WSN-IEEE 802.15.4 -MAC Protocol
- 1. AD HOC AND WIRELESS SENSOR NETWORKS Dr.Arun Chokkalingam Professor Department of Electronics and Communication RMK College of Engineering and Technology Chennai. WSN-IEEE 802.15.4 -MAC Protocol
- 2. Content • IEEE 802.15.4 MAC protocol,
- 3. Importance of MAC Protocols Medium access control (MAC) protocols: They coordinate the times where a number of nodes access a shared communication medium.
- 4. Types of MAC Protocols • Low Duty Cycle Protocols And Wakeup Concepts – – S-MAC, – The Mediation Device Protocol • Contention based protocols – PAMAS, • Schedule based protocols – LEACH, • IEEE 802.15.4 MAC protocol,
- 5. 802.15.4 General Characteristics • The standard covers the physical layer and the MAC layer of a low-Data rate Wireless Personal Area Network (WPAN). • Star or Peer-to-Peer operation. • Support for low latency devices. • Fully handshake protocol for transfer reliability. • Low power consumption. • combines both schedule-based as well as contention-based schemes. • The protocol is asymmetric in that different types of nodes with different roles are used.
- 6. The targeted applications for IEEE 802.15.4 are in the area of wireless sensor networks, home automation, home networking, connecting devices to a PC, home security, and so on.
- 7. Data rates of 250 kb/s Data rates of 20 kb/s. Data rates of 40 kb/s
- 8. Operating frequency bands • Most of these applications require only low-to-medium bitrates (up to some few hundreds of kbps), • The physical layer offers bitrates of • 20 kbps (a single channel in the frequency range 868 – 868.6 MHz), • 40 kbps (ten channels in the range between 905 and 928 MHz) • 250 kbps (16 channels in the 2.4 GHz) • There are a total of 27 channels available, but the MAC protocol uses only one of these channels at a time; it is not a multichannel protocol.
- 9. 05 2004Marco Naeve, Eaton Corp. Slide 9 Protocol Drivers Extremely low cost Ease of installation Reliable data transfer Short range operation • Reasonable battery life • Simple but flexible protocol
- 10. IEEE 802.15.4 MAC Upper Layers IEEE 802.15.4 SSCS IEEE 802.2 LLC, Type I IEEE 802.15.4 2400 MHz PHY IEEE 802.15.4 868/915 MHz PHY 802.15.4 Architecture
- 11. Network architecture and types & roles of nodes The standard distinguishes on the MAC layer two types of nodes: Full Function Device (FFD) can operate in three different roles: 1. PAN coordinator(PAN = Personal Area Network), 2. A simple coordinator 3. A device. Reduced Function Device (RFD) • operate only as a device.
- 12. Cont. • A device node must be associated to a coordinator node and communicates directly to the coordinator. (forming a star network.) • Coordinators can operate in a peer-to-peer fashion • Multiple coordinators can form a Personal Area Network (PAN).
- 13. A coordinator handles following tasks: • It manages a list of associated devices. • Devices are required to explicitly associate and disassociate with a coordinator using certain signaling packets. • It allocates short addresses to its devices. • All IEEE 802.15.4 nodes have a 64-bit device address. • When a device associates with a coordinator, it may request assignment of a 16-bit short address to be used subsequently in all communications between device and coordinator. • In the beaconed mode of IEEE 802.15.4, it transmits regularly frame beacon packets announcing the PAN identifier, a list of outstanding frames, and other parameters. • Furthermore, the coordinator can accept and process requests to reserve fixed time slots to nodes and the allocations are indicated in the beacon. •It exchanges data packets with devices and with peer coordinators.
- 14. Superframe Structure • We start with the beaconed mode of IEEE 802.15.4.
