Computer Network OverviewComputer Network Overview

Ikjun Yeom

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What is a computer network?What is a computer network?

• a collection of computing nodes and links connecting the nodes

• why we need it?– to transfer data to others

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 3 (FDDI)

H7 R3 H8

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What is the Internet?What is the Internet?

• originally means “Inter-network”• a network of networks• now means a network using IP (Internet

Protocol) for inter-networking• equivalent to IP networks• deployed in the whole world

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What is a network protocol?What is a network protocol?

• a pre-defined rule for data transfer• examples: IP, TCP, UDP, HTTP, RTP• why do we need it?

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Network LayeringNetwork Layering

• the current network protocols are implemented based on layering concept

• currently, there are five layers:– physical, data-link, network, transport and

application

• why ?

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Internet Protocol GraphInternet Protocol Graph

…

FTP HTTP NV TFTP

TCP UDP

IP

NET1 NET2 NETn

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Physical LayerPhysical Layer

Computer A Computer B

NICNetwork Interface CardEthernet CardLAN card

Wire

Physical Layer

Data

Physical LayerPhysical Layer

• Computing nodes– PC, PDA, Cellular phone, and embedded

systems– create packets, send and receive them.

• Links– physical medium to propagate data (packets)

from a node to the other– wired or wireless

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Computing NodesComputing Nodes

• General purpose computers such as PC, workstation, Laptop and PDA connected to a network.

• Either end-hosts or intermediate nodes (switch or router)

CPU

Cache

Memory I/O bus

Networkadaptor (To network)

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Physical Media - WiredPhysical Media - Wired

Twisted Pair (TP)• two insulated copper

wires– Category 3: traditional

phone wires, 10 Mbps ethernet

– Category 5 TP: 100Mbps ethernet

Fiber optic cable:• glass fiber carrying light

pulses• high-speed operation:

– 100Mbps Ethernet– high-speed point-to-point

transmission (e.g., 5 Gps)• low error rate

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Physical media - wirelessPhysical media - wireless

• no physical “wire”• categorized by transmission range:

– wide area links: CDMA and GSM– local area links: Wireless LAN– personal area links: Bluetooth, Zigbee

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Data Link LayerData Link Layer

Computer A Computer B Computer C

Data

DataDLLData link layer Data link layer

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Data Link LayerData Link Layer

• Framing• Unique ID• Medium Access Control (MAC)

– CSMA/CD (Carrier Sense Multiple Access/Collision Detection)

– CSMA/CA (Collision Avoidance)– Token Ring– TDMA (Time Division Multiple Access)

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Network LayerNetwork Layer

Data link layer

Router

IP

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Network LayerNetwork Layer

• Routing– RIP – Routing Information Protocol– OSPF – Open Shortest Path First

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IP Packet FormatIP Packet Format

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

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telnet

netscape

ftptelnet

netscape

ftp

Computer B

Data link layerIP

Computer A

Data link layerIPTCP TCP

Date

IP

MAC IPTCP

DateTCP

DateTCP

Date

Date

IP

MAC IPTCP

DateTCP

DateTCP

Date

Transport LayerTransport Layer

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Transport LayerTransport Layer

• End-to-end delivery• Transmission Control Protocol (TCP)

– Flow control – Advertised Window– Error control – Checksum– Reliable delivery – Loss detection and

retransmission

• User Datagram Protocol (UDP)

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Typical Limitations of the Typical Limitations of the NetworkNetwork

• Drop messages• Reorder messages• Deliver duplicate copies of a given

message• Limit messages to some finite size• Deliver messages after an arbitrarily long

delay

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UDP (User Datagram Protocol)UDP (User Datagram Protocol)

• Extending the host-to-host deliver service of the underlying network into a process-to-process communication service.

• Port number– Well-known port number– Port mapper

• Checksum – pseudoheader• Used for delay-sensitive applications

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UDP Header FormatUDP Header Format

SrcPort DstPort

Checksum Length

Data

0 16 31

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Source Port Destination Port

Sequence Number

Acknowledgement Number

Checksum Urgent Pointer

Options Padding

0 4 10 16 24 31

URG

ACK

PSH

RST

SYN

FIN

HeaderLength Reserved Window Size

Data

TCP SegmentTCP Segment