CHAP 1. Introduction

강의내용 - Stalling책 (10판)

• Chap. 1. 데이터통신, 데이터네트워크 및 인터넷

• Chap. 8. 다중화(Multiplexing)

• Chap. 10. 셀룰러 무선 네트워크

• Chap. 9. WAN 기술과 프로토콜 - 회선교환/패킷교환

• Chap. 2. 프로토콜 구조, TCP/IP, 인터넷 기반 응용

• 강의 내용

1) 역사적으로 개발된 여러가지 종류의 Network들

⇒ 전화망-음성, 컴퓨터 네트워크(WAN/LAN)-데이터

2) Network을 이해하는데 필요한 기본 개념들

3) Computer Network의 구조

Network

• History

- Road: 사람, 물자

- Mail: 편지

- Telegram: 전보

- Telephone: 음성 - 양방향(two-way), 실시간(real-time)

- Broadcasting: 방송 - 단방향(one-way)

- Computer Networks: 데이터 - LAN, WAN

- BcN (Broadband Convergence Network): 음성/데이터/방송 통합

• Why networks are needed?

- 인간은 '사회'를 구성: 조직화

⇒ 사람/물자/정보의 교환이 필요

⇒ 네트워크는 '공간'의 제약을 극복하려는 인간 노력의 산물들

(St. 10th ed.) pp.15 Fig. 1.3, pp.18 Fig. 1.4

(St. 7th ed.) pp.11 Fig. 1.1

⇒ 발신자(source)/전송기(transmitter)/전송시스템(transmission system)/수신기(receiver)/목적지(destination)

Example Networks

• Telephone system: PSTN → ISDN, xDSL (유선, wired)

cellular & PCS → 1G/2G/3G/4G/5G (무선, wirless)

• Computer networks:

1) WAN: ARPANet → MILNet & NSFNet → Internet

2) LAN: Ethernet, Token bus, Token ring, ATM, Wireless

• The future of the network: BcN, 초고속정보통신망, Internet 2

• Computer Network Architecture: "layered" structure

(St. 10th ed.) pp.25 Fig. 1.5, pp.28 Fig. 1.7

(St. 7th ed.) pp.20 Fig. 1.3, pp.12 Tab. 1.1

• History of Communication

1826:	Ohm의 법칙 - 전기의 법칙 I=V/R

1838:	Morse가 전(기통)신을 실험으로 증명

1864:	Maxwell이 전(자기)파의 방사 예언

1876:	Bell이 전화 발명

1887:	Hertz가 Maxwell의 이론 증명

- 스파크(빠르게 진동하는 불꽃)로부터 전자기파를 만들어 내는 실험에 성공.

1897:	Marconi가 무선 전신 시스템 개발

1906:  Forest가 3극 진공관 개발 - 신호의 증폭

1915:	Bell 시스템이 대륙횡단 전화 완성

1937:	Reeves가 펄스-부호 변조(PCM) 고안

1938:	TV방송 시작

World War Ⅱ: 레이더와 전파 시스템 개발

1948: 트랜지스터 개발;

Shannon의 “A Mathematical Theory of Communications" 발간

- 정보통신 이론의 기초

1950:	전화에 시분할 다중화(Time-division Multiplexing) 방식 적용

1956:	최초의 대서양횡단 전화 케이블 완성

1960～1970: Maiman이 레이저 개발(1960),

최초의 통신위성 TelstarⅠ발사(1962), 달에서 탐사의 TV 생중계,

실험적인 PCM 시스템, 실험적인 레이저 통신,

집적회로(IC), 디지털 신호 처리

컬러 TV

1970～1980: 상업용 위성중계 통신(음성, 디지털), 기가비트의 신호 속도,

대규모 집적회로(LSI, VLSI), 통신회로에서의 IC 구현,

대륙횡단 컴퓨터 통신망, 저손실 광섬유, 손쉬운 광통신 시스템,

패킷교환 디지털 데이터 시스템,

목성, 토성, 천왕성, 해왕성(1989)에서의 통신

1980～1990: 슈퍼컴퓨터 발전, 위성(하늘의 교환기) 발전,

이동 셀룰러 전화 시스템, 2 gigasamples/sec 디지털 발진기,

programmable 디지털 신호 프로세서

1990～현재: 칩의 암호화, 단일 칩 부호기와 복호기,

LAN(Ethernet)의 발전,

Bell 시스템 해체 이후 장거리 전화 서비스의 경쟁

전지구 위치 파악 시스템(GPS) 완성

지구 전체의 위성에 기반을 둔 셀룰러 전화와 종합 서비스

(GMPCS - Global Mobile Personal Communication by Satellite)

