Z-MAC: a Hybrid MAC for Wireless Sensor Networks Injong Rhee, Ajit Warrier, Mahesh Aia and Jeongki Min Dept. of Computer Science, North Carolina State University Presenter: Tim

Outline Introduction Design of Z-MAC Performance Evaluation Conclusion

What is Z-MAC? A Hybrid MAC –Combine the strengths of CSMA and TDMA while offsetting their weakness CSMA (Carrier Sense Multiple Access) –High channel utilization and low latency under low contention –Hidden terminal problem TDMA (Time Division Multiple Access) –No hidden terminal problem and high channel utilization under high contention –Not practical due to too many problems

Basic Idea of Z-MAC Each node owns a time slot. A node may transmit at any time slot. However, the owner has the higher priority to transmit data than the non-owners. When a slot is not in use by its owner, non-owners can steal the slot. Z-MAC behaves like CSMA under low contention and like TDMA under high contention.

Design of Z-MAC Setup phase Transmission Control Explicit Contention Notification Receiving Schedule of Z-MAC Local Time Synchronization

Setup Phase Including –neighbor discovery –slot assignment –local frame exchange –global time synchronization High overhead? –It runs only once during the setup phase and does not run until a significant change in the network topology

Setup Phase: Neighbor Discovery Steps –Every node periodically broadcasts a ping to its one-hop neighbors. –A ping message contains the current list of its one-hop neighbors. –Through the process, each node gathers the information of its two-hop neighbors. Implementation –Every node sends one ping at a random time in each second for 30 seconds. AB D C

Setup Phase: Slot Assignment Using DRAND to assign time slots to every node. –DRAND is a distributed implementation of RAND, used for TDMA scheduling or channel assignment for wireless networks. Ensuring no two nodes within a two-hop communication neighborhood are assigned to the same slot. The slot number assigned to a node does not exceed the size of its local two-hop neighborhood (δ). The running time and message complexity are also bounded by O(δ)

Setup Phase: Local Framing Each node needs to decide on the period in which it can use the time slot for transmission. The period is called the time frame of the node. Time frame rule –S i : the slot number assigned to node i –F i : the maximum slot number within node i’s two-hop neighborhood –Set node i’s time frame to be 2 a, where a satisfies 2 a-1 ≤ F i ≤ 2 a – 1. That is, node i uses the S i -th slot in every 2 a time slots.

Example 2 a-1 ≤ F i ≤ 2 a – 1 Node A a = 2 Node C a = 3

Transmission Control Two modes: low contention level (LCL) and high contention level (HCL). –Under LCL, non-owners are allowed to compete in any slot with low priority. –Under HCL, a node does not compete in a slot owned by its two-hop neighbors. To avoid being hidden terminal to the owners. A node is in HCL only when it receives an explicit contention notification (ECN) messages within the last t ECN period.

Transmission Rule Node i acquires data to transmit Is node i the owner? Take a random backoff within period T o Is the channel clear? Wait until the channel is not busy Is node i in LCL? Is the current slot owned by its two-hop neighbor? Wait for T o and performs a random backoff within a contention window [T o, T no ] Transmit data!!! Postpone its transmission until the time slot is (1)not owned by a two-hop neighbor or (2) owned by itself NO YES NO

Explicit Contention Notification ECN messages notify neighbors not to act as hidden terminals to the owner of each slot when contention is high. How to estimate two-hop contention? –According to noise level of the channel Low noise indicates low contention.

Explicit Contention Notification Steps: –As a transmitting node detects high contention, the node sends a unicast message, one-hop ECN, to the destination which is experiencing contention. If there are multiple destinations, it broadcasts a message with information about the multiple destinations. –Assume node j receives one-hop ECN. If node j is the destination, it then broadcasts the ECN, two-hop ECN, to its one-hop neighbors. If not, it simply discards the one-hop ECN. –When a node receives a two-hop ECN, it sets the HCL flags. ECN suppressing

Example

Local Time Synchronization Z-MAC adopts a technique from RTP/RTCP (real-time transport protocol). –The control message transmission rate is limited to a small fraction of session bandwidth. –In Z-MAC, a node sends one synchronization packet per every 100 data packets.

Local Time Synchronization Trust factor(β t ): –R drift : the max clock drift rate of each node –ε clock : the max acceptable clock error –I synch = ε clock / R drift : the min synchronization interval required to achieve the max clock error –α synch : the max weight applying to the new clock value received –S : the avg. rate at which a node receives or sends synchronization messages – How to get new clock value? –C avg : weighted moving avg. clock value –C new : newly received clock value –C avg = (1- β t )C avg + β t C new

Performance Evaluation Implementing Z-MAC in both ns-2 and Mica2/TinyOS. Comparing the performance of Z-MAC with that of PTDMA(ns-2), Sift(ns-2) and B-MAC(ns-2 and TinyOS). Three benchmarks –One-hop benchmark –Two-hop benchmark Two clusters, 7 and 8 sending nodes. –Multi-hop benchmark

Multi-hop Benchmark

Default settings

Throughput

Energy Efficiency

Conclusion Z-MAC has advantage over B-MAC under medium to high contention. It is good for application where expected data rates and two-contention are medium to high. Robust to topology changes and clock synchronization errors.

Thank you…