Wireless Ad-Hoc Real-Time Multicast Protocol (WARM)

1. Wireless Ad-Hoc Real-Time Multicast Protocol (WARM) • Streaming of real-time multicast traffic in wireless ad-hoc networks is a challenge • Simultaneous transmissions by nodes with common multicast member neighbors will result in collisions at those common neighboring nodes. • An intelligent scheduling mechanism is required. Furthermore, as nodes move, the scheme has to maintain the multicast structure for collision free streaming. • This is similar to providing QoS – Integrated Services  reserved bandwidth along a path from the source to a receiver.

2. Reference • G.D. Kondylis et al, “Multicasting Sustained CBR and VBR traffic in Wireless Ad Hoc Networks”, Proceedings of the IEEE International Conference on Communications (ICC), 2000. • On my website.

3. Wireless channel is broadcast in nature • One transmission can reach many recipients: ideal for multicasting • However, simultaneous transmissions of adjacent relays can lead to collisions • In a multicast scenario, ACKs cannot be used: because of the multiple receivers the same problem of collisions exists at the transmitter • The only way to achieve reliable communications (at the MAC layer) is to reserve bandwidth for the multicast session • Let us assume that the network size is moderate. • The problems of synchronizing nodes etc. are taken care of. • Why is this assumption required ? • Reservations imply we use either a TDMA or FDMA based method (sometimes CDMA).

4. Assumptions in WARM • A single source is transmitting to a multitude of receivers. • The receivers are told about the session via some advertisement or otherwise. • The onus is on the receiver to join the multicast session. • We have the bandwidth divided into channels – either in time (it could be frequency or code but..). • There is bandwidth set aside for the multicast session(which has to be spatially re-used appropriately). • Signaling is done in collision-free manner(a round-robin scheme is assumed) • Half-duplex transceivers – nodes cannot transmit and receive at the same time.

5. If nodes 2 and 3 (or 1) transmit simultaneously, node 6 will see a collision • Therefore, node 2 will have to use different “bandwidth” (TDMA slot / CDMA code / FDMA frequency band) • That’s a problem of coloring the graph with the minimum possible number of colors (color = TDMA slot, e.g.)

6. Main Principles of WARM • Joining multicast members should try to attach to already existing relays, before forcing other members to become relays • they should try to find colors already used by relays, where they can receive reliably • Relays have a set of “unusable” colors: colors that if used to transmit will cause collisions at some receiver. • This is with CBR in mind. • If the traffic is VBR, a set bandwidth is allocated and packets in excess will have to be relayed via random access.

7. The TDMA frame format Super-frame Random Access Random Access Reserved Reserved p1 p1 p0 p0 p1 0 1 0 1 Transmit Part Receive Part • Multiple transmit slots may be reserved. In this packet zero is received in slot 0 and packet 1 is received in slot 2. It transmits packet 1 in slots 0 and 1 (possibly for different children) and packet 0 in slot 6.

8. Packets in a frame that are in excess of the reserved number of slots are transmitted/received in the random-access portion of the frame. • This would happen in the case of the VBR traffic. • If reservation = mean source rate, then anything in excess of this mean source rate will have to be transmitted using the random access slots. • Parent node (in the multicast mesh) will inform its children of the specific random-access slots that they will have to listen to by appending the relevant info to packets transmitted during the reserved portion of bandwidth. • If other nodes can overhear this, (not always possible) they could potentially avoid these slots.

9. Signaling • Transmit scheduling for signaling between nodes is done in a round robin fashion on a separate channel. • This is not a requirement – if routing or other info is to be transmitted they need a control channel  MAC-routing interactions.

10. Signaling messages usually carry • The node’s unique ID, • The hop-count from the source (which helps a node to determine who to connect to) • A bit to indicate whether it is a transmit frame or receive frame that is to follow. • List the transmit and receive slots for the node (refreshing the reservations : soft state). • List the parents of the node. It is required that all parents be at the same level from the source. • Slots on which the node is precluded from transmitting or receiving to children (to parents). • This would essentially help when a node is searching for a new parent.

11. JOINing the Multicast Mesh and Maintenance • Depending upon what is transmitted in the Unusable Slots which is in fact how collisions (let us qualify what these mean later) are avoided a node can choose its parents (maybe more than one) to receive multicast data from the session. • If links break due to mobility (slots get jammed) then the receiving node can try and re-establish connectivity by using other usable slots or try and connect to other parents. • During the time of re-establishment, soft state empty packets may be sent to children so that they do not disconnect from the mesh thereby causing a chain reaction. • If the links are not re-established within certain time, then the children of the node who loses connectivity might have to attempt re-establishment and so on.

12. The Metrics for QoS • To re-iterate we are trying to stream real-time data. • For each session we set aside certain bandwidth that corresponds to its mean rate or peak rate (certain number of slots). • Packet losses may occur due to mobility or due to losses for VBR sources in the random access channels. • The performance is quantified in terms of throughput :percentage of received packets. If the structure is to be re-built often it results in a large packet loss. • The mesh structure allows nodes to identify new parents quickly when links fail.

