Z-MAC: a Hybrid MAC for Wireless Sensor Networks

Injong Rhee, Ajit Warrier, Mahesh Aia and Jeongki MinDept. 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.

A B

D

C

Setup Phase:Slot Assignment

• Using DRAND to assign time slots to every node.– DRAND is a distributed implementation of RAND, used for TDM

A 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).

0 1 2

0

3

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– 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 ev

ery 2a time slots.

Example

2a-1≤ Fi ≤ 2a – 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 an

y 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 tECN period.

Transmission RuleNode i acquires data to transmit

Is node i the owner?

Take a random backoff within period To

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 To and performs

a random backoff within a contention

window [To, Tno]

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

YES

YES

YES

NO

NO

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(βt):– Rdrift : the max clock drift rate of each node– εclock: the max acceptable clock error– Isynch = εclock / Rdrift : the min synchronization interval required to a

chieve the max clock error– αsynch: the max weight applying to the new clock value received– S : the avg. rate at which a node receives or sends synchro

nization messages–

• How to get new clock value?– Cavg : weighted moving avg. clock value – Cnew : newly received clock value– Cavg = (1- βt)Cavg + βt Cnew

}min{ , synchsynchsynct IS

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

Throughput

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…