High Throughput Route Selection in Multi-Rate Ad Hoc Wireless Networks Dr. Baruch Awerbuch, David Holmer, and Herbert Rubens Johns Hopkins University Department of Computer Science www.cnds.jhu.edu/archipelago

Overview Problem: Traditional Technique: New Technique: Goal: Route selection in multi-rate ad hoc network Traditional Technique: Minimum Hop Path New Technique: Medium Time Metric (MTM) Goal: Maximize network throughput Min has a number of problems in multi-rate networks. Path throughput and path stability Today I’m going to be motivating the need for a new routing metric And discussing techniques for maximizing throughput in multi-rate networks.

What is Multi-Rate? Ability of a wireless card to automatically operate at several different bit-rates (e.g. 1, 2, 5.5, and 11 Mbps) Part of many existing wireless standards (802.11b, 802.11a, 802.11g, HiperLAN2…) Virtually every wireless card in use today employs multi-rate

Advantage of Multi-Rate? Direct relationship between communication rate and the channel quality required for that rate As distance increases, channel quality decreases Therefore: tradeoff between communication range and link speed Multi-rate provides flexibility 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Add that MAC adaptively sets rate Add that multi-rate allows both longer range and higher speed (but not at same time) Multi-rate used by most existing standards (802.11abg HiperLANII, etc.) and likely to continue into the future. Very little research on the effects of multi-rate on ad hoc networks. Lucent Orinoco 802.11b card ranges using NS2 two-ray ground propagation model

Ad hoc Network Single Rate Example Which route to select? Destination Source

Ad hoc Network Single Rate Example Which route to select? Source and Destination are neighbors! Just route directly. Destination Source

Multi-rate Network Example Varied Link Rates Destination Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

Multi-rate Network Example Varied Link Rates Destination Throughput = 1.04 Mbps Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

Multi-rate Network Example Varied Link Rates Destination Throughput = 1.15 Mbps Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

Multi-rate Network Example Varied Link Rates Min Hop Selects Direct Link 0.85 Mbps Destination Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

Multi-rate Network Example Varied Link Rates Min Hop Selects Direct Link 0.85 Mbps effective Highest Throughput Path 2.38 Mbps effective Destination Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

Multi-rate Network Example Under Mobility Min Hop Path Breaks High Throughput Path Reduced Link Speed Reliability Maintained More “elastic” path Destination X Source 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

Challenge to the Routing Protocol Must select a path from Source to Destination Links operate at different speeds Fundamental Tradeoff Fast/Short links = low range = many hops/transmissions to get to destination Slow/Long links = long range = few hops/transmissions

Minimum Hop Path (Traditional Technique) A small number of long slow hops provide the minimum hop path These slow transmissions occupy the medium for long times, blocking adjacent senders Selecting nodes on the fringe of the communication range results in reduced reliability Min hop – for any arbitrary configuration, what is the least number of hops required to reach the destination. By selecting links which operate at the lowest speed, the MAC protocol has no flexibility in dealing with topological changes. This results in paths breaking from mobility.

How can we achieve high throughput? Throughput depends on several factors Physical configuration of the nodes Fundamental properties of wireless communication MAC protocol How do we know the throughput of a path? How can we achieve high throughput?

Wireless Shared Medium Transmission blocks all nearby activity to avoid collisions MAC protocol provides channel arbitration Carrier Sense Range Carrier Sense Range 1 2 The longer it takes to transmit each packet the less medium time is available to other senders in the network. This reduces network throughput.

Transmission Duration 4.55 Mbps 3.17 Mbps 1.54 Mbps 0.85 Mbps Time taken for full expected backoff, RTS, CTS, DATA, ACK exchange. (w/ 1500 byte payload) “Data” is actual application level payload, “Overhead” is everything else. Almost constant overhead regardless of link speed. higher link speed = shorter transmission time Short transmission time = high ratio of MAC overhead per packet Gives us an idea of how much medium time a packet sent at each rate consumes Medium Time consumed to transmit 1500 byte packet

Hops vs. Throughput Since the medium is shared, adjacent transmissions compete for medium time. Throughput decreases as number of hops increase. 1 2 3 Nodes need to alternate when sending along a path. Since they can’t send at the same time, throughput is reduced.

Effect of Transmission Source X X X X X X X Destination 1 2 3 4 5 6 7 8 Request to Send (RTS) Clear to Send (CTS) DATA ACK

Multi-Hop Throughput Loss (TCP) Inverse relationship = 1, 1/2, 1/3, 1/4, 1/5, etc.

