Computer Networks Lecture 3 PDF

Computer Networks Lecture 3 Network Layers Functions Figure 2.4 An exchange using the OSI model PDU (Protocol Data Unit) PDU = combination of data (payload) from the next higher layer and the control information of the current layer (Specific requests and instructions) ◼ Contr...

Computer Networks Lecture 3 Network Layers Functions Figure 2.4 An exchange using the OSI model PDU (Protocol Data Unit) PDU = combination of data (payload) from the next higher layer and the control information of the current layer (Specific requests and instructions) ◼ Control information are given in the header fields of the block except for the data link (has both header and trailer) ◼ Control information is read and executed ONLY by the peer layer on the receiving node Data Encapsulation Data encapsulation refers to the fact that layer N-1 carries in its data part the PDU of layer N without any knowledge of its content and its different parts. How Hosts Talk over a Network DATA Application Application needs to Application send data. Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical How Hosts Talk over a Network TCP Header DATA Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Transport layer establishes session with peer, assigns port number based on application. How Hosts Talk over a Network IP Header TCP Header DATA Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Network layer builds packet with source and destination IP address. Passes to data link layer for transmission How Hosts Talk over a Network Ethernet IP Header TCP Header DATA Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Data Link layer frames up packet and forwards data. Uses MAC address as destination address. How Hosts Talk over a Network Ethernet IP Header TCP Header DATA Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Data Link layer frames up packet and forwards data. Uses MAC address as destination address. How Hosts Talk over a Network IP Header TCP Header DATA Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Network layer looks at destination address. See’s the address as its own. Passes packet to TCP. How Hosts Talk over a Network TCP Header DATA Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical TCP identifies which application to pass data to by destination port number. How Hosts Talk over a Network DATA Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical Application processes data. Physical Layer The physical layer is responsible for movements of individual bits from one node to the next.  Physical Layer ◼ Deals with the actual communication media ◼ Defines the physical characteristics of interfaces and transmission media ◼ the way devices are connected to each other and to the link (topology) ◼ Shape, size and number of pins of connectors ◼ What voltages and currents are used ◼ The type of transmission media ◼ Transmission mode (duplex type) Important Terms: ◼ Representation of bits – encoding into Electrical or Optical signals ◼ Data Rate = transmission rate = bandwidth (number of bits sent per second) ◼ Synchronization of bits (sender and receiver clocks must be synchronized) ◼ Devices that operates at this level: Network card, hub, repeater Data Link Layer frames Data Link Data Layer Link Layer Physical bits Physical Layer Layer Note The data link layer is responsible for moving frames (data link PDU) from one node to the next. Data Link Layer ◼ Data Link layer PDU is called frame ◼ Transfers frames across direct connections Node- to-Node (hop-to-hop) delivery ◼ Framing: dividing the stream of bits into units called frames ◼ Physical Addressing: sender and receiver physical addresses (Local address – hardware address – NIC address – LAN address) Figure 2.8 Example 1 Network Layer Note The network layer is responsible for the delivery of individual packets from the source host to the destination host. Network Layer ◼ Network Layer PDU is called Packet or Datagram ◼ Source-to-destination (host-to-host) delivery of a PACKET possible across MULTIPLE networks. ◼ Logical addressing ▪ A unique global address that distinguishes each host connected to the Internet ▪ In the internet it is called IP address ▪ Routing ▪ connecting devices (routers ) route or switch the packets to their final destination. Figure 2.11 Example 2 Data Link Layer Figure 2.12 Transport layer Note The transport layer is responsible for the delivery of a message from one process to another. Transport Layer ◼ The transport layer is responsible for Process-to-Process delivery of the ENTIRE MESSAGE ◼ The network layer handles end-to-end (source-to-destination) delivery of INDIVIDUAL PACKETS; no relationship between packets, each one is considered independently but the transport layer ensures whole message arrives intact and in order ◼ Transport PDU is called SEGMENT ◼ Process addressing ◼ Port address Figure 2.12 Reliable process-to-process delivery of a message Addressing Figure 2.16 Pg 35 FOROUZAN03 Layer Addresses Figure 2.12 Session layer Session ◼ Setting up, managing, and terminating sessions (connections) between different applications ◼ Keeping different applications data separate from other application data ◼ Multiple file downloads requested by a particular FTP application ◼ Multiple telnet (remote connection to a device) connections from a single host ◼ Browsing many Web pages at the same time ◼ Dialog control ◼ Full/Half duplex: Processes either send and receive data at the same time or at different times ◼ Synchronization checkpoints ◼ Allow long transmissions to continue from where they were after a crash Figure 2.13 Presentation layer Presentation ◼ Translation ◼ Allows devices with different data representations to communicate (interoperability). ◼ Sender format ➔ common format ➔Receiver format ◼ Encryption/decryption ◼ Compression => Less bits (multimedia applications) Figure 2.15 Application layer Application ◼ User interface and Network services that allow user applications to access the network ◼ Internet Browser uses the HTTP application-layer protocol to access a WWW ◼ Telnet => remote host access ◼ File transfer, access and management ◼ Mail services TCP/IP Protocol Suite ◼ Developed in 1970s ▪ Is a suite of protocols named after the two most important protocols TCP and IP but includes other protocols such as UDP, etc ◼ Consists of Five layers: The first four lower layers correspond to the first four layers of the OSI model. The three top layers in the OSI model, however, are represented in TCP/IP by a single layer called the application layer. Figure 1.17 Layers in the TCP/IP protocol suite Message Segment Packet Frame Bits 38 Figure 2.12: TCP/IP and OSI model Original TCP/IP Model (4-layers)