telecommunication networks introduction

Telemedicina e eSaúde

Telecommunication Networks Introduction 1

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Telecommunication Networks IntroductionTelemedicina e e-Saúde

2019/20

Pedro Brandão

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References

• These slides are the companions of “Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009”oWith adaptations/additions by Rui Prior and Pedro Brandão

Telemed eSaúde 19/20 - Networks Intro - pbrandao

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Digital, Analogic


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Analogic Digital


Sampling

Quantification

Images from Wikipedia

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A few words on audio compression

• Audio signal sampling at a fixed rate:o telephone: 8 000 samples/sec

o CD: 44 100 samples/sec

• Each sample is quantized, rounded:o Ex.:, 28=256 possible values

• Each quantized value is represented by bitso 8 bits for 256 values

• example: 8 000 samples/sec, 256 quantized values 64 000 bps

• receiver converts bits back to analogue signal:o some quality reduction

• Example rates

• CD: 1.411 Mbps

• MP3: 96, 128, 160 kbps

• Internet telephony: 5.3 kbps and up


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A few words on video compression

• video: sequence of images displayed at constant rateo e.g. 24 images/sec

• digital image: array of pixelso each pixel represented by bits

• redundancyo spatial (within image)

o temporal (from one image to next)

Examples:

• MPEG 1 (CD-ROM) 1.5 Mbps

• MPEG2 (DVD) 3-6 Mbps

• MPEG4 (often used in Internet, < 1 Mbps)

Research:

• layered (scalable) videoo adapt layers to available bandwidth


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Introduction


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Introduction

Our goal:

• get “feel” and terminology

• approach:o use Internet as example

Overview:

• what’s the Internet?

• what’s a protocol?

• network edge; hosts, access net, physical media

• network core: packet/circuit switching, Internet structure

• performance: loss, delay, throughput

• protocol layers, service models


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Summary

1.1 What is the Internet?

1.2 Network edge end systems, access networks, links

1.3 Network core circuit switching, packet switching, network structure

1.4 Delay, loss and throughput in packet-switched networks

1.5 Protocol layers, service models


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What’s the Internet: under the “hood”


• millions of connected computing devices: hosts = end systems

o running network apps

• communication links

o fibre, copper, radio, satellite

o transmission rate = bandwidth

• routers: forward packets (chunks of data)

Home Network

Institutional Network

Mobile Network

Global ISP

Regional ISP

router

PC

server

Wirelesslaptop

Cellularphone

Wired links

Access points

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Network Equipment

Hub(out-of-date) LAN Switches Access Point

Router

Router

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Firewall

Modular server

Server

Network Equipment

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Data centre

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Data centre

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Cloud

But “Is the cloud

really just someone else's computer?” by Jack Wallen

“There is no cloud”, image and sentenceby Chris Watterson,

https://www.techrepublic.com/article/is-the-cloud-really-just-someone-elses-computer/

https://www.kcl.ac.uk/people/dr-christopher-j-watterson



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• protocols control sending, receiving of msgso e.g., TCP, IP, HTTP, Skype, Ethernet

• Internet: “network of networks”o loosely hierarchical

o public Internet versus private intranet

• Internet standardsoRFC: Request for comments

o IETF: Internet Engineering Task Force


Home network


Mobile network

Global ISP

Regional ISP

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What’s the Internet: a service view

• communication infrastructure enables distributed applications:oWeb, VoIP, email, games, e-commerce,

file sharing, telemedicine

• communication services provided to apps:o reliable data delivery from source to

destination

o “best effort” (unreliable) data delivery


Home network


Mobile network

Global ISP

Regional ISP

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What’s a protocol?

human protocols:

• “what’s the time?”

• “I have a question”

• introductions

… specific messages sent

… specific actions taken when messages received, or other events

network protocols:

• machines rather than humans

• all communication activity in Internet governed by protocols


protocols define format, order of messages sent and received among network entities, and actions taken on messages transmission, receipt

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What’s a protocol?


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a human protocol and a computer network protocol:

Hi

Hi

Got the

time?

It’s 14:00

TCP connectionrequest

GET https://mim.med.up.pt

<file>

time

TCP connection

response



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Summary







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A closer look at network structure:

• network edge: applications and hosts

• access networks, physical media:wired, wireless communication links

• network core: o interconnected routers

o network of networks


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The network edge:

• end systems (hosts):o run application programs e.g. Web, email

o at “edge of network”

• client/server modelo client host requests, receives service

from always-on server

o e.g. Web browser/server; email client/server

• peer-peer model:ominimal (or no) use of dedicated

servers

o e.g. BitTorrent, BitCoin, Spotify and Skype (up to a point)


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p2p

c/s

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Access networks and physical media

Q: How to connect end systems to edge router?

