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Understanding Computer Networks Basics & Configuration: A Comprehensive Guide

Explore the fundamentals of computer networks, OSI model layers, network devices, cable fabrication, VLANs, switch configuration, security protocols, and wireless networking. Learn about network concepts, terminology, and practical configurations for enhanced connectivity.

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Understanding Computer Networks Basics & Configuration: A Comprehensive Guide

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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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