1. Introduction and Overview
최양희
서울대학교 컴퓨터공학부
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Internet Timeline
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3000 BC : Abacus
1642 : Pascal : numerical wheel calculator
1822 : Babbage : difference engine
1945 : Eckert : ENIAC
1969 : first ARPANET connection
– UCLA, SRI, UCSB, Utah
• 1972 : CYCLADES (France)
• 1974 : V. Cerf : TCP
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Internet Timeline
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1977 : e-mail over Telenet
1980 : IBM : Bitnet
1981 : IBM : PC
1982 : Time
: Computer
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1984 : Domain Name System
1988 : Morris : Worm, and CERT
1989 : number of hosts exceeds 100,000
1991 : Al Gore : High Performance Computing Act
1991 : Tim Berners-Lee : WWW
1992 : Internet Society
1993 : Andreessen : Mosaic
1994 : Yahoo !
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인터넷 등장 배경
• 미 국방성의 요구: 많은 통신 시설이 파괴되더라도
계속 살아 남는 통신 네트워크를 고안할 것.
다양한 통신기술을 활용할 수 있을 것.
• 과학기술계의 결론
– 패킷 (packet) 방식의 통신네트워크
– 연결을 만들지 않음
• 전체의 70% 이상이 파괴되어도 계속 생존하여
통신기능 유지함
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패킷이란 ?
• Packet  Circuit 대응개념
• 보내야 할 정보를 취급하기 쉬운 작은 단위 (패킷이라 부름)로 쪼갬
– 예 : 소설을 엽서로 옮겨 적어 보낸다.
– 예 : 많은 화물은 여러 대의 트럭에 나누어 실어 보낸다
• 통신망은 패킷을 일일이 따로 구분하여 처리
– 엽서마다 보내는 이, 받는 이의 주소를 반복하여 적고 우체국은 엽서마다
이를 확인한다
• 받는 쪽에서 정보를 모아서 원래대로 재생
• 왜 패킷인가 ?
– 전송효율/ 네트워크 구축비용 절감 : 화물회사마다 전용차선이 있다면 낭비
– 다양한 트래픽을 쉽게 지원 : 전보, 편지, 음성, 영상, 컴퓨터 데이타
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패킷 통신의 예
A
A
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B
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B
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B
B
회선 공유
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A
A
A
B
B
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Internet Configuration
subnet
Packet
Host
subnet
subnet
Host
subnet
Router
Packet
subnet
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Internet structure:
network of networks
• roughly hierarchical
• national/international
backbone providers (NBPs)
local
ISP
regional ISP
NBP B
– e.g. BBN/GTE, Sprint, AT&T, IBM,
UUNet
– interconnect (peer) with each
NAP
other privately, or at public
Network Access Point (NAPs)
• regional ISPs
NAP
NBP A
regional ISP
local
ISP
– connect into NBPs
• local ISP, company
– connect into regional ISPs
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Protocols
• Protocol: rules for communication
– Message formats, timing
– Describes how a computer responds when a
message arrives
– Specifies how a computer handles errors or
other abnormal conditions
• All network services are described by
protocols
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What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
req.
Hi
TCP connection
reply.
Got the
time?
Get http://gaia.cs.umass.edu/index.htm
2:00
<file>
time
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Key Elements of a Protocol
• Syntax
– Data formats
– Signal levels
• Semantics
– Control information
– Error handling
• Timing
– Speed matching
– Sequencing
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A Three Layer Model
• Network Access Layer
• Transport Layer
• Application Layer
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Simplified File Transfer
Architecture
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Network Access Layer
• Exchange of data between the computer and the
network
• Sending computer provides address of
destination
• May invoke levels of service
• Dependent on type of network used (LAN, packet
switched etc.)
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Transport Layer
• Reliable data exchange
• Independent of network being used
• Independent of application
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Application Layer
• Support for different user applications
• e.g. e-mail, file transfer
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OSI
• Open Systems Interconnection
• Developed by the International
Organization for
Standardization (ISO)
• Seven layers
• A theoretical system delivered
too late!