• residential access nets

• institutional access networks (school, company)

• mobile access networks

Keep in mind:

• bandwidth (bits per second) of access network?

• shared or dedicated?


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1 Byte comprised of 8 bits



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Residential access: point to point

•Modem connectionoUses existing telephony infrastructure Home is connected to central office

oup to 56Kbps direct access to router (often less)

oCan’t surf and phone at same time: not “always on”

•DSL: Digital Subscriber LineoAlso uses existing telephone infrastructure

oUpstream and downstream bandwidth may be different

orates depend on the technology, with VDSL2 can go over 100 Mbps

odedicated physical line to telephone central office


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Residential access: cable modems

•HFC: hybrid fibre coaxialoasymmetric: up to 400 Mbps downstream, 20 Mbps upstream

onetwork of cable and fibre attaches homes to ISP router

ohomes share access to router unlike DSL, which has dedicated access

oUsually supplied by cable TV companies


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Cable Network Architecture: Overview


server(s)

Typically 500 to 5,000 homes

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home

cable headend

cable distribution

netw ork (simplif ied)

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Cable Network Architecture: Overview


home

cable headend

cable distribution

netw ork (simplif ied)

Channels

VIDEO

VIDEO

VIDEO

VIDEO

VIDEO

VIDEO

DATA

DATA

CONTROL

1 2 3 4 5 6 7 8 9

FDM (more shortly):

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Typically 500 to 5,000 homes



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Cable Network Architecture: fibre optics

•Active (AON) or passive (PON)


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Source: Images from Wikipedia

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Local Area Networks

• local area network (LAN) connects terminals to routers

• Ex.: Ethernet: o 10 Mbs, 100Mbps, 1Gbps, 10Gbps

oUsual configuration: terminals connect to Ethernet switch


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Wireless access networks

• shared wireless access network connects end system to routero via base station aka “access point”

• wireless LANs:o 802.11b/g (WiFi): 11 or 54 Mbps

o 802.11n: up to 600 Mbps

o 802.11ac : up to 6.77 Gbps

o 802.11ax : up to 11 Gbps

• wider-area wireless accesso provided by telecommunication operator

o ~14.4 Mbps over cellular system (EVDO, HSDPA)

o LTE up to 300 Mbps over wide area


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Mobile hosts

Router

Base station

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Home Networks

Typical home network components:

•DSL or cable modem

• router/firewall/NAT

• Ethernet

•Wireless access point


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To/from

centralmodem

router/

firewall EthernetWireless

Access point

Wireless

laptops



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Physical Media

• Bit: propagates betweentransmitter/receiver pairs

• physical link: what lies between transmitter & receiver

• guided media: o signals propagate in solid media:

copper, fibre, coax

• unguided media: o signals propagate freely, e.g., radio

Twisted Pair (TP)

• two insulated copper wires


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Physical Media: coax, fibre

Coaxial cable:

• two concentric copper conductors

• bidirectional

Fiber optic cable:

• glass fibre carrying light pulses, each pulse a bit

• high-speed operation:o high-speed point-to-point

transmission (e.g., 10’s-100’s Gbps)

• low error rate: repeaters spaced far apart; immune to electromagnetic noise


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Physical media: radio

• signal carried in electromagnetic spectrum

• no physical “wire”

• bidirectional

• propagation environment effects:o reflection oobstruction by objectso interference

Radio link types:

• terrestrial microwaveo e.g. up to 45 Mbps channels

• LAN (e.g., Wifi)o 11Mbps, 54 Mbps, 600 Mbps, 1300 Mbps

• wide-area (e.g., cellular)o 3G cellular: ~ 14.4 Mbpso 4G cellular: ~ 100 Mbpso 5G cellular: ~ 1.4 Gbit/s

• SatelliteoKbps to 45 Mbps channel (or multiple

smaller channels)o 270 msec end-end delayo geosynchronous versus low altitude


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Summary







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The Network Core

• mesh of interconnected routers

• the fundamental question: how is data transferred through net?o circuit switching: dedicated circuit per

call: telephone net

o packet-switching: data sent thru net in discrete “chunks”


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Network Core: Circuit Switching

End-end resources reserved for “call”

• link bandwidth, switch capacity

• dedicated resources: no sharing

• circuit-like (guaranteed) performance

• call setup requiredo Establishing connection

oData transfer

o disconnecting


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Network Core: Circuit Switching

