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Computer Networks & The InternetComputer Networks & The Internet

Lecture 2Lecture 2

Imran AhmedImran AhmedUniversity of Management & TechnologyUniversity of Management & Technology

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Agenda

• Network & its types• What’s the Internet?• What’s a protocol?• History• Network edge• Network core• Access net, physical media• Internet/ISP structure• Performance: loss, delay• Protocol layers, service models

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A closer look at network structure:

• network edge: applications and hosts

• network core: – routers– network of networks

• access networks, physical media: communication links

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The network edge:

• end systems (hosts):– run application programs

– e.g. Web, email

– at “edge of network”

• client/server model– client host requests, receives service

from always-on server

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

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

servers

– e.g. Gnutella, KaZaA

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Network edge: connection-oriented service

• Goal: data transfer between end systems

• Handshaking: setup (prepare for) data transfer ahead of time– Hello, hello back human

protocol

– Set up “state” in two communicating hosts

• TCP – Transmission Control Protocol– Internet’s connection-

oriented service

• TCP service [RFC 793]

• Reliable, in-order byte-stream data transfer– Loss: acknowledgements

and retransmission

• Flow control:– Sender won’t overwhelm

receiver

• Congestion control:– Senders “slow down

sending rate”, when network congested

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Network edge: connectionless service

• Goal: data transfer between end systems– Same as before!

• UDP – user Datagram Protocol [RFC768]:– Connectionless

– Unreliable data transfer

– No flow control

– No congestion control

• App’s using TCP:– HTTP (Web), FTP (file

transfer), Telnet (remote login), SMTP (email)

• App’s using UDP:– Streaming media,

teleconferencing, DNS, Internet telephony

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Agenda

• Network & its types• What’s the Internet?• What’s a protocol?• History• Network edge• Network core• Access net, physical media• Internet/ISP structure• Performance: loss, delay• Protocol layers, service models

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The Network Core

• mesh of interconnected routers

• the fundamental question: how is data transferred through net?– circuit switching:

dedicated circuit per call: telephone net

– 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 required

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Circuit Switching

A B

Source Destination

It’s the method used by the telephone network. A call has three phases:

1. Establish circuit from end-to-end (“dialing”),2. Communicate,3. Close circuit (“tear down”).

Originally, a circuit was an end-to-end physical wire. Nowadays, a circuit is like a virtual private wire: each

call has its own private, guaranteed data rate from end-to-end.

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Circuit Switching Telephone Network

Source“Caller”

Central Office“C.O.”

Destination“Callee”

Central Office“C.O.”

TrunkExchange

Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000

calls.

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Packet Switching

A

R1

R2

R4

R3

B

Source Destination

It’s the method used by the Internet. Each packet is individually routed packet-by-packet,

using the router’s local routing table. The routers maintain no per-flow state. Different packets may take different paths. Several packets may arrive for the same output link at

the same time, therefore a packet switch has buffers.

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Packet SwitchingSimple router model

R1Link 1

Link 2

Link 3

Link 4

Link 1, ingress Link 1, egress

Link 2, ingress Link 2, egress

Link 3, ingress Link 3, egress

Link 4, ingress Link 4, egress

ChooseEgress

ChooseEgress

ChooseEgress

ChooseEgress

“4”

“4”

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Why does the Internet usepacket switching?

1. Efficient use of expensive links:– The links are assumed to be expensive and scarce. – Packet switching allows many, bursty flows to share the

same link efficiently.– “Circuit switching is rarely used for data networks, ...

because of very inefficient use of the links” - Gallager

2. Resilience to failure of links & routers:– ”For high reliability, ... [the Internet] was to be a datagram

subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted” - Tanenbaum

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Some Definitions• Packet length, P, is the length of a packet in bits.• Link length, L, is the length of a link in meters.• Data rate, R, is the rate at which bits can be sent, in bits/second,

or b/s.1

• Propagation delay, PROP, is the time for one bit to travel along a link of length, L.

PROP = L/c.• Transmission time, TRANSP, is the time to transmit a packet of

length P. TRANSP = P/R.

• Latency is the time from when the first bit begins transmission, until the last bit has been received. On a link:

Latency = PROP + TRANSP.

