Week 2 Lecture 1
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1 Week 2 Lecture 1 Layers (basics) Dr. Fei Hu
description
Week 2 Lecture 1. Layers (basics) Dr. Fei Hu. Review last lecture. Dialup via modem up to 56Kbps direct access to router (often less) Can’t surf and phone at same time: can’t be “always on”. Communication Network example 1: Residential access: point to point access. - PowerPoint PPT Presentation
Transcript of Week 2 Lecture 1
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Week 2 Lecture 1
Layers (basics)Dr. Fei Hu
Review last lecture
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Communication Network example 1:Residential access: point to point access
• Dialup via modem– up to 56Kbps direct access to
router (often less)– Can’t surf and phone at same
time: can’t be “always on”
• ADSL: asymmetric digital subscriber line– up to 1 Mbps upstream (today typically < 256 kbps)
– up to 8 Mbps downstream (today typically < 1 Mbps)
– FDM: 50 kHz - 1 MHz for downstream
4 kHz - 50 kHz for upstream
0 kHz - 4 kHz for ordinary telephone
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ADSL
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Communication Network example 2:Residential access: cable modems
• HFC: hybrid fiber coax– asymmetric: up to 10Mbps upstream, 1 Mbps
downstream• network of cable and fiber attaches homes to ISP
router– shared access to router among home– issues: congestion, dimensioning
• deployment: available via cable companies, e.g., MediaOne
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Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5,000 homes
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork (simplified)
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork
server(s)
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Cable Network Architecture: Overview
home
cable headend
cable distributionnetwork
Channels
VIDEO
VIDEO
VIDEO
VIDEO
VIDEO
VIDEO
DATA
DATA
CONTROL
1 2 3 4 5 6 7 8 9
FDM:
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Communication Network example 3:Company access: local area networks
• company/univ local area network (LAN) connects end system to edge router
• Ethernet:
– shared or dedicated link connects end system and router
– 10 Mbs, 100Mbps, Gigabit Ethernet
• deployment: institutions, home LANs happening now
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Example 4:Wireless access networks (In week 4 –
some details!)• shared wireless access network
connects end system to router– via base station aka “access point”
• wireless LANs:– 802.11b (WiFi): 11 Mbps
• wider-area wireless access– provided by telco operator
– 3G ~ 384 kbps
• Will it happen??
– WAP/GPRS in Europe
basestation
mobilehosts
router
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Example 5: Wireless Sensor Networks
DeploySensors
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How to study Comm Networks?Sprint US backbone network – so complex !
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Communication Systems:How do I understand you?
Networks are complex! • many “pieces”:
– hosts– routers– links of various
media– applications– protocols– hardware,
software
Question: Is there any hope of organizing structure of
network?
Or at least our discussion of networks?
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Using “layers” – just like air travel
• a series of steps
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
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Organization of air travel: a different view
Layers: each layer implements a service– via its own internal-layer actions– relying on services provided by layer below
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
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Layered air travel: servicesCounter-to-counter delivery of person+bags
baggage-claim-to-baggage-claim delivery
people transfer: loading gate to arrival gate
runway-to-runway delivery of plane
airplane routing from source to destination
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Distributed implementation of layer functionality
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
airplane routing
airplane routing
Dep
art
ing
air
port
arr
ivin
g
air
port
intermediate air traffic sites
airplane routing airplane routing
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Why layering?Dealing with complex systems:• explicit structure allows identification, relationship of
complex system’s pieces
– layered reference model for discussion
• modularization eases maintenance, updating of system
– change of implementation of layer’s service transparent to rest of system
– e.g., change in gate procedure doesn’t affect rest of system
• layering considered harmful?
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For Communication Systems: The same thing …
• application: supporting network applications– FTP, SMTP, STTP
• transport: host-host data transfer– TCP, UDP
• network: routing of datagrams from source to destination– IP, routing protocols
• link: data transfer between neighboring network elements– PPP, Ethernet
• physical: bits “on the wire”
application
transport
network
link
physical
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Layers are actually implemented as …
Each layer takes data from above• adds header information to create new data unit• passes new data unit to layer below
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
source destination
M
M
M
M
Ht
HtHn
HtHnHl
M
M
M
M
Ht
HtHn
HtHnHl
message
segment
datagram
frame
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This course focuses on …
• The lowest layer -- Physical Layer
• It handles signal transmission (i.e. digital communications), including –
-- encoding/decoding ( 3 weeks)
-- Modulation (3 weeks)
-- A D or D A (2 weeks)
-- …
Let’s talk about higher layers
• Applications Layer:
• Execute application-level protocols– HTTP (Web)– FTP– SMTP / POP3 / IMAP (E-mail)– DNS
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Application Layer location
• application: supporting network applications– FTP, SMTP, STTP
• transport: host-host data transfer– TCP, UDP
• network: routing of datagrams from source to destination– IP, routing protocols
• link: data transfer between neighboring network elements– PPP, Ethernet
• physical: bits “on the wire”
application
transport
network
Data link
physical
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Client-server paradigmTypical network app has two
pieces: client and serverapplicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
Client:initiates contact with server (“speaks first”)typically requests service from server, Web: client implemented in browser; e-mail: in mail reader
request
reply
Server:provides requested service to cliente.g., Web server sends requested Web page, mail server delivers e-mail
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App Layer: Processes communicating
• process sends/receives messages to/from its socket
• socket analogous to door– sending process shoves
message out door
– sending process assumes transport infrastructure on other side of door which brings message to socket at receiving process
process
TCP withbuffers,variables
socket
host orserver
process
TCP withbuffers,variables
socket
host orserver
Internet
controlledby OS
controlled byapp developer
API: (1) choice of transport protocol; (2) ability to fix a few parameters
