model describe discrete functions that must occur at these layers.

Network Access layer of TCP/IP model does not specify which protocols to use when transmitting over a physical medium. It only describes the handover from the internet layer to physical layer. On the other hand, OSI Layer physical and data link layer discuss the basic process to access the media and the physical means to send data over a network.

OSI Layer 3

The network layer maps directly to the TCP/IP Internet layer. This layer is used to explain protocols that address and route messages from end to end an internetwork.

OSI Layer 4

The transport layer maps directly to the TCP/IP Transport layer. This layer explains general services and functions that provide ordered and reliable delivery of data between source and destination hosts.

The application layer of the TCP/IP model includes a number of protocols that provide specific functionality to a variety of end-user applications. The OSI Model Layers 5, 6, and 7 are used as references for application software developers and vendors to produce products that operate on networks.

Both the TCP/IP and OSI models are commonly used when referring to protocols at various layers. Because the OSI model separates the data link layer from the physical layer, it is commonly used when referring to these lower layers.

OSI is a generic, protocol-independent standard, acting as a communication gateway between the network and end user.TCP/IP model is based on standard protocols around which the Internet has developed. It is a communication protocol, which allows connection of hosts over a network.

OSI is a reference model around which the networks are built. Generally, it is used as a guidance tool.TCP/IP model is, in a way implementation of the OSI model.

OSI has 7 layers and TCP has 4 Layer model. following is the comparison model of both layer.

 

What is a computer

A computer is a device that accepts information and manipulates it for some result based on a program or sequence of instructions on how the data is to be processed. It has also the ability to retrieve and store data.

A computer is a machine, It is also called a PC. it looks like a television. It can show pictures like a TV. Can your TV do any homework for you? NO! but a computer can.

You want to do sums on this machine, you can do so, if your mummy wants to keep information about her monthly expenses, she can also do it, using this machine, IF your Daddy wants to do some office work , he can do it sitting at home just by connecting his PC at home to the PC in his office so does it mean that there is some kind of magic involved in machine for doing such job ? no, not at all, ALL these are the abilities that are built into the computer at the time of its manufacturing.

Types of Computer

Most of the people understand that computer is only in the shape of a personal computer such as a desktop or laptop. but, computers come in many shapes and sizes, and they also perform different functions. When you withdraw cash from an ATM, scan groceries at the store, or use a calculator, all are types of computers. Smart Phone, Smart TV, Game Console and many others are the types of computers. Personal computers come in two different styles,  PCs and Mac. Both are fully functional, but they have a different look and feel.

The PCs begin with the original IBM PC that was introduced in 1981. Other companies began creating similar computers, which were called IBM PC Compatible Today, this is the most common type of personal computer, and it typically includes the Microsoft Windows operating system.

The Macintosh was introduced in 1984, and it was the first widely-sold personal computer with a graphical user interface. All Macs are made by one company (Apple), and they almost always use the Mac OS X operating system.

Desktop Computer

Most of the people used desktop computers in the workplace, home, and school. Desktop computers are designed to be placed on a desk. The desktop computer is made up of different parts, that including the casing, monitor, keyboard, and mouse. The desktop computer is available, one is called desktop and the other is called the tower.

Laptop

The second type of computer, most of the people are familiar with the laptop. The Laptops are battery-powered computers that are more portable than desktops, allowing you to use them almost anywhere.

Tablets

The handheld computers that are even more portable than laptops. Instead of a keyboard and mouse, tablets use a touch-sensitive screen for typing and navigation.

Servers

A server is a computer that provides services to another computer on the same network or remote networks. The computer that a server program runs in is also frequently referred to as a server. That machine may be a dedicated server or used for other purposes as well.

 

Internet Protocol (IP)

Each host in a network and all interfaces of a router have a logical address called the IP address. All hosts in a network are grouped in a single IP address range which is called its net or subnet and each host having a unique address from that range. Each network has a different address range and routers that operate on layer 3 connect these different networks.

As IP receives segments from TCP or UDP, it adds a header with source IP address and destination IP address amongst other information. This PDU is called a packet. When a router receives a packet, it looks at the destination address in the header and forwards it towards the destination network. The packet may need to go through multiple routers before it reaches the destination network. Each router it has to go through is called a hop.

