Download logical topology

Basic LAN techniques
• IN common with all other computer based systems
networks require both HARDWARE and
SOFTWARE to function.
• Networks are often explained by reference to a
universal ‘model’ called the OSI (Open System
Interconnect) model.
• This model attempts to define the operation of
various hardware & software components f a
‘network’
OSI Model
• The OSI model is HIERARCHICAL with
seven layers, layer 1 being the bottom
(physical) layer.
Network
OS
Hardware
7
6
5
4
3
2
1
A functional network must have components operating at each
layer of the model.
The Network Interface Card
(NIC)
• At this point we shall concentrate on
ETHERNET version2 cards using 10/100
base T cabling systems. These cards adhere
to a standard known as IEEE 802.3u,
normally just called ETHERNET.
• NICs are available from around £6.00.
• Most modern motherboards have a built in
NIC (LAN connector)
NIC
• The NIC is often referred to as a LAYER 1
(physical layer) device although it actually
operates at layer 1 and 2 (data link).
• At layer 1 the NIC provides the means of physical
(electrical & mechanical) access to the network
media.
• At layer 2 the NIC provides Media Access and
Logical Link Control by having an embedded
ETHERNET chip on board.
NIC Operation
• This means that the Network Operating
System (NOS) does not need to implement
Ethernet.
• Layer 2 is divided into two sub layers called
» LOGICAL LINK CONTROL (LLC)
» MEDIA ACCESS CONTROL (MAC)
Logical Link Control
• The LOGICAL TOPOLOGY of the
network is controlled by the LLC sub layer.
• (Remember, it is possible to have for
example a physical star yet a logical ring
topology as in token ring ieee 802.5
systems)
MAC
• Media access control is the key to Ethernet
and all other shared media type networks
• Ethernet systems use a media access
technique called Carrier Sense Multiple
Access with Collision Detection
(CSMA/CD)
• Token passing (IBM) and CSMA/CA
(Apple) are other LAN possibilities.
MAC Addressing
• Each node on a network is uniquely identifiable on
a universal basis, much like people!
• This amazing feat is achieved by virtue of a two
part MAC address being ‘burnt’ into the NIC at
manufacture.
• This MAC address is a 48 bit (6 byte) binary
number, written as a 12 digit hexadecimal number
eg.
0067A7C11FB56
MAC address format
• The first 24 bits of the MAC address define
the MANUFACTURER of the device and
are allocated to the manufacturer.
• The last 24 bits are set by the manufacturer.
• Providing the manufacturer allocates a
unique 24 bit number correctly it is
impossible for any two NICs to have the
same MAC address.
Simplest Possible Network
• The simplest possible ‘network’ consists of
two PC’s fitted with NIC’s linked by a
cross-over cable
crossover cable
000103D0CF66
000103D0DO76
Each PC is uniquely identified by its’ MAC address
Software
• The NICs and cables used to construct this simple
LAN exist only at the lower layers of the OSI
model.
• We now need to add software components
covering the remaining layers.
• These components are supplied as part of the PC
operating system as is the case with MS or may be
supplied by an other vendor as is the case with
NOVELL.
OS / NOS
• The software components required to make the
network functional are often referred to as
‘Network Operating System (NOS)’ components,
as distinct from Operating System (OS)
components.
• All currently supported Microsoft offerings have
software components to allow at least peer to peer
capability.
Login
• When we configure the network software we
specify a ‘login’.
• This specifies the rules (protocol) at layer 5
(Session) and defines such things as how a
communication ‘session’ is initiated, managed and
terminated.
Minimum Software Needed
• A ‘driver’ for the NIC, supplied by the
manufacturer. This allows the NIC hardware to be
controlled by the PC an operates at the data link
layer.
• A ‘protocol’ such as TCP/IP, NetBeui, Ipx/Spx etc.
These are components which amongst other things
control how information is ‘routed’ in networks.
• Protocols are sometimes described as ‘sets of
rules’ for network operation.
• ‘Protocols’ operate at layer 3 & 4
Services
• These are ‘Application Layer’ services
provided by the NOS.
• Examples of services which must be
enabled are file and printer sharing
• Many application layer services are not
explicitly installed or enabled and are
essentially transparent to the user.
NIC communication
• All ETHERNET traffic is ultimately between
NICs, so it is important to study the nature of this
traffic.
• The actual electronic signals carried by the cables
are quite complex pulses, using MANCHESTER
II encoding. An exact understanding of the nature
of these pulses is not essential for a understanding
of networks as such.
NIC Communication
• These electronic signals are often shown on
diagrams as strings of 1’s and 0’s, representing the
streams of BINARY NUMBERS (bit streams),
flowing between NICs.
• It is important to understand that these streams of
bits are actually grouped into units called
ETHERNET FRAMES (FRAMES)
• All ETHERNET systems use the same FRAME
structure and are therefore compatible!
ETHERNET FRAME
• Again, a complete understanding of the
FRAME is not essential, the structure is
shown here for completeness and to aid
understanding.
ETHERNET FRAME
PC to PC Network
000103D0CF66
000103D0DO76
A more complex network
• With the addition of a HUB (a layer 1)
device, a much more complex local area
network may be constructed.
• The software requirements are the same as
for the simple PC to PC network outlined
earlier.
• CSMA/CD makes this topology possible !
Hub Based LAN
Switch Operation
The switch builds
up a table containing
the MAC address
and port number for
all devices connected.
if devices are removed
or added then the table
is updated.
Building The Switch Table
Port Name Node MAC
Switch Table
Port
1
MAC Address
A324BD3B
2
A324A1CD
3
A24D33D2
n
A32763E4
1
2
3
A
B
C
A324BD3B
A324A1CD
A24D33D2
4
n
X
A32763E4
Switch Advantages
• The major advantage of a switch is that it offers a
dedicated full bandwidth channel between any two
ports and thus any two communicating devices.
• In base t HUB or base 2/5BUS topologies the
bandwidth of the media is shared between all
communicating devices.
• Therefore the performance advantage of a switch
compared to a hub can be very pronounced.