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Internet Service in Developing RegionsThrough Network Coding
Mike P. Wittie, Kevin C. Almeroth, Elizabeth M. Belding, Department of Computer Science
UC Santa Barbara
Ivica Rimac, Volker HiltBell Labs
Alcatel-Lucent
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Networking and the Digital Divide
• The Digital Divide– Low penetration of Internet services– Higher price– Lack of adequate infrastructure
• Success of cellular deployments– No data services– High subscription price
• Rural mesh networks– Local communication patterns
• Goals: – Low cost data communications– Leverage cellular deployments– Cater to local communications
US Europe India Sub-saharan Africa
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50
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150
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250
300
350
400
Broadband price (USD/month)
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Multihop Cellular Networks (MCNs)
• Cellular network augmented by client-to-client Wi-Fi communications [Lin00] (A)
• Rural (sparse) MCNs– Large cell area– Large per-client spectrum usage
• Local traffic patterns (B):– Cannot use cell tower– Cannot form end-to-end paths
• Need: efficient opportunistic client-to-client forwarding in sparse MCNs
A. B.
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Delay Tolerant Networks (DTNs)
• Epidemic Routing [Vahdat00]– Bundled data forwarded during every
contact for eventual delivery– Flood scoping by hop-count or TTL
• PRoPHET [Lindgren04]– Transitive destination contact
probability as routing metric– Data forwarded up a routing metric
gradient
• But, high cost of flooding creates network congestion
• Cloud Routing (CR) [Wittie09]– Network and traffic state disseminated over a
control channel– Forwards a small set of data copies– Lower forwarding cost and higher network
throughput
• But, replication wastes network resources
S
D
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Intra-flow Network Coding (NC)
• Forwards randomly encoded data on each path
• With high probability, data arriving on multiple paths is innovative
• Codes are embedded in packets themselves [Chou03]
𝑛 bytes of data 𝐷𝑝×𝑛/𝑝 data matrix 𝐸𝑝×𝑝 encoding matrix, initially 𝐼 𝒗1×𝑝,𝒗𝑖 ∈𝒢ℱ(8) Coded piece: ሾ 𝒗𝐸 | 𝒗𝐷 ሿ1×(𝑝+𝑛/𝑝)
S
D
൦
𝒗1𝐸1𝒗2𝐸2⋮𝒗3𝐸3||||𝒗1𝐷1𝒗1𝐷1⋮𝒗1𝐷1
൪𝐺𝑎𝑢𝑠𝑠𝑖𝑎𝑛 𝑒𝑙𝑖𝑚.ሱۛ ۛ ۛ ۛ ۛ ۛ ۛ ۛ ۛ ۛ ሮ ൦ 𝐼 |||| 𝐷 ൪
𝑝×(𝑝+𝑛/𝑝)
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NC in DTNs
• Network Coding Probabilistic Routing (NCPR) [Widmer05]– Each node forwards floor(d)
coded pieces and additional coded piece with probability d-floor(d)
– Stops forwarding after ceil(d) coded pieces
– New innovative coded pieces reset forwarding cap
• But, tradeoff between high delivery rates and high load
• Need a more efficient mechanism for reliability
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D
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Semi-Innovative Set Routing (SISR)
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D
Linearly independent?
Coded pieces required to decode bundle: 𝑏= 𝑝
Redundant coded pieces: 𝑟= 𝑏4
Maximum coded pieces at node (bundle fraction): 𝑓= 𝑟
b rb – f f
SISR (scissor) forwards: small forwarding footprint (CR) fraction of data on each path
through NC (NCPR)
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Given a set of coded pieces 𝐶, ȁ'𝐶ȁ'= 𝑏+ 𝑟,𝑟≥ 1, we can construct a set of SISs over 𝐶, such that any subset of 𝐶 of size 𝑏 has full rank.
