Router modes and channel impairments

1. What does it mean for a wireless network to be operating in "infrastructure mode?"
Clients connected to a base station (an access point or router) are operating in infrastructure mode. They communicate indirectly through this access point or router which serves as a bridge to a wired network. Ad-hoc mode networks are networks that do not rely on a router or access point infrastructure. As a result, each client participates in the routing by forwarding data to the other connected clients.

2. What are the differences between the following types of wireless channel impairments:  path loss, multipath propagation, interference from other sources?
• Multipath propagation is when packet loss occurs due to electromagnetic waves reflecting off of ground objects which then take paths of different lengths between sender and receiver.
• Interference from other sources happens when there is interference from radio sources transmitting in the same frequency band. Wireless phones and wireless LANs can sometimes interfere with each other.
• Path loss is from decreasing signal strength due to passing through matter such as walls. The distance between sender and receiver factors into decreased signal strength as well.
3. Describe the role of beacon frames in 802.11.
Part of the 802.11 standard requires that the access point (AP) send out "beacon frames". Each frame includes the SSID and access point address. The wireless station scans all 11 channels checking for beacon frames that are being sent. The receiver can then select which access point to use. Typically the highest signal strength wins.

4. Why are acknowledgments used in 802.11 but not in wired Ethernet?
801.11 has a link-layer acknowledgment scheme. Frames sent from a wireless LAN station my not reach its destination and has no way of knowing if it did. When the destination receives a frame it pauses for a moment then sends back and acknowledgment. If the acknowledgment is not received after a fixed number of transmissions the transmitting station gives up. In wired Ethernet, the station can detect if the frame arrived with no collisions so no acknowledgment is necessary.

5. What are the differences between a master device in a Bluetooth network and a base station in an 802.11 network?
Bluetooth devices organize themselves into a "Piconet" of up to 8 slave devices. One of the devices will be the "master" device. The master's clock determines the Piconet time. The master can change any device from Parked to Slave. Blutooth is also an example of an Ad-hoc network.
A base station or access point is a receiver and transmitter acting as the hub of the WIFI network. Additionally, may also function as the gateway between a wired network and the wireless network. This is also an example of infrastructure mode.

6. What are three approaches that can be taken to avoid having a single wireless link degrade the performance of an end-to-end transport-layer TCP connection?
• Local recovery - Recovers from bit errors at the wireless link when they occur.
• TCP Sender awareness of wireless links - This will enable TCP to be aware of the wireless link where ordinarily it would not be. This will make the distinction between cognitive loss and caption loss occurring in the wired and wireless network.
• Split connection approaches - The sender and receiver is broken down into 2 transport layer connections 1 from the mobile host to the wireless AP, the other from the AP to the end point. This is widely used in cellular networks.