13. What is a collision ? • A collision is when the interference level is much higher than the required signal level. • One way of modeling collisions would be that if two nodes transmit such that they are both in the range of a third node, there is a collision at that node. • However, in reality, because of multiple distant transmissions, the interference level at a node due to the distant transmissions might be significant to cause degradation  collision. • In the following example such effects aren’t considered but are taken into account in the simulation studies reported in the paper. (collision if SIR < g).

14. Example(one hop interference only) • One color (slot) is not enough to fully connect the multicast session • Two slots (colors) can connect the session (but perhaps not every node is at the minimum hop count from the source.

15. Example of Results(full co-channel interference) • When source is mobile, packet dropping is more severe and is affected by speed more that when source is static.  Can you guess why ?

16. Power Management • Power Control is popular in cellular networks. • This would typically require that all nodes transmit such that the received power at the base station is the same. • It has been shown for both CDMA and TDMA systems that power control helps in reducing interference and improving throughput. • What is power control or management in an ad hoc network ? • Difficult to do  no centralized control. • Benefits  Improvement in throughput, reduction in power consumption. • Disadvantages  Creating unidirectional links : depends on density.

17. Reference • T.ElBatt et al, “Power Management for Throughput Enhancement in Wireless Ad Hoc Networks”, Proceedings of the IEEE International Conference on Communications (ICC), 2000. • My Web Site.

18. The Concept of Connectivity Connectivity of Each Node is 3.

19. The Key Idea • Each node varies its transmit power so as to define its connectivity. • What does this mean ? • Instead of using the constant power level as was done in all our previous discussions (except those on unidirectional links) a node uses the power level such that it reaches only a fixed set of neighbors. • What is the advantage ? • The zone of interference is reduced. Thus, frequency re-use is achieved. • What is the disadvantage ? • A packet has to potentially travel a large number of hops to reach a destination – does this increase bw consumption ?

20. The framework • Time Division Multiplexed Access with round-robin slots for transmitting signaling information. • Signaling Messages transmitted at maximum power carry routing and connectivity information. • Actual Data is transmitted by using Slotted Aloha medium access control protocol. • Power Based Routing is deployed. Two schemes investigated: • Same power within connectivity domain • Different powers for different nodes within the domain, I.e., power adaptability at a fine grained level.

21. How do we form domains ? • Remember, each node has its own slot to transmit signaling information. • This control info is transmitted at the largest power possible. • What is the received power ? • If transmit power is Pt, received power is • Pr = (z/d4) Pt. • Here, z is the shadowing co-efficient and the path loss is in accordance to a fourth power law. • Why is multi-path fading not considered ?

22. Each node records the received power from each of its neighbors within its maximum power range. • It orders these neighbors in accordance to an ascending rule. • It then identifies the number of neighbors to be included in its connectivity zone (K). • Thus, the domain is formed.

23. Power Control • We try two types. • One possible way is to use just the power level required to communicate with each node. • The second is to use the same power level for all nodes within the domain. • The first (intuitively) decreases interference further and increases the number of hops (perhaps). • The second makes links more robust and possibly reduces the number of hops to destination (by a little) but increases interference.

24. Are Unidirectional Links Formed here ? • The answer is well.. Yes and No. • What does this mean ? • Notice that the control info is always carried on bi-directional links. • This allows the formation of routing tables – all links are bi-directional as far as control signaling goes. • Data is however carried on unidirectional links. • We use the aloha protocol at MAC layer – thus it is not very good. • RTS/CTS might be used but they have same problems that we saw earlier.

25. Metrics of Performance • Average Power Consumption at a node. • Node throughput -- % of packets that were successfully transmitted. • End to end network throughput -- % of packets that actually reached their intended destinations.

26. Example Results • Network Throughput is maximized at an interim power level. A gain of nearly 8% in throughput. • When power adjustment is done within a cluster, the maximum is reached at a different point than when no power adjustment is done within the cluster. • This optimal connectivity range can be dynamically searched for.

27. Higher power savings (of the order of more than 50 milliwatts on the average) when power management is done. • Lower connectivity range results in lower power consumption. • From previous result and this one, we can see that when there we do power adjustments for reaching neighbors within a cluster, we maximize throughput at lower power.

28. Other forms of Power Management • Power management to ensure that network does not get partitioned. Use enough power to ensure complete connectivity by Ramanathan and R.Rosales Hein (BBN), Infocom 2000. • This is another paper on topology control. • Power Aware Routing  allow usage of minimum energy path. Allow nodes to sleep when they are not participating in routing – S.Singh and C.S.Raghavendra, “Power Aware Routing in Mobile Ad hoc Networks”, in Proceedings of ACM Mobicom, August 1998, Seattle . • SPAN in Mobicom 2001 – allow nodes to go to sleep when they are in dense parts of the network – power savings. by Hari Balakrishnan MIT.