디지털 통신망(ISDN) 개발, 개인 통신(PCS)의 실현

IoT(Internet of Things): 사물인터넷

Ex) 컴퓨터의 이해 (B. Williams): Computer and Communications

The Telephone System: PSTN (Public Switched Telephone Network)

• Terminology: Carrier(운반매체)의 물리적 구조

- signal(신호): data-문자, 숫자, 음성, 그림, 영상-의 물리적인 전달 수단

electric(전류, 전압)/electro-magnetic(전파)

analog(continuous signal)/digital(pulse signal - 0 or 1),

e.g., analog/digital TV

- frequency(주파수): Hz (cycles/sec)

high frequency ↔ low frequency

- bandwidth(대역폭): 통신에서 어떤 대역(band)의 넓이를 나타냄. 즉, 매체(medium, wired/wireless)가 전달할 수 있는 신호의 최고 주파수와 최저 주파수의 차.

(ex) 넓은 도로: broad band → high bandwidth

좁은 도로: narrow band → low bandwidth

- channel(통로): (특정 대역을 사용하는) data 통로

⇒ a (communication) channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel. (Wikipedia)

- multiplexing(다중화): 여러 사용자의 data를 동시에 전송.

⇒ 동시에 여러 채널 연결

(St. 10th ed.) pp.245 Fig. 8.1, pp.247 Fig. 8.2

(St. 7th ed.) pp.287 Fig. 8.1, pp.288 Fig. 8.2

(St. 6th ed.) 주파수분할 Multiplexing: pp.276 Tab. 8.1

500MHz 정도인 동축케이블의 대역폭에는 수십 개의 TV 신호가 FDM을 이용하여 동시에 전송 - Broadband coaxial cable

(St. 10th ed.) 시분할 Multiplexing: pp.256 Fig. 8.7

(St. 7th ed.) 시분할 Multiplexing: pp.299 Fig. 8.7

(ex)	TDMA(Time Division Multiple Access): 시분할의 examples

T1 Carrier

- Codec: PCM(Pulse Code Modulation)을 사용.

⇒ Analog voice signal을 digital code로 변환.

- (7+1) bit/sample voice, 24 voices are multiplexed.

⇒ 192bits/sample plus 1 control bit

- 8000 samples/sec (125 μsec/frame)

⇒ 193 bits/sample × 8000 frames/sec =1.544 Mbps

E1 Carrier

- 2.048 Mbps: 32 voices are multiplexed

SONET/SDH

810 octets × 8bits × 8000 frames/sec =51.84 Mbps

(St. 10th ed.) pp.258 Tab. 8.3, pp.259 Fig 8.9

(St. 7th ed.) pp.302 Fig. 8.9

(St. 10th ed.) pp.259 Tab. 8.4, pp.260 Fig 8.10

• PSTN Characteristics

- 음성전송: Telephone to Telephone

⇒ 음성(20 - 20,000 Hz) → Analog signal (300 - 3,400 Hz)

⇒ 전송한계: Bandwidth, Noise  30,894 bps (Shannons formular)

Ex) SKT, 한국통신(KT), 데이콤(LG U+), AT&T, MCI, US Sprint, Verizon

-	Analog (가입자 - 전화국) 및 Digital-sampling (전화국간)

⇒ analog transmission: 가입자 ↔ 지역

digital transmission: 지역 ↔ 중앙 ↔ 중앙 ↔ 지역

- Digital (가입자 - 전화국)

- Circuit switching: 회선점유 방식 for voice transmission

Once a call has been set up, a "dedicated path"(circuit) between both ends exists and will "continue to exist" until the call is finished.

