Chapter 12 Objectives • Become familiar with the fundamentals of network architectures. Chapter 12 • Be able to describe the ISO/OSI reference model and the TCP/IP standard. Network Organization and Architecture 2 12.1 Introduction 12.1 Introduction • Computer network – an interconnection of • Resource sharing across networks has taken computers and computing equipment using the form of multi-tier architectures having either wires or radio waves over small or large numerous disparate servers, sometimes far geographic areas. removed from the users of the system. • The network is a crucial component of today’s • If you think of a computing system as collection computing systems. of workstations and servers, then surely the network is the system bus of this configuration. 3 4
12.2 Early Business 12.2 Early Business Computer Networks Computer Networks • The first computer networks consisted of a mainframe • Hierarchical, polled network host that was connected to one or more front end processors. Predominant form in the 1960s and 1970s. • Front end processors received input over dedicated The front end processors lines from remote communications controllers poll each of the cluster connected to several dumb terminals. controllers, which in turn poll their attached • The protocols employed by this configuration were terminals proprietary to each vendor’s system. • One of these, IBM’s SNA (created in 1974) became the model for an international communications standard, the ISO/OSI Reference Model. 5 6 12.3 Early Academic and 12.3 Early Academic and Scientific Networks Scientific Networks • In the 1960s, the Advanced Research Projects Agency • A modern internetwork configuration funded research under the auspices of the U.S. Department of Defense. • Computers at that time were few and costly. In 1968, the Defense Department funded an interconnecting network to make the most of these precious resources. The network, DARPANet, had sufficient redundancy to withstand the loss of a good portion of the network. • DARPANet was the world’s first operational packet switching network, and the first to implement TCP/IP. • DARPANet later turned over to the public domain, and eventually evolved to become today’s Internet. 7 8
12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model • To address the growing tangle of incompatible • The OSI RM proprietary network protocols (also details were contains seven sometimes kept secret), in 1984 the ISO formed a protocol layers, committee to devise a unified protocol standard. starting with physical media • The result of this effort is the ISO Open Systems interconnections Interconnect Reference Model (ISO/OSI RM). at Layer 1, • The ISO’s work is called a reference model because through virtually no commercial system uses all of the features applications at precisely as specified in the model. Layer 7. • The ISO/OSI model does, however, lend itself to understanding the concept of a unified communications architecture. 9 10 12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model • The OSI model • The OSI model reduces complexity by defines only the breaking network communication into smaller functions of each simpler parts (layers). of the seven • Each layer performs a subset of the required layers and the communication functions. interfaces between them. • Each layer relies on the next lower layer to • Implementation perform more primitive functions. details are not part • Each layer provides services to the next of the model. higher layer. No layer skipping is allowed. • Changes in one layer should not require changes in other layers. 11 12
12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model • Flow of data through the OSI model • The Physical layer receives a stream of bits from the Data Link layer above it, encodes them and places them on the communications medium. • The Physical layer conveys transmission frames, called Physical End-to-end These layers only exist in the layers host processors at the ends of Protocol Data Units , or Physical the connection. PDUs. Each physical PDU carries an Device-to- These layers exist at the ends of address and has delimiter signal device layers the connection and also in the intermediate nodes that make up patterns that surround the payload , or the path. contents , of the PDU. 13 14 12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model • The Data Link layer is responsible for • At the originating computers, the taking the data and transforming it into Network layer adds addressing a frame with header. It negotiates information to the Transport layer frame sizes and the speed at which PDUs. they are sent with the Data Link layer • The Network layer establishes the at the other end. route and ensures that the PDU size – The timing of frame transmission is is compatible with all of the called flow control . equipment between the source and the destination. • Data Link layers at both ends acknowledge packets as they are • Its most important job is in moving exchanged. The sender retransmits PDUs across intermediate nodes. the packet if no acknowledgement is received within a given time interval. 15 16
12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model • The OSI Transport layer provides • The Session layer is responsible for end-to-end acknowledgement and establishing sessions between users. error correction through its It arbitrates the dialogue between two handshaking with the Transport layer communicating nodes, opening and at the other end of the conversation. closing that dialogue as necessary. – The Transport layer is the lowest layer • It controls the direction and mode of the OSI model at which there is any ( half-duplex or full-duplex) . awareness of the network or its • It also supplies recovery checkpoints protocols. during file transfers. • Transport layer assures the Session • Checkpoints are issued each time a layer that there are no network- block of data is acknowledged as induced errors in the PDU. being received in good condition. 17 18 12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model • The Presentation layer provides • The Application layer supplies high-level data interpretation meaningful information and services for the Application layer services to users at one end of above it, such as EBCDIC-to- the communication and ASCII translation. interfaces with system resources (programs and data files) at the • Presentation layer services are other end of the communication. also called into play if we use • HTTP and FTP are examples of encryption or certain types of protocols at this layer. data compression. 19 20
12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model • Common network applications include web browsing, e-mail, file • A way to remember the seven transfers, and remote logins. layers: • All that applications need to do is to send messages to the All People Seem To Need Presentation layer, and the lower Data Processing layers take care of the hard part. 21 22 12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model 23 24
12.4 Network Protocols I 12.4 Network Protocols I ISO/OSI Reference Model ISO/OSI Reference Model • Protocol data units (PDUs) MAC: Media Access Control LLC: Logical Link Control The LLC sublayer acts as an interface between the MAC sublayer and the Network layer. 25 26 12.4 Network Protocols I 12.5 Network Protocols II ISO/OSI Reference Model TCP/IP Architecture • TCP/IP is the de facto global data communications standard. • It has a lean 3-layer protocol stack that can be mapped to five of the seven in the OSI model. • TCP/IP can be used with any type of network, even different types of networks within a single session. 27 28
12.5 Network Protocols II 12.5 Network Protocols II TCP/IP Architecture TCP/IP Architecture • The IP Layer of the TCP/IP • The concept of the protocol stack provides datagram was essentially the same services fundamental to the as the Network layer of the robustness of OSI Reference Model. ARPAnet, and now, the Internet. • It divides TCP packets into protocol data units called • Datagrams can take datagrams , and then attaches any route available to routing information. them without human intervention. 29 30 12.5 Network Protocols II 12.5 Network Protocols II TCP/IP Architecture TCP/IP Architecture • Encapsulation/decapsulation of application data within the network stack. • IPv4 Packet Header 31 32
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