The IP address fields in the IP header play a very important role in sending a data through different hops. In fact, IP addresses are so important in a network that the next Chapter is entirely dedicated to IP Address. Following is IPv4 header and its important fields:-

 

                Figure 1-12 IPv4 Header

 

There are two versions of the Internet Protocol(Version 6 and Version 4). Version 4 is the most used version in today networking. Figure 1-12 shows the header structure of IPv4. The following fields make up the header:-

Version 

IP version number. For IPv4 this value is 4.

Header Length

This specifies the size of the header itself. The minimum size is 20 bytes. The figure does not show the rarely used options field that is of a variable length. Most IPv4 headers are 20 bytes in length.

DS Field

The Differentiated Services field is used for marking packets. Different Quality-Of-Service (QoS) levels can be applied to different markings. For example, data belonging to voice and video protocols have no tolerance for delay. The DS field is used to mark packets carrying data belonging to these protocols so that they get priority treatment through the network. On the other hand, peer-to-peer traffic is considered a major problem and can be marked down to give in best effort treatment.

Total Length

This field specifies the size of the packet. This means the size of the header plus the size of the data.

Identification

When IP receives a segment from TCP or UDP; it may need to break the segment into chunks called fragments before sending it out to the network. Identification fields serve to identify the fragments that make up the original segment. Each fragment of a segment will have the same identification number.

Flags

Used for fragmentation process.

Fragment Offset

This field identifies the fragment number and is used by hosts to reassemble the fragments in the correct order.

Time to Live

The Time to Live (TTL) value is set to the originating host. Each router that the packet passes through reduces the TTL by one. If the TTL reaches 0 before reaching the destination, the packet is dropped. This is done to prevent the packet from moving around the network endlessly.

Protocol

This field identifies the protocol to which the data it is carrying belongs. For example, a value of 6 implies that the data contains a TCP segment while a value of 17 signifies a UDP segment. Apart from TCP and UDP, there are many protocols whose data can be carried in an IP packet.

Header Checksum

This field is used to check for errors in the header. At each router and at the destination, a cyclic redundancy check performed on the header and the result should match the value stored in this field. If the value does not match, the packet is discarded.

Source IP address – This field stores the IP address of the source of the packet.

Destination IP address – This field stores the IP address of the destination of the packet.

Transmission control protocol(TCP)

Transmission Control Protocol (TCP)

TCP is basically designed to the TCP/IP suite and hence the name of the model. When application layer sending a large amount of data, it sends the data to transport layer for TCP or UDP to transport it across the network. TCP first sets up a connection between the source and the destination in a process called three-way handshake. Then it breaks down the data into segments, adds a header to each segment and sends them to the Internet layer.

The TCP header is 20 to 24 bytes in size and the format is shown in Figure 1-9. It is not necessary to remember all fields or their size but most of the fields are discussed below.

             Figure 1-9 TCP header

When the Application layer sends data to the transport layer, TCP sends the data across using the following sequence:

Connection Establishment

TCP uses a process called three-way handshake to establish a connection with the destination. The three-way handshake uses the SYN and ACK flags in the Code Bits section of the header. This process is necessary to initialize the sequence and acknowledgment number fields. These fields are important for TCP and will be discussed in the following. 

 Figure 1-10 TCP three-way handshake

As shown in Figure 1-10, the source starts the three-way handshake by sending a TCP header to the destination with the SYN flag set. The destination responds back with the SYN and ACK flag sent. Examine in the figure that destination uses the received sequence number plus 1 as the Acknowledgement number. This is because it is assumed that 1 byte of data was contained in the exchange. In the final step, the source responds back with only the ACK bit set. After this, the data flow can commence.

Data Segmentation

The size of data that can be transmitted across in a single Internet layer PDU is limited by the protocol used in that layer. This limit is called the maximum transmission unit (MTU). The application layer may send data much larger than this limit; hence TCP has to break down the data into smaller chunks called segments. Each segment is limited to the MTU in size. Sequence numbers are used to identify each byte of data. The sequence number in each header signifies the byte number of the first byte in that segment.

Flow Control

The source starts sending data in groups of segments. The Window bit in the header determines the number of segments that can be sent at a time. This is done to avoid irresistible the destination. At the start of the session the window in small but it increases over time. The destination host can also decrease the window to slow down the flow. Hence the window is called the sliding window. When the source has sent the number of segments allowed by the window, it cannot send any further segments till an acknowledgment is received from the destination. Figure 1-11 shows how the window increases during the session. Notice the Destination host increasing the Window from 1000 to 1100 and then to 1200 when it sends an ACK back to the source.

