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Quiz ECE 8
4/29/2009
Name: Student ID: (Please write clearly and be concise; you don't get extra credits for lengthy answers). 1. Why is CSMA/CD not suitable for very high-speed Ethernets such as Gigabit Ethernet? (5 points)
The efficiency of CSMA/CD is in reverse ratio to the normalized delay-bandwidth product a=tpropR/L = dR/vL, where d is the LAN cable length, R is the transmission speed, v is the fixed signal propagation speed, and L is the frame length. CSMA/CD works well when a is small (a <1). If we want to use CSMA/CD in very high-speed LANs (R is very large), we have to either reduce the cable length and/or increase the minimum frame length in order keep to a small. Otherwise, CSMA/CD will not be efficient. Intuitively, the cable length cannot be reduced too much so as to accommodate enough stations, so we have to greatly increase the minimum frame length for very high transmission speeds. This means a lot of padding overhead for short data packets to be transmitted in most applications.
2a Is CSMA/CD suitable for wireless LANs? Why or why not? (5 points)
No. The reasons are
Collision detection is not practical on a wireless network, as a transmitting station cannot effectively distinguish incoming weak signals from noise and the effects of its own transmission Hidden terminal problem Exposed terminal problem