Chapt 1 Introduction 1

Introduction to the Technology of Data Communications

• Consider computer networks as a structure for the transmission of integrated data of all types - voice/video/telephony, etc.

• What are different types of computer networks?• What are the uses of computer networks?• What hardware do computer networks use?• What software do computer networks use?• Other factors (ethical, legal, performance,

security)

Chapt 1 Introduction 2

Defining Computer Networks• Autonomous computers

– Hosts/ stations /devices– Interconnected by communication subsystem

• Links – fiber, copper, microwave, radiowave, infrared

• Nodes - Routers/ bridges/ gateways

– Distributed control– Built over layers of hardware and software– Distributed system is coherent to users

Chapt 1 Introduction 3

Usage of computer networks• Communications

– Email, instant messaging, telephony– Social networking– E-commerce– E-education– Games

• Resource Sharing– Data (search engines assist in finding the data)– Printers and other peripherals– Load sharing– Storage

• Home Networks• Data Input (e.g., sensor networks)

More uses of computer networks

• RFID (Radio Frequency Identification)– Product identification

– Pet and child identification

– No battery or activity required

• GPS• Cloud computing• Convergence of telephony, radio and television

networks, computer usage and storage

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Some advantages of computer networks

• Reliability/ Availability – Redundancy provides advantages over centralized

systems

• Scalability• Opportunities for geographically challenged

people• Time and effort simplified by working from home• Discrimination is harder

– Age, race, sex are not evident

Chapt 1 Introduction 6

Some disadvantages of computer networks

• Security– Privacy is more easily compromised – Intrusion is easier– Authentication is more easily compromised

• Decentralized control is harder– Ex: Child pornography and bomb making information

can be found on the World Wide Web

• Other social issues– Infringement of patents and copyrights for songs and

movies is easier through networks and the web – Easy access to gambling, pornography

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Classifications of networks• By Geography (i.e., distance covered)

– WAN – Wide Area Network– MAN – Metropolitan Area Network– LAN – Local Area Network– PAN – Personal Area Network– (also SAN- Storage Area Network )

• By Media– Guided/ wired – copper, fiber– Wireless

• By Technology– Circuit Switching – Packet Switching (computer networks)

• By Control– Client-server– Peer-to-Peer

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Storage Area Networks– High speed, short distance, large amount of data

• Includes disks, tape libraries, and CD arrays

• Fibre Channel (updated from HIPPI); parallel interface

– Large databases have special transmission & storage needs; SANs integrate all storage devices on network

• High data speed

• Reliability/ redundancy

– Storage devices are directly available on the network; they do not require access through servers; do not use Internet protocols

• Example: direct storage-to-storage communication used for backup and replication

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Some issues in Network Hardware

• Broadcast or point-to-point media:– Broadcast – signal sent to all stations within range

• Wireless LANs• Coaxial cable with multiple taps• Typical bus or hub based LANS

– Point to point• Connection between pairs of machines using switches, routers

• Broadcast/multicast/unicast addressing for hardware

• Simplex/Duplex/Half-Duplex channels

Chapt 1 Introduction 10

Network Topology

• Graph – redundant, irregular links – WANs

• Star – hub or switch based LANs, local loop in WANs

• Bus – multiple taps on copper

• Ring – circular bus

• Tree – attached bus or star segments

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History of Local Area Networks• 1970s – Alohanet (random access packet

radio)

• 1970s - Ethernet – Metcalfe and Boggs– DEC, Intel, Xerox standardized and made non-

proprietary (public domain)

– Random access (carrier sense) with collision detection

• 1980s -IEEE802 standards: 802.3, 802.5, 802.11• 1990s, 2000s -Fast, Gigabit Ethernet• 2000s, 2010s -Wireless LANs & PANs

Chapt 1 Introduction 12

NIC (adapter) functioning in a LAN

Wired LANs, MANs, WANs• Fiber networks are mainly:

– Internet infrastructure using graph topology– MAN infrastructure, the local loop with FIOS

• Cable networks– Typically MANs using coaxial for local loop

• Copper twisted pair networks– Ethernet LANs over 90% of wired LAN market– Switch or bus or tree topology

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Sample LAN topology

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Wireless Networks• Developed mainly for mobile users• Types

– Wireless LANs, PANs, MANs, WANs• Microwave links for satellite, MANs • Cellular phones• Access Points – 802.11 LANs in infrastructure mode• Peer-to-peer – Bluetooth; 802.11 in ad hoc mode

• Switching between cells for mobile users– Frequency reuse

• Different technical needs than wired LANS (reliability, security are more vulnerable)