Figure 1-11 TCP Sliding Window and Reliable delivery

 Reliable Delivery with Error recovery

 When the destination receives the last segment in the agreed window, it has to send an acknowledgment to the source. It sets the ACK flag in the header and the acknowledgment number is set to the sequence number of the next byte expected. If the destination does not receive a segment, it does not send an acknowledgment back. This tells the source that some segments have been lost and it will retransmit the segments. Figure 1-13 shows how windowing and acknowledgment are used by TCP. Notice that when the source does not receive acknowledgment for the segment with sequence number 2000, it retransmits the data. Once it receives the acknowledgment, it sends the next sequence according to the window size.

 Ordered Delivery

TCP transmits data in the order it is received from the application layer and uses the sequence number to mark the order. The data may be received at the destination in the wrong order due to network conditions. Thus TCP at the destination orders the data according to the sequence number before sending it to the application layer at its end. This order delivery is part of the benefit of TCP and one of the purposes of the Sequence Number.

Connection Termination

After all, data has been transferred, the source initiates a four-way handshake to close the session. To close the session, the FIN and ACK flags are used.

Was this lawyer-turned-WWII-spy the basis for James Bond?

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On the other hand, we denounce with righteous indignation and dislike men who are so beguiled and demoralized by the charms of pleasure of the moment, so blinded by desire, that they cannot foresee the pain and trouble that are bound to ensue; and equal blame belongs to those who fail in their duty through weakness of will, which is the same as saying through shrinking from toil and pain. These cases are perfectly simple and easy to distinguish. In a free hour, when our power of choice is untrammelled and when nothing prevents our being able to do what we like best, every pleasure is to be welcomed and every pain avoided. But in certain circumstances and owing to the claims of duty or the obligations of business it will frequently occur that pleasures have to be repudiated and annoyances accepted. The wise man therefore always holds in these matters to this principle of selection: he rejects pleasures to secure other greater pleasures, or else he endures pains to avoid worse pains.

Well Known Port Numbers

Port Numbers

A host in a network can send traffic to or receive traffic from different hosts at the same time. The system has no way to know which data belongs to which application. TCP and UDP solve this problem by using port numbers in their header. Common application layer protocols have been assigned port numbers in the range of 1 to 1024.TCP and UDP on the receiving host know which application to send the data to based on the port numbers received in the headers.

On the source host, each TCP or UDP session is assigned a random port number above the range of 1024. So that returning traffic from the destination can be identified as belonging to the originating application. A combination of the IP address, Protocol (TCP or UDP) and the Port number forms a socket at both the sending and receiving hosts. in view of the fact that each socket is unique, an application can send and receive data to and from multiple hosts.

Figure 1-8 shows two hosts communicating using TCP to a single host. Notice that the hosts A and host C are sending traffic to host B. Both A and C are sending traffic destined to Port 80 but from different source ports. Host B is able to handle both the connections at the same time because of the combination of IP address, Port numbers and Protocols makes each connection different.
Figure 1-8 Multiple Sessions using Port Numbers

Following table shows the transport layer protocol and port numbers used by different common application layer protocols.

Well-known Port Numbers

POP (Post Office Protocol) is an Internet standard that defines an email server (the POP server) and a way to retrieve mail from it (using a POP client).

 

