CSE 561: Computer Communication and Networks

Class Notes: Nov 6, 2002

Administration

• No class on Monday

• Friday: Risk Assessment Report

Wireless Network

• Do wireless matter at all? What will the wireless world be 10 years from now?

• Vision #1: Dense short range networks, all devices have computers, thus form a sensor net.
• Vision #2: Cooperative ISP, everyone would antenna on rooftop, no monopoly.
• Vision #3: Widely use in robots, military, etc..
• Vision #4: Information everywhere, PDA in every pocket. Real-time vast information access.
• Vision #5: Widely use in communication.

• How can all these happen? What are the ways of deployment?

• Ubiquitous base stations.   Reliability and performance choice.
• Cooperation wireless routing.    Flexibility choice. 
• Fad.
• Cooperative micro LAN + fixed WAN. Well-rounded choice.

• Media access

• Carrier sense
• Latency of wire/wireless

• As number of stations increases throughput decreases

• Collisions increase, time wasted in resolving the collisions
• Can decrease collisions by randomizing the delay (although based on the assumption that a sender can sense when a transmission by another sender starts and finishes). Use exponential backoff for randomizing delay

• Wireless : Sender can't always detect whether others are transmitting

• Receiver tells sender to backoff. Problem: Receiver has to respond to every sender
• Use RTS/CTS

• MACAW

• Uses RTS/CTS. Not essential if senders can hear one another or when contention is low
• Communication is unpredictable in wireless networks : Use ACK for every data packet transmitted
• Is link layer ACK essential ?
  • depends on loss rate
  • best effort retransmission

• Important considerations not included in MACAW

• Malicious nodes
• compete unfairly by jamming
• Soln: receiver can detect and block
• Power

• Other approaches

• Use directional antennas to solve interference
• TDMA/CDMA/FDMA

• GPSR

• Use geographical info for routing
• Contradiction : Hop-by-hop routing would be taken when devices are not moving. Otherwise, use base stations. So, in any case no need of geography based routing