[컴퓨터네트워크] Computer Network - Chapter 1. Introduction

 

컴퓨터 네트워크 수업을 듣고 배웠던 내용을 다시 정리하고자 합니다.

여기서는 Chapter 1. Introduction 부분을 정리하며 컴퓨터 네트워크에 대한 기본적인 개념들 위주로 정리하였습니다.

 

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Chapter 1. Introduction

Chapter 1: roadmap

What is the Internet?

What is a protocol?

Network edge : hosts, access network, physical media

Network core : packet/circuit switching, internet structure

Performance : loss, delay, throughput

Security

Protocol layer, service models

History

 

Internet : “network of networks”

• hosts = end systems
• running network apps at Internet’s “edge”
• Packet switches : forward packets (chunk of data) ex) routers, switches
• Communication links ex) fiber, copper, radio, satellite
• Networks : collection of devices, routers, link : managed by an organization

protocol : 사전에 약속된 규약 (protocol define the format, order of messages sent and received among network entities, and actions taken on message transmission, receipt

 

A closer look at Internet structure

Network edge

access networks, physical media : wired, wireless communication links

Network core : interconnected routers, network of networks

 

Access networks : cable-based access

frequency division multiplexing (FDM) : different channels transmitted in different frequency bands

 

Wireless access network (18p)

• Wireless local area networks (WLANs)
• Wide-area cellular access network

Host : sends packets of data

host sending function :

• takes application message
• breaks into smaller chunks, known as packets, of length L bits
• transmits packet into access network at transmission rate R

packet transmission delay = time needed to transmit L-bit packet into link = $$ \frac{L(bits)}{R(bits/sec)}$$ 

 

 

Links : physical media

 

bits : propagates between transmitter / receiver pairs

physical link : what lies between transmitter & receiver

guided media : signals propagate in solid media : copper, fiber, coax

unguided media : signals propagate freely ex) radio

 

Twisted pair(TP)

• two insulated copper wires

Coaxial cable

• two concentric copper conductors
• bidirectional
• broadband

Fiber optic cable

• glass fiber carrying light pulses, each pulse a bit
• high-speed operation (high speed point to point transmission)
• low error rate

Wireless radio

• signal carried in various “bands” in electromagnetic spectrum
• no physical “wire”
• broadcast, “half-duplex”

Network core : packet/circuit switching, internet structure

• packet-switching : hosts break application-layer messages into packets

Two key Network core functions

1. Routing (global action) - determine source-destination paths taken by packets
2. forwarding (local action) - move arriving packets from router’s input link to appropriate router output link

Packet-switching : store-and-forward

• packet transmission delay : takes L/R seconds to transmit(push out) L-bit packet into link at R bps
• store and forward : entire packet must arrive at router before it can be transmitted on next link

ex) one-hop numerical example
L = 10 Kbits
R = 100 Mbps
one-hop transmission delay = 0.1 msec

 

Packet-switching : queueing

 

Queueing occurs when work arrives faster than it can be serviced

• Packet queuing and loss : if arrival rate (in bps) to link exceeds transmission rate (bps) of link for some period of time
• packets will queue, waiting to be transmitted on output link
• packets can be dropped (lost) if memory (buffer) in router fills up

 

Alternative to packet switching : circuit switching

 

end-end resources allocated to, reserved for “call” between source and destination

• dedicated resources : no sharing
• circuit-like (guaranteed) performance
• circuit segment idle if not used by call (no sharing)
• commonly used in traditional telephone networks

Frequency Division Multiplexing (FDM)

• optical, electromagnetic frequencies dived into (narrow) frequency bands
• each call allocated its own band, can transmit at max rate of that narrow band

Time Division Multiplexing (TDM)

• time divided into slots
• each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band (only) during its time slot(s)

ex ) 1 Gb/s link
each user : 100 Mb/s when "active"
active 10% of time

Q ) how many users can use this network under circuit-switching and packet switching?

circuit-switching : 10 users
- packet switching : with 35 users, probability > 10 active at same time is less than .0004 *

Q ) how did we get value 0.0004 ?
HW problem (for those with course in probability only)

