DCN_Ch_01 - Network Models_Sep 05,2023.pdf

Data Communications & Networking Introduction to Data Communications Dr. Ramakrishna Dantu College of Business Network Models The OSI Model Layer 4: Transport Layer • The transport layer deals with endto-end issues  That is, procedures for entering and departing from the network • Transport la...

Data Communications & Networking Introduction to Data Communications Dr. Ramakrishna Dantu College of Business Network Models The OSI Model Layer 4: Transport Layer • The transport layer deals with endto-end issues  That is, procedures for entering and departing from the network • Transport layer controls the reliability of communication through  Segmentation  Flow Control  Error Control The OSI Model Layer 4: Transport Layer • Transport layer is responsible for:  Segmentation  breaking a large data into smaller packets (if needed),  Flow control  performing flow control to ensure that no computer is overwhelmed by the number of messages it receives  Error control  ensuring that all the packets have been received,  eliminating duplicate packets The OSI Model Layer 4: Transport Layer Segmentation The OSI Model Layer 4: Transport Layer • Flow Control The OSI Model Layer 4: Transport Layer • Error control The OSI Model Layer 4: Transport Layer • Protocols and Services  Transport layer protocols are  Transmission Control Protocol (TCP)  User Datagram Protocol (UDP)  Transport layer performs two types of transmission services  Connection-oriented Transmission o Done using TCP  Connectionless Transmission o Done using UDP The OSI Model Layer 4: Transport Layer • UDP Vs. TCP  UDP is faster than TCP, because  UDP does not provide any feedback  TCP provides feedback so that lost data can be retransmitted  UDP is used where it doesn't matter if we received all data  E.g., Online video streaming, Songs, Games, VOIP  TCP is used where full data delivery is must  E.g., WWW, Email, FTP, etc., The OSI Model Layer 3: Network Layer • Key functions of network layer  Logical addressing  Routing  Path determination The OSI Model Layer 3: Network Layer • It is the layer where routers reside • Transport layer passes data segments to Network Layer • Network layer works for the transmission of received data segments from one computer to another located in different networks • Data units in the network layer are called packets • Key functions of network layer are  Logical addressing  Routing  Path determination The OSI Model Layer 3: Network Layer • Logical addressing The OSI Model Layer 3: Network Layer • Routing The OSI Model Layer 3: Network Layer • Path Determination  Layer 3 devices use protocols such as:  OSPF – Open Shortest Path First  BGP – Border Gateway Protocol  IS-IS – Intermediate System to Intermediate System To determine the best possible path for data delivery The OSI Model Layer 2: Data Link Layer • Logical addressing (Network Layer)  Sender's and receiver's IP address are assigned to each segment to form a data packet • Physical addressing (Data Link Layer)  MAC address of sender and receiver are assigned to each data packet to form a frame  MAC address is a 12 digit alpha-numeric number embedded in the NIC of a computer v MAC = Media Access Control The OSI Model Layer 2: Data Link Layer • Data Link Layer is embedded as software in the Network Interface Card (NIC) of the computer • It provides a means for transferring data from one computer to another via a local media • Local media includes:  copper wire, fiber optics, or air for radio signals • Data Link Layer performs two basic functions:  it allows the upper layers of OSI model to access media  controls how data is placed and received from the media using such as  Media Access Control (MAC)  Error Detection The OSI Model Layer 2: Data Link Layer The OSI Model Layer 2: Data Link Layer • Consider two distant hosts:  A desktop and a Laptop communicating with each other • As laptop and desktop are connected to two different networks  they will be using network layer protocols (E.g., IP) to communicate with each other • Desktop is connected to router R1 via an Ethernet cable • Laptop is connected to router R2 via a wireless link • Router R1 and R2 are connected via a satellite link The OSI Model Layer 2: Data Link Layer • Desktop tries to send some data to laptop • Based on the medium used to connect desktop to router R1, data link layer adds some data in the head and tail sections of the IP packet and converts it to a frame (E.g., Ethernet frame) • Router R1 receives this frame, decapsulates it to an IP Packet and then encapsulates it again to a frame so that it can cross the satellite link to reach router R2 The OSI Model Layer 2: Data Link Layer • Router R2 again decapsulates the received frame and encapsulates it again to form a wireless data link frame • Laptop receives this wireless data link frame, decapsulates it, and forwards IP packet to network layer • Finally data arrives at the application layer • Application layer protocols make the received data visible on computer screen • Higher level layers are able to transfer data over the media with the help of data link layer The OSI Model Layer 2: Data Link Layer • Media Access Control • Error Control Image Source: https://www.configrouter.com/examining-csma-cd-7302/ The OSI Model Layer 2: Data Link Layer • Media Access Control  Data link layer also controls how the data is placed and received from the media  The technique used to get the frame on and off the media is called Media Access Control  There may be a number of devices connected to a common media  If two or more devices connected to same