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 Mbps 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 : –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 – Ray Tomlinson at BBN in 1972

Internetworking and new and proprietary networks : 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 : 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 : –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, , 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 ATM 이 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