WPAN/WBANs.Zigebee.m

IEEE 802.15 WG

• Specify WPAN standard

• Wireless Personal Area Network

• TG 1 802.15.1: WPAN/Bluetooth
  • defines PHY and MAC of Bluetooth
  • standard issued in 2002 and 2005
• TG 2: 802.15.2: coexistence
  • coexistence of WPANs with other networks in unlicensed band;
  • EEE 802.15.2-2003 published in 2003 and then ”hibernated”.(休眠)
• high rate WPAN
  • 802.15.3-2003 is a MAC and PHY standard for high-rate (11 to 55 Mbit/s) WPANs;
  • 802.15.3a: UWB PHY… no agreement when choosing PHY (MB-OFDM vs. DS-UWB);
  • 802.15.3b-2005: improve implementation and interoperability of the MAC;
  • 802.15.3b-2009: mm-wave-based PHY, 57-64Ghz unlicensed band, >2Gbps
• TG 4: Low Rate WPANs
  • long battery life, low data rate, low complexity;
  • 802.15.4 standard released in May 2003;
  • many networks runs on top of 802.15.4: ZigBee, 6LoWPAN, WirelessHART, etc.
• Enhancements of 802.15.4
  • 802.15.4a-2007: additional PHYs, e.g. UWB pulsed radio;
  • 802.15.4-2006: clarification of the original standard;
  • IEEE 802.15.4c: adaptation to unlicensed bands in China;
  • IEEE 802.15.4d: adaptation to unlicensed bands in China;
  • IEEE 802.15.4e: enhancements for industrial apps, e.g. channel hopping;
  • IEEE 802.15.4f: active RFID systems;
  • IEEE 802.15.4g: smart utility networks: large networks with a lot of end systems.
• TG 5 Mesh networking
  • two parts: low rate and high rate mesh networks;
    • low rate: IEEE 802.15.4-2006 MAC
    • high rate: IEEE 802.15.3/3b MAC;
  • common features: network initialization, addressing, multihop unicasting;
  • low rate: multicasting, broadcasting, portability, trace route and energy saving
• TG 6 Body Area Network
  • low-power short range standard, draft in 2011
• TG 7: visible light communication
• work in progress

ZigBee

• developed by ZigBee Alliance
• on top of IEEE 802.15.4
• Particular implementation of those features specified in IEEE standard
• Toptology in
  • Centralized star
  • Cluster-tree-based
  • full mesh(requires additional routing protocol)
• Specifics:
  • low-rate (even compared to Bluetooth)
  • extremely low power consumption
  • example of applicability: sensor networks

Comparison with other technologies

IEE 802.11x technolgies

• 3x more expensive than Bluetooth

• 5x the power consumption of Bluetooth

  

Reason for Zigbee

• low cost, high reliability, very long battery life
• high security, self-healing properties, larger number of nodes supported

• ease of deployment, guaranteed delivery, route optimization

• NOT CHOOSING
  • Very specific apps
  • BLE Exists

ZigBee application

• Wireless sensor networks

ZigBee Protocol Overview

IEEE 802.15.4 PHY

• Three low power unlicensed radios:
  • 2.4Ghz: 250Kbps(EU) 16 channels (ch11-ch26);
  • 915Mhz: 40Kbps(US) 10 channels(ch1 - ch10)
  • 868Mhz: 20Kbps(Europe and Japan) 1 channel (ch0)
• Cahnnels and modulation in 2.4Ghz
  • 16 channels , each 5Mhz wide ch 11-26
  • actual throughput, 50% of 250Kbps due to overheads
    • overheads: addressing, security, error control
  • DSSS (direct sequence spread spectrum ) channel access
  • O-QPSK modulation
• Other responsiblities of PHY
  • detecting transmissions from new nodes
  • assessing quality of links with other nodes

IEEE 802.15.4 MAC

• Functionlity
  • CSMA/CA
  • max.length of packet is 127bytes (2 bytes for CRC)
  • guarantees? transmissions
• Two modes of operation
  • Acknowledge
  • Unacknowledge
• How ACK mode is implemented
  • setting ACK bit in a forward packet
  • if set: receiver ACKs correct reception
  • if no: ACK is received with some time, retransmission

