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'