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Network edge
- hosts: clients and servers
- servers often in data centers
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Access networks
- physical media
- wired
- wireless
- communication links
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Network core
- interconnected router
- network of networks
- Signal carried in electromagnetic spectrum
- No physical "wire"
- Propagation environment effects:
- reflection
- obstruction by objects
- interference
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Shared wireless access network connects end systems to router
- via base station aka "access point"
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Wireless LANS:
- within building (100ft)
- 802.11 (WiFi): up to 54Mbps transmission rate
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Wide-area wireless access:
- provided by telco (cellular) operator, > 10km
- between 1 and 10Mbps
- 3G, 4G: LTE
- Today, the number of mobile phone subscribers (> 5.5 billion) now exceeds that of wired phone subscribers!
- Advantages: laptops, PDAs, mobile phone promise anywhere, anytime untethered Internet access
- Two important (but different) challenges
- Wireless: communication over wireless link
- mobility: handling the mobile user who changes point of attachment to network
- Wireless Hosts
- Base station
- Wireless link
- Infrastructure Mode:
- base station connects mobiles into wired network
- Ad Hoc Mode:
- no base stations
- nodes can only transmit to tether nodes within link coverage
- nodes organize themselves into a network route among themselves
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Decreased signal strength: radio signal attenuates as it propagates through matter (e.g. passing through wall)
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Interference from other sources: standardized wireless network frequencies (e.g., 2.4GHz) shared by other devices (e.g., phone); devices (motors) interfere as well
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Multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times
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SNR: signal-to-noise ratio
- larger SNR - easier to extract signal from noise (a "good thing")
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SNR versus BER (bit error rate)
- given physical layer: increase power -> increase SNR -> decrease BER
- given SNR: choose physical layer that meets BER requirement, giving highest throughput
- SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)
* channel partitioning
- divide channel into smaller "pieces" (time slots, frequency, code)
* random access
- channel not divided, allow collisions
- "recover" from collisions
* "taking turns"
- nodes take turns but nodes with more to send can take longer turns
- Used for cellular, satellite network
- Unique "code" assigned to each user
- All users share same frequency, but each user has own "chipping" sequence (i.e., code) to encode data
- Encoded signal = (original data) * (chipping sequence)
- Decoding: inner-product of encoded signal and chipping sequence
- Multiple users transmit data simultaneously
- CDMA Encode/Decode