15-441: Computer Networking Lecture 25: Wireless. Ad-Hoc Networks Sensor networks
15-441: Computer Networking Lecture 25: Wireless, Ad-Hoc Networks, Sensor Networks
Scenarios and Roadmap Point to point wireless networks EXample: Your laptop to CMU wireless Challenges Poor and variable link quality (makes TCP unhappy) Many people can hear when you talk Pretty well defined Ad hoc networks(wireless++) Rooftop networks(multi-hop, fixed position) Mobile ad hoc networks Adds challenges: routing, mobility Some deployment some research Sensor networks(ad hoc++) Scatter 100s of nodes in a field bridge /etc Adds challenge: Serious resource constraints Current, popular, research Lecture25:11-302006 2
Lecture 25:11-30-2006 2 Scenarios and Roadmap • Point to point wireless networks • Example: Your laptop to CMU wireless • Challenges: • Poor and variable link quality (makes TCP unhappy) • Many people can hear when you talk • Pretty well defined. • Ad hoc networks (wireless++) • Rooftop networks (multi-hop, fixed position) • Mobile ad hoc networks • Adds challenges: routing, mobility • Some deployment + some research • Sensor networks (ad hoc++) • Scatter 100s of nodes in a field / bridge / etc. • Adds challenge: Serious resource constraints • Current, popular, research
Wireless Challenges(review) Need to share airwaves rather than wire Dont know what hosts are involved Host may not be using same link technology No fixed topology of interconnection ·| nterference Other hosts: collisions, capture, interference The environment(e.g, microwaves+ 802. 11) Mobility - Things change often Environmental changes do too How do microwaves work? Relate to 802. 11 absorption Other characteristics of wireless Noisy→ lots of losses ·S|ow Multipath interference Lecture25:11-302006 3
Lecture 25:11-30-2006 3 Wireless Challenges (review) • Need to share airwaves rather than wire • Don’t know what hosts are involved • Host may not be using same link technology • No fixed topology of interconnection • Interference • Other hosts: collisions, capture, interference • The environment (e.g., microwaves + 802.11) • Mobility -> Things change often • Environmental changes do too • How do microwaves work? Relate to 802.11 absorption. • Other characteristics of wireless • Noisy → lots of losses • Slow • Multipath interference
Wireless bit-Errors Router Computer 1 Computer 2 Loss→ Congestion 0 Loss t Congestion Wireless Lecture25:11-302006
Lecture 25:11-30-2006 4 Wireless Bit-Errors Router Computer 1 Computer 2 23 2 Loss → Congestion 21 0 Loss → Congestion Wireless
TCP Problems Over noisy links Wireless links are inherently error-prone Fading, interference, attenuation -> LoSS errors Errors often happen in bursts TCP cannot distinguish between corruption and congestion TCP unnecessarily reduces window, resulting in low throughput and high latency Burst losses often result in timeouts What does fast retransmit need? Sender retransmission is the only option Inefficient use of bandwidth Lecture25:11-302006
Lecture 25:11-30-2006 5 TCP Problems Over Noisy Links • Wireless links are inherently error-prone • Fading, interference, attenuation -> Loss & errors • Errors often happen in bursts • TCP cannot distinguish between corruption and congestion • TCP unnecessarily reduces window, resulting in low throughput and high latency • Burst losses often result in timeouts • What does fast retransmit need? • Sender retransmission is the only option • Inefficient use of bandwidth