FIT1047 Tutorial 9 – Sample Solution
Topics and goals
• Data Link Layer LAN & WLAN, Email
• Assignment 2 Discussion
Instructions
The tasks are done in groups of 2 to 3 students. But for Online Classes it will be done individually.
Task 1: Basic knowledge: Physical layer
How long is a bit? This sounds like a silly question, but we can actually compute it.
The speed of light is approximately 3 × 108 m/s. But signals in copper cables are slower, they travel with approximately 70% of the speed of light.
Now if we assume that bits are transmitted sequentially through a copper cable (one after the other), and if we know how many bits are transmitted per second, we can calculate how long the signal is (in meters) for each bit!
Compute the length of a bit for the original Ethernet, which ran at 10 Mbit/s.
0.7 × 3 × 108 m/s = 21 m/bit 10 × 106 bits/s
Task 2: Wireless LAN
a) What is the basic building block of an 802.11 WLAN?
The basic building block of an 802.11 WLAN is the Basic Service Set, which consists of wireless stations executing the same MAC protocol and competing for access to the same- shared wireless medium.
b) Define an extended service set.
Two or more basic service sets interconnected by a distribution system is an extended service set. An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system. The extended service set appears as a single logical LAN to the logical link control (LLC) level.
c) What is a Distribution System? Can the Distribution System be Wireless Network?
A Distribution System connects two or more BSS blocks. Distribution System may or may not be wireless. A DS can be a switch, a wired network or a wireless network.
Task 3: Email
a) Explain these email protocols: SMTP, POP & IMAPS
Simple Mail Transfer Protocol (SMTP): The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission. As an Internet standard, SMTP was first defined in 1982 and updated in 2008 to Extended SMTP additions, which is the protocol variety in widespread use today. Handles transfer of text messages between email client and mail server, and between mail servers.
Post Office Protocol (POP): Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server POP version 3 (POP3) is the version in common use. POP3 supports download-and-delete from a remote mailbox. Although most POP3 clients have an option to leave mail on the server after download, they generally connect, retrieve all messages, store them on the client system, and delete them from the server.
Internet Message Access Protocol (IMAP): Internet Message Access Protocol (IMAP) is an Internet standard protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection.
By contrast, the Internet Message Access Protocol (IMAP) normally leaves all messages on the server). Supports multiple clients to be connected simultaneously to same mailbox
b) What is MIME?
MIME is Multi-Purpose Internet Mail Extensions. SMTP can handle only plain text emails or ASCII characters. So, to support other data types and attachments like other file types, MIME specifies an encoding scheme to support character sets (e.g. Unicode) to send emails with non-latin characters and support non-text attachments.
Hence, MIME supports multi-part message bodies MIME is an extension to the RFC 822 framework that is intended to address some of the problems and limitations of the use of SMTP (Simple Mail Transfer Protocol) or some other mail transfer protocol and RFC 822 for electronic mail.
Task 4: Data link layer / Ethernet
This activity is a continuation of the packet sniffing we did in the previous activity. Now you will use Wireshark to capture the live network traffic into and out of your computer.
1. Open Wireshark and start capturing packets from the appropriate network inter- face. In the Monash labs, that would usually be an Ethernet interface. You can select that interface on the main Wireshark screen, or select Interfaces from the Capture menu.
2. The Wireshark window will now start filling up with packets. Open the following URL in a web browser:
http://users.monash.edu/~guidot/test.html
3. Answer the following questions, based on the contents of the Ethernet frame containing the HTTP GET message.
a) What is the value of the Ethernet address of your computer? (Hint: on Windows, use the ipconfig /all command line tool)
b) What is the destination address in the Ethernet frame? Is this the Ethernet address of www.csse.monash.edu?
The destination MAC address belongs to the router that your computer is connected to, not to www.csse.monash.edu.
c) How many bytes from the very start of the Ethernet frame does the ASCII “G” in “GET” appear in the Ethernet frame?
4. Next, answer the following questions, based on the contents of the Ethernet frame containing the first byte of the HTTP response message.
a) What is the value of the Ethernet source address? Is this the address of your computer, or of www.csse.monash.edu?
The source MAC address belongs to the router that your computer is con- nected to, not to www.csse.monash.edu.
b) What is the destination address in the Ethernet frame? Is this the Ethernet address of your computer?
The destination MAC address should be your own computer.
c) How many bytes from the very start of the Ethernet frame does the ASCII “O” in “OK” (i.e., the HTTP response code) appear in the Ethernet frame?
Task 5: Assignment 2:
Discuss the second assignment requirements.