Semi-Innovative Sets (SISs)
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D
SIS 𝑠 is a set of linearly independent coded pieces
𝐶
SIS1SIS2SIS3
every possible union of 𝑠𝑖,𝑠𝑗
𝑠1,𝑠2,⋯,𝑠 |𝐶|𝑏/2ඈ s1 s2 s3
b rb – f f
f
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Semi-Innovative Sets (SISs)
S
D
SISs can be constructed to tolerate any number of losses 𝑙 = ≥𝑓,ۂ�������𝑟/𝑓ہ������� 𝑟
𝑙 = 3 → 𝐶= 2𝑏,𝑓= 13 SIS1
every possible union of 𝑠𝑖,𝑠𝑗 SIS2SIS3SIS4SIS5SIS6
b rb rf ff b – f r-2f
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SISR in an MCN
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D
While the number of SISs grows
exponentially as ቆቒ𝑏+𝑟𝑏/2ቓ2 ቇ, each
node only needs to maintain
ቒ𝑏+𝑟𝑏/2ቓ− 1 SISs
n2 n3
When 𝑑 coded pieces are delivered, the global encoding adjusts accordingly
D
SIS1SIS2SIS3SIS4SIS5SIS6
ሺ𝑥,𝑦ሻ,𝒗𝐸
Embedded codes disseminated over the control channel to announce forwarding progress
b rb rSIS1SIS2SIS3SIS4SIS5SIS6
d
n1
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SISR Cloud Progress Example
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Evaluation Setup• Want to compare SISR with CR and NCPR
– NCPR – flooding and network coding– CR – small set of bundle copies– SISR – network coded bundle + redundancy
• Configuration details:– Area, node density and mobility models a rural community– Single flow between a node pair at different distances– Interested in evaluating:
• Bundle forwarding cost • End-to-end delay• Control channel load
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Forwarding Cost
• Forwarding cost – the amount of data forwarded
in the network before delivery
• NCPR – high cost of flooding• CR – high cost of replication• SISR – lowest cost
– Fraction of data on each path– Improvements for multiple
simultaneous flows
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Overhead of Control Traffic
• Control channel load– Position updates– Bundle progress notifications– Data encoding vectors (SISR only)
• Cellular channel gain– Bundle size minus control traffic
• Prevalence of position updates• Higher gain for multiple flows• Gain higher for CR, but SISR
easier on client resources
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A. B.
Conclusions and Future Work
• Introduced Semi-Innovative Set Routing (SISR)
• End-to-end management of NC and forwarding mechanisms
– Only innovative data forwarded– Tolerates any number of losses
• 2X reduction in forwarding cost– Lower cost of infrastructure and data
services– Make data services affordable for more
clients
• Future work:– Adaptation to different
network settings– Directional mesh
networks with smart antennas
– Different ratios of data and control traffic propagation speeds
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D
b rSIS1SIS2SIS3SIS4SIS5SIS6
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Q & A
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Backup Slides
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Evaluation Setup• Want to compare SISR with
CR and NCPR• Configuration details:
– Area, node density and mobility models a rural community
– Single flow between random node pair
– NCPR – d configured for 100% delivery at 6km
– CR – lower forwarding cost at delay comparable to larger clouds
– SISR – lowest delay at 6km
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Bundle Delay
• Delay– end-to-end forwarding delay of
entire bundle (all coded pieces)
• SISR - last copy delay• NCPR – nodes use up
forwarding allowance before delivery
• CR – first copy delay
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Multihop Cellular Networks (MCNs)
• Cellular network augmented by client-to-client Wi-Fi communications [Lin00]
• MCNs can:– Reduce cellular channel load
(A)– Extend cell coverage (B, C)
• MCNs make cellular infrastructure go further
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MCNs in Developing Regions
• Sparse MCNs– Fewer clients and larger cell area– Larger per-client spectrum usage
• Local data communications– Regional caches (B)– Opportunistic client-to-client
communications (C)
• Our focus: opportunistic client-to-client forwarding in sparse MCNs
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