computer networks i
Post on 01-Jan-2016
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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
• Network (routers/L3switches): packet
• Transport: segment
• Session
• Presentation
• Application: data
Signals
• Duplex
• Bandwidth
• Throughput
• Delay/Latency
• Cyclic Redundancy Check
• Manchester
• Orthogonal Frequency Division Multiplexing
• Spectrum Analyzer
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
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
Manchester
OFDM
Spectrum Analyzer
• Allows to study signals– Frequency domain– SNR measures
Physical Layer
• Signals are sent through the chosen medium
• Fiber Optic
• Unshielded Twisted Pair (UTP)
• Wireless
• Hubs
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
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
UTP FabricationSRC Straight Cross-over Rollover
1 1 3 8
2 2 6 7
3 3 1 6
4 4 4 5
5 5 5 4
6 6 2 3
7 7 7 2
8 8 8 1
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
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
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
Virtual Local Area Networks (VLAN)• Segmentation of the network
• Breaks broadcasts domains
• Needs a router for different vlan communications
• Increase in security
Spanning Tree Protocol (STP)
• Creates a spanning tree of the switches topology
• Breaks loops to prevent broadcast storms
• Should be always enabled
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
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
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#
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
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
Switch Configuration (Linksys)File: /etc/config/network#### VLAN configuration config switch eth0 option vlan0 "0 1 2 3 5*" option vlan1 "4 5"
Wireless Networking
• Channels allowed by the FCC (default 6)
• Authentication and association– WEP– WPA
• Add-Hoc vs Infrastructure
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
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
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
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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