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Computer Networking 숙명여자대학교 컴퓨터 과학과 최 종원 What is the Internet? • • • • • • • A world-wide computer network hosts or end systems network application program protocol communication link router packet switching Service Description • The Internet allows distributed applications running on its end-systems to exchange data with each other • The Internet provides two services to its distributed applications: a connection-oriented service and a connectionless service. • Currently the Internet does not provide a service that makes promises about how long it will take to deliver the data from sender to receiver. Soom Good Hyperlinks • • • • Internet Engineering Task Force (IETF): The World Wide Web Consortium (W3C): The Association for Computing Machinery (ACM) the Institute of Electrical and Electronics Engineers (IEEE): • Connected: An Internet Encyclopedia: • Media History Project: What is a Protocol? • A Human Ananlogy : Fig 1.2 • Network Protocols – a protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event The Network Edge • End Systems, Clients and Servers • Figure 1.3• Connectionless andConnection-Oriented Service – Connection-Oriented Services • • • • reliable data transfer Flow control congestion control TCP • Connectionless Service • UDP The Network Core • Figure 1.4circuit switching • packet switching • Message Switching Routing in Data Networks • Classes of packet-switched networks – – – – datagram networks and virtual circuit networks route according to host dest addr or to vc number datagram net : Internet vc network : X.25, frame relay and ATM • VC network – a path – vc numbers, one # for each link along the path – entries in VC number translation table in each packet switch • Why vc numbers are changed? – The length of the vc field is reduced – a network management function is simplified • Datagram network – ananlogous to postal services – do not maintain connection-state information in the switches Access Networks and Physical Media • Access network - the physical link(s) that connect an end system to its edge router – residential access network – institutional – mobile • Residential access networks – use of a modem over POTS – twisted-pair phone line – Narrowband ISDN • all digital transmission of data – ADSL and HFC • adsl uses frequency division multiplexing – a hig-speed downstream channel, 50kHz-1MHz – a medium-speed upstream channel, 4kHz to 50kHz – an ordinary POTS two-way telephone channel, 0 to 4kHz • data rate : a function of the distance between the home modem and the ISP modem • HFC : extensions of the cable network – cable modem, 10-Base T Ethernet port – the rates are shared among the homes • Company access network – Lan is used to connect an end system to an edge router – twisted-apir copper wire or coxial cable – shared Ethernet and switched Ethernet • Mobile access networks – use radio spectrum to connect a mobile end system to a base station – cellular digital packet data Physical Media • Guided media and unguided media • twisted pair copper wire – UTP : category 3 and 5 • coaxial cable : baseband and broadband • fiber optics • terrestrial and satellite radio channels Delay and Loss in Packet-Switched Networks • What can happen to a packet as it travels • suffers from several different types of delays at each node along the path • nodal processing delay, queing delay, transmission delay, propagation delay Types of Delay • Processing delay : The time required to examine the packet’s header and determine where to direct the packet – the time needed to check for bit-level errors in the packet • Queuing delay : the time to wait to be trasmitted onto the link • Transmission delay : the amount of time required to transmit all of the packet’s bits into the link • Propagation delay : the time required to propagate from the beginning of the link to the next router • traffic intensity : La/R – L : number of bits of packet – a : average rate at which packets arrive to the queue – R : transmission rate • Packet loss : with no place to store packets, a router will drop that packet, lost. Protocol layers and their service models • Layered architecture : protocol stack • see fig 1.23 • layer functions : error control, flow control segmentation and reassembly, multiplexing connection setup The Internet Protocol Stack • See fig 1.24 : 5 layers • application layer : responsible for supporting network applications : http, smtp, ftp 등 • transport layer : responsible for transporting application-layer messages between the client and server sides of an application – TCP, UDP • Network layer : responsible for routing datagrams from one host to another – a protocol that defines the fields in the IP datagrams as well as how the end systems and routers act on these field : IP protocol – routing protocols that determine the routes that datagrams take between sources and destinations : many routing protocols : RIP, OSPF, IDRP 등 • Link layer : responsible for moving a packet from one node to the next node in the route – ethernet, ppp, atm, frame relay • Physical layer : responsible for moving the individual bits within the frame from one node to the next Network Entities and Layers • End systes and packet switches : network entities • two types of packet switches : routers and bridges • see fig 1.25 Internet Backbones, NAPs and ISPs • Internet topology : complex, hierarchical • NBP : internetMCI, SprintLink, PSINet, Uunet 등 - 1.5Mbps --- 622Mbps and higher • each NBP : has hubs, regional ISP tap into it • NAP : NBP interconnect • local ISP -- regional ISP • new tiers and branches can be added to the Internet topology like Lego construction A Brief History of Computer Networking and the Internet • Development and demonstration of early packet switching principles : 1961-1972 – – – – – – – – computer 등장 - time shared computers need to share among geographically distibuted users traffice was likely to be “bursty” Kleinrock, Baran, Davies 등이 Packet switching 연구 Robers : Advanced Research Projects Agency net 4 nodes ; Interface Message Processor : switching NCP : host-to-host protocol e-mail : Ray Tomlinson at BBN in 1972 Internetworking and new and proprietary networks : 1972-1980 • Additional packet-switching nework : – – – – ALOHAnet, a satelliet network Telenet : a BBN commercial net Tymnet Transpac : a French packet-switching net • Robert Metcalfe : 1973 Ethernet 연구 • Vinton Cerf and Robert Kahn : create a network of networks : open net architecture • Open Network Architecture – – – – Minimalism, autonomy best effort service stateless routers decentralized control • proprietary network architectures : – DEC : DECnet 1975 – Xerox : XNS architecture – IBM : SNA architecture A Proliferation of Networks : 1980-1990 • • • • • Time of tremendous growth BITnet, CSNET 1986, NSFNET 1983 NCP ---> TCP/IP France : Minitel : based on X.25 VC Commercialization and the Web : The 1990s • Continued evolution and the soon-to-arrive commercialization of the Internet • NSFnet : serve as a backbone network • Release of the WWW : CERN Time Berners-Lee : 1989-1991 – based on hypertext concept from Bush(1940s) and Ted Nelson(1960s) • Marc Andreesen : Mosaic : Netscape Comm Asynchronous Transfer Mode Networks • Goals of ATM : to transport real-time audio and video as well as text, e-mail, and image files. • ATM Forum and ITU : for BISDN • ATM standards – call for packet switching with virtual circuits – define how applications directly interface with ATM • Deployed within telephone net and Intenet backbone • unsuccessful in extending itself all the way to PCs and W/S area • ATM이 성공하지 못할 것이라는 의견 – TCP/IP protocol suite was integrated into all of the popular operating systems – Companies began to transact commerce over the Internet – Residential Intenet access became very inexpensive – many wonderful application were developed for TCP/IP networks – low-cost high-speed LAN tech developed Principle Characteristics of ATM • ATM standards defines protocols from transport layer to physical layer • uses packet switching with fixed-length packets of 53 bytes : cells – each cell : 5 bytes of header, 48 bytes of payload • uses VC : virtual channels – VCI : in the header • provides no retransmissions on a link-by-link basis • provides congestion control on an end-to-end basis • can run over just about any physical layer Overview of the ATM Layers • Protocol stack – ATM adaptation layer(AAL) – ATM layer – ATM physical layer • ATM physical layer : deals with voltages, bit timings, and framing on the physical dedium • ATM layer : the core of the ATM standard, defines the structure of the ATM cell • AAL : roughly analogous to the transport layer in the Internet Protocol stack : support many different types of services – AAL 5 : allows tcp/ip to interface with atm