Analysis General Model of ad hoc network throughput Multi-rate transmission graph Interference graph Flow constraints General Throughput Maximization Solution is NP Complete Derived an optimal solution under a full interference assumption

New Approach: Medium Time Metric (MTM) Assigns a weight to each link proportional to the amount of medium time consumed by transmitting a packet on the link Existing shortest path protocols will then discover the path that minimizes total transmission time

MTM Example 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 2.5ms = 2.5 4.55 Mbps 1 13.9ms = 13.9 0.85 Mbps

MTM Example 11 Mbps 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 2.5ms 2.5ms = 5.0 2.36 Mbps 1 13.9ms = 13.9 0.85 Mbps

MTM Example 11 Mbps 11 Mbps 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 + 11 2.5ms 2.5ms 2.5ms = 7.5 1.57 Mbps 1 13.9ms = 13.9 0.85 Mbps

MTM Example 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 + 11 + 11 2.5ms 2.5ms 2.5ms 2.5ms = 10.0 1.18 Mbps 1 13.9ms = 13.9 0.85 Mbps

MTM Example 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 + 11 + 11 + 11 2.5ms 2.5ms 2.5ms 2.5ms 2.5ms = 12.5 0.94 Mbps 1 13.9ms = 13.9 0.85 Mbps

MTM Example 11 Mbps Source Destination 1 Mbps Path Medium Time Metric (MTM) Path Throughput Link Rate 11 + 11 + 11 + 11 + 11 + 11 2.5ms 2.5ms 2.5ms 2.5ms 2.5ms 2.5ms = 15 0.78 Mbps 1 13.9ms = 13.9 0.85 Mbps

MTM Example Medium Time Usage Destination Source Link Throughput Destination 11 Mbps 2.5ms 4.55 Mbps 3.17 Mbps 5.5 Mbps 3.7ms 1.54 Mbps 2 Mbps 7.6ms 0.85 Mbps 1 Mbps 13.9ms Source Path Medium Time Metric (MTM) Path Throughput 11 Mbps 5.5 Mbps 1 13.9ms = 13.9 ms 0.85 Mbps 2 Mbps 1 Mbps

MTM Example Medium Time Usage Destination Source Link Throughput Destination 11 Mbps 2.5ms 4.55 Mbps 3.17 Mbps 5.5 Mbps 3.7ms 1.54 Mbps 2 Mbps 7.6ms 0.85 Mbps 1 Mbps 13.9ms Source Path Medium Time Metric (MTM) Path Throughput 11 Mbps 5.5 + 2 3.7ms 7.6ms = 11.3 ms 1.04 Mbps 5.5 Mbps 1 13.9ms = 13.9 ms 0.85 Mbps 2 Mbps 1 Mbps

MTM Example Medium Time Usage Destination Source Link Throughput Destination 11 Mbps 2.5ms 4.55 Mbps 3.17 Mbps 5.5 Mbps 3.7ms 1.54 Mbps 2 Mbps 7.6ms 0.85 Mbps 1 Mbps 13.9ms Source Path Medium Time Metric (MTM) Path Throughput 11 + 2 2.5ms 7.6ms = 10.1 ms 1.15 Mbps 11 Mbps 5.5 + 2 3.7ms 7.6ms = 11.3 ms 1.04 Mbps 5.5 Mbps 1 13.9ms = 13.9 ms 0.85 Mbps 2 Mbps 1 Mbps

MTM Example Medium Time Usage Destination Source Link Throughput Destination 11 Mbps 2.5ms 4.55 Mbps 3.17 Mbps 5.5 Mbps 3.7ms 1.54 Mbps 2 Mbps 7.6ms 0.85 Mbps 1 Mbps 13.9ms Source Path Medium Time Metric (MTM) Path Throughput 11 + 11 2.5ms 2.5ms = 5.0 ms 2.38 Mbps 11 + 2 2.5ms 7.6ms = 10.1 ms 1.15 Mbps 11 Mbps 5.5 + 2 3.7ms 7.6ms = 11.3 ms 1.04 Mbps 5.5 Mbps 1 13.9ms = 13.9 ms 0.85 Mbps 2 Mbps 1 Mbps

MTM Definition

Advantages It’s an additive shortest path metric Paths which minimize network utilization, maximize network capacity Global optimum under complete interference Single flow optimum up to pipeline distance (7-11 hops) Excellent heuristic in even larger networks Avoiding low speed links inherently provides increased route stability Can be applied to existing link state and distance vector protocols.

Disadvantages MTM paths require more hops More transmitting nodes Increased contention for medium Results in more load on MAC protocol Only a few percent reduction under the simulated conditions Increase in buffering along path However, higher throughput paths have lower propagation delay

Sounds great but… Do faster paths actually exist? There needs to be enough nodes between the source and the destination to provide a faster path Therefore performance could vary as a function of node density When density is low: MTM = Min Hop

Performance Increase vs. Node Density in Static Random Line

MTM Throughput Increase Under 802.11MAC -NS2 Network Simulations -20 TCP Senders and receivers -Random Waypoint mobility (0-20m/s) -DSDV Protocol modified to find MTM path

Reference Baruch Awerbuch, David Holmer, Herbert Rubens, The Medium Time Metric: High Throughput Route Selection in Multi-rate Ad Hoc Wireless Networks, Technical Report, Johns Hopkins University, October 2004.