(St. 10th ed.) pp.281 Fig. 9.1

(St. 7th ed.) pp.356 Fig. 10.1

(St. 10th ed.) pp.283 Fig. 9.2, pp.284 Fig. 9.3

(St. 7th ed.) pp.359 Fig. 10.2, pp.360 Fig. 10.3

(St. 10th ed.) pp.287 Fig. 9.5, pp.288 Fig. 9.6

(St. 7th ed.) pp.363 Fig. 10.5, pp.364 Fig. 10.6

• Cellular Network

- 이동전화/PCS(개인통신시스템)/무선인터넷

- a geographic region is divided up into "cells", each using some set of frequences.

⇒ relatively small cells

⇒ less power(0.6 watts) is need for a celluar phone, which leads to smaller and cheaper devices.

(St. 10th ed.) pp.314 Fig. 10.1, pp.315 Fig. 10.2, pp.318 Fig. 10.5

- "reuse" transmission frequencies in nearby (but not adjacent) cells.

⇒ At the center of each cell is a ‘base station’ (기지국) to which all the telephones in the cell transmit.

⇒ Finding locations high in the air to place base station antennas is a major issue

(ex) The Roman Catholic church

- all the base station are connected to a device called an MTSO(Mobile Telephone Switching Office) or MSC(Mobile Switching Center).

⇒ The MTSO is connected to a telephone system end office (PSTN).

- 1G/2G/3G/4G

(St. 10th ed.) pp.326 Tab. 10.1, pp.329 Fig. 10.9

- hand-off: 통화 중의 기지국간 이동

⇒ The telephone is informed of its "new" base station, and if a call is in progress, it will asked to switch to a new channel (because the old frequency is not used in any of the adjacent 6 cells)

Computer Networks: WAN

• ARPANet (Advanced Research Project Agency of the U.S. DoD)

- ‘68 리클리더 & 테일러 논문: computer network for data communication & resouce sharing

- '69/11/17: 시험망 연결

⇒ leased line(전용선)과 IMP를 이용한 packet-switched network 개발

⇒ 구조: point-to-point network,

⇒ 패킷교환망

(St. 10th ed.) pp.292 Fig. 9.10, pp.297 Fig. 9.13

(St. 7th ed.) pp.378 Fig. 10.11, pp.383 Fig. 10.14

- IMP(Interface Message Processor):

․ made by BBN사 (Bolt, Beranek & Newman Inc.)

․ Honeywell DDP-316 minicomputers with 12K 16-bit words of core memory.

․ routing 기능: store-and-forward the packets.

⇒ A host could send messages of up to 8063 bits to its IMP.

⇒ IMP would then break these	up into packets of at most 1008 bits and forward them independently toward the destination.

⇒ Each packet was received in its entirety before being forwarded, so the subnet was the first electronic store-and-forward packet- switching network.

․ 초기: 56kbps leased line → T1(1.544Mbps)/E1급 사용

4 hosts/IMP → 10's hosts/IMP

⇒ ‘Router’ (현재)

- Growth of the ARPANet:

⇒ ‘71, MIT와 Harvard 포함 20개 연결.

⇒ ‘74, 62개 연결.

⇒ ‘81, 200개 연결.

⇒ '83, separate the military portion (about 160 IMPs) into a separate subnet, MILNet.

⇒ the remaining research subnet and NSFNet were interconnected

→ the Internet (the collection of networks)

- store-and-forward network:  For 'data' transmission

⇒ no path is established in advance. (회선 공유 방식)

① message switching: each msg is received in its entirety, inspected for errors, then retransmitted.

⇒ no limit on message size

⇒ a router must have a big buffer (disk) to buffer long messages, and a single message may tie up a router-router line for many minutes.

(ex) telegrams

② packet switching: divide a message into 'packets', and send seperately.

