Transcript Computer Networking

Computer Networking
숙명여자대학교
컴퓨터 과학과
최 종원
What is the Internet?
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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
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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
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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
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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
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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 :
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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
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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
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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