- 15. Optional Frame Structure 15ms * 2n where 0 n 14 GTS 3 GTS 2 Network beacon Transmitted by PAN coordinator. Contains network information, frame structure and notification of pending node messages. Beacon extension period Space reserved for beacon growth due to pending node messages Contention period Access by any node using CSMA-CA Guaranteed Time Slot Reserved for nodes requiring guaranteed bandwidth. GTS 1 0123456789101112131415 Slot Battery life extension Contention Access Period Contention Free Period
- 16. • The coordinator of a star network operating in the beaconed mode organizes channel access and data transmission with the help of a superframe structure. • The superframe is subdivided into an active period and an inactive period. • During the inactive period, all nodes including the coordinator can switch off their transceivers and go into sleep state. • The nodes have to wake up immediately before the inactive period ends to receive the next beacon. The inactive period may be void. • The active period is subdivided into 16 time slots. • The first time slot is occupied by the beacon frame • remaining time slots are partitioned into a Contention Access Period (CAP) followed by a number (maximal seven) of Guaranteed Time Slots (GTSs). Superframe structure
- 17. Cont • The length of the active and inactive period as well as the length of a single time slot and the usage of GTS slots are configurable. • The coordinator is active during the entire active period. • The associated devices are active in the GTS phase only in time slots allocated to them; in all other GTS slots they can enter sleep mode. • In the Contention Access Period (CAP), a device node can shutdown its transceiver. • Coordinators do much more work than devices and the protocol is inherently asymmetric.
- 18. Guaranteed Time Slots (GTS) Management GTS request packets GTS request packets ACK ACK the device is required to track the coordinator’s beacons for some specified time to approve GTS GTS descriptor GTS descriptor Wait for approval
- 19. Guaranteed Time Slots (GTS) Management • The coordinator allocates GTS to devices only by sending appropriate request packets during the CAP. • Flag in the request indicates whether the requested time slot is a transmit slot or a receive slot. • In a transmit slot, the device transmits packets to the coordinator . • in a receive slot the data flows in the reverse direction. • Another field in the request specifies the desired number of time slots in the GTS phase. • The coordinator answers the request packet in two steps: • An immediate acknowledgment packet confirms that the coordinator has received the request packet properly but contains no information about success or failure of the request. • After receiving the acknowledgment packet, the device is required to track the coordinator’s beacons for some specified time
- 20. Cont • If the coordinator has insufficient resources, it generates a GTS descriptor for (invalid) time slot zero, indicating the available resources in the descriptors length field. • the device may consider renegotiation. • it concludes that the allocation request has failed. • A GTS is allocated to a device on a regular basis until it is explicitly deallocated. • The deallocation can be requested by the device by means of a special control frame. • After sending this frame, the device shall not use the allocated slots any further. • The coordinator can also trigger deallocation based on certain criteria. Specifically, the coordinator monitors the usage of the time slot: If the slot is not used at least once within a certain number of superframes, the slot is deallocated. • The coordinator signals deallocation to the device by generating a GTS descriptor with start slot zero.
- 21. Operational Models Of IEEE 802.15.4
- 22. Data Transfer Procedures • first assume that a device wants to transmit a data packet to the coordinator If the device has an allocated transmit GTS, it wakes up just before the time slot starts and sends its packet immediately without running any carrier-sense or other collision-avoiding operations. Allocated transmit GTS coordinator Device
- 23. Cont • Second assume when the device does not have any allocated GTS slots it sends its data packet during the CAP using a slotted CSMA protocol Transmit during CAP coordinator Device
- 24. Cont • data transfer from the coordinator to a device If the device has allocated a receive GTS and when the packet/acknowledgment/IFS cycle fits into these, the coordinator simply transmits the packet in the allocated time slot without further coordination. Receive GTScoordinator Device
- 25. Cont • when the coordinator is not able to use a receive GTS. The handshake between device and coordinator • The coordinator announces a buffered packet to a device by including the devices address into the pending address field of the beacon frame.
- 27. Slotted CSMA-CA protocol The variable NB - counts the number of backoffs, CW- indicates the size of the current congestion window, BE- is the current backoff exponent. CCA - Clear Channel Assessment Upon arrival of a new packet to transmit, these variables are initialized with NB 0, CW 2, and BE macMinBE (with macMinBE being a protocol parameter), respectively.
- 28. Nonbeaconed mode/ unslotted CSMA-CA protocol • In the nonbeaconed mode, the coordinator does not send beacon frames nor is there any GTS mechanism. • The lack of beacon packets takes away a good opportunity for devices to acquire time synchronization with the coordinator. • All packets from devices are transmitted using an unslotted CSMA-CA protocol. • the device performs only a single CCA (Clear Channel Assessment) operation. If this indicates an idle channel, the device infers success. • Coordinators must be switched on constantly but devices can follow their own sleep schedule. • Devices wake up for two reasons: (i) to send a data or control packet to the coordinators, (ii) to fetch a packet from the coordinator
Editor's Notes
- #11: <month year>