⇒ tight upper limit on packet size

⇒ less buffer size (memory), and no user can monopolize any transmission line very long (miliseconds)

⇒ well suited for "interactive" traffic

(ex) PSDN(Packet Switched Data Network) ↔ PSTN (circuit switching)

- 비교:

Circuit switching	Packet switching
- statically reserves the requird channel in advance - any unused channel on an allocated circuit is wasted - no reodering - no buffering	- acquires and releases a channel as needed - a channel may be utilized by other packets - packets may be delivered in wrong order → reordering is needed - buffering is needed

(St. 10th  ed.) pp.299 Fig. 9.14, pp.299 Fig. 9.15, pp.300 Tab. 9.1

(St. 7th  ed.) pp.385 Fig. 10.15, pp.386 Tab.10.2

- 두가지 packet switching:

(St. 10th ed.) pp.294 Fig. 9.11, pp.295 Fig. 9.12,

(St. 7th ed.) pp.380-pp.381 Fig. 10.12, Fig. 10.13, pp.441 Fig. 12.2

(St. 10th ed.) ATM: pp.302 Fig. 9.16, pp.305 Fig. 9.17

• The Internet: packet switching network의 확장

- Growth continued "exponentially".

⇒ connect existing networks to the Internet

(ex)	SPAN: NASA's space physics network

HEPNET: a high energy physics network

BITNET: IBM's mainframe network

EARN: a European academic network

and many other

⇒ How the glue is possible? TCP/IP (only official protocol for the Internet)

- a computer is on the Internet if it runs the TCP/IP protocol, has an IP address, and has the ability to send IP packets to all the other compters on the Internet.

(Ex) PC: 네트워크 설정 - 인터넷 프로토콜 (TCP/IP)

- services: academic, government & industrial users:

⇒ ① Info. Sharing: E-mail, News, file transfer(ftp), WWW, distributed DB

② Resouce Sharing: remote login(telnet), network printer

⇒ '90,

① PC: many PC's have ability to call up an Internet service provider's router using a modem, be assigned a temporary IP address, and send IP packets to other Internet hosts. (SLIP/PPP, Ethernet)

② WWW(GUI): invented by CERN physicist Tim Berners-Lee

→ new, ‘nonacademic’ users for commercial uses (전자 상거래 등)

⇒ audio/video stream: Voice over IP, Broadcasting 등

Computer Networks: LAN

• Medium 공유 구조: NOT point-to-point network ('store-and-forward')

⇒ Broadcasting - packets sent by any computer are received by all the others.

⇒ 패킷방송망

• Ethernet

- '73, R. Metcalfe at MIT: Ph.D. thesis - decribed his research on LANs.

⇒ '76, R. Metcalfe at Xerox with D. Boggs implement the Ethernet LAN. (ether - a medium for electro-magnetic radiation (wave))

⇒ a 2.94Mbps system to connect over 100 computers on a 1 km cable.

- Intel built a single chip controller for Ethernet.

⇒ LAN card (Wikipedia)

- medium/channel access protocol

․ ALOHA: Terminals and a host via a wireless network.

① Transmit whenever a terminal wants.

② If collision, wait a random period of time, and retransmit.

․ CSMA/CD (Carrier Sensed Multiple Access/Collision Detection)

① Having all the computers listen to the cable

② If the cable is idle, any computer can transmit.

If collision, they all stop, wait a random period of time, and try again.

⇒ distributed control (no central controller)

⇒ In theory, there is no upper bound on the time a computer might "have to wait" to send a message.

⇒ starvation may occur.

⇒ for office protocol

(St. 10th ed.) pp.381

• Token bus

- factory automation at GM: conveyer belt moving at a "fixed" rate.

⇒  정해진 시간 내에 message가 전달되어야 함.

⇒  CSMA/CD does not have this property making it unsuited for real-time systems.

- 구조: p.287 fig.4-25

- medium access protocol: the computers took turns in round-robin way.

① When a "logical" ring is initialized, the highest numbered station may send the first message.

② After it is done, it passes permission to its immediate neighbor by sending the neighbor a special control message called a "token".

③ The token propagates around the logical ring with only the token holder being permitted to transmit messages.

