PC Construction and Maintenance Week 7

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27 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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PC Construction and
Maintenance

Week 7

Networks

Introduction to networks


In computing, the term Network generally means
connecting two or more machines together


Networking covers a wide area of computing.


Networking is becoming increasingly important,
even in the home environment


The cost of networking hardware has dropped
considerably

Why have networks?


The main reason is to share resources
between computers


Many computers can share a printer


Internet access can be shared amongst a
number of machines


Access to information and resources shared
across a network is often useful in practice

Type of networks


We are concerned with building a small LAN
(Local Area Network) in order to enable file and
printer sharing


Also, a connection to the internet is desirable, for
the download of drivers, patches, BIOS upgrades
etc


The word internet literally means between
networks


The internet is really just a large collection of
networks, themselves networked together

The theory behind networks


Network theory could itself encompass a whole
course


We are going to explore the tip of the ice
-
berg of
network theory


We only need to know the key points in order to
have enough knowledge to understand how a
small home or business network functions


There are a just few fundamental facts of
computer networking that interest us

Network Theory


Key Points


All networks are based on a layered model. The
ISO 7
-
layered model is the theoretical ideal model
of a computer model


Real networks are based on a simplified models of
the theoretical ideal. But always retain the
“layered” quality.


We are interested primarily in TCP/IP networks.
TCP/IP is the protocol that runs the Internet


We need at the very least to know the essential
properties of the layered model in order to
configure and troubleshoot basic networks

ISO 7
-
layer reference model


7. Application


6. Presentation


5. Session


4. Transport


3. Network


2. Data Link (Hardware Interface)


1. Physical Hardware Connection

ISO layers explained


Physical Layer:


Transmitting data bits. Voltages,
timing factors, cable standards, etc.

Data Link Layer:


Managing basic transmission
circuit.


Error detection and correction; message
delimitation.

Network Layer:


Addressing and routing of
packets.

Transport Layer:


Establish, maintain, and
terminate logical connections. Generating
addresses; breaking packets: eliminating duplicate
packets; flow control.

Session Layer:


Initiating, maintaining, terminate
logical session.

Presentation Layer:


Display, formatting,
encryption

Application Layer:


Applications.

The TCP/IP model



The Physical Layer


For ordinary networks, there are two common
ways of hooking up several computers


One way is to use BNC cabling, provided the
network cards have a BNC connector on them.


The most common way at the present time is to
use UTP (Unshielded Twisted Pair) cable, in
combination with a Hub

BNC based network


All machines are daisy chained together via BNC
cabling and T
-
pieces


There is no need for a hub


If a single wire is disconnected, then the whole
network fails


A terminator is plugged in to both ends of the
network


This sort of arrangement is known as “BUS
topology”


A 10
-
Mbit BNC network is known as 10
-
BASE2

BUS topology (BNC networks)

UTP based network


Several machines are connected together via a hub


Several hubs can be chained together to form a
larger networks


For the simple case of a two
-
computer network,
you don’t need a hub, you can just use a
‘crossover cable’


UTP based networks are fast and reliable


If one wire is disconnected, then only one
computer is affected. The remaining network still
functions correctly


UTP networks are known as 10BASE
-
T, or
100BASE
-
T, depending on the speed

Star Topology (UTP based
networks)

UTP Cable Wiring

UTP Cable Wiring

UTP Cable Wiring Reference


1 White/Orange


2 Orange


3 White/Green


4 Blue


5 White/Blue


6 Green


7 White/Brown


8 Brown

UTP cables


A standard UTP network cable is known as a
straight cable, or a patch cable


If the pairs #2 and #3 are swapped with each
other, then we have a crossover cable


Straight cables are the most common, and are used
to connect a PC to a hub


Crossover cables are used to connect a PC to a PC
(2 PC network) or , a hub to another hub


Receive/Transmit swapping


In the 10
-
base T case. One twisted pair is for
receiving a signal (Rx), and the other for
transmitting (Tx)


The hub swaps these over internally so that (Tx)
of machine A connects to (Rx) of machine B


Straight cables do not swap Tx and Rx, crossover
cables do.


