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 (AC

M)• the Institute of Electrical and Electronics Engineer

s (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 m

essages 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 pac

ket 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 syst

em 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 t

he 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 que

ue– 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 datagr

ams 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 netw

ork 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) a

nd 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 A

TM

• 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 th

e Internet– Residential Intenet access became very inexpen

sive– 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 l

ayer

• 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