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COMPUTER NETWORKS-1 (BTCS-403) By Baljinder Singh

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1 COMPUTER NETWORKS-1 (BTCS-403) By Baljinder Singh
Assistant Professor (I.T) Beant College of Engg. & Technology, Gurdaspur

2 CONTENTS Introduction Computer Networks OSI & TCP/IP Reference Model
Layering, TCP/IP, Internet Addresses, DNS, Client Server Model, Port Numbers. Link Layer Ethernet & IEEE 802( SLIP, PPP).


4 Computer Network A computer n/w is a group of communicating entities that uses a common network protocol to exchange data and share resources with each other over a communication medium. These communicating entities can be differentiated into end systems or intermediate systems. End System(Computer, Telephone, Laptop, etc…) Intermediate System(Repeater, Hub, Bridges, Router)

5 Contt… Computer Network can be classified into different types on the basis of the…. Geographical Extent(LAN, MAN, WAN). Physical Interconnection of hosts(Network Topologies(Bus, Ring, Star, Tree, Mesh). Type of communication path used Manner of data transmission among hosts over this communication path.

6 Uses of Computer Networks
Resource Sharing Communication Medium Reliability Scalability Money Saving E-Commerce Online Banking Etc.

7 Business Applications of Networks
A network with two clients and one server.

8 Business Applications of Networks (2)
The client-server model involves requests and replies.

9 Mobile Network Users Wireless Mobile Applications No
Desktop PCs in Offices Yes A Laptop used in a hotel room to access internet Network in an older unwired building Portable Office / Mobile Office/ Internet on laptops through Wireless connectivity in a Campus

10 Types of Networks Local Area Networks Metropolitan Area Networks
Wide Area Networks Wireless Networks

11 Broadcast Networks Types of transmission technology Broadcast links
Point-to-point links

12 Classification of interconnected processors by scale.

13 Local Area Networks Two broadcast networks (a) Bus (b) Ring

14 Metropolitan Area Networks
A metropolitan area network based on cable TV.

15 Wide Area Networks Relation between hosts on LANs and the subnet.

16 Wide Area Networks (2) A stream of packets from sender to receiver.

17 Wireless Networks Categories of wireless networks:
System interconnection Wireless LANs Wireless WANs

18 Wireless Networks (2) (a) Bluetooth configuration (b) Wireless LAN

19 Network Software Protocol Hierarchies Design Issues for the Layers
Connection-Oriented and Connectionless Services Service Primitives The Relationship of Services to Protocols

20 Network Software Protocol Hierarchies
Layers, protocols, and interfaces.

21 Design Issues for the Layers
Addressing Error Control Flow Control Multiplexing Routing

22 Services to Protocols Relationship
The relationship between a service and a protocol.

23 Reference Models The OSI Reference Model The TCP/IP Reference Model

24 Reference Models The OSI Reference Model.

25 Network Communication Protocols
In order to standardize these protocols and also to provide a framework for their development, some standard reference models were developed. These network models are based on the concept of layering. The n/w protocols are structured into set of layers, with each layer responsible for a different fact of communication. A layered architecture lies in the ability to independently design, build or modify the functions and services of each layer without effecting the other layers. It reduces the complexity of n/w operations by dividing them into simpler components It also enable multi-vendor integration by dividing standard interfaces.

26 Protocols & Standards A protocol is a set of rules that govern data communication. A protocol define what is communicated, how it is communicated & when to communicated. Key elements are. 1. Syntax 2. Semantics 3. Timing Syntax refers to structure or format of the data, order they presented. 1st 8-bit address of sender 2nd 8-bit address of receiver Rest of stream of message Semantics refers to the meaning of each section of bits, how is particular pattern interpreted. Timing refers two things:- 1. When data should be sent 2. How fast it can be send.

27 Contt… Standards are essentials in creating and maintaining an open and competitive market for equipment manufacturers and also in guaranteeing national & international inter operability of data & telecommunications technology & processes. There are two types de-facto(By fact or by convention) de-jure(By law by regulation) RFCs(Request for Comment) All the official standards in the internet community are published as RFC. Additionally there are lot of RFCs that are not official standards but published for information purposes. Each RFC ranges from 1 page to almost 200 pages. Each RFC is identified by a number.


