Download Network Models: A Comprehensive Guide to TCP/IP and OSI Models and more Study notes Topology in PDF only on Docsity!
Dr. Gihan NAGUIB 1
Network
Models
Chapter 2 Behrouz A. Forouzan “Data communication and Networking” Fourth edition Dr. Gihan NAGUIB 2
Layered Tasks
WeWe useuse thethe concept concept ofof layers layers inin ourour daily daily lifelife.. AsAs anan example, example, letlet usus consider consider twotwo friends friends who who communicate communicate through through postal postal mail mail.. The The process process ofof sending sending aa letter letter toto aa friend friend would would bebe complex complex ifif there there were were nono services services available available from from thethe post post office office.. Process of sending a letter Process of sending a letter Sender, Receiver, and Carrier Hierarchy: Tasks must be done in order given in hierarchy Services : each layer at the sending site uses the services of the layer below it.
Dr. Gihan NAGUIB 3 Sending a letter Dr. Gihan NAGUIB 4 Internet layers The The layers layers inin thethe TCP/IP TCP/IP protocol protocol suite suite isis made made ofof fivefive layers layers:: physical physical,, data data link link,, network network,, transport transport,, and and application application.. Layer: A grouping of related tasks involving the transfer of information .Each layer addresses an essential networking tasks
Dr. Gihan NAGUIB 7
Organization of the layers
Network support layer : Deal with the physical aspects of moving data from one device to another such as :electrical specifications , physical connections, physical addressing, transport timing and reliability. Includes Layers 1 , 2 and 3 User support layer: Application layer Layer 4 (transport layer) links the two subgroups to insure that what the lower layers have transmitted is in form the upper layer can use Dr. Gihan NAGUIB 8 An exchange using the Internet model
Dr. Gihan NAGUIB 9 At sender: at each layer, a header can added to the data unit. At layer 2 a trailer is added as well. When formatted data unit passes through physical layer it is changed into an electromagnetic signal and transported along physical link. Encapsulation: A packet (header and data ) at level 5 is encapsulated in a packet in level 4 , and so on. The data portion of packet at level N- 1 carries the whole packet (data and header/trailer) from level N. At destination: A data then move back up through the layer a headers and trailers attached to data at the corresponding layer are removed (decapsulated) and action appropriate to that layer are taken. (unwrapped/decapsulated) At layer 5 the message is again in form appropriate to the application and is made available to user. Dr. Gihan NAGUIB 10
Function of the layers
The physical layer is responsible for movements of individual bits from one hop (node) to the next.
1. Physical layer
Dr. Gihan NAGUIB 13
- Data link layer hop to- hop delivery The data link layer is responsible for moving frames from one hop (node) to the next. Dr. Gihan NAGUIB 14 Duties of data link layer Framing: Divide the stream of bits received from network layer into data units called frames Physical addressing: It adds a header to the frame to define the sender and receiver of the frame. If the frame for a system outside the sender’s network the receiver address: is the address of the connecting device that connects the network to next one (Router/switch). Flow control: It imposes a flow control mechanism , if the data rate at the receiver is less than produced by sender the data link layer imposes a flow control to avoid overwhelming the receiver
Dr. Gihan NAGUIB 15 Duties of data link layer Error control: Add mechanisms to detect and retransmit damaged or lost frames. Prevent also duplication of frames. Error control is normally achieved through a trailer added to the end of frame. Access control: When two or more devices than one devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at given time. Dr. Gihan NAGUIB 16 Hop-to-hop delivery
Dr. Gihan NAGUIB 19 Example 1 :Physical addresses Dr. Gihan NAGUIB 20
- Network layer source to destination delivery The network layer is responsible for the delivery of individual packets from the source host to the destination host across multiple network. If two system are connected to the same link (network), no need for this layer.
Dr. Gihan NAGUIB 21 Duties of network layer ( internetwork layer) Logical addressing In contrast to physical addressing implemented by data link layer handling the addressing problem locally. Net work layer adds unique identifier ( IP or logical address) to the packet. These unique identifier( as tel. no, each tel. has unique number ) enable special devices called router to make sure the packet get to correct system. Routing: provide the routing mechanism for the router which route the packet to their final destination. Routers : devices used when independent networks are connected to create an internetworking ( network of networks) Dr. Gihan NAGUIB 22 Source-to-destination delivery
Dr. Gihan NAGUIB (^) 25 Example 2 : IP addresses (logical address) Dr. Gihan NAGUIB (^) 26 Transport layer ( process-to-process delivery) The transport layer is responsible for the delivery of a message from one process to another. From application layer To application layer
Dr. Gihan NAGUIB 27 Duties of transport layer
- Port addressing (Service-point addressing) Computer often run several process ( running programs) at the same time, so the process to process delivery means delivery from a specific process on a computer to specific process to the other. The transport layer header must include Port address Port address: 16 - bit addresses represented by decimal number range from 0 - 65535 to choose among multiple processes on the destination host Destination port No is needed for delivery Source port no is needed for replay. Dr. Gihan NAGUIB 28 IP (logical address )and port address play different roles in selecting the final destination
IP: defines the host among the different hosts in
the world
After host is selected , the port address defines
one of the processes on this particular host. Note:Note:
Dr. Gihan NAGUIB 31 5 .Connection control The transport layer can be either connection less or connection oriented
- Connection oriented Makes a connection with the transport layer at the destination machine first before delivering the packers. When the connection established a sequence of packets from source to the destination can be sent one after another on the same path and in sequential order. When all packets of message have been delivered, the connection is terminated This makes the sending transport layer ensure that the message arrives at the receiving transport layer without error ( damage, loss or duplication Duties of transport layer Dr. Gihan NAGUIB 32 Duties of transport layer Connection Less ( as Internet) It sends the data, but does not establish and verify a connection between hosts before sending data. Treats each packet independently, the packets in a message may or may not travel the same path to their destination.
Dr. Gihan NAGUIB 33 Reliable process-to-process delivery of a message The network layer gets each packet to the correct computer; the transport layer gets the message to the correct processes on that computer Dr. Gihan NAGUIB (^) 34 The following figure 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. Example
Dr. Gihan NAGUIB 37 Relationship of layers and addresses in TCP/IP Dr. Gihan NAGUIB (^) 38
Application layer
The application layer enables the user to access the network
Dr. Gihan NAGUIB 39 The application layer is responsible for providing services to the user such as Mail services File transfer and access Remote log-in Accessing the web (WWW)
- Telnet: A service that enables users on the internet to log onto remote systems from their own host system.
- HTTP: Hyper text transfer protocol used for network file transfers in WWW environment
- SMTP: Simple mail transfer protocol used to send electronic mail on the internet. Note:Note: Dr. Gihan NAGUIB (^) 40 Summary of duties
