Case Studies

Networked Systems 3 Lecture 3

Lecture Outline

• Case Studies: Network Design Choices • The Telephone Network

• The Internet

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The Telephone System

• Public switched telephone network (PSTN) • Voice phones

• Fax machines

• Dial-up modems

• Ignoring (for now): • Mobile phones, VoIP

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History and Development

• 1876: Alexander Graham Bell

• Bell Telephone Company → AT&T

• National telephone monopolies • Strong governmental regulation

• Slow pace of innovation and service change

• Liberalisation, competition, and opening of the local loop

Alexander Graham Bell

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Basic Concepts

• Multi-level circuit switched network • Analogue circuits transport speech to exchange

• Sampled at exchange, digital circuits in the core

• Optimised for speech traffic • Only a single service provided: convey speech

data

• Circuit capacity based on speech characteristics

• Network dimensioned using typical call duration

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Physical and Link Layers

• Single twisted pair cable forms the local loop

• Analogue circuit, band limited to 300 – 3,400Hz • Acceptable quality speech; not suitable for music

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Network Layer

• Local loop terminates at exchange

• Structured hierarchical circuit switching and addressing to route call to destination

• Calls block if no capacity at intermediate circuit

• Structured addressing: +44 141 330 4256Country

ExchangeCustomer

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Transport Layer

Analogue speech signal is digitised at the exchange: 7 bits × 8000 samples/second = 56kbps channel (US & Japan) 8 bits × 8000 samples/second = 64kbps channel (Elsewhere)

Channel 1 Channel 2 Channel 3 Channel 24

Frame 1 (193 bits)

Each frame comprises: 1 framing bit, 24 channels (7 data bits, 1 control bit)

24 voice channels

Outside US & Japan: 32 voice channels,E1 line (2.048 Mbps)

T1 line (1.544 Mbps)

Multiplexing continues at higher rates Synchronous Digital Hierarchy (SDH)

All digital circuits in the phone system are defined as synchronous multiples of the voice channel rate

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Encoded as audio tones sent over the voice path

Digital path extended to edge

Primary service• Voice telephony

• Fax

• Video conferencing

• Data circuits

Applications

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Telephony Standards

• International Telecommunications Union • http://www.itu.int/

• Governmental-level body: part of The United Nations

• Formal representation and voting process • Companies send representatives to national standards bodies (e.g.

BSI, ANSI, DIN); national standards bodies cast their country’s vote at the ITU plenary meeting

• Cycle of formal comments on technical protocols between plenary and national standards bodies

• Liaisons with other standards bodies (e.g. IETF, W3C)

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Design Choices

• Circuit switched network • Potential blocking; high quality guaranteed if accepted

• Traditionally strong reliability guarantees

• Highly optimised for voice telephony

• Inflexible architecture, bureaucratic standards • Stability and reliability preferred over flexibility

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The Internet

• Interconnected set of global networks, running a common network layer • The Internet Protocol (IP)

• Supporting technology for application protocols • World Wide Web (HTTP)

• Email (SMTP)

• Instant Messaging (Jabber, etc.)

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History and Development

• 1965: Packet switching • Paul Baran (RAND),

Donald Davies (NPL)

• 1969: ARPA funding • First link: UCLA – SRI

• 1973: First non-US sites

• 1983: Switch to IPv4

• 1990: World Wide Web • Tim Berners-Lee

ARPA network map December 1972

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10026(Asia Netcom)10026(Asia Netcom)

4755(TATACOMM)4755(TATACOMM)

8395(COMSTAR)8395(COMSTAR)

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38809(NXGNET)38809(NXGNET) 3904 (Hutchison)9304 (Hutchison)

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20485(JSC)20485(JSC)

1273(CW)1273(CW)5459(London IX)5459(London IX)

1239(Sprint)1239(Sprint) 12989(HWNG)12989(HWNG)3549(Global Crossing)3549(Global Crossing)

4320(Embratel)4320(Embratel)

577(Bell Canada)577(Bell Canada)7843(Road Runner)7843(Road Runner)

2828(XOXO)2828(XOXO)3561(Savvis)3561(Savvis)

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A S - l e v e l I N T E R N E T G R A P H

I P v 4 & I P v 6 I N T E R N E T T O P O L O G Y M A P J A N U A RY 2 0 0 9

Source: CAIDA

Basic Concepts

• Global inter-networking protocol

• Hour glass protocol stack • Single standard network layer protocol (IP)

• Packet switched network, best effort packet delivery

• Uniform network and host addressing

• Uniform end-to-end connectivity (subject to firewall policy)

• Range of transport & application layer protocols

• Range of link-layer technologies supported

IP

TCPSMTP

HTTP RTPSIP

HTMLMIMESDP Codecs

Wi-Fi

EthernetADSL

SONET

UDP

WirelessTwisted Pair

Optical FibrePhysical

Data Link

Network

Transport

Session

Application Presentation

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Lower Layers

• IP runs on any data link/physical layer • Ethernet, ADSL, Wi-Fi, optical fibre, carrier pigeon…

• Anything that can deliver packets, can support IP

• No requirement for synchronous circuits

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The Internet Protocol (IP)

• Gives each host a globally unique address

• Delivers packets from one host to another • Best effort delivery – discards packets on failure

• No performance guarantees

• Agnostic of packet contents – except firewalls

• Provides uniform network connectivity

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The Internet Transport Layer

• Hide vagaries of IP layer • UDP: unreliable packet (“datagram”) delivery service, with no guarantee

of reception

• TCP: reliable, in-order, byte stream service

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Applications

• End-to-end argument

• Flexible, supports wide range of applications

• Intelligence at edge of the network; dumb core • Innovation happens at end hosts

• Core network doesn’t know or care what application data is being transported

• Allows rapid change, deployment of new protocols

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Internet Standards (1)

• Internet Engineering Task Force • Volunteer standards body; open membership

• Mailing lists; 3 physical meetings per year

• Standards and work-in-progress drafts freely available to all: • http://www.ietf.org/

• http://www.rfc-editor.org/

• Primary focus: network and transport layers (IP, UDP, TCP), session and presentation layer protocols to support applications (e.g. HTTP, SMTP, SIP), and routing

Data Link

Network

Transport

Session

Presentation

Application

Physical

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Internet Standards (2)

APP GEN INT

O&M

RAIRTGSEC

TRN

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Design Choices

• Packets rather than circuits

• Single generic best-effort network layer • Generic packet delivery service

• Easy to implement on any link-layer

• The end-to-end argument • Transparent network: not optimal for any application

• Application flexibility at the expense of performance

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Flexibility vs. optimality?

Benefits of convergence

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Questions?

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