Lecture # 02 Network Models Course Instructor: Engr. Sana Ziafat

 Set of rules that govern data communication  Protocol Defines: 1. What is communicated 2. How it is communicated 3. When it is communicated

 protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt a human protocol and a computer network protocol: Hi Got the time? 2:00 TCP connection req. TCP connection reply. Get time

 Syntax – format or structure of data  Semantics – meaning of each section of bits  Timing – when and how fast data should be sent

human protocols:  “I have a question”  introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols:  machines rather than humans  all communication activity in Internet governed by protocols

An example from the everyday life Hierarchy? Services

 To reduce complexity of communication task by splitting it into several layered small tasks  Functionality of the layers can be changed as long as the service provided to the layer above stays unchanged  makes easier maintenance & updating  Each layer has its own task  Each layer has its own protocol

 OSI reference model  TCP/IP

 Open System Interconnection  7 layers 1. Create a layer when different abstraction is needed 2. Each layer performs a well define function 3. Functions of the layers chosen taking internationally standardized protocols 4. Number of layers – large enough to avoid complexity

 Layer x on one machine communicates with layer x on another machine - called Peer-to-Peer Processes.  Interfaces between Layers Each interface defines what information and services a layer must provide for the layer above it. Well defined interfaces and layer functions provide modularity to a network  Organizations of the layers  Network support layers : Layers 1, 2, 3  User support layer : Layer 5, 6, 7 It allows interoperability among unrelated software systems  Transport layer (Layer 4) : links the two subgroups

Transporting bits from one end node to the next physical connection

 Duties performed by Physical layer are: 1. Physical characteristics of interfaces and media 2. Representation of bits 3. Data rate 4. Synchronization of bits 5. Topology 6. Line configuration 7. Transmission mode

The physical layer is responsible for movements of individual bits from one hop (node) to the next. Note

 Network adapter  Repeater  Network hub  Modem

Transporting frames from one end node to the next one logical connection - framing - physical addressing - flow control - error control - access control

The data link layer is responsible for moving frames from one hop (node) to the next. Note

 Network Devices: -Bridge, Switch, ISDN Router, Intelligent Hub, NIC, Advanced Cable Tester

 The network layer is responsible for the delivery of individual packets from the source host to the destination host.

End-to-End packet delivery  From the original source to a destination Needed when 2 devices are attached to different networks  What is the network definition here? Main duties: 1. Logical addressing 2. Routing

Data Link Network layer

The network layer is responsible for the delivery of individual packets from the source host to the destination host. Note

 Router works as the post office and network layer stamps the letters (data) for the specific destinations.  Protocols: These protocols work on the network layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF.  Network Devices: Network devices including Router, Frame Relay device and ATM switch devices work on the network layer.

 Process-to-Process delivery of the entire message ◦ From the original source to a destination  Needed when several processes (running programs) active at the same time  Note: What is a process  Main tasks: ◦ Service Point addressing ◦ Segmentation and reassembly ◦ Connection control ◦ Flow control ◦ Error control

 The transport layer is responsible for the delivery of a message from one process to another.

The transport layer is responsible for the delivery of a message from one process to another. Note

33 The session layer is responsible for dialog control and synchronization. Note

 The session layer allows a system to add check points or synchronization points.  This layer allow two systems to enter in to dialog

 Presentation layer is concerned with syntax and semantics of information exchanged between two systems.  Provides translation, encryption and compression to data  It is a best layer for cryptography.

The presentation layer is responsible for translation, compression, and encryption. Note

 The application layer is responsible for providing services to the user.

 Services provided by Application layer: - File transfer, Access - Mail services

40  Enables user to access the network  Provides services to a user   Remote file access and transfer (Telnet, FTP)  Access to WWW (HTTP)

41 The application layer is responsible for providing services to the user. Note

 Protocols: FTP, DNS, SNMP, SMTP, FINGER, TELNET, TFTP, BOOTP and SMB protocol are operated on the application layer.

 A convenient aid for remembering the OSI layer names is to use the first letter of each word in the phrase:  All People Seem To Need Data Processing

 Low-level protocols define the electrical and physical standards to be observed, bit- and byte-ordering and the transmission and error detection and correction of the bit stream  High-level protocols deal with the data formatting, including the syntax of messages, the terminal to computer dialogue, character sets, sequencing of messages

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

In Figure on next slide a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver. Example 1

Figure Physical addresses

As we will see in later lectures, most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below: Example 2 07:01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address.

Figure on next slide shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection. Example 3

Figure IP addresses

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 4

Figure Port addresses

Example 5 As we will see in later chapters, a port address is a 16-bit address represented by one decimal number as shown. 753 A 16-bit port address represented as one single number.

58  Chapter 1 (B. A Forouzan)  Section 1.4  Chapter 2 (B. A Forouzan)  Section 2.1, 2.2,2.3, 2.4, 2.5