05-03-2010, 05:40 PM
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Z-MAC: A Hybrid MAC for Wireless Sensor Networks
Presented By:-
Subodh Kumar Gond
Roll. No. 08EC6415
M.Tech. 1st Year
Telecommunication System Engineering
Introduction
What is MAC
-Controlling access to the channel
What is Z-MAC
- A Hybrid MAC which Combine the strengths of CSMA and TDMA while offsetting their weakness.
- The main feature of Z-MAC is its adaptability to the level of contention in the network .
CSMA (Carrier Sense Multiple Access)
¢ It is popular because of its simplicity , flexibility and robustness.
¢ It does not require much infrastructure support.
¢ High channel utilization and low latency under low contention.
¢ Hidden terminal problem.
TDMA (Time Division Multiple Access)
No hidden terminal problem.
High channel utilization under high contention.
Complexity of scheduling.
Need synchronization.
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.
The priority is implemented by adjusting the initial contention window size.
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
Neighbor Discovery
¢ 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.
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 (d).
-The running time and message complexity are also bounded by O(d).
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
¢ Si: the slot number assigned to node i.
¢ Fi: the maximum slot number within node i™s two-hop neighborhood.
¢ Set node i™s time frame to be 2a , where a satisfies
2a-1= Fi < 2a “ 1. That is, node i uses the Si-th slot in every 2a time slots.
E.g., 5 neighbors, you choose a = 3, and your slots are 1,9,17, ¦
Transmission Control
Two modes: low contention level (LCL) and high contention level (HCL).
A node is in HCL only when it receives an explicit contention notification (ECN) messages within the last tECN period.
¢ 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
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
¢ To receive acknowledgement from the one hope receiver.
¢ According to noise level of the channel
¢ Low noise indicates low contention.
Performance Evaluation
Comparisons with B-MAC, default MAC of Mica2.
Measured throughput, fairness, and energy efficiency for Three benchmarks.
- One-hop benchmark
- Two-hop benchmark
- Two clusters, 7 and 8 sending nodes.
- Multi-hop benchmark
Conclusion
The MAC dynamically adjusts behavior between CSMA and TDMA depending on the level of contention in the network.
Suitable for applications where expected data rates and two hop contention are medium to high.