29 Communication Architecture
Strategy for connecting host computers and other communicating equipment. Defines necessary elements for data communication between devices. A communication architecture, therefore, defines a standard for the communicating hosts. A programmer formats data in a manner defined by the communication architecture and passes it on to the communication software. Separating communication functions adds flexibility, for example, we do not need to modify the entire host software to include more communication devices.

30 Layer Architecture Layer architecture simplifies the network design.
It is easy to debug network applications in a layered architecture network. The network management is easier due to the layered architecture. Network layers follow a set of rules, called protocol. The protocol defines the format of the data being exchanged, and the control and timing for the handshake between layers.

31 Open Systems Interconnection (OSI) Model
International standard organization (ISO) established a committee in 1977 to develop an architecture for computer communication. Open Systems Interconnection (OSI) reference model is the result of this effort. In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture. Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards.

32 OSI Reference Model The OSI model is now considered the primary Architectural model for inter-computer communications. The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network. The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems . This separation into smaller more manageable functions is known as layering.

33 OSI Reference Model: 7 Layers
OSI Model OSI Reference Model: 7 Layers

34 Layer Responsible For: 7.) Application Provides Services to User Apps 6.) Presentation Data Representation 5.) Session Communication Between Hosts 4.) Transport Flow Ctrl, Error Detection/Correction 3.) Network End to End Delivery, Logical Addr 2.) Data Link Media Access Ctrl, Physical Addr 1.) Physical Medium, Interfaces, Puts Bits on Med.

35 Layer Example 7.) Application HTTP, FTP, SMTP 6.) Presentation ASCII, JPEG, PGP 5.) Session BOOTP, NetBIOS, DHCP, DNS 4.) Transport TCP, UDP, SPX 3.) Network IP, IPX, ICMP 2.) Data Link Ethernet, Token Ring, Frame Relay 1.) Physical Bits, Interfaces, Hubs

36 OSI: A Layered Network Model
OSI Model OSI: A Layered Network Model The process of breaking up the functions or tasks of networking into layers reduces complexity. Each layer provides a service to the layer above it in the protocol specification. Each layer communicates with the same layer’s software or hardware on other computers. The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network. The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications. Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

37 The OSI Model Layer Name Description Examples Physical Link Network
Transport Session Presentation Application Unreliable Wire, Telco Line Reliable Across Physical Line Unreliable Thru Multi-Node Network Reliable End to End Sync Points and Dialogs Data Representation & Syntax User Level Processing RS232, T1, 802.x LAPB, HDLC X.25 Pkt, IP TCP ISO Session ISO Presentation Telnet, FTP, Mail

38 The OSI Model The Standards Layer Name Standards Physical Link Network
Transport Session Presentation Application X.400, Telnet, FTP, Mail ASN.1, X.409 X.225 X.224, TCP X.25 Pkt, Q.931, IP X.25 LAPB, Q.921 LAPD, ISO 3309 HDLC Reliable Across Physical Line RS232, V.35, EIA530, X.21, T1, E1 Unreliable Wire, Telco Line Unreliable Wire, Telco Line

39 OSI Model Physical Layer Provides physical interface for transmission of information. Defines rules by which bits are passed from one system to another on a physical communication medium. Covers all - mechanical, electrical, functional and procedural - aspects for physical communication. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

40 OSI Model Data Link Layer Data link layer attempts to provide reliable communication over the physical layer interface. Breaks the outgoing data into frames and reassemble the received frames. Create and detect frame boundaries. Handle errors by implementing an acknowledgement and retransmission scheme. Implement flow control. Supports points-to-point as well as broadcast communication. Supports simplex, half-duplex or full-duplex communication.

