Transcript slides - Fei Hu

Week 2 Lecture 1
Layers (basics)
Dr. Fei Hu
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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
Typically 500 to 5,000 homes
cable headend
cable distribution
network (simplified)
home
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Cable Network Architecture: Overview
cable headend
cable distribution
network (simplified)
home
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Cable Network Architecture: Overview
server(s)
cable headend
cable distribution
network
home
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Cable Network Architecture: Overview
FDM:
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Channels
cable headend
cable distribution
network
home
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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
router
base
station
• wider-area wireless access
– provided by telco operator
– 3G ~ 384 kbps
• Will it happen??
– WAP/GPRS in Europe
mobile
hosts
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Example 5: Wireless Sensor
Networks
Sensors
Deploy
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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
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
• a series of steps
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Organization of air travel: a different view
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
Layers: each layer implements a service
– via its own internal-layer actions
– relying on services provided by layer below
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Layered air travel: services
Counter-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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ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
arriving airport
Departing airport
Distributed implementation of layer functionality
intermediate air traffic sites
airplane routing
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
application
transport
network
link
physical
– PPP, Ethernet
• physical: bits “on the wire”
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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
source
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
destination
application
Ht
transport
Hn Ht
network
Hl Hn Ht
link
physical
M
message
M
segment
M
M
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)
-- …
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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
application
transport
network
Data link
physical
– PPP, Ethernet
• physical: bits “on the wire”
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Client-server paradigm
Typical network app has two
pieces: client and server
application
transport
network
data link
physical
Client:
initiates contact with server (“speaks
first”)
typically requests service from server,
Web: client implemented in browser; email: in mail reader
request
Server:
provides requested service to client
e.g., Web server sends requested Web page, mail
server delivers e-mail
reply
application
transport
network
data link
physical
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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
host or
server
host or
server
process
controlled by
app developer
process
socket
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
controlled
by OS
API: (1) choice of transport protocol; (2) ability to fix a few
parameters
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