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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