• TCP/IP is the de facto standard
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• A layer model
• Each layer performs a subset of
the required communication
functions
• Each layer relies on the next
lower layer to perform more
primitive functions
• Each layer provides services to
the next higher layer
• Changes in one layer should
not require changes in other
layers
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OSI Layers
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Internet protocol stack
• application: supporting network
applications
– ftp, smtp, http
• transport: host-host data transfer
– tcp, udp
• network: routing of packets from source
to destination
– ip, routing protocols
• link: data transfer between neighboring
network elements
application
transport
network
link
physical
– ppp, Ethernet
• physical: bits on the wire
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OSI v TCP/IP
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Some Protocols in TCP/IP Suite
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Access Technologies
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Dialup via modem
DSL: digital subscriber line : ADSL, VDSL
Cable Modem
LAN (wired, wireless : IEEE 802), WiBro (mobile WiMax) 802.16e
Wireless Mesh
Bluetooth
Satellite
Cellular (2, 2.5, 3, 4G)
FTTH
Sensor network
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Data Link Layer
• two physically connected devices:
– host-router, router-router, host-host
• unit of data: frame
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
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application
transport
network
link
physical
data link
protocol
phys. link
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network
link
physical
Hl Hn Ht
frame
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M
Link Layer Services
• Framing, link access:
– encapsulate datagram into frame, adding header,
trailer
– implement channel access if shared medium,
– ‘physical addresses’ used in frame headers to identify
source, destination
• different from IP address
• Reliable delivery between two physically
connected devices:
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Link Layer Services (more)
• Flow Control:
– pacing between sender and receivers
• Error Detection:
– errors caused by signal attenuation, noise.
– receiver detects presence of errors:
• signals sender for retransmission or drops frame
• Error Correction:
– receiver identifies and corrects bit error(s) without
resorting to retransmission
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Stop and Wait
• The source transmits one frame
• The destination receives one frame and replies
with an acknowledgement
• The source waits for ACK before sending the next
frame
• The destination can stop the flow by not sending
ACK
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Sliding Windows Flow Control
• Allow multiple frames to be in transit
• Receiver has buffer (size = W frames)
• Transmitter can send up to W frames without
ACK
• Each frame is numbered
• ACK includes the number of next frame expected
• Sequence number
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Error Control
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Detection and correction of errors
Lost frames
Damaged frames
Automatic repeat request
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Error detection
Positive acknowledgment
Retransmission after timeout
Negative acknowledgement and retransmission
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Automatic Repeat Request
(ARQ)
• Stop and wait
• Go back N
• Selective reject (selective retransmission)
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Stop and Wait Diagram
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Go Back N
• Based on sliding window
• If no error, ACK as usual with the number of the
next frame expected
• Use window to control the number of outstanding
frames
• If error, reply with rejection
– Discard the bad frame and all future frames until the
error frame is received correctly
– Transmitter must go back and retransmit the bad frame
and all subsequent frames
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Go Back N :
Diagram
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Ethernet : CSMA/CD
• Carriers Sense Multiple
Access with Collision
Detection
• IEEE 802.3
– Random Access
• Stations access medium
randomly
– Contention
• Stations contend for time
on medium
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1. If medium idle, transmit,
otherwise, step 2
2. If busy, listen for idle,
then transmit
3. If collision detected, jam
then cease transmission
4. After jam, wait random
time then start from step
1
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Hubs
• Physical Layer devices: essentially repeaters operating at
bit levels: repeat received bits on one interface to all other
interfaces
• Hubs can be arranged in a hierarchy (or multi-tier design),
with backbone hub at its top
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IEEE 802.11 Wireless LAN
• IEEE 802.11 standard:
– MAC protocol
– unlicensed frequency spectrum: 900Mhz, 2.4Ghz
• Basic Service Set (BSS)
– wireless hosts
– access point (AP)
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