• Developed to transmit voice

• “Intelligence” resides on the network

• Dedicated resources per calloNo sharing (capacity wasted)

oGuaranteed performance

• Good resource usage for voice callso Someone is always talking most of the

time…

• For datao Inactive line most of the time

• Dividing link bandwidth into “pieces”o Frequency division (FDM)

o Time division (TDM)


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Circuit Switching: FDM and TDM


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FDM

frequency

time

TDM

frequency

time

4 users

Example:



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Numerical example

•How long does it take to send a file of 640 000 000 bits from host A to host B over a circuit-switched network?oAll links are 153.6 Mbps

oEach link uses TDM with 24 slots/s

o500 ms to establish end-to-end circuit


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100 s + 500 ms = 100,5 s

* In practice we need to add propagation delay… More on that later

153.6 𝑀𝑏𝑝𝑠

24= 6.4 𝑀𝑏𝑝𝑠

640 000 000 𝑏𝑖𝑡𝑠

6.4 𝑀𝑏𝑝𝑠= 100 𝑠

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Network core: packet switching


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Network core: packet switching

Each packet is dealt with independently:

• No relation with preceding packets

• Each node chooses next hop per packet

• Packets for the same destination do not necessarily follow the same route (although they usually do)

• Packets may arrive out of ordero Destination re-orders them

• Packets may be dropped or corrupted in transit

• Source and destination are responsible for dealing with losses and corruption


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Network Core: Packet Switching

idea: divide end-to-end data flow in small pieces (packets)

• Packets of different users share network resources

• each packet uses full link bandwidth

• resources used as needed

resource contention:

• aggregated resource demand can exceed resources available

• congestion: packets queue, wait for link use

• store and forward: packets move one hop at a timeo Node receives complete packet before

forwarding


Bandwidth division into “pieces”

Dedicated allocation

Resource reservation

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Packet Switching: Statistical Multiplexing

• Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing.

• In TDM: each host gets same slot in revolving TDM frame.


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A

B

CEthernet100 Mb/s

1.5 Mb/s

D E

Statistical multiplexing

queue of packetswaiting for output

link

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Packet-switching: store-and-forward

• takes L/R seconds to transmit (push out) packet of L bits on to link at R bps

• store and forward: entire packet must arrive at router before it can be transmitted on next link

• delay = 3L/R (assuming zero propagation delay)

Example:

• L = 7.5 Mbits (~0.94 Mbytes)

• R = 15 Mbps

• Transmission delay = 1.5 s


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R R R

L



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Packet switching versus circuit switching

• 1 Mb/s link

• Each user:o100 kb/s when “active”

oactive 10% of time

• circuit switching: o10 users

• packet switching : oWith 35 users, probability > 10 active at same time is less than 0.000424


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N users

1 Mbps

Packet switching allows more

users to use network

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Packets vs. Circuits

• Great for bursty datao resource sharing

o simpler, no call setup

• excessive congestion:packet delay and lossoprotocols needed for reliable

data transfer, congestion control


Is packet switching a “slam dunk winner?”

Q: How to provide circuit-like behavior?• bandwidth guarantees needed for audio/video apps• still an unsolved problem• Over-provisioning is the current answer…

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Internet: network of networks

Internet is composed of

• Backbones: infrastructures to interlink networks, like NSFNET, in the USA, GÉANT in Europe, as well as commercial operators backbones like PT/Altice

• Regional networks, connecting for example universities and research institutes, e.g. FCCN

• Commercial networks, e.g. internal usage or for supplying services to customers, like internet connections, e.g. PT/Altice, NOS, Vodafone…

• Local networks


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RCTS (Rede Ciência, Tecnologia e Sociedade)

Education and research national

network

Communication platform to provide

researchers, professors and students

the specific network resources needed


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Diagram

https://www.fccn.pt/institucional/rcts/

https://www.fccn.pt/wp-content/uploads/2019/09/mapaRCTS.2019.pdf



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GÉANT

• Pan-European network of research and education

• Interconnects RCTS and its 25 European Points of Presence (POP)o 50 million users at 10 000

institutions across Europe.

• Core at 100Gbps (with develfor 8Tbps), connections starting at 155Mbps


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GÉANT Connection


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New map, withfew changes

https://www.geant.org/Resources/#maps

https://www.geant.org/Resources/Documents/GEANT_at_the_Heart_of_Global_Research_and_Education_Networking_Sept_2019.pdf



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Internet: network of networks

• A packet flow crosses several networks

• Visual Traceroute


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Domestic network

Institution network

Mobile network

Global ISP

Regional ISP

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Summary







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http://en.dnstools.ch/visual-traceroute.html



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How do losses and delays occur?

