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An Adaptive, High Performance MAC for Long-Distance Multihop
Wireless Networks
Presented by Jason Lew
An Adaptive, High Performance MAC for Long-Distance Multihop Wireless Networks
Authors:• Sergiu Nedevschi, UC Berkeley
• Rain K. Patra, UC Berkeley
• Sonesh Surana, UC Berkeley
• Sylvia Ratnasamy, Intel Research Berkeley
• Lakshminarayanan Subramanian, NYU
• Eric Brewer, UC Berkeley
Overview
Background Improvement JazzyMac Design JazzyMac Evaluation Conclusion
Background:WiLD Networks
Multi-hop WiFi long-distance networks are able to provide connectivity to thinly-populated or rural regions
Networks have long links and use directional antennas
Therefore, there are long propagation delays and increased collision probability
Background:Using SynOp to Avoid Interference
a) Mix-Tx-Rx• Not feasible because of the physical proximity between the radios and
the presence of antenna side-lobes b) SynRx
• Must satisfy (1) c) SynTx
• Must satisfy (1)
isolationRR ThSPP || 21
(1)
Background:Using SynOp to Avoid Interference
Thus,• SynOp can allow multiple adjacent WiLD links to
simultaneously use the same wireless channel if:
• Links are separated by large enough α AND
• Radio transmit powers satisfy (1)
isolationRR ThSPP || 21
(1)
Background:MAC Protocols for WiLD Links
CSMA-based MAC protocols do not perform well in networks with long distance links• Links are much too long to carrier sense
• TDMA is a better option
Background:MAC Protocols for WiLD Links
(a) is an example of a bipartite network used in long distance WiLD Links• A transmits on all links
• Then, B transmits on all links
Improvement: Improving Throughput
Adapt to traffic demand• Allocate time slots
according to demand Allow neighboring,
overlapping transmissions• If A only needs a portion
of transmission time to B, allow B to transmit to D during A’s slot
Improvement: Improving the Bandwidth-Delay Tradeoff
Bandwidth vs. delay tradeoff is fixed with fixed slots• Fix by using dynamic slot adaptation
• Low utilization link small TDMA slots lower per-hop delay
• High utilization link large TDMA slots higher bandwidth
JazzyMac Design: Features
Adaptive slots Allow parallel neighboring-but-
independent transmissions Generalized topologies (not limited to
bipartite)
JazzyMac Design: Protocol
Each node has a mode of operation• mode is either Tx or Rx
Each link AB has a token • Only the node (A or B) holding the token can transmit
• Each token has timeout value
• Controls when the node with the token can transmit over the link
Network-wide parameter, max_slot, bounds the maximum transmission slot length
JazzyMac Design: 4 Protocol Rules
Token Exchange Rule:• After transmitting, node tells connected node when it
will be able to receive traffic
• Passes this along with token
Mode Rule:• A node in receive mode can move to transmit mode
only when it has the token for all of its links
• A node in transmit mode can move to receive mode when it has release the tokens for all of its links
JazzyMac Design: 4 Protocol Rules
Transmission Rule:• Node can transmit over link only if:
• Node is in transmit mode AND
• Node holds token
• Token is valid
Slot Rule:• Node can transmit on link for no longer than max_slot
JazzyMac Design: Operation
t = 0:• A has Token AB and
Token AC
• Can transmit on both links
JazzyMac Design: Operation
t = 15:• A’s Tx to B ends
• Token AB is passed to B
• Token AB is passed with timeout = 35 because A’s Tx to C needs more time
JazzyMac Design: Operation
t = 15:• B has Token AB and
Token BC
• Token AB is not valid because it was passed with timeout = 35
• B transmit to C only
JazzyMac Design: Operation
t = 50:• A releases Token AC
• A moves into Rx mode
• Token AB is now valid
• B can now transmit over AB
JazzyMac Design: Operation
t = 60:• C transitions to Tx
JazzyMac Design: Dealing with Loss
Packet loss can cause the loss of link tokens• Implementations of JazzyMac can piggyback tokens
or send multiple copies of the token
Loss will still occur rarely• Solution: add sequence number to each token
• Increment every time token is exchanged
• After timeout, every node resends tokens
• Duplicate tokens ignored; lost tokens recovered
• Drawback: token retransmissions can interfere with other token retransmissions or other packets
JazzyMac Evaluation Improves maximum throughput by 15-100% in typical
topologies Improvements are consistent across many topologies and
traffic patterns Reduces gap between network throughput from practical
approaches and the optimal theoretical network throughput (assuming idealized transmission slots)
Dynamic slot sizing improves the delay-throughput trade-off
Throughput increases of as large as 80% for asymmetric (non-bipartite) traffic distributions
JazzyMac Evaluation: Performance in Random Topologies
JazzyMac Evaluation: Performance in Random Topologies
JazzyMac Evaluation: Performance in Random Topologies
JazzyMac Evaluation: Performance in Random Topologies
JazzyMac Evaluation: Effect of Traffic and Topology
JazzyMac Evaluation: Effect of Traffic and Topology
JazzyMac Evaluation: Effect of Traffic and Topology
JazzyMac Evaluation: Effect of Traffic and Topology
Conclusion
WiLD networks provide services to many users in rural areas around the world
JazzyMac can improve throughput, reduce latency, and enable operation on general topologies
JazzyMac outperforms existing WiLD MAC protocols and can operate on any network topology
An Adaptive, High Performance MAC for Long-Distance Multihop
Wireless Networks