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Chapter 1 Communication
Networks and ServicesNetwork Architecture and Services
Telegraph Networks & Message SwitchingTelephone Networks and Circuit Switching
Computer Networks & Packet SwitchingFuture Network Architectures and Services
Key Factors in Network Evolution
1
Chapter 1Communication
Networks and Services
Network Architecture and Services
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Communication Services & Applications
A communication service enables the exchange of information between users at different locations.Communication services & applications are everywhere.
E-mail server
Exchange of text messages via servers 3
Communication Services & Applications
A communication service enables the exchange of information between users at different locations.Communication services & applications are everywhere.
Web Browsing
Web server
Retrieval of information from web servers 4
Communication Services & Applications
Instant Messaging
Direct exchange of text messages
A communication service enables the exchange of information between users at different locations.Communication services & applications are everywhere.
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Communication Services & Applications
Telephone
Real-time bidirectional voice exchange
A communication service enables the exchange of information between users at different locations.Communication services & applications are everywhere.
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Communication Services & Applications
Cell phone
Real-time voice exchange with mobile users
A communication service enables the exchange of information between users at different locations.Communication services & applications are everywhere.
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Communication Services & Applications
Short Message Service
Fast delivery of short text messages
A communication service enables the exchange of information between users at different locations.Communication services & applications are everywhere.
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Many other examples!
Peer-to-peer applicationsNapster, Gnutella, Kazaa file exchangeSearching for ExtraTerrestrial Intelligence (SETI)
Audio & video streamingNetwork gamesOn-line purchasingText messaging in PDAs, cell phones (SMS)Voice-over-Internet
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Services & Applications
Service: Basic information transfer capabilityInternet transfer of individual block of informationInternet reliable transfer of a stream of bytesReal-time transfer of a voice signal
Applications build on communication servicesE-mail & web build on reliable stream serviceFax and modems build on basic telephone service
New applications build on multiple networksSMS builds on Internet reliable stream service and cellular telephone text messaging
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What is a communication network?
The equipment (hardware & software) and facilities that provide the basic communication serviceVirtually invisible to the user; Usually represented by a cloud
CommunicationNetwork
EquipmentRouters, servers, switches, multiplexers, hubs, modems, …
FacilitiesCopper wires, coaxial cables, optical fiberDucts, conduits, telephone poles …
How are communication networks designed and operated?11
Communication Network Architecture
Network architecture: the plan that specifies how the network is built and operatedArchitecture is driven by the network servicesOverall communication process is complexNetwork architecture partitions overall communication process into separate functional areas called layers
Next we will trace evolution of three network architectures: telegraph, telephone, and computer networks 12
Network Architecture Evolution
1.0E+00
1.0E+02
1.0E+04
1.0E+06
1.0E+08
1.0E+10
1.0E+12
1.0E+14
1850 1875 1900 1925 1950 1975 2000
Telegraphnetworks
Telephonenetworks
Internet, Optical& Wireless networks
Info
rmat
ion
trans
fer
per s
econ
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Next Generation
Internet
?
13
Network Architecture Evolution
Telegraph NetworksMessage switching & digital transmission
Telephone NetworksCircuit SwitchingAnalog transmission → digital transmissionMobile communications
InternetPacket switching & computer applications
Next-Generation InternetMultiservice packet switching network
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Chapter 1Communication
Networks and ServicesTelegraph Networks &
Message Switching
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Telegraphs & Long-Distance Communications
Approaches to long-distance communicationsCourier: physical transport of the message
Messenger pigeons, pony express, FedExTelegraph: message is transmitted across a network using signals
Drums, beacons, mirrors, smoke, flags, semaphores…Electricity, light
Telegraph delivers message much sooner16
Optical (Visual) TelegraphClaude Chappe invented optical telegraph in the 1790’sSemaphore mimicked a person with outstretched arms with flags in each handDifferent angle combinations of arms & hands generated hundreds of possible signalsCode for enciphering messages kept secretSignal could propagate 800 km in 3 minutes! 17
Message SwitchingNetwork nodes were created where several optical telegraph lines met (Paris and other sites)Store-and-Forward Operation:
Messages arriving on each line were decodedNext-hop in route determined by destination address of a messageEach message was carried by hand to next line, and stored until operator became available for next transmission
Northline
Southline
Westline
Eastline
NetworkNode
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Electric Telegraph
William Sturgeon Electro-magnet (1825)Electric current in a wire wrapped around a piece of iron generates a magnetic force
Joseph Henry (1830)Current over 1 mile of wire to ring a bell
Samuel Morse (1835)Pulses of current deflect electromagnet to generate dots & dashes Experimental telegraph line over 40 miles (1840)
Signal propagates at the speed of light!!!Approximately 2 x 108 meters/second in cable 19
Morse code converts text message into sequence of dots and dashesUse transmission system designed to convey dots and dashes
Morse Code
Morse Code
Morse Code
Morse Code
A · — J · — — — S · · · 2 · · — — —
B — · · · K — · — T — 3 · · · — —
C — · — · L · — · · U · · — 4 · · · · —
D — · · M — — V · · · — 5 · · · · ·
E · N — · W · — — 6 — · · · ·
F · · — · O — — — X — · · — 7 — — · · ·
G — — · P · — — · Y — · — — 8 — — — · ·
H · · · · Q — — · — Z — — · · 9 — — — — ·
I · · R · — · 1 · — — — — 0 — — — — —
Digital Communications
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Switches
Message
Destination
SourceMessage
Message
Message
Electric telegraph networks explodedMessage switching & Store-and-Forward operationKey elements: Addressing, Routing, Forwarding
Optical telegraph networks disappeared
Electric Telegraph Networks
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Operator 25-30 words/minutebut a wire can carry much more
Baudot multiplexer: Combine 4 signals in 1 wireBinary block code (ancestor of ASCII code)
A character represented by 5 bitsTime division multiplexing
Binary codes for characters are interleavedFraming is required to recover characters from the binary sequence in the multiplexed signal Keyboard converts characters to bits
Baudot Telegraph Multiplexer
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Baudot Telegraph Multiplexer
…A2D1C1B1A1
PaperTape
Printer
PaperTape
Printer
PaperTape
Printer
PaperTape
Printer
BaudotDemultiplexer
Keyboard
BaudotMultiplexer
5 bits / character
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Elements of Telegraph Network Architecture
Digital transmissionText messages converted into symbols (dots/dashes, zeros/ones)Transmission system designed to convey symbols
MultiplexingFraming needed to recover text characters
Message SwitchingMessages contain source & destination addressesStore-and-Forward: Messages forwarded hop-by-hop across networkRouting according to destination address
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Chapter 1Communication
Networks and ServicesTelephone Networks and
Circuit Switching
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Bell’s TelephoneAlexander Graham Bell (1875) working on harmonic telegraph to multiplex telegraph signalsDiscovered voice signals can be transmitted directly
Microphone converts voice pressure variation (sound) into analogous electrical signalLoudspeaker converts electrical signal back into sound
Telephone patent granted in 1876Bell Telephone Company founded in 1877
Signal for “ae” as in cat
Microphone Loudspeakeranalog
electricalsignalsound sound
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Bell’s Sketch of Telephone
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Signaling
Signaling required to establish a callFlashing light and ringing devices to alert the called party of incoming callCalled party information to operator to establish calls
Signaling + voice signal transfer
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The N2 Problem
For N users to be fully connected directlyRequires N(N – 1)/2 connections Requires too much space for cablesInefficient & costly since connections not always on
N = 1000N(N – 1)/2 = 499500
1
2
34
N
29
Telephone Pole Congestion
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Circuit Switching
Patchcord panel switch invented in 1877Operators connect users on demand
Establish circuit to allow electrical current to flow from inlet to outlet
Only N connections required to central office1
23
N – 1
N
31
Manual Switching
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Strowger SwitchHuman operators intelligent & flexible
But expensive and not always discreetStrowger invented automated switch in 1888
Each current pulse advances wiper by 1 positionUser dialing controls connection setup
Decimal telephone numbering systemHierarchical network structure simplifies routing
Area code, exchange (CO), station number
...
0
9
0
9...
0
9
0
9
...
1st digit 2nd digit . . .
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Strowger Switch
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Telephone subscribers connected to local CO (central office)
Tandem & Toll switches connect CO’s
Hierarchical Network Structure
TandemCO
Toll
CO COCO
CO
TandemCO = central office
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Network selects route;
Sets up connection;
Called party alerted
Telephonenetwork
Pick up phone
Dial tone.
Dial number
Exchange voice signals
1.
2.
3.
4.
5.
Telephonenetwork
Telephonenetwork
Telephonenetwork
Telephonenetwork
Telephonenetwork
Hang up.6.