Application Protocol	Transport Protocol	Port Number	Description
HTTP	TCP	80	HTTP is the most commonly used protocols over the Internet and private networks. HTTP is the main protocol that is used by all web browsers and is thus used by any client that uses files located on these servers.
FTP (control)	TCP	21	FTP is one of the protocols which is regularly used for the file transfer on the Internet and within private networks. An FTP server provides the ability to easily transfer files from one system to another. With little networking knowledge, anybody can set up an FTP server. FTP control is handled on TCP port 21 and its data transfer can use TCP port 20 as well as dynamic ports depending on the specific configuration.
FTP (data)	TCP	20
SSH	TCP	22	SSH is used to connect and manage network device remotely. It is typically used as a secure substitute to Telnet which does not support secure connections.
Telnet	TCP	23	Telnet is the primary method used to manage network devices remotely. Like SSH it does not provide a secure connection, it simply provides a basic unsecured connection. Many lower level network devices support Telnet and not SSH as it required some additional processing. Caution is important when connecting to a device using Telnet over a public network as the login credentials will be transmitted in the clear.
DNS	TCP, UDP	53	The DNS is used to translate domain names into IP addresses, typically it is used for network routing. It converts the alphabetic names into numeric IP addresses. For example, when a Web address (URL) is typed into a browser, DNS servers return the IP address of the Web server associated with that name.
SMTP	TCP	25	SMTP has two primary functions, transfer mail (email) from source to destination between mail servers and end users email to a mail system.
DHCP	UDP	67/68	DHCP is used on networks for assigning IP address automatically. A DHCP server can be set up by an administrator or engineer with a pool of IP addresses that are available. When a client device is turned on it will send a request for assigning an IP address to the local DHCP server, the local server then assigns an IP address to a client device. This assignment is not on a permanent basis, all IP addresses are assigned on a lease basis. If an address renewal is not requested and the lease expires the address will be put back into the poll for assignment.
TFTP	UDP	69	TFTP offers a method of file transfer without the session establishment. It is used where user authentication and directory visibility are not required. TFTP is used by devices to upgrade software and firmware, this includes Cisco and other network vendors’ equipment.
POP2	TCP, UDP	109	Post Office Protocol is an Internet standard that defines an email server (the POP server) and a way to retrieve mail from it. POP has three version POP1, POP2, and POP3. POP was designed to allowing a client to retrieve the complete contents of a server mailbox and then deleting the contents from the server by a simple way.
POP3	TCP, UDP	110

 

PERSONAL PROTECTIVE EQUIPMENT'S ( PPE)

 Foot and leg protection

1. 
   

   There are many types of foot and leg protection. Often more than one type is worn by many workers within one organization or department. Some examples include.

   
   

Safety boots or shoes

• Clog,


• Foundry boots.


• Anti-static footwear


• Conductive footwear


• Gaiters


• Ballistic trousers (chainsaw use)

Safety footwear can possess features such as:

• Steel toe-caps.


• Midsole protection.


• Slip resistance for a variety of situations.


• Insulation against wet, extreme heat.


• Good insulation against heat.


• Shock absorbers.

Typical hazards posed to feet and legs include:

• 
  

  Wet and cold.

  
  


• 
  

  Chemicals/ molten metal

  
  


• 
  

  Impact,

  
  


• 
  

  Slipping.

  
  


• 
  

  cuts.

  
  


• 
  

  Puncture,

  
  


• 
  

  Electrostatic buildup.

  
  


• 
  

  Hand and arm protection

  
  

Protection of the hand and arms can be achieved using.

• 
  

  Chemical protection gloves of various grades and materials.

  
  


• 
  

  Contact/ gripper gloves.

  
  


• 
  

  Wrist cuffs.

  
  


• 
  

  wrist cuffs,

  
  


• 
  

  contact/gripper gloves

  
  


• 
  

  mitten

  
  

Typical hazards include:

• 
  

  chemical

  
  


• 
  

  impact

  
  


• 
  

  Contamination and diseases.

  
  


• 
  

  Puncture/ cuts.

  
  


• 
  

  Extremes of temperature

  
  


• 
  

  Abrasions

  
  

Selecting suitable hand protection

Specialist advice should be sought from the manufacturer or other competent people when selecting gloves to form the wide range available. There are two main points to consider.

• That they give adequate protection against a hazard.


• That they fit well and are comfortable to wear.

https://www.britsafe.org/training/health-safety-and-environmental-management-training-courses

Note – If you think your friends would find this useful, Please share it with them I’d really appreciate it.

Common Protocol in Networking

What are protocols?

When two humans make conversation with each other, they may have to use the same language but they generally understand each other without having to adhere to rigid rules of grammar or formal language frameworks. Computers, on the other hand, have everything openly defined and structured. If computers wish to communicate with one another, they have to know in advance exactly how information is to be exchanged and precisely what the format will be. Therefore, standard methods of transmitting and processing various kinds of information are used and these methods are called "protocols". Protocols are established by international agreement and ensure that computers everywhere can talk to one another. There are many protocols for different kinds of information and functions. This article will discuss some of the common protocols.

