Computer Networks, lab. 6
IFE
The aim of the laboratory is getting familiar with basic networking and to have some
practice with using simple equipment. Each student should practice preparing twisted pair
cable and test it with the aid of dedicated tester. Properly prepared cables are needed to create
connections between pairs of computers. Connected computers will be configured afterwards.
To test the created simplest-case computer network the previously learned software tools will
be applied. The first section of this document presents some simplified background.
1. Computer networks structures, net topologies
Fundamentals:
OSI reference model:
Layer 7
Application layer
Layer 6
Presentation layer
Layer 5
Session layer
Layer 4
Transport layer
Layer 3
Network layer
Layer 2
Data link layer
Layer 1
Physical laser
Simplified communication model:
Basic networking terms:
Communication protocol – set of rules and conventions needed for information exchange
and distribution among computers connected with the aid of transmission medium .
Frame – an information unit, which source and destination in the OSI model is the Data link
layer. It consists of a header, data and ending.
Packet - an information unit, which source and destination in the OSI model is the transport
layer.
Datagram - an information unit, which source and destination in the OSI model is the
network layer.
Segment - an information unit, which source and destination in the OSI model is the
application layer.
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1.2. Local Area Networks LAN
LAN (Local Area Network) consists of both hardware and software. Most important LAN
elements are as follows:
√ transmission medium, eg. twisted pair, coaxial cable, optical fiber,
√ information transmission control mechanism – it prevents conflicts occurring
between users simultaneously tending to access the transmission medium
√ network interface – interface of devices connected to the network, it conducts
necessary tests and converts coded information.
LAN networks enables easy access for multiple users sharing the same resources
(devices, data and applications), exchange files, users communication with the aid of
electronic mail and other applications.
LAN types:
Nowadays there are three most popular cable LAN technologies:
 Ethernet technology,
 Token-Ring technology,
 FDDI (Fiber Distributed Data Interface) technology
Ethernet – a family of LAN technologies; three major are:
 Ethernet and IEEE 802.3 – several sets of LAN specifications, working at max with 10
Mb/s.
 Ethernet 100 Mb/s (Fast Ethernet) - LAN specification, working at max with 100 Mb/s.
 Ethernet 1000 Mb/s (Gigabit Ethernet) - LAN networks specifications 1000 Mb/s (1
Gb/s).
Token-Ring. Technology originaly developed in IBM laboratories in seventies; stall widely
used. Workstations are connected directly to MAU (Multistation Access Unit) device, which
join them in one common ring.
FDDI. FDDI (Fiber Distributed Data Interface) technology is now used in WAN networks. Its
biggest advantage is huge bitrate (100 Mb/s) and high reliability. Optical fiber is the
transmission medium. There is a double ring topology in this approach. Primary Ring is used
for data transmission, and the Secondary Ring is not busy during normal work. In the LAN
networks FDDI is stall too expensive comparing to other solutions.
Przykładowe metody komunikacji – sposób dostępu do medium komunikacyjnego sieci:
CSMA/CD (Carrier Sense Multiple Access with Collision Detection) – Device is transmitting
only when it has exclusive medium access. When two devices begin to transmit at the same
time, the collision occurs. Then devices stop transmission after detecting such situation. After
waiting a random period of time the transmission is reactivated. This method causes limiting
the effective bus length to 2500 meters (with longer segments this mechanism no longer
works).
Token Ring – The dedicated packed is employed to transfer information. It is called Token.
Device tending to start the transmission have to get this packet. It changes one bit inside it
(converts into SFS frame - start of frame) and sends information. Token is then passed from
one workstation to another according to topology. Logical ring is related to physical one.
Changing of priority is also possible.
Token Bus – This mechanism is similar to the previous one, except Token is transferred in
different sequence. Order is dependent on the device address, and not the physical location.
Logical ring is not related to physical one. Nowadays this method is not very popular.
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1.3. Networks topologies in Physical layer.
Physical layer - IEEE 802.3
10 Base 2
10 Base 5
10 Base T
10 Base F
10 Broad 36
100Base X
100VGAnyLAN
Coaxial cable (RG58 A/U) max segment length 185m. Transmission
bitrate 10 Mb/s. CSMA/CD method.
Coaxial cable max segment length 500m,
Twisted pair segment length 100m. Twisted pair types: UTP, STP, ScTP,
FTP. Transmission bitrate 10 Mb/s. CSMA/CD method.
Optical fibre cables max segment length 2000 m. Single and multimode
optical fibres. Transmission bitrate 10 Mb/s. CSMA/CD method.
Coaxial cable (RG59/A/U/CATV) max segment length 3600 m. Mostly
used for broadband transmission. Transmission bitrate 10 Mb/s. CSMA/CD
method.
Twisted pair max segment length 100m. Transmission bitrate 100 Mb/s.
CSMA/CD method.
Twisted pair segment length 50m. Transmission bitrate 100 Mb/s. Access
upon request with priority method. 802.12 standard.
Abbreviations:
XXX – stands for network transmission bitrate in Mbps
„Base” – stands for Baseband
„Broad” – stands for Broadband
YY – means: segment length times 100 m
Example:10 Base 5 means Baseband network with transmission bitrate up to 10
Megabits per second with max segment length of 200 meters.
XXX Base YY -
1.3. Cables types and categories
Coaxial cable (nonsymmetrical cable) – similar to a usual tv antenna cable, RG58 A/U, RG58
C/U.