41 OSI Model Network Layer Implements routing of frames (packets) through the network. Defines the most optimum path the packet should take from the source to the destination Defines logical addressing so that any endpoint can be identified. Handles congestion in the network. Facilitates interconnection between heterogeneous networks (Internetworking). The network layer also defines how to fragment a packet into smaller packets to accommodate different media.

42 OSI Model Transport Layer Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers. Ensures that the data units are delivered error free. Ensures that data units are delivered in sequence. Ensures that there is no loss or duplication of data units. Provides connectionless or connection oriented service. Provides for the connection management. Multiplex multiple connection over a single channel.

43 OSI Model Session Layer Session layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications. This layer requests for a logical connection to be established on an end-user’s request. Any necessary log-on or password validation is also handled by this layer. Session layer is also responsible for terminating the connection. This layer provides services like dialogue discipline which can be full duplex or half duplex. Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint.

44 OSI Model Presentation Layer Presentation layer defines the format in which the data is to be exchanged between the two communicating entities. Also handles data compression and data encryption (cryptography).

45 OSI Model Application Layer Application layer interacts with application programs and is the highest level of OSI model. Application layer contains management functions to support distributed applications. Examples of application layer are applications such as file transfer, electronic mail, remote login etc.

46 OSI Model OSI in Action A message begins at the top application layer and moves down the OSI layers to the bottom physical layer. As the message descends, each successive OSI model layer adds a header to it. A header is layer-specific information that basically explains what functions the layer carried out. Conversely, at the receiving end, headers are striped from the message as it travels up the corresponding layers.

47 TCP/IP Model OSI & TCP/IP Models


49 Introduction The TCP/IP protocol suite allows computers of all sizes, from many different computer vendors, running totally different operating systems, to communicate with each other. What started in the late 1960s as a government-financed research project into packet switching networks has, in the 1990s, turned into the most widely used form of networking between computers. It forms the basis for what is called the worldwide Internet, or the Internet, a wide area network (WAN) of more than one million computers that literally spans the globe.

50 Transport Layer (TCP/UDP)
TCP/IP Model Application Layer Application programs using the network Transport Layer (TCP/UDP) Management of end-to-end message transmission, error detection and error correction Network Layer (IP) Handling of datagrams : routing and congestion Data Link Layer Management of cost effective and reliable data delivery, access to physical networks Physical Layer Physical Media

51 TCP/IP Model Message Segment Datagram Frame Bit 5 4 3 2 1

52 Layering Networking protocols are normally developed in layers, with each layer responsible for a different facet of the communications. A protocol suite, such as TCP/IP, is the combination of different protocols at various layers. TCP/IP is normally considered to be a 4-layer system, as shown in Figure Figure The four layers of the TCP/IP protocol suite. Application Telnet, FTP, , etc. Transport TCP, UDP Network IP, ICMP, IGMP Link Device driver and interface card

53 TCP/IP Layering

54 Conttd… Each layer has a different responsibility.
The link layer, sometimes called the data-link layer or network interface layer, normally includes the device driver in the operating system and the corresponding network interface card in the computer. Together they handle all the hardware details of physically interfacing with the cable (or whatever type of media is being used). The network layer (sometimes called the internet layer) handles the movement of packets around the network. Routing of packets, for example, takes place here. IP (Internet Protocol), ICMP (Internet Control Message Protocol), and IGMP (Internet Group Management Protocol) provide the network layer in the TCP/IP protocol suite.

55 Conttd… 3. The transport layer provides a flow of data between two hosts, for the application layer above. In the TCP/IP protocol suite there are two vastly different transport protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). 4. The application layer handles the details of the particular application. There are many common TCP/IP applications that almost every implementation provides: Telnet for remote login, FTP, the File Transfer Protocol, SMTP, Simple Mail Transfer protocol, for electronic mail, SNMP, the Simple Network Management Protocol,

56 Fig :- Two hosts on a LAN running FTP.

57 Fig :- Two n/w connected with router
FTP client TCP IP Host A Host B FTP Protocol FTP server TCP Protocol TCP Router IP IP eth drv t.r. drv Token Ring Driver Ethernet Driver Token Ring Ethernet

58 Internet Addresses Every interface on an internet must have a unique Internet address (also called an IP address). These addresses are 32-bit numbers. Instead of using a flat address space such as 1, 2, 3, and so on, there is a structure to Internet addresses. These 32-bit addresses are normally written as four decimal numbers, one for each byte of the address. This is called dotted-decimal notation. For example, the class B address of system is The easiest way to differentiate between the different classes of addresses is to look at the first number of a dotted-decimal address.

