1 Low Latency Multimedia Broadcast in Multi-Rate Wireless Meshes Chun Tung Chou, Archan Misra Proc. 1st IEEE Workshop on Wireless Mesh Networks (WIMESH), Sep

2 Outline Introduction Impact of Multi-Rate Links on Efficient Broadcasting Optimal Network-Wide Broadcast in a Multi-Rate Wireless Mesh Network Heuristic for Low Latency Broadcast Tree Simulated Performance Studies Conclusion and Comments

3 Introduction We introduce the problem of efficient routing and packet distribution for broadcast traffic flows in multi-rate, multi-channel, multi-radio WMN. The broadcast latency: Computed as the maximum delay between the transmission of a packet by a source node and its eventual reception by all the intended receivers. Our goal is to minimize this worst-case path latency.

4 Contributions of This Paper Optimal of efficient packet broadcasting is often achieved by having an intermediate node perform multiple broadcasts, each of which is directed towards a different subset of child nodes. Optimal traffic distribution topology, and the best broadcasting strategy, can be highly sensitive to the bit rate of the broadcast flow. Designing modifications to existing wireless broadcast algorithms, such that the exploit the wireless broadcast advantage, as well as the multi-rate nature of individual links.

5 Outline Introduction Impact of Multi-Rate Links on Efficient Broadcasting Optimal Network-Wide Broadcast in a Multi-Rate Wireless Mesh Network Heuristic for Low Latency Broadcast Tree Simulated Performance Studies Conclusion and Comments

8 Impact of Multi-Rate Links on Efficient Broadcasting

9 Outline Introduction Impact of Multi-Rate Links on Efficient Broadcasting Optimal Network-Wide Broadcast in a Multi-Rate Wireless Mesh Network Heuristic for Low Latency Broadcast Tree Simulated Performance Studies Conclusion and Comments

10 Optimal Network-Wide Broadcast in a Multi-Rate WMN Model assumptions One radio per node, common channel. A node can multicast at different data rates. Neighbors: all the nodes that can be reached using the lowest rate. Interference range: 1.7 Centralized scheduling No two multicasts will interfere with each other.

11 Optimal Network-Wide Broadcast in a Multi-Rate WMN The key decisions in the optimization problem are : Whether a node should multicast and if so, to which of its neighbors; The timing of these multicasts. A node can only multicast a packet after it has received. Some multicasts cannot take place the same time because they interfere with each other. Theorem 1: the minimum latency network-wide broadcast problem in a multi-rate WMN is NP- hard.

12 Outline Introduction Impact of Multi-Rate Links on Efficient Broadcasting Optimal Network-Wide Broadcast in a Multi-Rate Wireless Mesh Network Heuristic for Low Latency Broadcast Tree Simulated Performance Studies Conclusion and Comments

13 Heuristic for Low Latency Broadcast Tree We decompose the algorithm into two logically independent steps: Topology Construction (Broadcast Incremental Bandwidth, BIB) The aim is to compute a broadcast tree T. Transmission Scheduling Note that the scheduling heuristic is independent of the Topology Construction algorithm and can take any spanning tree as its input.

14 A. The BIB Topology Construction Algorithm We first compute the tree from a source node s to V – {s}. The BIB algorithm use a modified version of Prim ’ s algorithm, greedily adding links to an existing tree such that the incremental cost is minimized. BIB primarily aims to choose high-rate links. The transmission rate R(x) of node x is given by the slowest downstream link, i.e.,

P x : parent of node x, already part of tree. Dynamic modification of the link cost

UpdateCost((P x,x),y)

17 B. The Scheduling of Transmissions Directed graph (tree) G b = (V b, E b ). Let V b = {b 1,b 2, …,b k } be the set of all the branch points in the broadcast tree T. b 1 is the source node. t(b i ): minimum transmission time it takes the node b i to transmit a packet to all its children. Undirected conflict graph G c = (V c, E c ).

18 B. The Scheduling of Transmissions A valid schedule is one which meets the following constraints: The objective is to find a valid schedule which minimizes the broadcast latency:

19 B. The Scheduling of Transmissions D(b i ): the set of all descendants of b i in G b. P(b i,x): the set of nodes on the path from b i to x. w(b i ): the time needed to reach all the descendants of b i in T in the absence of interference. w(b i ) is a lower bound. We define f(q i ) as follows: An estimate of the downstream latency to reach all the descendants of q i in the worst possible scenario, where the node q i can only transmit after all the qualified nodes that interferes with q i have transmitted.

Q = {q 1,q 2, …,q m }: qualified node e(b i ): earliest possible multicast times for b i f(bi): priority measure PTIME: permissible time

21 Outline Introduction Impact of Multi-Rate Links on Efficient Broadcasting Optimal Network-Wide Broadcast in a Multi-Rate Wireless Mesh Network Heuristic for Low Latency Broadcast Tree Simulated Performance Studies Conclusion and Comments

22 Simulated Performance Studies We will study 3 heuristics. BIB. SPT: shortest path tree given by Dijkstra ’ s algorithm. CDS (connected dominating set) Assumes that all broadcasts are at the lowest rate. The broadcast tree is computed using a greedy approximation of the minimum CDS.

23 A. Small, Regular grid topology For each given topology, we compute the worst case delay:

BIB6.5 SPT17.5 CDS11

25 B. Heuristic performance in random topology We use randomly generated topologies of different sizes. For each size, we generate 100 topologies whose nodes are uniformly randomly distributed in a square of 1 km 2. The results are given in Figure Good performance for delay also means good performance for throughput and vice versa. SPT uses the most number of multicasts per tree. CDS fails to exploit the higher transmission rates.

29 Outline Introduction Impact of Multi-Rate Links on Efficient Broadcasting Optimal Network-Wide Broadcast in a Multi-Rate Wireless Mesh Network Heuristic for Low Latency Broadcast Tree Simulated Performance Studies Conclusion and Comments

30 Conclusions We have introduced a novel type of WMN operation where A node can multicast at the link layer to different subsets of neighbors at different transmission rates. We propose a heuristic which takes both wireless multicast advantage and multi-rate into consideration. Simulation studies show that significant gain can be achieved by exploiting multiple rates available.

31 Comments Although the authors say that good performance for delay also means good performance for throughput, the relationship between low latency and high throughput should be examined thoroughly and carefully.