Connection set up
Information transfer
Connection release
Three Phases of a Connection
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Computer Connection ControlA computer controls connection in telephone switchComputers exchange signaling messages to:
Coordinate set up of telephone connectionsTo implement new services such as caller ID, voice mail, . . .To enable mobility and roaming in cellular networks
“Intelligence” inside the networkA separate signaling network is required
SignalingComputer
Switch connectsInlets to Outlets. .
.
. . . Voice
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Digitization of Telephone Network
Pulse Code Modulation digital voice signalVoice gives 8 bits/sample x 8000 samples/sec = 64x103 bps
Time Division Multiplexing for digital voiceT-1 multiplexing (1961): 24 voice signals = 1.544x106 bps
Digital Switching (1980s)Switch TDM signals without conversion to analog form
Digital Cellular Telephony (1990s)Optical Digital Transmission (1990s)
One OC-192 optical signal = 10x109 bpsOne optical fiber carries 160 OC-192 signals = 1.6x1012 bps!
All digital transmission, switching, and control38
Digital Transmission Evolution
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1.0E+14
1850 1875 1900 1925 1950 1975 2000
Morse
T-1 CarrierSONETOpticalCarrier
Info
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ion
trans
fer
per s
econ
d
WavelengthDivision
Multiplexing ?
Baudot
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Elements of Telephone Network Architecture
Digital transmission & switchingDigital voice; Time Division Multiplexing
Circuit switchingUser signals for call setup and tear-downRoute selected during connection setupEnd-to-end connection across networkSignaling coordinates connection setup
Hierarchical Network Decimal numbering systemHierarchical structure; simplified routing; scalability
Signaling NetworkIntelligence inside the network
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Chapter 1Communication
Networks and Services
Computer Networks & Packet Switching
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Computer Network Evolution Overview
1950s: Telegraph technology adapted to computers 1960s: Dumb terminals access shared host computer
SABRE airline reservation system1970s: Computers connect directly to each other
ARPANET packet switching networkTCP/IP internet protocolsEthernet local area network
1980s & 1990s: New applications and Internet growthCommercialization of InternetE-mail, file transfer, web, P2P, . . .Internet traffic surpasses voice traffic
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What is a protocol?
Communications between computers requires very specific unambiguous rulesA protocol is a set of rules that governs how two or more communicating parties are to interact
Internet Protocol (IP)Transmission Control Protocol (TCP)HyperText Transfer Protocol (HTTP)Simple Mail Transfer Protocol (SMTP)
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A familiar protocol
“Do you have a first name or street?”
Caller
“What name?”“Simpson”
Caller replies
System replies with number
System replies
System replies
System replies
Operator replies
“Thank you, please hold”
“Evergreen Terrace”
Caller replies
“Springfield”
Dials 411
“What city”?
Caller replies
Operator replies
Caller waits
“Thank you, please hold”Caller waitsCaller dials
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Terminal-Oriented Networks
Early computer systems very expensiveTime-sharing methods allowed multiple terminals to share local computerRemote access via telephone modems
Host computer
Terminal
Terminal. .
.
TerminalModem ModemTelephoneNetwork
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Dedicated communication lines were expensiveTerminals generated messages sporadicallyFrames carried messages to/from attached terminalsAddress in frame header identified terminalMedium Access Controls for sharing a line were developedExample: Polling protocol on a multidrop line
Medium Access Control
Host computer
TerminalTerminal . . . Terminal
Terminals at different locations in a cityMust avoid collisions on inbound line
Polling frames & output frames
input frames
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Statistical MultiplexingStatistical multiplexer allows a line to carry frames that contain messages to/from multiple terminalsFrames are buffered at multiplexer until line becomes available, i.e. store-and-forwardAddress in frame header identifies terminalHeader carries other control information
CRC Information Header
Header Information CRC
Host computer
Terminal
Terminal
. . .