Telnet 

Telnet is a terminal protocol used to access the resources of a remote host. A host, called the Telnet server, runs a telnet server application (or daemon in Unix terms) that receives a connection from a remote host called the Telnet client. This connection is presented to the operating system of the telnet server as though it is a terminal connection connected directly (using keyboard and mouse). It is a text-based connection and usually provides access to the command line interface of the host. Remember that the application used by the client is usually named telnet also in most operating systems. You should not confuse the telnet application with the Telnet protocol. Telnet is application layer protocol.

HTTP 

HTTP is also in application layer protocol. It is the foundation of the World Wide Web. It is used to transfer Web pages and such resources from the Web Server or HTTP server to the Web Client or the HTTP client. When you use a web browser such as Internet Explorer, Google Chrome or Firefox, you are using a web client. It uses HTTP to transfer web pages that you request from the remote servers.

File Transfer Protocol (FTP)

File Transfer Protocol (FTP) lives up to its name and provides a method for copying files over a network from one computer to another. More generally, it provides for some simple file management on the contents of a remote computer. It is an old protocol and is used less than it was before the World Wide Web came along. Today, Its primary use is uploading files to a Web site. It can also be used for downloading from the Web but, more often than not, downloading is done via HTTP. Sites that have a lot of downloading (software sites, for example) will often have an FTP server to handle the traffic. If FTP is involved, the URL will have FTP: at the front.

SMTP

Simple Mail Transfer Protocol is used to send e-mails. When you configure an email client to send emails you are using SMTP. The mail client acts as an SMTP client here. SMTP is also used between two emails servers to send and receive emails. However, the end client does not receive emails using SMTP. The end clients use the POP3 protocol to do that.

TFTP

Trivial File Transfer Protocol is a stripped down version of FTP. Where FTP allows a user to see a directory listing and perform some directory related functions, TFTP only allows sending and receiving of files. It is a small and fast protocol, but it does not support authentication. Because of this inherent security risk, it is not widely used.

DNS

Every host in a network has a logical address called the IP address. These addresses are a bunch of numbers. When you go to a website such as www.cisco.com you are actually going to a host which has an IP address, but you do not have to remember the IP Address of every WebSite. This is because Domain Name Service (DNS) helps map a name such as www.google.com to the IP address of the host where the site resides. This obviously makes it easier to find resources on a network. When you type in the address of a website in your browser, the system first sends out a DNS query to its DNS server to resolve the name to an IP address. Once the name is resolved, an HTTP session is established with the IP Address.

DHCP

As you know, every host requires a logical address such as an IP address to communicate in a network. The host gets this logical address either by manual configuration or by a protocol such as Dynamic Host Configuration Protocol (DHCP). Using DHCP, a host can be provided with an IP address automatically. To understand the importance of DHCP, imagine having to manage 5000 hosts in a network and assigning them IP address manually! Apart from the IP address, a host needs other information such as the address of the DNS server it needs to contact to resolve names, gateways, subnet masks, etc. DHCP can be used to provide all these information along with the IP address.

TCP

TCP will be discussed in detail in next lesson.

IP

IP(Internet protocol) will be discussed in coming lesson and also be discussed in its own chapter in detail.

UDP

User Datagram Protocol (UDP) is used together with IP when small amounts of information are involved. It is simpler than TCP and lacks the flow-control and error-recovery functions of TCP. Thus, it uses fewer system resources. Different between UDP and TCP is that UDP is sending data without ack but TCP is ever required ack. 

ICMP

A different type of protocol is Internet Control Message Protocol (ICMP). It defines a small number of messages used for diagnostic and management purposes. It is also used by Ping and Traceroute.

personal protective equipments (PPE)

Definition:

PPE includes all equipment designed to be worn or held by a person to protect against one or more risks. Some of the common examples of PPE are

• Apron 


• clothing for adverse weather conditions


• gloves


• safety footwear 


• safety helmets


• eye protectors


• breathing apparatus


• safety harnesses

It is good practice for a company to eliminate or reduce the risk by some other means before resorting to PPE. where there are no other effective means of protecting worker the provision and use of PPE vital. The step must be taken to control risk at their source to prevent the need for PPE, since:

PPE as a Last Report

its good practice advises employers to:

• identify and assess the risk


• ensure the most appropriate means of reducing risk at an acceptable level


• consider the hierarchy of risk control measure before providing PPE to the worker.