1. Note that a kilobit/second, kb/s, is 1000 bits/second, not 1024 bits/second.

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Packet Switching: Store-and-forward

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Packet Switching

Host A

Host B

R1

R2

R3

A

R1

R2

R4

R3

B

TRANSP1

TRANSP2

TRANSP3

TRANSP4

PROP1

PROP2

PROP3

PROP4

Source Destination

“Store-and-Forward” at each Router

( )i ii

TRANSP PROP Minimum end to end latency

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Agenda

• Network & its types• What’s the Internet?• What’s a protocol?• History• Network edge• Network core• Access net, physical media• Internet/ISP structure• Performance: loss, delay• Protocol layers, service models

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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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Internet access technologies

• Previously, most people use 56K dial-up lines to access the Internet, but a number of new access technologies are now being offered.

• The main new access technologies are:– Digital Subscriber Line (DSl,ADSL)– Cable Modems– Local Area Networks– Wireless Networks

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Access networks: DSL & ADSL

• Digital Subscriber Line (DSL) is one of the most used technologies now being implemented to significantly increase the data rates over traditional telephone lines.

• Historically, voice telephone circuits have had only a limited capacity for data communications because they were constrained by the 4 kHz bandwidth voice channel.

• Most local loop telephone lines actually have a much higher bandwidth and can therefore carry data at much higher rates.

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Access networks: DSL & ADSL

• DSL services are relatively new and not all common carriers offer them.

• Two general categories of DSL services have emerged in the marketplace. – Symmetric DSL (SDSL) provides the same

transmission rates (up to 128 Kbps) in both directions on the circuits.

– Asymmetric DSL (ADSL) provides different data rates to (up to 640 Kbps) and from (up to 6.144 Mbps) the carrier’s end office. It also includes an analog channel for voice transmissions.

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DSL ArchitectureLocal Carrier End Office

Line Splitter

Customer Premises

Telephone

DSL Modem

Hub

Computer Computer

Local Loop

MainDistribution

Frame

CustomerPremises

CustomerPremises

VoiceTelephoneNetwork

DSL AccessMultiplexer

ATM Switch

ISP POP

ISP POP

ISP POP

ISP POP

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Access networks: Cable modems

• One potential competitor to DSL is the “cable modem” a digital service offered by cable television companies which offers an upstream rate of 1.5-10 Mbps and a downstream rate of 2-30 Mbps.

• A few cable companies offer downstream services only, with upstream communications using regular telephone lines.

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Cable Modem Architecture

Cable Company Distribution Hub

Cable Splitter

Customer Premises

TV

Cable Modem

Hub

Computer Computer

SharedCoaxCable

System

Combiner

CustomerPremises

CustomerPremises

TV VideoNetwork

Cable ModemTermination

System

ISP POP

Cable CompanyFiber Node

Optical/ElectricalConverter

Downstream

Upstream

Router

Cable Company

Fiber Node

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Access networks: Local area networks

• Company/univ. local area network (LAN) connects end system to edge router

• Ethernet:– Shared or dedicated link, connects end systems

and router– 10 Mbs, 100mbs, Gigabit Ethernet etc.

• Details will be available in future

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Access networks: Wireless networks

• Shared wireless access network connects end system to router via base stations aka “access point”

• Wireless Lans:– 802.11b (WiFi): 11 Mbps

• Wide-area wireless access:– Provided by telecommunication companies

– WAP/GPRS etc.

• Satellite:– Up to 50 Mbps channels or multiple smaller channles

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Physical Media

• Bit: propagates between transmitter/rcvr pairs

• Physical link: What lies between transmitter & receiver

• Guided media:– Signals propagates in solid

media; copper, fiber, coax.

• Unguided media:– Signals propagates freely,

e.g., radio

Twisted Pair (TP)

• Two insulated copper wires:– Category 3: traditional

phone wires, 10 Mbps (Ethernet)

– Category 5: 100 Mbps (Ethernet)

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Physical Media

Coaxial cable:• Two concentric copper

conductors

• Bidirectional

• Baseband:– Single channel on cable

• Broadband:– Multiple channel on cable

Fiber optic cable:• Glass fiber carrying light

pulses, each pulse a bit

• High-speed operation:– High-speed point-to-point

transmission (e.g., 5 Gps)

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

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Summary

• We studied about network edge: end systems etc.

• Circuit switching & packet switching.

• Internet access technologies: DSL, cable modem etc.

• Physical medias: Twisted pair, coaxial pair & fiber optics.