Packet switching vs Circuit switching

packet-switching : hosts break application-layer messages into packets

• Forwarding : local action : move arriving packets from router’s input link to appropriate router output link
• Routing : global action : determine source-destination paths taken by packets
• store-and-forward
• Queueing : packets can be dropped if memory in router fills up

circuit-switching : end-end resources allocated to, reserved for “call” between surce and destination

• dedicated resources : no sharing
• circuit segment idle if not used by call(no sharing)
• commonly used in traditional telephone networks

Is packet switching a “slam dunk winner”? → great for “bursty” data - sometimes has data to send, but at other times not (어떨 땐 보낼 데이터가 있고, 어떨 땐 보낼 데이터가 없는 상황에서는 좋다)

• resource sharing
• simpler, no call setup

excessive congestion possible : packet delay and loss due to buffer overflow

• protocols needed for reliable data transfer, congestion control

How to provide circuit-like behavior with packet-switching ?

“It’s complicated” We’ll study various techniques that try to make packet witching as “circuit-like” as possible

 

Internet structure : a “network of networks”

 

hosts connect to Internet via access Internet Service Providers (ISPs)

access ISP in turn must be interconnected (any two hosts can send packets to each other)

resulting network of networks is very complex (evolution driven by economics, national policies)

Performance : loss, delay, throughput

packets queue in router buffers, waiting for turn for transmission

• queue length grows when arrival rate to link exceeds output link capacity
• packet loss occurs when memory to hold queued packets fills up

— packet being transmitted (transmission delay)

— packets in buffers (queueing delay)

Packet delay : four sources
transmission delay
queueing delay
processing delay
propagation delay

 

$$ d_{nodal} = d_{proc} + d_{queue} + d_{trans} + d_{prop} $$

$$ d_{proc} $$

nodal processing

• check bit errors
• determine output link
• typically < microsecs

$$ d_{queue} $$

queueing delay

• time waiting at output link for transmission
• depends on congestion level of routers
• $$ d_{trans} $$

transmission delay

• L : packet length (bits)
• R : link transmission rate (bps)

L/R

$$ d_{prop} $$

propagation delay

d : length of physical link

s : propagation speed

d/s

❗ queue의 length가 증가할수록 queueing delay가 늘어남. 계속 queueing delay가 늘어나면 결국 packet이 쌓일 수 밖에 없고, packet loss 가 발생

packet loss

• queue의 overflow로 인한 packet loss
• 무선 네트워크인 경우 ) bit error로 인한 packet loss
• 이러저러한 이유로 길을 못찾아서 없어지는 경우

Packet queueing delay (revisited)

a : average packet arrival rate

L : packet length (bits)

R : link bandwidth (bit transmission rate)

$$ La/R $$

 

packet loss

queue (aka buffer) preceding link in buffer has finite capacity

packet arriving to full queue dropped (aka lost)

lost packet may be retransmitted by previous node, by source end system, or not at all

 

Throughput

throughput : rate (bits/time unit) at which bits are being sent from sender to receiver

instantaneous : rate at given point in time

average : rate over longer period of time

→ 이 두 경우 end-end throughput은 같다

 

Security

packet interception

• packet “sniffing”
• (broadcast media, promiscuous network interface reads/records all packets)

fake identity

• injection of packet with false source address

denial of service

• attackers make resources(server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic

Lines of defense

authentication

confidentiality

integrity checks

access restrictions

firewalls

 

Protocol layers, service models

layers : each layer implements a service

 

why layering?

• explicit structure allows identification, relationship of system’s pieces
• modularization eases maintenance, updating of system

Layered Internet protocol stack

• application : supporting network applications <Application exchanges messages to implement some application service using services of transport layer>
• transport : process-process data transfer <Transport-layer protocol transfer M from one process to another, using services of network layer>
• network : routing of datagrams from source to destination <Network-layer protocol transfers transport-layer segment from one host to another, using link layer services>
• link : data transfer between neighboring network elements < Link-layer protocol transfers datagram from host to neighboring host, using network-layer services>
• physical : bits “on the wire”

 

 

 

 

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이해가 쉽게 풀어서 쓰고 싶어서.. 계속 편집할 예정.

 

 

 

 

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Computer Network 의 다른 글도 보고 싶으면 밑의 포스팅을 참고해주세요

 

 

[컴퓨터네트워크] Computer Networks