⇒ If there are n stations and it takes T sec to send a message, no message will ever have to wait more than nT sec to be sent.

⇒ Giving a "deterministic" worst-case performance rather than a "statistical" one.

⇒ No collision

⇒ for manufacturing automation protocol

• Token ring

- IBM's Zurich Lab.

⇒ p.293 fig.4-28 (4th ed.)

⇒ a "physical" ring is not really a broadcast medium, but a collection of individual point-to-point links that happen to form a circle.

⇒ a ring is also "fair", and has a known upper bound on channel access.

- medium access protocol: the computers took turns in round-robin way.

① token circulates around the ring whenever all computers are idle.

② when a computer wants to transmit a message, it is required to seize the token and removes it from the ring before transmitting.

⇒ No collision

• Later

- IEEE(Institute of Electrical and Electronics Engineer) approved LAN standards

① IEEE 802.3: CSMA/CD LAN (based on Ehernet backed by DEC, Xerox & Intel)

② IEEE 802.4: Token bus LAN backed by GM

③ IEEE 802.5: Token ring LAN backed by IBM

- IEEE 802.11b/g: Wireless LAN

• Bridge/LAN Switch

- LAN과 LAN의 연결

- 'Store-and-forward' packets

(St. 10th ed.) pp.363 Fig. 11.11, pp.365 Fig. 11.12

(St. 7th ed.) pp.573 Fig.15.8

• Backbone LANs

- FDDI(Fiber Distributed Data Interface): a high performance fiber optic token ring LAN running at 100Mbps over distance up to 200km with up to 1000 stations connected.

- High speed Ethernet

Ex) Fast Ethernet (100Mbps), Gigabit Ethernet

(St. 10th ed.) pp.382~

- ATM switch

Ex) 155Mbps ATM switch: ATM Hub들을 연결해 줌.

ATM

card

ATM

card

ATM Hub

ATM Switch

ATM Hub

• Interconnected LANs, WANs & PSTN

(St. 10th ed.) pp.28 Fig. 1.7

e.g. KNUE: LAN - KREN (교육망)

1,100Mbps (KT: 1,000Mbps + Dacom : 100Mbps)

http://eits.knue.ac.kr

KREONET

- TCP/IP is used for the integration of all

⇒ "one" big computer network

⇒ the Internet

The Future of the Network

• Hybrid network: the integration of the PSTN and the IP network.

⇒ BcN (Broadband Convergence Network)

⇒ high-speed data transport for science and industry

⇒ It will offer VOD, live TV, full motion multimedia  E-mail, CD-quality music, and many other services that have not yet been thought of.

• 'Internet2' Project in the U.S.

Computer Network Architecture

• 위에서 여러가지 종류의 Network들을 살펴봄 ⇒ Computer Network의 ‘구조’는?

• H/W: Packet Switching Network - LANs (local/backbone) & WANs

S

S

R

R

R

R

R

R

R

S

R

S

KNUE

leased line

NSFNet

U. C. at

Berkeley

U. C. at

L. A.

210.93.99.64

211.34.193.6

KREN

(서울대)

KORNet

(KT)

Ex) KNUE: KT/Dacom에 연결 - 1,100Mbps (KT: 1,000Mbps + Dacom: 100Mbps)

• S/W: 어떻게 H/W를 운영할 것인가? ⇒ protocol(규약)

Ex) 국가(H/W) - 헌법(S/W)

- "Layered" Structure

OS	DBMS	산업구조

command/user program

process/file control

H/W driver

H/W

query processor

DB engine

file manager

disk controller

disk

OS

운수회사

자동차회사

부품회사

재료(철강,프라스틱)회사

광산, 석유회사

Why? ① Partition the N/W design into several smaller manageable problem.

⇒ divide-and-conquer

② Make it simpler to replace the implementation of one layer with a completely different implementation

⇒ abstraction

- Why?

