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Computer Networks I

Computer Networks I. By: Ing. Hector M Lugo-Cordero, MS. What is a network?. Collection of computers interconnected to share resources A network does not mean Internet access Exposes security issues. OSI Model Layers. Physical (repeaters/hubs): signals Data Link (bridges/switches): frame

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Computer Networks I

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  1. Computer Networks I By: Ing. Hector M Lugo-Cordero, MS

  2. What is a network? • Collection of computers interconnected to share resources • A network does not mean Internet access • Exposes security issues

  3. OSI Model Layers • Physical (repeaters/hubs): signals • Data Link (bridges/switches): frame • Network (routers/L3switches): packet • Transport: segment • Session • Presentation • Application: data

  4. Signals • Duplex • Bandwidth • Throughput • Delay/Latency • Cyclic Redundancy Check • Manchester • Orthogonal Frequency Division Multiplexing • Spectrum Analyzer

  5. Parameters • Duplex: communication channel direction • Half-duplex: communication can flow in one direction at a given time • Full-duplex: communication can flow in both directions at the same time • Bandwidth: theoretical capacity of the communication channel • Throughput: actual capacity of the communication channel • Delay/Latency: the time that takes the network to deliver a packet from source to destination

  6. CRC • Detects the presence of errors so that a retransmission can be asked for • Ethernet uses a fixed polynomial for the CRC computation known as CRC32 • x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1

  7. Manchester

  8. OFDM

  9. Spectrum Analyzer • Allows to study signals • Frequency domain • SNR measures

  10. Physical Layer • Signals are sent through the chosen medium • Fiber Optic • Unshielded Twisted Pair (UTP) • Wireless • Hubs

  11. Hubs • Work the physical layer • Built with pure hardware • Amplify the signal and retransmit it to all ports except the one that sent the data • Expand collision domain and broadcast domain

  12. UTP Fabrication • Straight cable: used to communicate different devices • Cross-over cable: used to communicate devices of the same nature • PC and Routers are the exception • Rollover cable: used to communicate with the devices using the console

  13. UTP Fabrication

  14. Data Link Layer • Translates bits to signals and schedules the access to the medium • Composed of two sub-layers • Logical Link Control (LLC – IEEE 802.2) • Medium Access Control (MAC) • IEEE 802.3 – Ethernet • IEEE 802.11 – Wireless • IEEE 802.15 – Bluetooth • Switches work at this layer with MAC address

  15. MAC Addresses • Identifies uniquely a node in the network • This address should be private (unknown to other users, but not nodes) • 48 bit number • MM:MM:MM:SS:SS:SS (hex) • M is manufacturer’s id • S is serial number

  16. Switches • Able to create virtual circuit • Break collision domains but enlarge the broadcast domains • Have more intelligence than hubs • Can create network segments for privacy • Ports can be access or trunk

  17. Virtual Local Area Networks (VLAN) • Segmentation of the network • Breaks broadcasts domains • Needs a router for different vlan communications • Increase in security

  18. Spanning Tree Protocol (STP) • Creates a spanning tree of the switches topology • Breaks loops to prevent broadcast storms • Should be always enabled

  19. Switch Configuration (Cisco) • Connect rollover cable from serial in PC to console at the switch • Open hyper terminal connection with 9600 baud, data bits 8, parity none, stop bits 1, flow control none • Tipically known as: 9600-8N1

  20. Switch Configuration (Cisco) • This steps erase everything to factory defaults • SW>enable #enter from user to privileged • SW#show running-config • delete vlan.dat • erase startup-config • reload

  21. Switch Configuration (Cisco) • This steps configure the ports of the switch • SW#configure terminal • SW(config)#interface FastEthernet 0/0 • SW(config-if)#switchport mode access • SW(config-if)#switchport access vlan 10 • SW(config-if)#exit #end goes to begin • SW(config)#interface range FastEthernet 0/0-15 • SW(config-if)#switchport mode access • SW(config-if)#switchport access vlan 20 • SW(config-if)#end • SW#

  22. Switch Configuration (Cisco) • This steps configure a trunk (multi vlans) • SW(config-if)#switport mode trunk • SW(config-if)#switport trunk allowed vlan add 10 • SW(config-if)#switport trunk allowed vlan add 20

  23. Switch Configuration (Cisco) • This steps configure port security • SW(config-if)#switchport port-security maximum 1 • SW(config-if)#switchport port-security violation shutdown • SW(config-if)#switchport port-security mac-address 001f.453a.1234

  24. Switch Configuration (Linksys) File: /etc/config/network #### VLAN configuration config switch eth0 option vlan0 "0 1 2 3 5*" option vlan1 "4 5"

  25. Wireless Networking • Channels allowed by the FCC (default 6) • Authentication and association • WEP • WPA • Add-Hoc vs Infrastructure

  26. Infrastructure Networks • Access networks are wired LAN with access points • Nodes connect to access points to access the wired distribution system • A bridge from IEEE802.11 to IEEE802.3 is needed

  27. Ad-Hoc Networks • Interconnected fully wireless • Multi-hop network • Nodes can either be client or server • Extend the range of normal WLAN • Can reach places were wires can’t

  28. Setting a Wireless Access Network • What you need • Access point (IEEE802.11 to IEEE802.3 bridge) • Wireless internet card • The right technology • IEEE802.11a • IEEE802.11b • IEEE802.11g • IEEE802.11n • Security • IEEE802.11i • WPA • No SSID broadcast • MAC filtering

  29. Wireless Configuration (Linksys) File: /etc/config/wireless config wifi-device wl0 option type broadcom option channel '6' option disabled '0' config wifi-iface option device wl0 option network 'wlan' option mode 'adhoc' option ssid 'OLSR' option encryption none option hidden '0'

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