Personal protective equipment

EYE PROTECTION:

The eye is particularly vulnerable to injury from impact of foreign bodies and, as the are also the only son entrance to the brain, the use of suitable eye protection may not merely save a person`s sight but also their life.

typical hazards to the eye include:

• projectiles


• foreign bodies e.g rust and dust


• chemicals/ molten metal splashes


• gas and vapors


• radiation

it is possible to classify eye protection into four main groups,

• safety spectacles 


• eye-shields.


• safety goggles


• face shields (visors)

safety spectacles:

Spectacles are made up in some countries, (mainly Europe) to meet the requirements of certain standards and are standards and generally tested to low energy impact resistance level ( denoted by the letter F marked on the lens.

For those employees who normally wear prescription spectacles safety version are available. these are tested to increased robustness level only. CE mark must be present on the frame and lens.

Eye shields:

Shields is similar to safety spectacles nut are heavier and designed either a frameless one-piece molded lens.

Safety Goggles:

Basically, there are two styles of the gaggle, panoramic with curved lens and a wide field of vision and the more traditional box type.

• impact.


• chemical splash


• dusts


• gases

Face shields:

 Shields with varying degrees of green filter are also available for welding 

these provide protection against ultra-violet and strong light.

Georgia lawyer charged with involuntary manslaughter in the shooting death of his wife

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Internetworking Models - OSI and TCP/IP

As the importance of computers grows, vendors recognized the require for networking them. They produced a variety of protocols whose specifications were not made public. Hence each vendor had different ways of networking computers and these ways were not compatible with each other. This means that computers of one vendor could not be networked with another vendor’s computers. Slowly these specifications were made public and some inter-vendor compatibility was created but this still represented too many complications.

Open System Interconnection(OSI) Model

In 1977 the International Organization for Standardization (ISO) started working on an open standard networking model that all vendors would support to promote interoperability. This standard was published in 1984 and was known as the Open Systems Interconnection (OSI) Model.

The OSI reference model was created to support communication between devices of various vendors. It also promotes communication between disparate hosts such as hosts using different operating platforms. Keep in mind that you are very unlikely to ever work on a system that uses protocols conforming to the OSI reference model.  But it is essential to know the model and its terminology because other models such as the TCP/IP model are often compared to the OSI reference model. Hence the discussion on this model will be limited compared to the discussion on the TCP/IP model.

The OSI reference model, like most other network models, divides the functions, protocols; and devices of a network into various layers. The OSI reference model has seven such layers that can be divided into two groups. The upper layers (Layers 7, 6 and 5) define how applications interact with the host interface, with each other, and the user. The lower four layers (Layers 4, 3, 2 and 1) define how data is transmitted between hosts in a network. Figure 1-6 shows the seven layers and a summary of their functions.

The layered approach provides many benefits, some of which are:

•  Communication is divided into smaller and simpler components


• Since it is a layered approach, the vendors write to a common input and output specification per layer.  The guts of their products function in between the input and output code of that layer.


• Changes in one layer do not affect other layers. Hence development in one layer is not bound by limitations of other layers. For example, wireless technologies are new but old applications run seamlessly over them without any changes.


• It is easier to normalize functions when they are divided into smaller parts like this.

It allows various types of hardware and software, both new and old to communicate with each other seamlessly

The following section describes the 7 layers in detail.

Application Layer

This Layer provides the interface between the software application on a system and the network. Remember that this layer does not include the application itself, but provides services that an application requires. One of the easiest ways to understand this layer’s function is to look at how a Web Browser such as Internet Explorer or Firefox works is the application. When it needs to fetch a web page, it uses the HTTP protocol to send the request and receive the page contents.  This protocol resides at the application layer and can be used by an application such as IE or FF to get web pages from web servers across the network. On the other side, the web server application such as Apache or IIS interacts with the HTTP protocol on the Application layer to receive the HTTP request and send the response back.

Presentation Layer

This layer presents data to the Application layer. The Presentation Layer is also responsible for data translation and encoding. It will take the data from the Application layer and translate it into a generic format for transfer across the network. At the receiving end, the Presentation layer takes in generically formatted data and translates into the format recognized by the Application layer. An example of this is a JPEG to ASCII translation. The OSI model has protocol standards that define how data should be formatted. This layer is also involved in data compression, decompression, encryption, and decryption.

