King Fahd University of Petroleum Minerals College of

King Fahd University of Petroleum & Minerals College of Computer Science & Engineering Information & Computer Science Department ICS 343 Fundamentals of Computer Networks Network Models These slides are based on: Chapter 2, Data Communications and Networking, 4 th Edition

Objectives • • • Describe layered tasks Introduce the OSI model Introduce the TCP/IP protocol suite 2. 2

Layered Tasks We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office. 2. 3

Layered Tasks Figure 2. 1 Tasks involved in sending a letter 2. 4

OSI Model Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970 s. 2. 5

OSI Model Figure 2. 2 Seven layers of the OSI model 2. 6

Peer-to-peer processes At the sending device each layer adds its own information to the message it receives from the layer above it and passes the whole package to the layer just below it. At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it. We need an interface between each pair of adjacent layers. Each interface defines what information and services a layer must provide for the layer above it. Thus, provide modularity. 2. 7

Layers Interaction Figure 2. 3 The interaction between layers in the OSI model 2. 8

Exchange between layers Figure 2. 4 An exchange using the OSI model 2. 9

Encapsulation n Data portion of a packet at level N – 1 carries the whole packet (data + header + trailer if any) from level N n n Level N – 1 is not aware of which part of the transmitted packet is data and which part is the header or trailer. For level N – 1, the whole packet coming from level N is treated as one integral unit. This is called Encapsulation. 2. 10

Physical Layer n n n Deals with mechanical and electrical specifications of the interface and transmission media. Defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur. It is concerned about: n Physical characteristics of interfaces and media n Representation of bits n Data rate: Transmission rate – number of bits sent per second n Synchronization of bits n Line Configuration n Physical Topology n Transmission Mode The physical layer is responsible for transmitting individual bits from one node to the next. 2. 11

Physical Layer Figure 2. 5 Physical layer 2. 12

Data Link Layer n n n Transforms the physical layer, a raw transmission facility, to a reliable link. Makes the physical layer appear error-free to the upper layer (network layer). n Framing: divide the stream of bits from network layer into frames n Physical addressing n Flow control: Prevent overwhelming the receiver. n Error control: Detect and retransmit damaged or lost frames; Prevent duplication of frames; using trailer. n Access control: Determine which device has control over the link at any given time. The data link layer is responsible for transmitting frames from one node to next. 2. 13

Data Link Layer Figure 2. 6 Data link layer 2. 14

Example A node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection: 2. 15

Hop-to-hop delivery Figure 2. 7 Hop-to-hop delivery 2. 16

Network Layer n n n Responsible for the source-to-destination delivery of a packet possibly across multiple networks. Data link layer oversees the delivery of the packet between two systems on the same network. Network layer ensures that each packet gets from its point of origin to its final destination. n If two systems are connected to the same link, there is usually no need for a network layer. 2. 17

Network Layer Figure 2. 8 Network layer 2. 18

Network Layer n n Logical addressing: Physical addressing in data link layer handles the addressing problem locally, this is why logical addressing is needed. Routing: When independent networks or links are connected to create an internetwork (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. 2. 19

Example We want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic. 2. 20

Source-to-destination delivery Figure 2. 9 Source-to-destination delivery 2. 21

Transport Layer n n n Process-to-Process delivery Network layer oversees host-to-destination delivery of individual packets, it does not recognize any relationship between those packets. Ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the process-to-process level. 2. 22

Transport Layer n n n The transport layer is responsible for delivery of a message from one process to another. Port addressing: indicating a process Segmentation and reassembly: divide into segments having sequence number. Connection control: connection-oriented and connectionless Flow control: end to end rather than across a single link. Error control: End to end rather than across a single link. Sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). 2. 23

Transport Layer Figure 2. 10 Transport layer 2. 24

Process-to-process delivery Figure 2. 11 Reliable process-to-process delivery of a message 2. 25

Example Figure in next slide shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination. 2. 26

Example (cont) 2. 27

Session Layer n Dialog control: Allows two systems to enter into a dialog, either half or full duplex. n Terminal to mainframe is half duplex. n Synchronization: Add check points (synchronization points) into a stream of data. 2. 28

Session Layer Figure 2. 12 Session layer 2. 29

Presentation Layer n Presentation layer n Translation: Interoperability between different encoding methods. n Encryption: For privacy. n Compression: Reduces the number of bits contained in the information; useful in video, audio, … 2. 30

Presentation Layer 2. 3 1

Application Layer n n Enables the user, whether human or software, to access the network. Provides user interfaces and support for services such as electronic mail, remote file access and transfer, access to WWW, and so on. 2. 32

Summary of Layers Figure 2. 15 Summary of layers 2. 33

TCP/IP Protocol Suite 2. 34 The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-tonetwork, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application.

TCP/IP and OSI Models Figure 2. 16 TCP/IP and OSI model 2. 35

Addressing Four levels of addresses are used in an internet employing the TCP/IP protocols: physical, logical, port, and specific. 2. 36

Addresses in TCP/IP Figure 2. 17 Addresses in TCP/IP 2. 3 7

Addresses in TCP/IP Figure 2. 18 Relationship of layers and addresses in TCP/IP 2. 38

Note The physical addresses will change from hop to hop, but the logical addresses usually remain the same. 2. 39

The end Important to do at home : - read chapter 2 of the textbook