Terminal
Multiplexer
Frame
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Error Control ProtocolCommunication lines introduced errorsError checking codes used on frames
“Cyclic Redundancy Check” (CRC) calculated based on frame header and information payload, and appended Header also carries ACK/NAK control information
Retransmission requested when errors detected
Header Information CRC
CRC Information Header
Terminal
48
Tree Topology NetworksNational & international terminal-oriented networks Routing was very simple (to/from host)Each network typically handled a single application
New York City
San Francisco
Chicago AtlantaT
TT
49
Computer-to-Computer Networks
As cost of computing dropped, terminal-oriented networks viewed as too inflexible and costlyNeed to develop flexible computer networks
Interconnect computers as requiredSupport many applications
Application ExamplesFile transfer between arbitrary computersExecution of a program on another computerMultiprocess operation over multiple computers
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Packet Switching
Network should support multiple applicationsTransfer arbitrary message sizeLow delay for interactive applicationsBut in store-and-forward operation, long messages induce high delay on interactive messages
Packet switching introducedNetwork transfers packets using store-and-forwardPackets have maximum lengthBreak long messages into multiple packets
ARPANET testbed led to many innovations51
ARPANET Packet Switching
Packet Switch
Packet Switch Packet
Switch
Packet Switch
Packet Switch
Message
Packet 1
Packet 2
Packet 1Packet 1
Packet 2 Message
Host generates messageSource packet switch converts message to packet(s)Packets transferred independently across network
Destination packet switch delivers messageDestination packet switch reasembles message
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ARPANET Routing
Packet Switch
Packet Switch Packet
Switch
Packet Switch
Packet Switch
Packets header includes source & destination addressesPacket switches have table with next hop per destination
No connection setup prior to packet transmission
Routing tables calculated by packet switches using distributed algorithm
PacketHdr
Dest: Next Hop:
xyz abc
wvr edf
Routing is highly nontrivial in mesh networks
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Other ARPANET Protocols
Packet Switch
Packet Switch Packet
Switch
Packet Switch
Packet Switch
Congestion control between source & destination packet switches limit number of packets in transit
Error control between adjacent packet switches
Flow control between host computers prevents buffer overflow
Error Control
Congestion Control
Flow Control
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ARPANET ApplicationsARPANET introduced many new applicationsEmail, remote login, file transfer, …Intelligence at the edge
UCLA RAND TINKER
USC
NBS
UCSB
HARV
SCD
BBN
STAN
AMES
AMES McCLELLAN UTAH BOULDER GWC CASE
CARN
MITRE
ETAC
MIT
ILLLINC
RADC
55
Ethernet Local Area Network
In 1980s, affordable workstations availableNeed for low-cost, high-speed networks
To interconnect local workstationsTo access local shared resources (printers, storage, servers)
Low cost, high-speed communications with low error rate possible using coaxial cableEthernet is the standard for high-speed wired access to computer networks
56
Ethernet Medium Access ControlNetwork interface card (NIC) connects workstation to LANEach NIC has globally unique addressFrames are broadcast into coaxial cableNICs listen to medium for frames with their addressTransmitting NICs listen for collisions with other stations, and abort and reschedule retransmissions
Transceivers
57
The InternetDifferent network types emerged for data transfer between computersARPA also explored packet switching using satellite and packet radio networksEach network has its protocols and is possibly built on different technologiesInternetworking protocols required to enable communications between computers attached to different networksInternet: a network of networks
58
Internet Protocol (IP)Routers (gateways) interconnect different networks Host computers prepare IP packets and transmit them over their attached networkRouters forward IP packets across networks Best-effort IP transfer service, no retransmission
Net 1 Net 2
Router 59
Addressing & RoutingHierarchical address: Net ID + Host IDIP packets routed according to Net IDRouters compute routing tables using distributed algorithm
GG
G
GG
G
Net 1
Net 5
Net 3
Net 4Net 2
H
H H
H
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Transport ProtocolsHost computers run two transport protocols on top of IP to enable process-to-process communicationsUser Datagram Protocol (UDP) enables best-effort transfer of individual block of information Transmission Control Protocol (TCP) enables reliable transfer of a stream of bytes
Internet
Transport
Protocol
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Names and IP Addresses
Routing is done based on 32-bit IP addressesDotted-decimal notation
128.100.11.1Hosts are also identified by name
Easier to rememberHierarchical name structuretesla.comm.utoronto.edu
Domain Name System (DNS) provided conversion between names and addresses
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Internet Applications
All Internet applications run on TCP or UDPTCP: HTTP (web); SMTP (e-mail); FTP (file transfer; telnet (remote terminal)UDP: DNS, RTP (voice & multimedia)TCP & UDP incorporated into computer operating systemsAny application designed to operate over TCP or UDP will run over the Internet!!!
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Elements of Computer Network Architecture
Digital transmissionExchange of frames between adjacent equipment
Framing and error controlMedium access control regulates sharing of broadcast medium.Addresses identify attachment to network or internet. Transfer of packets across a packet networkDistributed calculation of routing tables
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Elements of Computer Network Architecture
Congestion control inside the network Internetworking across multiple networks using routersSegmentation and reassembly of messages into packets at the ingress to and egress from a network or internetwork End-to-end transport protocols for process-to-process communicationsApplications that build on the transfer of messages between computers.Intelligence is at the edge of the network.
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