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
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Physical (repeaters/hubs): signals
Data Link (bridges/switches): frame
Network (routers/L3switches): packet
Transport: segment
Session
Presentation
Application: data
Signals
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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 Fabrication
SRC
Straight
Cross-over
Rollover
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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: 96008N1
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)
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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
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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
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IEEE802.11a
IEEE802.11b
IEEE802.11g
IEEE802.11n
• Security
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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'