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Networks

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Computer Networks

• “An interconnected collection of autonomous end-systems”

• End Systems: hosts, PCs, dedicated computers, network

components

• Connection can be via various means: copper wire, fiber optic

cables, microwaves, satellites, ham radio.

• Distributed Systems:

– Existence of multiple autonomous computers is transparent (not visible), machines managed automatically

• Network:

– Users explicitly log onto a machine, machines managed personally

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Network Hardware

• Broadly speaking : two types of transmission technology

1) Broadcast Networks

2) Point-to-Point Networks

Broadcast (multiaccess) Point-to-point

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Broadcast Networks

• Single communication channel that is shared by all the machines on the network

• Share wire, frequency etc.

• Packets (small messages) sent by one mechanism is received by the others.

• Each machine checks the address field in the provided message. If the address is the same as receiving machine’s address, packet is processed, otherwise it is ignored.

• Some arbitration mechanism is needed when more than one machine want to send message at the same time.

• Multicasting: transmission to a subset of machines.

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Point-to-Point Networks

• Consists of many connections between individual pair of machines.

• To go from source machine to destination machine, packet may have to visit intermediate machines.

• Various routes (multiple paths) possible Routing algorithms are required.

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Connecting Networks- Repeaters -

• Copies bits from one network to another

• Does not look at (interpret) bits

• Allows the extension of a network beyond physical length limitations

REPEATER

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Connecting Networks- Bridges -

• Copies frames from one network to another

• Can operate selectively – does not copy all forms

• Extends the network beyond physical length limitations

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Connecting Networks - Routers -

• Switching Elements, packet switching nodes, intermediate systems, data switching exchanges

• Copies packets from one network to another

• Makes decisions about what route (path) a packet should take

• If two routers not connected by direct link want to communicate, they do so by using intermediate routers

• Subnets in which intermediate routers can store a packet and forward it is called point-to-point or store-and-forward, packet switched.

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Subnet

LAN

host

LAN

LAN LAN

Router

SUBNET

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Connecting Networks- Gateways -

• Operates as a router

• A gateway is a host that is connected to two or more physical networks simultaneously and is configured to switch packets between them

• Can perform

– Data conversions

– Translation: connect different application protocols

– encryption

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Hardware vs. Software

• Repeaters are typically hardware devices

• Bridges can be implemented in hardware or software

• Routers and gateways are typically implemented in software so that they can be extended to handle new protocols

• Many workstations can operate as routers or gateways

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Local Area Networks (LANs)

• LANs connect computers that are physically close together

• LANs are restricted in size

• Worst case transmisson time is bounded and known in advance

• High speed

• Transmission technology: multi-access (broadcast)

• Speeds

– Ethernet: 10 Mbps

– Token Ring: 16 Mbps

– FDDI: 100 Mbps

– Fast Ethernet: 100 Mbps

– Gigabit Ethernet: 1 Gbps

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Classification of Networks• Smaller, geographically localized networks tend to use broadcasting. Larger

networks are usually point-to-point.

Interprocessor Distance Processors located in the same: Example

1m system multicomputer

10m

100m

1km

Room

Building

Campus

LAN

10km City Metropolitan Area

Network (MAN)

100km

1000km

Country

Continent

Wide Area

Network (WAN)

10000km Planet The Internet

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Transmission Media

• Can be classified as:

– Guided : waves are guided along a solid medium such as copper wire, optical fiber etc.

– Unguided : wireless transmission, atmosphere, outer space, transmit electromagnetic signals, radio, laser etc.

• Design issues in transmission systems:

– Data rate, distance, bandwidth, transmission impairments, interference, number of receivers etc.

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Twisted Pair (guided trans. media)

• Oldest, least expensive, most widely used

• Telephones connected to telcos by twisted pair (subscriber loops)

• Can transmit both analog and digital signals:

– For analog: amplifiers needed every 5 to 6 km

– For digital: repeaters required every 2 to 3 km

• Limited distance, bandwidth compared to others

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Twisted Pair (guided trans media)

• 2 kinds:

1) Unshielded (UTP, 10 Base T) is ordinary telephone wire, least expensive transmission media for LANs

2) Shielded: shield twisted pair with a metalic braid/sheating to reduce interference, more expensive

• UTP kinds:

– Category 3: 3 or 4 twists per foot, (data rates: 16 Mbps)

– Category 5: 3 to 4 twists per inch (data rates: 100 Mbps)

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Coaxial Cable (guided trans. media)

• Can be used over longer distances and supports more stations on a shared line than twisted pair.

• Bandwidth possible depends on cable length. For 1 km cables 1-2 Gbps feasible.

