Question

1. In a wireless LAN using infrastructure mode with N nodes (not including the AP (access point)), describe the network’s communication topology. What is the average number of communication hops between any two end nodes in this WLAN.

2. In 802.15.4 beacon enabled wireless network, if there are multiple consecutive collisions during backoff process in CAP, what does it indicate? What can you do to address this problem?

Answer 1

1. The wireless LAN using infrastructure mode with N nodes is the ring topology as it does not have any access point router. A ring topology is a network configuration where device connections create a circular data path. Each networked device is connected to two others, like points on a circle. Together, devices in a ring topology are referred to as a ring network.

2. If multiple consecutive collision during the backoff process is when multiple sender meant to send to receiver or vice-versa, it impair the transmission reliability of emergency traffic under high traffic loads, which may result in loss of high valued medical information. To address this problem we use CSMA/CA, CSMA algorithms attempt to break symmetries of failing transmissions being restarted at almost the same time by using randomised binary exponential backoff procedures. While wired devices can listen during their own transmissions and employ CSMA with collision detection (CSMA/CD), stations in wireless networks usually cannot listen to their own transmissions, and consequently colliding transmissions can only be detected after they have been completed. Thus wireless devices use CSMA with collision avoidance (CSMA/CA or CSMA-CA).

So, the CSMA/CA is used to prevent against collision.

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