Session Layer

In a host, different applications or even different instances of the same application might request data from across the network. It is the Sessions layer’s responsibility to keep the data from each session separate. It is responsible for setting up, managing and tearing down sessions. its also provides dialog control and coordinates communication between the systems.

Transport Layer

Where the upper layers are related to applications and data within the host, the transport layer is concerned with the actual end-to-end transfer of the data across the network. This layer establishes a logical connection between the two communicating hosts and provides reliable or unreliable data delivery and can provide flow control and error recovery. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). These protocols will be discussed in great detail later in this chapter.

Network Layer

To best understand what the Network layer does, consider what happens when you write a letter and use the postal service to send the letter. You put the letter in an envelope and write the destination address as well as your own address so that an undelivered letter can be returned back to you.

In network terms, this address is called a logical address and is unique in the network. Each host has a logical address. When the post office receives this letter. it has to ascertain the best path for this letter to reach the destination. Similarly, in a network, a router needs to determine the best path to a destination address. This is called path determination. Finally, the post office sends the letter out the best path and it moves from the post office to post office before finally being delivered to the destination address. Similarly, data is moved across network mainly by routers before being finally delivered to the destination.

All these three functions – logical addressing, path determination and forwarding – are done at the Network Layer. Two types of protocols are used for these functions – routed protocols are used for logical addressing and forwarding while routing protocols are used for path determinations. There are many routed protocols and routing protocols available. Some of the common ones are discussed in great detail later in the book. Routers function in this layer. Remember that routers only care about the destination network. They do not care about the destination host itself. The task of delivery to the destination host lies on the Data Link Layer.

Data Link Layer

The Network layer deals with data moving across networks using logical addresses. On the other hand, the Data Link layer deals with data moving within a local network using physical addresses. Each host has a logical address and a physical address. The physical address is only locally significant and is not used beyond the network boundaries (across a router). This layer also defines protocols that are used to send and receive data across the media. You will remember from earlier in the chapter that only a single host can send data at a time in a collision domain or else packets will collide and cause a host to back off for some time. The Data Link layer determines when the media is ready for the host to send the data and also detects collisions and other errors in received data. Switches function in this layer.

Physical Layer

This layer deals with the physical transmission medium itself. It activates, maintains and deactivates the physical link between systems (host and switch for example). This is where the connectors, pinouts, cables, electrical currents etc. are defined. Essentially this layer puts the data on the physical media as bits and receives it in the same way. Hubs work at this layer.

Data Encapsulation

In the previous sections, you learned about various layers of the OSI reference model. Each layer has its distinct function and it interacts with the corresponding layer at the remote end. For example, the transport layer at the source will interact with the transport layer of the destination. For this interaction, each layer adds a header in front of the data from the previous layer. This header contains control information related to the protocol being used at that layer. This process is called encapsulation. This header and the data being sent from one layer to the next lower layer is called a Protocol Data Unit (PDU). Figure 1-7 shows how data gets encapsulated as it travels from layer 7 down to layer 1.

Data Encapsulation Process

As shown in Figure 1-7, The Application layer adds its protocol dependent header to the data and creates the Layer 7 PDU. Which is then passed down to the Presentation Layer. This layer then adds its header to the Layer 7 PDU to create the Layer 6 PDU and sends it down to the Session layer. This goes on till Layer 2 receives the Layer 3 PDU. Layer 2 adds a header and a trailer to the Layer 3 PDU to create the Layer 2 PDU that is then sent to Layer 1 for transmission. Data transmission in electrical signals it’s just 0s and 1s 

At the receiving end, Layer 1 takes the data off the wire and sends it to Layer 2. Here the Layer 2 header and trailer are examined and removed. The resulting Layer 3 PDU is sent to Layer 3. Layer 3, in turn, examines the header in the PDU and removes it. The resulting Layer 4 PDU is sent to Layer 4. Similarly, each layer removes the header added by the corresponding layer at the source before sending the data to the upper layer. Finally, the Application layer removes the Layer 7 header and sends the data to the application. This process of examining, processing and removing the header is known as decapsulation.