Twisted pair (symmetrical cable) – usually a copper cable consisting of several pairs of wires
twisted together in pairs (2 up to even few thousands pairs of twisted wires) put in the same
shield. Other names for twisted pair cables depending on max bitrate are for example:
10BASE-T, 100BASE-T or 1000BASE-T. Nowadays it is a most popular transmission
medium. Twisted pair cables are higly reliable and low cost. Cables classification:
Category 1 – used in telephony networks
Category 2 - data transmission up to 4 Mb/s,
Category 3 - data transmission up to 10 Mb/s (10BaseT), 3-5 twists/feet
Category 4 - data transmission up to 16 Mb/s (TR), 5-8 twists/feet
Category 5 - data transmission up to 100-150 Mb/s (Fast Ethernet, FDDI, CDDI, 100VGanyLAN), 8-10 twists/feet
There are basically 2 types of twisted pair cables: shielded (STP, FTP) and unshielded
(UTP). Some common abbreviations are listed below:
UTP – Unshielded Twisted Pair
STP – Shielded Twisted Pair
ScTP – Screened Twisted Pair
FTP – Foil tape screen Twisted Pair
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In general, the shielded ones are covered with shield and are better protected, which provides
low transmission losses and gives better resistance to interference. However UTP cables are
still widely used. Max length of UTP segments are about 100 m, whereas for STP it is 250 m.
These limits can be easily extended with the aid of repeaters. The impedance of the twisted
pair is 100 ohms in both cases. RJ-45 connector is needed to plug twisted pair cable into
computer network card interface. RJ11-14 connectors are used in telephony networks. There
are some pictures with cables and connectors below:
It is STP cable category 5 on the right.
RJ45 connector unshielded; with 8 pins
RJ11 telephonic connector with 4 pins
100BaseT network adapter, 1x RJ45 connector versus 2x RJ45 socket
Transmission media types in networks of different logical topologies:
10Base T
10Base 2
10 Base 5
Twisted Pair
Coaxial
Coaxial
Topology
Star, Tree
Bus
Bus
Segment
length
100 m
185 m
457 m
Devices per
segment
32
30
100
Devices per
network
1024
1024
1024
Media
2. Connecting computers with twisted pair cable
2.1. Cable and connector structure
The easiest way of constructing local area network is connecting two computers
equipped with network interface cards with the aid of cable. One can do it with coaxial cable
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or with the twisted pair cable. Type of cable depends on the computer network cards type.
There are computers equipped with the twisted pair enabled cards in the laboratory. Therefore
in order to connect two computers the 1 meter long, cross-over cable should be prepared.
Then the correctness of fabricated cables will be tested with dedicated equipment. Schematic
construction of RJ-45 connector is presented on the figure below:
Usual twisted pair pin-wires connections:
1.white-orange
2. orange
3. white-green
4. blue
5. white-blue
6. green
7. white-brown
8. brown
In the twisted pair transmission medium there are different types of wires pairs
assigned to different tasks. It has significant importance when trying to connect two
computers directly without any patch panel, hub or switch, etc. The reason is that the one pair
is used for sending data and another one is supposed to expect receiving frames, therefore
connecting two computers with the ordinary twisted pair cable would cause a situation when
both computers would expect frames on the same pair of wires. To eliminate need for using
patch panel or other equipment one should switch appropriate pairs of wires to create the so
called cross-over cable. There are two pairs of wires switched in this type of cable. On the one
end of cable we prepare the connector in a usual way with standard positions of wires like for
the computer-patch panel or computer-hub, etc connection. On the other end, we change the
position of two pairs of wires, in a manner that signal send from one computer on the 1 and 2
pins could be received by the other computer on the 3 and 6 pins. The correct way of
preparing the cross-over cable is presented in the figure below:
Cross-over connector pin scheme:
1. white-green
2. green
3. white-orange
4. blue
5. white-blue
6. orange
7. white-brown
8. brown
2.1. Practical tips
The following procedure can be helpful during mounting connectors with the cables.
1. Cut the end of the cable to make it “smooth” at the cross section.
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2. Remove 1 cm of the external isolation shield. Do not remove individual wires isolation.
3. Form the desired configuration of wires according to schemes provided above.
4. Push the cable into the connector to the very bottom.
5. Put connector together with the cables hold deep in the connector into the squeezing device.
6. Squeeze the device – that’s it!
3. Local network connection configuration
The properties of each installed network card interface are presented in the Network
Connections dialog window. This window can be invoked through the Control Panel or by the
Network Neighborhood context menu (depending on the Windows version). To configure
Local Area Connection, one should right click and choose Properties option. Dialog window
Local Area Connection Properties presents information about network card, clients, services
and protocols.
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Computer Networks, lab. 6
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In order to link the communication based on the TCP/IP protocol, one should
configure the properties of this protocol. First, mark the connection element: Internet Protocol
(TCP/IP) and press Properties button. The TCP/IP Internet protocol Properties window
enables the manual configuration of IP address settings, subnet mask and default gateway. In
standard, the IP address is automatically assigned by the DHCP (Dynamic Host Configuration
Protocol) server, but it is enough to switch the option button to change it from dynamic to
static mode.
During laboratory classes one can choose IP class C addresses with appropriate subnet
mask. Working in pairs, the task is to configure two cooperating workstations with the
following settings:
Computer #1 IP address: 192.168.1.2
Computer #2 IP address: 192.168.1.3
Subnet mask: 255. 255. 255.0
Default gateway: 192.168.1.1
The prepared and saved configuration should be tested – communication between
computers should be checked. In order to do that ping application employment should be
enough. However, the full network interface configuration can be also presented with the aid
of ipconfig or ifconfig tools.
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