DT008/3
Communications Systems 1
Laboratory 3
Objective
This lab will investigate the set up of simple networks based on the Ethernet LAN standard, using
the following networking components:
Hubs: a device to which can be connected multiple hosts. Frames transmitted by one host are
forwarded to all ports of the hub, and therefore to all connected hosts.
LAN Switches: a device to which can be connected multiple hosts. Frames transmitted by one
host are forwarded to the port of the device to which the host identified by the layer 2 address of
the frame is attached.
Category 5/5E (Enhanced) Cables: A cable that consists of 4 twisted pair conductors
(Unshielded Twisted Pairs, UTP), used for data transmission. Each end of the cable is terminated
with an RJ-45 connector. The twisting of the pairs protects to some degree the signals carried by
the cable from unwanted interferences. Category 5 cable is typically used for 10 or 100 Mbps
Ethernet LANs, with only 2 pairs effectively used. Category 5 E is a higher specs cable designed
for higher transmission speeds.
In order to connect 2 PCs, or 2 Hubs or 2 Switches, the cable must be a crossover cable rather
than a straight through: the receive and transmit pairs are crossed to allow transmission.
Ping: the ICMP protocol will be used to test the connectivity between PCs (ping). The ICMP data
are encapsulated as follows:
Ethernet MAC
header
IP header
ICMP header
ICMP Data
Ethernet MAC
trailer
Message capture with Wireshark: some fields in the Ethernet frame are generated in HW in the
Network Interface Card just before transmission on line, and are not displayed by the Wireshark
analyser. These fields are the FCS, pre-amble and SFD, (and padding, on the transmitter side, if
needed in the payload).
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During most of the laboratory you will need to interconnect 3 PCs. You should agree to work with
1 or 2 partners at the beginning of the session, and select a group of 3 PCs you will be able to
use. There can be a single joint report for the group.
Exercise 1: Hub based network
In this part of the laboratory, you will need to work with 3 interconnected PCs.
You will observe the traffic using the Wireshark network protocol analyser.
Build the network with 3 PCs
At the beginning of this exercise, you should have 3 PCs, 3 straight through cables and 1 hub
(3COM Office Connect Dual Speed 5).
Connect the 3 PCs to the hub using the straight through cables.
Configure the IP settings
For each PC:
-
from the “Control Panel” select “Network and Internet - View Network Status and Tasks".
In the panel on the left, click on "Change Adapter Settings".
-
Right click on Local Area Connection and select properties. Then, highlight “Internet
Protocol Version 4 (TCP/IP)” , and click on “Properties”.
-
select “Use the following IP address”; set the IP information as follows, then click OK to
close the panel.
Host
IP Address
Subnet Mask
Default Gateway
PC 1
192.168.1.1
255.255.255.0
N/A
PC 2
192.168.1.2
255.255.255.0
N/A
PC 3
192.168.1.3
255.255.255.0
N/A
-
open a Command Prompt window and use the ipconfig/all command to check that the
values displayed are consistent with the values entered above, and to find the MAC
(Physical) address of the PC.
List in the Results sheet the 2 addresses for each PC.
=>Results Sheet
Test Reachability
You should start the Network Protocol Analyser “Wireshark” from the desktop of each PC. Enter
icmp in the display filter and Apply.
In the Wireshark menu click on Capture, and click on start.
From one PC, in the command prompt window, try to ping the IP address of the 2 other
computers with only one request per ping and 81 bytes of data.
Stop the capture after the sending PC has received all replies.
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Observe the traffic captured by the analyser on each PC.
On one of the PCs, identify a captured message that illustrates the “shared medium” operation of
the Hub. Justify the choice, identify the PC on which you are making this observation, and list the
source & destination addresses in the message.
 Results Sheet
Observe the packet header details window. Report the number of bytes in the frame (“bytes on
wire”) and find the number of Header bytes added by each layer. Show the breakdown of the
number of bytes in the frame, by listing the payload and all headers and their length in bytes.
 Results Sheet
Exercise 2: Switch based network
Build the network with 3 PCs
At the beginning of this exercise, you should have 3 PCs, 3 straight through cables and 1 switch
(Cisco Catalyst Switch).
Connect the 3 PCs to the switch using the straight through cables.
Verify the IP settings
Open a Command Prompt window; use the ipconfig command to check the values of addresses
are same as entered in exercise 1.
Each PC should send a default ping to each of the 2 other PCs to verify the connectivity.
Test Reachability
You should start the Network Protocol Analyser “Wireshark” from the desktop of each PC. Enter
icmp in the display filter and Apply.
In the Wireshark menu click on Capture, and click on start.
From same PC as in Exercise 1, send a single request ping (1 requests, 32 bytes) to each of
the 2 other PCs.
Stop the capture when all replies have been received.
Observe the traffic captured by the analyser on each PC.
Explain the difference between the observed messages in this test and in exercise 1.
 Results Sheet
Effect of payload size
In the Wireshark menu, click on Capture, and click on start. For this test clear the icmp filter.
From the command prompt window, try to ping from a PC the IP address of one of the 2 other
computers, using only 1 request per ping and 1472 bytes.
Then ping from a PC the IP address of one of the 2 other computers, using only 1 request per
ping and 1473 bytes.
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Calculate the size of the total payload that must be carried by the Ethernet LAN in each case,
taking into account the IP and ICMP headers.
 Results Sheet
Note: a packet too big for encapsulation in a single Ethernet frame will be fragmented, which
leads to the display of packets identified as IP Fragments by Wireshark.
Observe the messages captured by Wireshark for each of the 2 requests (ping 1472 and ping
1473), and explain how many frames are used in each case. Give the size of the frames.
 Results Sheet
When a payload is too small, it is “padded” by adding bytes to reach the size of the minimum
Ethernet frame. Find an example of such padded frame in the captured results. Explain how
much padding has been added to this frame.
 Results Sheet
Exercise 3
Connecting a management PC to the Switch
1. Connect the switch’s console port using a roll-over cable to the serial (COM) port on PC,
Rollover cable
2. Open “Hyper terminal” by clicking: Start > Programs > Accessories > Communication > Hyper
terminal.
3. Enter connection name, e.g.: Cisco
4. Select the COM port
5. Under “bits per second” select 9600 and leave all other fields.
6. Enter password if prompted
(Passwords to be given during lab)
Note: When first connecting, the lowest access mode, called the EXEC mode is entered. From
this mode, the user can only use a limited numbers of unprivileged commands.
It is necessary to enter privileged mode to enable more commands. This is done using the
command enable. Privileged mode will usually be password protected.
7. Type enable and enter the password to enter Privileged mode.
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From the command prompt, you can enter the command show mac-address-table dynamic, to
visualize the MAC address table (forwarding database) created by the switch.
First enter the following command to clear the forwarding database.
Switch# clear mac-address-table dynamic
Then send a ping from PC1 to PC2 only; use the show command to look at the MAC address
table.
Explain how the switch uses the Source Address and the Destination Address contained in an
Ethernet frame.
 Results Sheet
NOTE: AT THE END OF THE LABORATORY, RE-OPEN THE LOCAL AREA CONNECTION
PROPERTIES FOR IPV4, AND RE-CHECK THE BOX “OBTAIN AN IP ADDRESS
AUTOMATICALLY”.
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