Device types

• FFD (Full Function Device)
  • capable of all the features and always “on”
  • routing/coordination/network formation
  • can talk to other FFDs and RFDs
  • FFDs require more power
• RFD (Reduced Function Device)
  • Sometimes called leaf nodes
  • simple netoworking functions
  • end systems in a sensor network
  • can talk to FFD only

Logical entities

Full Function Device (FFD)	Reduced Function Device (RFD)
Found in any topology	Found only in start topology
Can be a network co-ordinator	Cannot be a network co-ordinator
Can talk to any type of device	Talks only to FFD
Usually main powered	Usaully battered powered

• Network Coordinator
  • FFD, one per work
  • Create a network , assign channel/address
  • adds new devices to a network
  • constant power supply
  • sometimes serves as a gateway
  • a node may join if the coordinator is up
  • if down, already existing node may continue to network
• Router Functionality
  • FFD devices serving as a relay node
  • range extension
  • constant poewr suplly
  • sores packets sent to sleeping nodes
  • can be used to access the network
• End Device
  • FFD/RFD
  • low power consumption
  • sleeping modes are defined
  • communication through routers

Network Topologies

Star Topology

Advantage	Disadvantage
small delay due to single hop	single point of failure(co-coodinator)
	end devices cannot communicate directly

Cluster-tree Topology

• two levels of hierarchy
• more nodes can be added via routers
• large coverage areas
• several paths in-between end nodes

Mesh Topology

• extension of cluster-tree topology
• connections to deveices at different layer feasible

• RFD are still unable communicate directly

• DELAY CAN BE REDUCED but COMPLEXITY of ROUTING is HIGH

Access methods

• non-beacon access
  • transmit at anytime when channel is idle
  • “free-for-all” environment
• beacon-based access
  • coordinator generates a superframe identified at beacon time
  • all nodes are synchronized
  • nodes transmit only i its designated time slot
  • superframe may contain common slot when stations compete
  • in-between: could go sleeping

Creating a network

• Initialization for coordinator

  • a node searches for coordinators on all channels;
  • if no coordinators, starts its own one using unique 16-bits PAN ID;

• Initialization for end nodes:

  • scanning all available channels;
  • can detect router and coordinator with the same PAN ID
  • if yes, device with strongest SNR is chosen;
    • end devices sends “can i join”
    • address is allocated if there is place for a new node

• Parameters set by a coordinator:

  • max number of child devices allowed per router
  • max number of hops from the co-ordinator to the most distant device

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Network Example

Addressing

• Three types of IDs
  • MAC address (64-bits ID)
  • network address(16-bits ID)
  • name of device

Unicasting and broadcasting

• Usage of address
  • while joining: extended MAC address
  • while connected: short network address
• Unicast
  • network address is used as destination address in MAC header
  • message is routed in the network
  • destination accepts the message, others drop
  • destination answers with ACK
  • the process is a bit more complex: local ACKs
    • 

• Broadcasting is used when

  • joining or rejoining network
  • discovering routes in the network;
  • should be minimized
• Broadcasting
  • MAC address is 0XFFFF
  • all active devices receive and analyse the message.
  • all active FFD devices retransmit it
• ACKing broadcast message
  • no explicit active ACKs
  • passive ACKing: listening whether all neighbors retransmitted
    • if not repeat the transmission

Routing and route discovery

• General consideration
  • star topology no need routing
  • cluster-tree and mesh topologies need routing
  • more than one approach
• Cluster-tree topology
  • tree-routing
    • works fine for small networks
  • route discovery
    • work when network is unstable or large

• Mesh topology

• route discovery is only possible with AODV

• Tree routing

  • use tree hierarchial structure
  • first decision: whether to go up or down in hierarchy
  • examining
    • if destination is a descendant, the device sends the packet to a child;
    • otherwise, send it to a parent
  • upon reception by a node
    • accepts if the destination is a directly connected child
    • otherwise: sends to a parent

  BAD: path could be longer than needed

  GOOD: quite stable as tree structure is guaranteed

Sleeping modes

General facts

• reduce power consumptions
• still retain network address while sleeping
• parent device buffers packets while child is asleep
• upon wake up it checks whether there are some in store

Two types of sleeping mode

• Cyclic sleep
• additional modes : can be controled , pin sleep