media send data simultaneously, there may be collisions of data packets resulting in loss of data  To avoid this situation, data link layer keeps an eye on when the shared media is free so that devices can transmit data for the receiver  This is called Carrier Sense Multiple Access (CSMA) • Error Control  Tail of each frame contains bits which are used to check for errors in the received frame  Errors occur due to certain limitations of the media used for transmitting data The OSI Model Layer 1: Physical Layer • Till now, data from application layer has been  segmented by transport layer,  placed into packets by network layer, and  framed by data link layer • This is a sequence of binary 0's and 1's • Frame check sequence (FCS) is an error-detecting code added to a frame in a communication protocol The OSI Model Layer 1: Physical Layer • Physical layer converts this binary sequence into signals and transmits over the local media • It can be:  an electrical signal (for copper cable),  light signal (optical fiber), and  radio signal (in case of air) • Signal generated by physical layer depends on the type of media used to connect two devices The Internet Model The Internet Model Overview The Internet Model Overview • This is the network model that dominates current hardware and software. • Development  OSI model was developed by formal committees, but the Internet model evolved from the work of thousands of people who developed pieces of the Internet. • Standards  The OSI model is a formal standard that is documented in one standard, but the Internet model has never been formally defined  It has to be interpreted from a number of standards. • The Internet model collapses the top three OSI layers into one layer. The Internet Model The Physical Layer • Similar to the OSI Model • It is the physical connection between the sender and receiver. • Its role is to transfer a series of electrical, radio, or light signals through the circuit. • The physical layer includes all the:  hardware devices (e.g., computers, modems, and switches) and  physical media (e.g., cables and satellites). • The physical layer specifies the type of connection and the electrical signals, radio waves, or light pulses that pass through it. The Internet Model The Data Link Layer • The data link layer is responsible for moving a message from one computer to the next computer in the network path from the sender to the receiver. • It performs the same three functions as the data link layer in the OSI model.  First, it controls the physical layer by deciding when to transmit messages over the media.  Second, it formats the messages by indicating where they start and end.  Third, it detects and may correct any errors that have occurred during transmission. The Internet Model The Network Layer • Network Layer:  Performs the same functions as the network layer in the OSI model.  First, it performs routing, in that it selects the next computer to which the message should be sent.  Second, it can find the address of that computer if it doesn’t already know it. The Internet Model The Transport Layer • Transport Layer  Very similar to the transport layer in the OSI model. Performs two key functions:  First, it is responsible for linking the application layer software to the network and establishing end-to-end connections between the sender and receiver.  Second, it is responsible for breaking long messages into several smaller messages to make them easier to transmit and then recombining the smaller messages back into the original larger message at the receiving end.  The transport layer can also detect lost messages and request that they be resent. The Internet Model The Application Layer • It is the application software used by the network user • It includes much of what the OSI model contains in the  Application layer,  Presentation layer, and  Session layer • It is the user’s access to the network. • By using the application software, the user defines what messages are sent over the network. Message Transmission Using Layers Data Communication Networks Message Transmission Using Layers Data Communication Networks Message Transmission Using Layers • Generally, all messages sent in a network pass through all layers. • All layers except the physical layer create a new Protocol Data Unit (PDU) as the message passes through them. • The PDU contains information that is needed to transmit the message through the network. • The word packet is also used to sometime to mean a PDU. • Figure shows how a message requesting a Web page would be sent on the Internet. Data Communication Networks Message Transmission Using Layers • Application Layer  User message (E.g., an HTTP request such as www.csus.edu)  Rules of HTTP creates HTTP Packet.  The Web browser fills in the necessary information in the HTTP packet, drops the user’s request inside the packet, then passes the HTTP packet (containing the Web page request) to the transport layer. Data Communication Networks Message Transmission Using Layers • Transport Layer  The transport layer uses a protocol called TCP (transmission control protocol),  TCP protocol rules create its own PDUs.  TCP is responsible for  Breaking the large files into smaller packets and  Opening a connection to the server for the transfer of a large set of packets.  The transport layer places the HTTP packet inside a TCP PDU (which is called a TCP segment), fills in the information needed by the TCP segment, and passes the TCP segment (which contains the HTTP packet containing the message) to the network layer. Data Communication Networks Message Transmission Using Layers • Network Layer  The network layer uses a protocol called IP (Internet Protocol), which has its rules and PDUs.  