If the number of “swaps” between the source and
destination machines is odd, then they are wired
correctly, else they will not talk to each other

The datalink layer


The datalink layer deals with transfer of binary
data from one network interface card to another


The dominant datalink protocol used at the current
time in a LAN environment is
Ethernet


Some other datalink protocols are still found on
LANs (e.g. token ring), but these seem to be dying
out



Because of widespread use of Ethernet
equipment, it is now very cheap

Ethernet Networks


In the Ethernet environment, every network card
on the planet has a unique MAC address


The Media Access Code is used to address
different devices on the network


On a simple network, any device on the network
can address any other device within a given
subnet
using its MAC address


Devices cannot address other devices on other
subnets by using a MAC address


This is because the datalink protocol is non
-
routable


The network layer


IP is the network protocol that drives the Internet
(simply IP=InteRnet Protocol)


It became widely used on UNIX systems to start
off with, but has now spread to virtually every
other operating system


IP is needed to ‘surf the web’


Unlike the datalink layer, IP is a routable protocol
and therefore can be used to build large networks
(e.g. the Internet)

IP architecture


In the IP scheme of things, the network as a whole
is treated as an interconnected set of subnets


Each subnet allows communication within itself
using the aforementioned datalink protocol


Subnet A is connected to Subnet B via a device
known as a router


Each machine on the network (even on the public
internet) must have a unique IP address

IP addresses


IP addresses have a four byte form and are
normally written using a dotted notation


For example 150.204.51.254


One some networks, static I.P addresses are used,
which means that each machine handles it’s own
IP configuration locally


Some networks employ protocols to allow an IP
address to be assigned dynamically at boot time


Configuring IP under windows
95


As a test case, we will go through the steps
of configuring IP networking under
windows 95


Firstly, we need to have a network adapter
available


We need to check using device manager
that a network card is installed

Installing the network card driver


If windows has automatically recognised the card,
then we will see it in device manager


If not, we may need to use the “add hardware
wizard”


Sometimes, windows can see the card, but doesn’t
know what type it is. In this case, it will show up
as “Unknown device”, next to a question mark
symbol


If this happens, remove the unknown device from
device manager and reboot the PC


Windows should then properly detect the card on
the next boot

Installing the network
components


Once we have the card install correctly, we
need to add the TCP/IP component


Right
-
click on Network
-
Neighbourhood (Or
select network) in Control Panel, and then
click on “Add” to add a protocol (You will
need the windows CD in the drive)


The protocol is Microsoft
-
> TCP/IP


TCP/IP Parameters


After adding the TCP/IP protocol, we need to
configure the relevant settings to run the machines
on our network


The main setting is IP address (I have reserved a
separate address for each group, written on a list)


The other settings are Subnet Mask,Gateway,
DNS servers and WINS servers

Subnet mask


For our network, the subnet mask is always
255.255.252.0


The subnet mask defines how we choose to
split our physical subnet into logical subnets


The non
-
zero (left hand portion) defines the
network part of the address


The zero portion (right hand portion)
defines the host part of the address

Gateway


For our network, the address of the gateway
(a.k.a the router) is 150.204.51.254


The router is just a box that bridges the gap
between our network and the outside world


The network could work as a self contained
entity without a router (e.g. as in a home
network)


The router is needed to access the internet

DNS servers


DNS servers are just other computers on our
network that translate things like web
-
site names
into IP addresses


Without them, we could theoretically still browse
the internet, but we would have to refer to
websites by number, rather than by name


The address of the three DNS servers on our
network are 150.204.51.1 150.204.51.5
150.204.51.17

WINS Servers


WINS servers are used to translate between
Microsoft networking names and IP addresses


We need to configure WINS servers to allow are
machines to use Microsoft network resources such
as shared printers and files


Our WINS servers are 150.204.13.108 and
150.204.40.1

Completing the configuration


Any other protocols on the system, such as
NetBUI and IPX should be removed on PCs on
our network


After rebooting, the machine should be enabled
for TCP/IP


This can be tested by launching Internet explorer


If that fails, then try “ping 150.204.51.254” to
bounce a packet of data off the router from the
DOS prompt.

Notes on home networking


In the home environment, there are typically no
WINS or DNS servers available


In that case, it is still possible to run TCP/IP over
the network


Its just that local file configurations are used
instead of network servers


Local file configuration becomes impractical
when working with a large number of machines
though


Microsoft’s custom protocols may sometimes be
more suited to a small home network (but not for
internet sharing)