• Ease of installation, mobility

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Network Software• Architecture & Modularity

– Layers and hierarchies

• Protocol – what is it?– Peer protocols (fig. 1-13)

• Layer interface• Services provided to the next higher layer

– Services available at SAPs (Service Access Point)

• Cost of calls between layers• IDUs and ICIs, SDUs and PDUs

– Interface Data Unit; Interface Control Information– Protocol Data Unit= Service Data Unit + Control Info

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Circuit switching

• Traditional telephone system

• Connection is established/released

• Bandwidth is pre-allocated for entire connection

• Same path is used for entire connection

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Packet switching

• Bandwidth is allocated on demand• Requires storing of data units during bursts of traffic

• May be connection-oriented or connectionless• Message is broken into some maximum size unit called a

packet (cell) at the network layer• Requires identification on each data unit

• May have same path (virtual circuit) or different path (datagram switching) for each packet

• Packet is switched at each router• Store and forward network• Utilizes speed and code conversion

Chapt 1 Introduction 19

Connection-oriented Service

• Model was telephone system • Connection is established, released

– Overhead in time, transmission• Each PDU needs smaller connection ID rather

than entire source and destination address• Quality of Service (options), authentication may

be negotiated for entire connection • PDUs are delivered in sequence• Sample protocol: ATM• PDUs are sent on same route in network layer

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Connectionless Service

• Model was smail, telegraph systems• Each PDU is sent on its own• Full address on each data unit• PDUs may arrive out of order• May or may not be acknowledged• May or may not provide error correction• PDUs may use different routes in network layer • Sample protocol: IPv4

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Basic network Services

• Send /receive/ reply

• Reliable or unreliable– Reliable (acknowledged & corrected, sequenced)

– Unreliable (no ack, no sequencing)

• Connection–oriented or connectionless– Connection-oriented : Connect/disconnect/send/receive

– Connectionless: Send/receive

• Service to higher layer is implemented by protocol

OSI Reference Model of ISO

• Top (7th) Layer -Application Layer– Provides interface to users; does user “work”

• 5th & 6th – Presentation & Session Layers– Services are not differentiated by TCP/IP

• 4th Transport layer– Forwards user PDUs (segment) to network

• These protocols are in software at hosts

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OSI Reference Model• 3rd Network Layer (PDU is packet)

– Functions include Routing, Network administration, network addressing

• 2nd Data Link Layer (PDU is frame)– Framing, Media access for broadcast media

• 1st Physical Layer (bit)• Media dependent and media independent sublayers• Determines what is a bit (timing, coding, clock

synchronization)

• Implemented in firmware on NIC in host and on routersChapt 1 Introduction 23

OSI model concepts• Services• Interfaces• Protocols

Each layer defines what services it offers to higher layer (such as send) and the interface – how and where to reach service (such as SAP). Protocols – rules for communication - are interchangeable.Virtual communication via peer protocols

• Why study OSI model?

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TCP/IP Reference Model

• Application Layer

• Transport Layer

• Internet Layer

• Link layer/ network interface layer (physical + data link layer)

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Development of Internet

• Why the Arpanet developed (cold war)

• How the Arpanet developed (RFCs)

• Protocols developed by students– TCP/IP Reference Model

• TCP, UDP, IPv4, IPv6

• Telnet, FTP, SMTP, DNS• ICMP and other control protocols

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Comparison• SNA (IBM)

– Proprietary system

– Homogeneous system

– Central control

• Arpanet– DARPA wanted distributed control (cold war)

– 4 node network of University sites Dec. 1969• Digital packet switching

– IP designed to connect different types of computers

– DNS (Domain Name System) added as system grew

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NSFNET & ANSNET

• Expansion of the Arpanet• NSF built backbone networks by the 1980s

(NSFNET)• Gave it to ANSNET as the first step

towards commercialization• NSPs & ISPs in the 1990s for home &

business use– WWW with graphical browser (Mosaic,

Netscape, Explorer, Firefox, Bing, Chrome)

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Other Protocols

• SNA – IBM WAN

• ATM, SONET – wired WAN

• X.25 – wired WAN

• Novell NetWare, Windows NT

• Ethernet, Token Rings – wired LANS

• 802.11(b, a, g, n)ac, ad, etc. – wireless LAN

• Bluetooth – wireless PAN

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Standards and Standards Organizations

• Advantages of standards– Allows manufacture of interchangeable

components

• Disadvantages of standards– Standards are frequently obsolete by the time

they are issued

• IEEE, EIA, ISO, IETF, etc., etc., etc., etc.