59 Conttd… Every interface on an internet must have a unique IP address, there must be one central authority for allocating these addresses for networks connected to the worldwide Internet. That authority is the Internet Network Information Center, called the InterNIC. The InterNIC assigns only network IDs. The assignment of host IDs is up to the system administrator Figure :- Ranges for different classes of IP addresses. Class Range A to B to C to D to E to

60 IP Address Structure - Class-full
Address format 32 bits Network address Host address Class A network=8 bits Class B network=16 bits 1 Class C network=24 bits 1 1 Class D (multicast) 1 1 1 Class E (reserved) 1 1 1 1

61 Conttd… There are three types of IP addresses:
Unicast (Destined for a single host), Broadcast (Destined for all hosts on a given network), Multicast (Destined for a set of hosts that belong to a multicast group)

62 Domain Name System Although the network interfaces on a host, and therefore the host itself, are known by IP addresses, humans work best using the name of a host. In the TCP/IP world the Domain Name System (DNS) is a distributed database that provides the mapping between IP addresses and hostnames. For now we must be aware that any application can call a standard library function to look up the IP address (or addresses) corresponding to a given hostname. Similarly a function is provided to do the reverse lookup-given an IP address, look up the corresponding hostname.

63 Encapsulation When an application sends data using TCP, the data is sent down the protocol stack, through each layer, until it is sent as a stream of bits across the network. Each layer adds information to the data by pretending headers (and sometimes adding trailer information) to the data that it receives. The unit of data that TCP sends to IP is called a TCP segment. The unit of data that IP sends to the network interface is called an IP datagram. The stream of bits that flows across the Ethernet is called a frame. A physical property of an Ethernet frame is that the size of its data must be between 46 and 1500 bytes.

64 Conttd…

65 Conttd…

66 Client Server Model Most networking applications are written assuming one side is the client and the other the server. The purpose of the application is for the server to provide some defined service for clients. We can categorize servers into two classes: 1. Iterative 2. Concurrent An iterative server iterates through the following steps. I1. Wait for a client request to arrive. I2. Process the client request. I3. Send the response back to the client that sent the request. I4. Go back to step I1. The problem with an iterative server is when step I2 takes a while. During this time no other clients are serviced.

67 Conttd… A concurrent server, on the other hand, performs the following steps. Cl. Wait for a client request to arrive. C2. Start a new server to handle this client's request. This may involve creating a new process, task, or thread, depending on what the underlying operating system supports. How this step is performed depends on the operating system. This new server handles this client's entire request. When complete, this new server terminates. C3. Go back to step Cl. The advantage of a concurrent server is that the server just spawns other servers to handle the client requests. Each client has, in essence, its own server. Assuming the operating system allows multiprogramming, multiple clients are serviced concurrently.

68 Port Numbers Servers are normally known by their well-known port number. For example, every TCP/IP implementation that provides an FTP server provides that service on TCP port 21. Every Telnet server is on TCP port 23. Every implementation of TFTP (the Trivial File Transfer Protocol) is on UDP port 69. Those services that can be provided by any implementation of TCP/IP have well-known port numbers between 1 and The well-known ports are managed by the Internet Assigned Numbers Authority (IANA).


