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