CECS 474 COMPUTER NETWORKING INTEROPERABILITY
Purpose: In this lab you will become acquainted with the basic functionality of a IEEE 802.11 protocol. You will work with your Team to create an ad hoc WiFi network and configure a typical home network 802.11g/n/ac router for an infrastructure network.
Instructions: As a team, you will answer the questions concerning wireless networks. Each Team will be assigned a 802.11 router and documentation and/or software. Your Team is responsible for the router and returning all documentation to the instructor at the end of each lab. Turn in only one copy of this assignment for your entire group via BeachBoard.
Your answers should describe/discuss the experiments you have performed and information you have gathered.

Be sure to cite your references.
 Do not simply quote answers from the wireless NIC documentation, explain what the answer means.
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You should use the time in lab to experiment with wireless networks and research their behavior.
If you need additional information to answer questions, you may use:
 The wireless NIC and access point documentation,
 This handout, publications, and the World Wide Web (but not Wikipedia)
Background: Typically, home network routers include the following network interfaces: WAN port
(RJ-45); 4 Ethernet (802.3) ports (RJ-45); 802.11 (WiFi) antenna. The protocols running on the router usually include: DHCP, NAT, a simple firewall, and (sometimes) UPnP (Universla Plug and Play) port mapping.
The most popular wireless LAN technology is the IEEE 802.11 standard. The designers of 802.11 wanted to use a protocol as close to Ethernet as possible. Unfortunately, the characteristics of wireless transmission requires WiFI to use a modified Ethernet protocol for it’s Media Access Control
(MAC).
Informal Protocol Name
Standard
Media Access Control (MAC)
Ethernet
IEEE 802.3
CSMA/CD
WiFi
IEEE 802.11
CSMA/CA
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CECS 474 COMPUTER NETWORKING INTEROPERABILITY
CSMA/CA
In the Ethernet protocol, a station must listen to the transmission media before they send and continue listening during the entire transmission of the frame to determine whether a collision occurs. Unfortunately, in WiFi it cannot be guaranteed that a station can hear all transmissions made by other stations on the network.
Two situations arise which make a direct CSMA/CD implementation impossible. These situations result in a modification of the CSMA/CD to become CSMA/CA.
Situation 1: The hidden node problem occurs (A cannot see D in Figure 1). Using the standard
Ethernet protocol, the hidden node problem will result in collisions that the sender cannot recognize.
FIGURE 1: Example Wireless Network with operating in an ad hoc fashion.
Situation 2: The exposed node problem occurs (B can send to A; simultaneously with C sending to D in Figure 1). Using the standard Ethernet protocol, these simultaneous transmissions would not be allowed (but we want them).
As a result, the IEEE 802.11 MAC Protocol must be modified. It is changed to Multiple Access with
Collision Avoidance (CSMA/CA). 802.11 uses Collision Avoidance to handle these two situations.
Refer to the Chapter 16 course notes. Here is the basic strategy:
 The sender and receiver exchange control frames before the sender actually transmits data so that all nearby nodes know that transmission is about to begin.
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Sender sends a request to send (RTS) frame to the receiver. The frame includes the length of time the sender needs to transmit its data frame (or “hold the medium”).
 Receiver replies with a clear to send (CTS) frame that echoes the length field back to the sender and anyone else who receives it.
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If a node sees the CTS frame, it must not transmit during the time interval indicated in the length field (solving the hidden node problem).
 If a node sees the RTS frame but not the CTS frame, it is free to transmit (solving the exposed node problem).
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CECS 474 COMPUTER NETWORKING INTEROPERABILITY
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The receiver sends an ACK to the sender after successfully receiving a frame. All nodes wait for this ACK before trying to transmit.
If two senders transmit RTS frames at the same time, a collision happens. These collisions cannot be detected, but because the RTS frames are destroyed no CTS frames are ever sent. The sender’s timeout while awaiting the CTS frames, and must retransmit.
Ad hoc vs. Infrastructure Mode
In ad hoc mode, all nodes communicate directly with each other as in Figure 1 above. In
infrastructure mode, nodes communicate via an access point (AP) as in Figure 2.
FIGURE 2: Access points connected to a distribution network.
Access points can be simply connected to a wired network infrastructure, or connected in a more sophisticated way by a distribution system. The distribution system runs at Layer 2 forming a Wireless
LAN (WLAN). The WLAN is similar to a Switched Ethernet LAN.
Exercises: WiFi Introduction
Perform any tasks specified and then answer the following questions about Wireless LANs:
1.
What are the three MAC address of the wireless 802.11 NICs you are using on your workstations?
Are these addresses static or configurable? What version of 802.11 are they capable of supporting?
2.
What is the major difference between CSMA/CD and CSMA/CA? Why was it necessary to modify the traditional CSMA/CD protocol for use in wireless networks? Explain in your own words.
3.
What is an ad-hoc wireless LAN? Explain in your own words.
4.
Get an ad-hoc wireless LAN working in our lab. How did you configure it? Describe the topology, data paths, computers used and transmission rates of your network.
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CECS 474 COMPUTER NETWORKING INTEROPERABILITY
5.
What is an infrastructure wireless LAN? Explain in your own words.
6.
Get an infrastructure wireless LAN working in our lab using one of the 802.11g Routers. How did you configure the network? Describe the topology, data paths, computers used and transmission rates of your network.
7.
What is the transfer rate of your 802.11g CSMA/CA wireless network? Where did you get this information? Is it the theoretical or actual rate? How does the theoretical transfer rate compare with the actual transfer rate you can measure in lab?
8.
Using a laptop computer with an 802.11g NIC, attach it to the wireless router you are using. Now increase the distance between the router and your laptop by walking down the hall or outside the building. Observe the data transfer rate of your connection continuously while you perform this exercise. How does the actual transfer rate you measure compare with the theoretical transfer rate? Describe your results.
9.
Is there a limit to the number of stations that can be attached to a wireless LAN? (You may want to look this one up on a manufacturers website.)
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