70 Introduction The purpose of the link layer in the TCP/IP protocol suite is to send and receive IP datagrams for the IP module ARP requests and replies for the ARP module RARP requests and replies for the RARP module TCP/IP supports many different link layers, depending on the type of networking hardware being used: Ethernet Token ring FDDI (Fiber Distributed Data Interface) RS-232 serial lines

71 Ethernet & IEEE 802 Encapsulation
Ethernet is a standard published in 1982 by DEC(Digital Equipment Corporation) & XEROX Corp.. It is a LAN with TCP/IP using access method CSMA?CD. It uses 48-bit MAC Address. Operating speed is from 10Mbps. IEEE committee published a slightly different standards. 802.3 covers entire set of CSMA/CD networks. 802.3 covers entire set of Token Bus networks. 802.3 covers entire set of Token Ring networks. 802.2 standard is common to all & define logical link control. Combination of & defines a new standard that is different from true Ethernet by DEC & XEROX

72 IEEE 802.2/802.3 Encapsulation(RFC 1042)

73 SLIP- Serial Line IP SLIP stands for Serial Line IP. It is a simple form of encapsulation for IP datagrams on serial lines. SLIP has become popular for connecting home systems to the Internet, through the ubiquitous RS-232 serial port found on almost every computer and high-speed modems. The following rules specify the framing used by SLIP. The IP datagram is terminated by the special character called END (0xc0).To prevent any line noise before this datagram from being interpreted as part of this datagram, most implementations transmit an END character at the beginning of the datagram too. (If there was some line noise, this END terminates that erroneous datagram, allowing the current datagram to be transmitted. That erroneous datagram will be thrown away by a higher layer when its contents are detected to be garbage.)

74 Conttd.. If a byte of the IP datagram equals the END character, the 2-byte sequence 0xdb, 0xdc is transmitted instead. This special character, 0xdb, is called the SLIP ESC character, but its value is different from the ASCII ESC character (0xib). If a byte of the IP datagram equals the SLIP ESC character, the 2-byte sequence 0xdb, 0xdd is transmitted instead.

75 Conttd.. Figure :-SLIP encapsulation

76 Conttd… SLIP is a simple framing method. It has some deficiencies that are worth noting. 1.Each end must know the other's IP address. There is no method for one end to inform the other of its IP address. 2.There is no type field (similar to the frame type field in Ethernet frames). If a serial line is used for SLIP, it can't be used for some other protocol at the same time. 3.There is no checksum added by SLIP (similar to the CRC field in Ethernet frames). If a noisy phone line corrupts a datagram being transferred by SLIP, it's up to the higher layers to detect this.

77 Compressed SLIP SLIP lines are often slow (19200 bits/sec or below) and frequently used for interactive traffic (such as Telnet and Rlogin, both of which use TCP), there tend to be many small TCP packets exchanged across a SLIP line. To carry I byte of data requires a 20-byte IP header and a 20-byte TCP header, an overhead of 40 bytes. Recognizing this performance drawback, a newer version of SLIP, called CSLIP (for compressed SLIP). CSLIP normally reduces the 40-byte header to 3 or 5 bytes. It maintains the state of up to 16 TCP connections on each end of the CSLIP link and knows that some of the fields in the two headers for a given connection normally don't change. Of the fields that do change, most change by a small positive amount. These smaller headers greatly improve the interactive response time.

78 PPP: Point-to-Point PPP, the Point-to-Point Protocol, corrects all the deficiencies in SLIP. PPP consists of three components. 1. A way to encapsulate IP datagram on a serial link. PPP supports either an asynchronous link with 8 bits of data and no parity (i.e., the ubiquitous serial interface found on most computers) or bit-oriented synchronous links. 2. A link control protocol (LCP) to establish, configure, and test the data-link connection. This allows each end to negotiate various options. 3. A family of network control protocols (NCPs) specific to different network layer protocols, currently exist for IP, the OSI network layer, DECnet, and AppleTalk. The IP NCP, for example, allows each end to specify if it can perform header compression, similar to CSLIP.

79 Figure :- Format of PPP frames.
Conttd… Figure :- Format of PPP frames.

80 Review of Lecture We have learned today the following topics
Introduction Computer Networks OSI & TCP/IP Reference Model Layering, TCP/IP, Internet Addresses, DNS, Client Server Model, Port Numbers. Link Layer Ethernet & IEEE 802( SLIP, PPP).


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