CCN Activity Session for Section 5.2 – Wireless Networking
For this sheet, we’ll be continuing our exploration of wireless networking.
Question
Power is a precious resource in mobile devices, and thus the 802.11 standard provides power- management capabilities that allow 802.11 nodes to minimise the amount of time their sense, transmit, and receive functions and other circuitry need to be “on”. Explain in brief how a node communicates with an AP to perform power management.
When entering sleep state, device sets power management bit to 1 to notify AP
• Wake just before next beacon frame (around 100ms)
• AP buffers frames for sleeping nodes
• Next beacon frame includes list of nodes with buffered frames
o If no frames waiting return to sleep state
o If frames waiting, node polls AP Can result in sleep up to 99% of time
• Active 250 microseconds (wake-up time) to 1ms per 100ms Discussion
What is the role of a GSM network’s base station controller (BSC)?
The spectrum available in a cell is divided into channels. Base Station Controllers (BSC) allocate channels to subscribers in the cell, without an allocated channel, a device cannot communicate with the network.
Discussion
Why are acknowledgements used in 802.11 but not in wired Ethernet?
Collisions cannot be detected in wireless environments in the same way as they can in wired Ethernet. As such, an acknowledgement must be used for a sender to be sure its message was received.
Question
If IEEE 802.11 RTS and CTS frames (control frames) were the same length as standard data frames, would there be any advantage to using them? Explain your answer.
No, there wouldn’t be any advantage. Suppose there are two stations that want to transmit at the same time, and they both use RTS/CTS. If the RTS frame is as long as a DATA frames, the channel would be wasted for as long as it would have been wasted for two colliding DATA frames. Thus, the RTS/CTS exchange is only useful when the RTS/CTS frames are significantly smaller than the DATA frames.
Question
What is the difference between Bluetooth and Zigbee in terms of data rates? How does this link to their main function?
Recent versions of Bluetooth offer speeds of the order of tens of Mbit/s (theoretical 24Mbit/s for Bluetooth 3 + HS), Zigbee by comparison runs in the tens or hundreds of kbit/s. The high speeds associated with Bluetooth are linked to its design aim of replacing personal media cables etc. in a
very local area, while Zigbee is designed for low data rates at low power, potentially over a reasonable range.