• Widespread use: television distribution, long distance telephone transmission, short-run computer system, local area networks

• Varieties: thin, thick coaxial

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Fiber Optics (guided trans. media)

•Physiscs: light ray is refracted (bent) when passing from one medium to another)

•Above a critical angle of incidence, light is refracted back into silica (hence trapped inside fiber) and can travel for many kilometers

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Fiber Optics (guided trans. media)

• Lower attenuation

• Fibers thinner and lighter weight (considerably)

• Data Rates: 1 Gbs, 100 Gbps, 1 Tbps

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Fiber Optics (guided trans. media)

• Electromagnetic isolation (i.e. not affected by external electromagnetic fields) no interference

• Difficult to tap, therefore better security

• Enables greater spacing between repeaters

• Multimode fiber: allows many different rays incident at different angles to travel

• Singlemode fiber: employs fiber core which has small distance (of the order of a wavelength) and allows only a single ray to pass. Singlemode fiber is more expensive but can be used for longer distances.

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Global Wireless Network Standards

• IEEE 802.15 (Bluetooth) for the Personal Area Network (PAN)

• IEEE 802.11 (Wi-Fi) for the Local Area Network (LAN)

• IEEE 802.16 (WiMax) for the Metropolitan Area Network (MAN)

• IEEE 802.20 standard for the Mobile Broadband Wireless Access (MBWA)

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Bluetooth

• Can link up to 8 devices in 10-m area

• Can transmit up to 722 Kbps in the 2.4 GhZ band

• Low power requirements

• Radio based communication

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Bluetooth

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Wi-Fi

• Three standards:

– 802.11a: up to 54 Mbps (10 – 30 meters)

– 802.11b : up to 11 Mbps (30 – 100 meters)

– 802.11g : up to 54 Mbps (30 – 100 meters)

• Infrastructure mode: Devices use access point to communicate with wired network

• Ad-hoc mode (peer-to-peer): Wireless devices communicate directly with each other

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Wireless LAN

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The Telephone System

• Telephone was patented by Alexander Graham Bell, 1876.

• Evolution of the structure of telephone system:

(a) Fully-interconnected network.(b) Centralized switch.(c) Two-level hierarchy.

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The Telephone System• Basic Bell system model remained essentially intact up to now.

• Presently, we have highly redundant multilevel hierarchy.

• Hierarchy: Home, End Office, Toll Office, Primary Office, Sectional Office, Regional Office

• Transmission media used:

– Local loops consist of twisted pair

– Between switching offices: coaxial cables, microwaves, fiber optics

• Transmission:

– Local loops: analog signaling

– Trunks: mostly digital

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Structure of Telephone System

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Local Loop

• Between home and the end office

• Modems can be used to connect computer at home

• Baud Rate: Signalling speed – the number of times per second that signal changes its value (i.e. its voltage)

– b-baud line does not necessarily transmit b bits/sec

– each signal may convey several bits

– Example: if voltages 0,1,2,3,4,5,6,7 are used, each signal can convey 3 bits. So bit rate: 3 X baud rate

• DSL (Digital subscriber lines) : Use existing telephone lines to transmit signals. 500Kbps-1.5Mbps possible.

– ADSL is asymmetric DSL : download and upload speeds are different. Upload speed is slower than download speed.

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Trunks and Multiplexing

• Cost of installing and maintaining both high-bandwidth and low bandwidth trunk the same (essentially)

• Therefore, choose high-bandwidth trunk installation

• To efficiently use high bandwidth, multiplex many conversations over a single trunk

• Multiplexing schemes:

– Frequency Division Multiplexing

– Wavelength Division Multiplexing

– Time Division Multiplexing

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Example of Time Division Multiplexing

• T1 Carrier (1.544 Mbps):

– 24 voice channels multiplexed together

– Analog signals are sampled on a round robin basis

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Multiplexing T1 streams into higher carriers

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Layered Network Software Architecture Model

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Protocol Hierarchies

•Design issues for the layers: addressing, error control, flow control, multiplexing, routing

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OSI Reference Model

• Network architecture model based on a proposal developed by International Standards Organization (ISO)

• OSI: Open Systems Interconnection

• Standardized protocols for network layers

• OSI has 7 layers

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OSI LayersLayer Responsibility

1) Physical Layer Transmission of raw bits over a communication channel

2) Data Link Layer

Data Link Control

MAC Sublayer

- Provide an error free communication link. Issues: framing, addressing

- Medium Access Control needed by multiaccess communications. MAC provides DLC with virtual wires on multiaccess networks

3) Network Layer -Path selection between end-systems (routing)

-Subnet flow control

- Translation between different network types

4) Transport layer -Provides virtual end-to-end links between peer processes. Fragmentation and reassembly. End-to-end flow control

5) Session Layer - Establishes, manages and terminates sessions between applications. Service location lookup.

6) Presentation Layer - Data encryption, data compression, data conversion

7) Application Layer - Anything not provided by other layers

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TCP/IP Reference Model

• First used in ARPANET (Advanced Research Projects Agency) sponsored by US Department of Defense

• Now used in worldwide Internet

• Goal: ability to connect multiple networks in a transparent way

• Requirements: Connections should remain intact as long as source and destination machines are functioning even if some of the intermediate machines, links are out of operation

• Internetwork Layer (Internet) : Packet switching network based on a connectionless internetwork layer.

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TCP/IP Reference Model

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Protocols and networks in the TCP/IP model

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Why OSI Model/Protocols were not implemented

• Bad timing

• Bad technology

• Bad implementations

• Bad politics