․ Abstraction: independent from underlying architecture. (가상통신 제공)

중역 A

비서 A

FAX A

중역 B

비서 B

FAX B

PSTN

계약서

가상통신

실제경로

계층 3

계층 2

계층 1

․ 보안성: 사용자로부터 system을 보호

․ 확장성: 새로운 기술을 도입시 해당 layer만 교체.

(St. 10th ed.) pp.43 Fig. 2.9

- Most networks are organized as a series of "layers".

⇒ no. of layers, the name of each layer, the contents of each layer, the function of each layer 'differ' from network to network.

(ex) OSI reference model: 7 layers

․ a proposal developed by ISO

․ NOT a network architecture

⇒ because it does NOT specify the exact services and protocols to be used in each layer

․ it just tells what each layer should do

(ex) TCP/IP reference model: 4 layers

(ex) Novell Netware reference model: 5 layers

(St. 10th ed.) pp.33 Fig. 2.1, pp.34 Fig. 2.2, pp.36 Fig. 2.3

(St. 7th ed.) pp.30 Fig. 2.4, pp.31 Fig. 2.5, pp.36 Fig. 2.7

- The purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented.

⇒ 'Abstraction'

- Each layer passes data and control information to the layer immediately below it until the lowest layer is reached.

⇒ Clean-cut 'interfaces' bet. the layers and well-understood 'functions' of each layer are required.

(St. 10th ed.) pp.37 Fig. 2.4, pp.42 Fig. 2.8

(St. 7th ed.) pp.37 Fig. 2.8, pp.40 Fig. 2.10, pp.42 Fig. 2.11

• 각 Layer의 기능 (OSI 7 layer)

- Application layer: services to users

(ex) E-mail(SMTP), file transfer(ftp), virtual terminal(telnet), WWW(HTTP) 등

* Presentation layer: syntax and semantics of the 'information'(정보) transmitted.

(ex) encryption(암호화), encoding/decoding, multimedia data 전송 등

* session layer: manage "sessions" bet. two machines.

(ex) dialog

- Transport layer: process-to-process comm.

(ex) TCP, UDP

- Network layer: controlling the operation of the network

⇒ host-to-host comm.	즉, network 전체에서 data를 전달.

⇒ routing, congestion control, internetworking

(ex) IP

□NIC에서 32-bit IP address 부여

□network: KREONet, KREN, KORNet(KT), BORANet(Dacom)등

□subnetworking

- Data link layer: 두 인접한 host/IMP(router) 간에 frame들을 전달.

⇒ framing, error detection & correction (free of transmission errors),

flow control

(ex) X.25, Frame Relay, ATM 등

* Medium access sublayer: LAN에서 medium을 사용하는 방법을 규정.

(ex)  CSMA/CD, Token Ring, ATM 등

- Physical layer: Transmitting raw bits over a communication channel.

⇒ 물리적인 electric/electromagnetic signal을 전달.

(ex) wired(유선), wireless(무선)

(St. 10th ed.) pp.36 Fig. 2.3

(St. 7th ed.) pp.34 Fig. 2.6, pp.35 Tab. 2.2

• 전송방식(규약, protocol): NOT a reference ‘model’

- 사전에 ‘미리 정의된’ 데이터 전송에 관한 ‘약속’

(ex) - TCP/IP - Internet

- SNA - IBM

- DECNet - DEC

- IPX/SPX - Novell Netware

TCP/IP protocol suite (or just TCP/IP)

• A "family" of protocols was specified as the standard for the ARPANet and associated DoD network in the early 1980s.

⇒	최초의 컴퓨터통신망인 ARPANet에 채택 (1983)

⇒	Unix에 구현 - 개방성

⇒	Internet의 표준 protocol	e.g. English

• Characteristics

․ It is NOT vendor-specific.

․ It has been implemented on everything from PC to the supercomputers.

․ It is used for both LANs and WANs.

․ It is used by many different government agencies and commercial sites, NOT just DARPA-funded networks.

• Why INTER+NET?

서로 다른 구조의 Network들을 TCP/IP로 통합

⇒ ‘Integration’

• Data Communication on the IP Network

: 각 host의 TCP/UDP를 사용하여 IP network을 통해 데이터를 교환.

