Download Exercise3 - University of North Carolina Wilmington

UNIVERSITY OF NORTH CAROLINA WILMINGTON
MIS 323 – BUSINESS TELECOMMUNICATIONS
Data Link Layer Exercise
In this lab, you will be examining the data link properties of a typical networked computer. Recall that this
layer converts user messages to the "ones and zeroes" that will eventually flow through the media.
Data link layer messages (frames)
The data link layer performs three key functions: media access control, error detection and correction,
and message delineation (identifying the start and end of the message). Each network adapter also has a
unique hardware data link layer address. Because these addresses are set in hardware, each data link layer
address is permanent.
Each network adapter also has one or more network protocols for transferring data between like network
adapters. The data link layer is connected to the media in one of two ways: multipoint connections (as is the
case with LANs) and point to point connections (i.e., phone lines to an ISP). The most popular multipoint
data link protocol is Ethernet. Computers basically listen for existing conversions and if the line if free, start
their own conversation. Errors can occur if two or more computers try to send messages at the same time
since all messages flow over the same bus (Errors can also be caused by external factors such as power
fluctuations).
Ethernet frames have a format similar to the following:
Recall that the lower layers "encapsulate" messages from higher level layers, treating the entire message
from these layers as their own data. So in the case of a Web message using a TCP/IP and
Ethernet network, we have:
Initially, a computer only knows its own data link address. In order for two computers on the same
Ethernet network to communicate, the sender must know the data link address of the receiving computer. In
this case, the sender "looks up" the address by sending a message to everyone on the network called a
broadcast message. The process is similar to that of an instructor taking roll on the first day of class. As he
calls each name, everyone hears each name but only the person with the matching "address" responds. The
instructor makes a notation in a log and from then on can "send messages" directly to the desired student by
looking up the name in the table of addresses. Each computer also builds a table of data link addresses for
future reference in much the same way.
Tools
Versions of the Windows operating system include a number of tools for examining data link layer properties
and functions. Note that some of these utilities are "command line" programs so you'll have to open a DOS
window. You can use the Start | Run sequence then type "cmd".

Windows Control Panel - The network icon in the Windows Control Panel can provide useful
information on the data link layer protocols. The Control Panel provides information on the network
interface card (NIC) properties and protocols in use by the computer (under “Network and Internet”).
Once you have clicked the View network status and tasks, you can find out more information about your
adapters by clicking on the “Change adapter settings” link on the left side of the screen.

Address Resolution Protocol (ARP) - For examining the table of datalink addresses of other computers
currently stored in computer's memory (use "arp -a" at the command prompt)

IPconfig (IP configuration utility) - For examining the computer's own data link address (use "ipconfig /all"
at the command prompt).

Netstat - Provides information on the data transfer statistics (including errors encountered) by the
network adapter (use "netstat -e" at the command prompt). The tool refers to normal messages as
"unicast" and broadcast messages as "non-unicast".
Name: __________________________
Data Link Layer Exercise
1. What is the data link address of your computer? (ipconfig /all)
2. List other data link addresses known to your computer (max. 5)? (arp -a)
3. Approximately how many bytes have been sent and received by your computer this session? (netstat -e)
4. How many errors has your computer encountered this session? (netstat -e)
5. How many broadcast (i.e., non-unicast) messages has your computer processed this session?
(netstat -e) What is the purpose of a broadcast message?
Using the Wireshark capture file provided on my website, answer the following questions about the data
link layer frames. This is a web request that occurred from my laptop (IP 172.20.104.114) to the
UNCW webserver (IP 152.20.9.40):
For the captured HTTP packet on line no. 119, provide the following information concerning the Ethernet Frame:
Destination Address:
MAC address:
____________________
NIC manufacturer: ____________________
Source Address:
MAC address:
____________________
NIC manufacturer: ____________________
Frame Type is also included with the information above. For Ethernet II frames, this field contains a hexadecimal
value that is used to indicate the type of upper layer protocol in the data field. There are numerous upper layer
protocols supported by Ethernet II.
What is the frame type listed and what does this indicate?
When information was being transferred back and forth, a problem occurred with the packet on line no. 225.
Answer the following questions concerning the problem:
What layer did this problem occur in (e.g. Application, Data Link, Physical, etc.)?
What was the problem? Describe what this problem tells us about the transmission?
Did this occur on a HTTP request (i.e. message going to the web server at UNCW) or on an HTTP
response (i.e. message coming back to my laptop)?