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Computer Networks

Course: CIS 3003Fundamental of

Information Technology

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Objectives

• OSI model• Local area network (LAN)• Topologies of LAN• Wireless LAN

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OSI Model

• Open Systems Interconnection model

• Layer abstractions• Protocol/technology at

each layer can be changed with minimal impact to other layers

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OSI Model

• Hardware: Fiber optic, twisted pair cables

• Link: 802.11• Network: routing

protocols• Transport: TCP/IP, UDP• Application: HTTP, FTP• User: Internet Explorer,

Firefox

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OSI Model

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Accessing LAN

• Network Interface Controller (NIC) • Each NIC has unique physical address – MAC-48, EUI-48, EUI-64;

• Ethernet NIC address has 6 byte or 48 bits– First 3 bytes: manufacture identifier– Last 3 bytes: device unique identifier; serial no.

• NIC breaks data into smaller frames and add control information

• NIC sends data over physical medium.

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Ethernet Data Format

• Ethernet preamble: 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011

• Source and destination address are NIC addresses

• Padding: minimum data size is 46 bytes• FCS – error checking

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Local Area Network (LAN)

• Confined to a local area such as building or home.

• Shared access to devices such as printer, internal servers.

• Logical Topologies: – Ring– Star– Bus

• Wired and wireless

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Token Ring

• A token is passed from one computer to next.

• Whichever computer has the token can transmit data.

• Computers can be connected to a hub that provides a ring topology.

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Token Ring

• Deterministic through use of token.

• One fail device can break an entire ring unless hub bypasses the device.

• Use special designed cable and network card; more expensive.

• Token management.

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FDDI

• Fiber Distributed Data Interface

• Connected by fiber optic cables.

• Dual ring topology – redundancy fault tolerance

• 100 Mbps: faster than token ring but not faster than latest Ethernet (100 and 1000 Mbps).

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Token Ring Frame

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Bus

• One computer transmits, all computers can hear the information

• Need a medium access control mechanism to prevent two computers transmit at the same time.

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Bus

• One computer transmits, all computers can hear the information

• Need a medium access control mechanism to prevent two computers transmit at the same time.

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Bus

• Computers can be connected to a hub that provides a bus topology.

• Hub is a simple device. • Add more computers to

the bus increases chance of collision.

• Cost advantage of hub over switch is diminished.

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CSMA/CD & CSMA/CA

• Carrier Sense Multiple Access with Collision Detection

• Used in LAN• Node senses the medium

first. • When node detects

collision, it stops transmission and retransmits after a random delay.

• Carrier Sense Multiple Access with Collision Avoidance

• Used in wireless• RTS/CTS packets

– A node sends Request to Send packet to a receiver.

– The receiver replies with a Clear to Send if it does not sense a busy wireless channel.

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Switched Network

• Each computer connects directly to a switch

• Switch manages the data to be sent out to the network.

• Switch performs faster than hub due to lack of collision.

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WLAN

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WLAN Standards

• IEEE 802.11a — Wireless technology operating at a frequency of 5 GHz with a maximum data rate of 54 Mbps

• IEEE 802.11b — Wi-Fi wireless technology operating at a frequency of 2.4 GHz with a maximum data rate of 11 Mbps

• IEEE 802.11g — Backward compatible with 802.11b, operating at a frequency of 2.4 GHz and a maximum data rate of 54 Mbps

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WLAN Standards

• IEEE 802.11i—Standards for wireless security mechanisms

• IEEE 802.15—Known as WPAN, or wireless personal area networks; blue tooth.

• IEEE 802.16—WiMAX, an emerging technology that seeks to provide high-speed wireless access over much longer distances than implementations of the IEEE 802.11 standards

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Security

• Information transmitted over air; no physical security as in a wired network.

• Signal can be overheard by other Wi-Fi enabled computers within the range.

• Rogue access point, jamming wireless channel, overhearing.

• Turn on WEP/WPA encryption, change default access point password and SSID, which shows models of the access point.

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Summary

• Local area networks can have a ring, bus, or switch topology.

• Token ring provides a deterministic method to access shared medium. However, it has the complexity of token management.

• Bus hubs used to cost less than a switch but there are increased collision when more devices connected to a bus.

• Switches are inexpensive and provide easy setup and faster speed than a bus topology.

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Summary

• WLAN is similar to a bus topology where every node shares the same wireless medium.

• WLAN employs RTS/CTS to avoid collision. • WLAN is less secure than a switch network.• There are many ways to protect against

attacks in a WLAN.