TCP/IP Model

During the same time period (1973 to 1985) another effort by the Defense Advanced Research Projects Agency (DARPA) was underway to create an open standard network model. This network model came to be known as the TCP/IP Model. By 1985, the TCP/IP model started gaining more importance and support from vendors and ultimately replaced the OSI model.

 

The TCP/IP model was on the path of development when the OSI standard was published. The TCP/IP model is not same as OSI model. OSI is a seven-layered standard, but TCP/IP is a four-layered standard. The OSI model has been very important in the growth and development of TCP/IP standard, and that is why much OSI terminology is applied to TCP/IP. Both models are open standard networking models.  However, the TCP/IP model has found more acceptance today and the TCP/IP protocol suite is more commonly used. Just like the OSI reference model, the TCP/IP model takes a layered approach. In this section, we will look at all the layers of the TCP/IP model and various protocols used in those layers.

The TCP/IP model is a reduced version of the OSI reference model consisting of the following 4 layers:

• Application Layer


• Transport Layer


• Internet Layer


• Network Access Layer

 The functions of these four layers are comparable to the functions of the seven layers of the OSI model. Figure 1-7 shows the comparison between the layers of the two models.

The following sections discuss each of the four layers and protocols in those layers in detail.

Figure 1-7 Comparison of TCP/IP and OSI models

As we can see from the above figure, presentation and session layers are not there in TCP/IP model. Also, note that the Network Access Layer in TCP/IP model combines the functions of Datalink Layer and Physical Layer.

Application Layer of TCP/IP model

The application layer is the topmost layer of TCP/IP model. The application layer is present on the top of the Transport layer. Application layer defines TCP/IP application protocols and how host programs interface with Transport layer services to use the network.

The Application Layer of the TCP/IP Model consists of various protocols that perform all the functions of the OSI model’s Application, Presentation, and Session layers. This includes interaction with the application, data translation and encoding, dialogue control and communication coordination between systems.

Application layer includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol),SMTP (Simple Mail Transfer Protocol) , DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.

Transport Layer of TCP/IP model

Transport Layer is the third layer of the TCP/IP model. The position of the Transport layer is between the Application layer and Internet layer. The purpose of Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data. The main protocols included at Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

TCP/IP transport layer’s function is same as the OSI layer’s transport layer. It is concerned with end-to-end transportation of data and setups up a logical connection between the hosts.

TCP is a connection-oriented and reliable protocol that uses windowing to control the flow and provides ordered delivery of the data in segments. On the other hand, UDP simply transfers the data without the bells and whistles. Though these two protocols are different in many ways, they perform the same function of transferring data and they use a concept called port numbers to do this. Port number will be discussed in next article.

Internet Layer of TCP/IP model

Once TCP and UDP have segmented the data and have added their headers, they send the segment down to the Network layer. The destination host may reside in a different network far from the host divided by multiple routers. It is the task of the Internet Layer to ensure that the segment is moved across the networks to the destination network

Internet layer pack data into data packets known as IP datagrams, which contain source and destination address (logical address or IP address) information that is used to forward the datagrams between hosts and across networks. The Internet layer is also responsible for the routing of  IP datagrams.

Packet switching network depends on a connectionless internetwork layer. This layer is known as Internet layer. Its job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. At the destination side data, packets may appear in a different order than they were sent. It is the job of the higher layers to rearrange them in order to deliver them to proper network applications operating at the Application layer.

The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol).

The Internet layer of the TCP/IP model corresponds to the Network layer of the OSI reference model in function. It provides logical addressing, path determination, and forwarding.

Network Access Layer of TCP/IP model

The Network Access Layer is the first layer of the TCP/IP model. Network Access Layer defines details of how data is physically sent through the network. It is already including how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire. The protocols of  Network Access Layer are Ethernet, Token Ring, FDDI, X.25, Frame Relay etc.

LAN architecture is the most popular among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media when Ethernet operates in a shared media.

IN CSMA/CD Access Method, every host has equal access to the medium and can place data on the wire when the wire is free from traffic or in the idle position. When a host wants to place data on the wire. It will check the wire to find out whether another host is already using the medium. If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance; they will collide with each other, destroying the data. If the data is destroyed during transmission. This data will need to be retransmitted. After the collision, each host will wait for a small interval of time and again the data will be retransmitted.