IP selects the next stop on the message’s route through the network.  It places the TCP segment inside an IP PDU (called an IP packet), and passes it to the data link layer.  The IP packet contains the TCP segment, which, in turn, contains the HTTP packet, which, in turn, contains the message. Data Communication Networks Message Transmission Using Layers • Data Link Layer  If you are connecting to the Internet using a LAN, your data link layer may use a protocol called Ethernet, which also has its own rules and PDUs.  The data link layer formats the message with start and stop markers, adds error checks information, places the IP packet inside an Ethernet PDU, which is called an Ethernet frame  It instructs the physical hardware to transmit the Ethernet frame, which contains the IP packet, which contains the TCP segment, which contains the HTTP packet, which contains the actual message. Data Communication Networks Message Transmission Using Layers • Physical Layer  The physical layer is network cable connecting your computer to the rest of the network.  The computer will take the Ethernet frame (complete with the IP packet, the TCP segment, the HTTP packet, and the message) and send it as a series of electrical pulses through the cable to the server. Data Communication Networks Message Transmission Using Layers • When the server gets the message, this process is performed in reverse. • The physical hardware  translates the electrical pulses into computer data and passes the message to the data link layer. Data Communication Networks Message Transmission Using Layers • The data link layer:  uses the start and stop markers in the Ethernet frame to identify the message  checks for errors and, if found one, requests that the message be resent.  If there is no error, the data link layer strips off the Ethernet frame and passes the IP packet (with the TCP segment, the HTTP packet, and the message) to the network layer. Data Communication Networks Message Transmission Using Layers • The network layer  checks the IP address and, if it is destined for this computer, strips off the IP packet and passes the TCP segment (which contains the HTTP packet and the message), to the transport layer. • The transport layer  processes the message, strips off the TCP segment, and passes the HTTP packet to the application layer for processing. Data Communication Networks Message Transmission Using Layers • The application layer (i.e., the Web server)  reads the HTTP packet and the message it contains (the request for the Web page) and processes it by generating an HTTP packet containing the Web page you requested. • Then the process starts again as the page is sent back to you. Data Communication Networks Pros and Cons of Using Layers • Three important aspects:  First,  many different software packages and many different PDUs that operate at different layers to successfully transfer a message.  Second,  for successful communication, each layer in one computer must be able to communicate with its matching layer in the other computer.  Third,  the layered network model is somewhat inefficient. Data Communication Networks Pros and Cons of Using Layers • Multiple software packages and multiple PDUs  This is called encapsulation  The PDU at a higher level is placed inside the PDU at a lower level so that the lower-level PDU encapsulates the higher-level one.  Advantages  It is easy to develop new software, because all one has to do is write software for one level at a time.  The developers of Web applications, for example, do not need to write software to perform error checking or routing, because those are performed by the data link and network layers. o Developers can simply assume those functions are performed and just focus on the application layer.  It is simple to change the software at any level (or add new application protocols), as long as the interface between that layer and the ones around it remains unchanged. Data Communication Networks Pros and Cons of Using Layers • Each layer in one computer must be able to communicate with its matching layer in the other computer.  For example, the physical layer connecting the client and server must use the same type of electrical signals to enable each to understand the other  or there must be a device to translate between them.  Ensuring that the software used at the different layers is the same as accomplished by using standards.  A standard defines a set of rules, called protocols, that dictate exactly how hardware and software that conform to the standard are required to operate.  Any hardware and software that conform to a standard can communicate with any other hardware and software that conform to the same standard. Without standards, it would be virtually impossible for computers to communicate. Data Communication Networks Pros and Cons of Using Layers • A layered network model is somewhat inefficient.  There are several layers, each with its own software and PDUs, sending a message involves many software programs (one for each protocol) and many PDUs.  Total amount of data and Processing Power  The PDUs add to the total amount of data that must be sent (thus increasing the time it takes to transmit), and  The different software packages increase the processing power needed in computers.  Because the protocols are used at different layers and are stacked on top of one another, the set of software used to understand the different protocols is often called a protocol stack. Thank You