Packets queue in router buffers

• If packet arrival rate exceeds output link capacityo packets queue, wait for turn


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A

B

Packet being transmitted (delay)

Packets queuing (delay)

Free (available) buffer: arriving packets dropped (loss) if no free buffers

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Four sources of packet delay

1. Node processing : o Check bit errors

oDetermine output link

2. Queuingo time waiting at output link for

transmission

o depends on congestion level of router


A

B

Node processingQueuing

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Four sources of packet delay

3. Transmission delay

• R= link bandwidth (bps) (rate)

• L= packet length (bits)

• Time to send bits into link = L/R

4. Propagation delay

• d = length of physical link (distance)

• s = propagation speed in medium (~2x108 m/s)

• Propagation delay = d/s


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A

B

Note: s and R are verydifferent quantities!propagation

transmission

Example

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Caravan analogy

• cars “propagate” at 100 km/hr

• toll booth takes 12 sec to service car (transmission time)

• car ≈ bit; caravan ≈ packet

• Q: How long until caravan is lined up before 2nd toll booth?

• Time to “push” entire caravan through toll booth onto highway = 12 × 10 = 120 sec

• Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr

• A: 62 minutes


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toll booth

toll booth

ten-car caravan

100 km 100 km

http://www.ccs-labs.org/teaching/rn/animations/propagation/



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Nodal delay

• dproc = processing delay

o Typically a few microseconds or less

• dqueue = queuing delay

o depends on congestion

• dtrans = transmission delay

o = L/R, significant for low speed links

• dprop = propagation delay

oA few microseconds to hundreds of milliseconds

proptransqueueprocnodal ddddd

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Packet loss

• queue (buffer) preceding link has finite capacity

• packet arriving to full queue dropped (lost)

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


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A

B

Packet being transmitted

packet arriving to

full buffer is lost

Buffer(waiting area)

Packet being propagated



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Throughput

• Throughput: rate (bits/time unit) at which bits transferred between sender/receiveroinstantaneous: rate at given point in time

oaverage: rate over longer period of time


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link capacityRs bits/sec

Rc bits/sec)server, withfile of F bits

to send to client

server sends bits (fluid) into pipe

pipe that can carryfluid at rateRc bits/sec

pipe that can carryfluid at rateRs bits/sec

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Throughput

•Rs < Rc What is average end-end throughput?

• Rs > Rc What is average end-end


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link on end-to-end path that constrains end-to-end throughput

bottleneck link

Rc bits/sRs bits/s

Rc bits/sRs bits/s



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Throughput: Internet scenario

• Q: throughput per connection? omin(Rc,Rs,R/10)

• in practice: Rc or Rs is often bottleneck


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10 connections (fairly) share backbone bottleneck link R bits/sec

Rs

Rs

Rs

Rc

Rc

Rc

R

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Summary







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Protocol layers

•Networks are complex!!!!

•Many “pieces”:ohosts

orouters

oLinks of various types

oapplications

oprotocols

ohardware, software


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Sending “snail” mail

• Each layer uses the lower adding something:oAddress “precision”

oExtra services (reliability, receipt acknowledgement)


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Registered

Street, Number

Postal Code

Reception acknowledge

Street, Number

Postal Code Postal Code

Registered

Street, Number

Postal Code

Reception acknowledge

Regional Dist. Local Dist.



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Why layering?

Dealing with complex systems:

• explicit structure allows identification, relationship of complex system’s pieces

olayered reference model for discussion

• modularization eases maintenance, updating of system

ochange of implementation of layer’s service transparent to rest of system

oe.g., change in gate procedure doesn’t affect rest of system


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Internet protocol stack

• Application: network applicationso FTP, SMTP, HTTP

• Transport: transferring data between processeso TCP, UDP

• Network : routing of datagrams between source and destinationo IP, routing protocols

• Link: data transfer between neighbouring network elementso PPP, Ethernet

• Physical: bits “on the wire”


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Physical

Link

Network

Transport

Application



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source

application

transport

networklink

physical

HtHn M

segment

datagrama

destination

HtHnHl M

HtHn M

Ht M

M

network

link

physical

link

physical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

EncapsulationM

HtHt M

Hn

frame

application

transport

networklink

physical

67message

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The endIntroduction to communication networks