• 각 layer의 기능

① Application layer: services to the users.

② Transport layer: process-to-process comm.

③ Internet layer: routing, congestion control, internetworking.

⇒ host-to-host comm.

④ Network Access + Physical layer: error detection & correction, delivery of physical signal.

⇒  node-to-node comm.	(host-to-gateway, gateway-to-gateway)

(St. 10th ed.) pp.33 Fig. 2.1, pp.34 Fig. 2.2

(St. 7th ed.) pp.28 Fig. 2.2, pp.46 Fig. 2.12

(St. 10th ed.) pp.36 Fig. 2.3, pp.37 Fig. 2.4

(St. 7th ed.) pp.49 Fig. 2.14, pp.51 Fig. 2.15

(St. 10th ed.) pp.38 Fig. 2.5, pp.42 Fig. 2.8

(St. 7th ed.) pp.20 Fig. 1.3

1. Application Layer

- SMTP, FTP, TELNET, HTTP 등

2. Transport Layer: TCP and UDP

① Connection-oriented transfer: TCP

․ First, the server (process) is started, then, sometime later a client (process) is started and establish a "virtual connection" with a server by using a three-way handshake.

ex) 전화.

․ "Reliable" data delivery service with end-to-end error detection and correction.

ex) SMTP, FTP, TELNET, POP(post office protocol), HTTP 등.

⇒ (in /etc/services in Unix/Linux)

② Connectionless transfer: UDP

․ The client does not establish a connection with the server and the server does not have to accept a connection from a client.

⇒ The client just sends a "datagram" to the server on which the address of the server is written.

ex) 편지, 전보

․ NO error detection and correction.

⇒ Unreliable datagram delivery service.

ex) echo, talk 등. (in /etc/services)

(St. 10th ed.) pp.40 Fig. 2.6

(St. 7th ed.) pp.40 Fig. 2.10

- 데이터그램 패킷교환은 미리 경로를 구성하지 않음.

⇒ 길이가 짧은 메시지의 경우 가상회선 패킷교환보다 빠름.

⇒ 그러나 데이터그램이 독립적으로 경로배정되므로 각 노드에서의 처리시간이 더 오래 걸림.

⇒ 긴 메시지의 경우 가상회선 패킷교환이 더 낫다.

3. Network Layer (Internet Layer in TCP/IP): IP

- IP is the protocol that provides the 'packet delivery' service for TCP and UDP.

⇒ "routing", internetworking, congestion control

TCP

UDP

H/W

interface

IP

process

host

H/W

interface

H/W

interface

IP

gateway(router)

H/W

interface

H/W

interface

IP

gateway(router)

TCP

UDP

H/W

interface

IP

process

host

WAN

LAN

LAN

- Every IP packet contains the "source" addr. and the "dest." addr. so that each packet can be routed and delivered "independently."

⇒ Reassembling packets must be provided by the upper layer. (TCP & UDP)

	0		8	16		24 31
	version	head. len.	type of service	total length
	identification			flags	fragmentation offset
	time to live		protocol	header checksum
	"source addr."
	"destination addr."
	options					padding

- IP header format

(St. 10th ed.) pp.41 Fig.2.7

(St. 7th ed.) pp.47 Fig. 2.13

Ex) IP Networks in Korea

4. Data Link & Physical Layer (Network Access Layer in TCP/IP)

- The protocol in this layer provide the means to deliver frames to the other devices on a "directly attached" network.

- 다양한 physical connections

① LAN: Ethernet(CSMA/CD), FDDI(token ring), ATM 등.

WAN: PSDN(packet switching) - X.25, ATM 등.

② Physical media

․ Wired: twisted pair, coaxial cable(baseband/broadband), optical fiber 등.

․ Wireless: radio, infrared, microwave(terrestrial/satellite) 등.

- There are "many" network access protocols - one for each physical network standard.

⇒ As new H/W technology appear, "new" network access protocols must be developed so that TCP/IP networks can use the new H/W.

ex) ATM on optical fiber network.

Computer Network의 ‘용어’와 ‘기본 개념’의 이해