Sensor Networks: Lessons 17 - RIS · One of the oldest proprietary industrial sensor/actuator and device communication buses in use. Its topology is a double ring (main trunk) with

Sensor Networks: Lessons 17

Prof. Sabato [email protected]+390817683845

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The Media Access Control (MAC) data communication protocol sub-layer, also known as the Medium Access Control, is a sub-layer of the Data Link Layer specified in the OSI model. It provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multi-point network. The hardware that implements the MAC is referred to as a Medium Access Controller.

Sensor Networks - Prof. Sabato Manfredi

MEDIA ACCESS CONTROL

INDUSTRIAL PROTOCOL

HART (Highway Addressable Remote Transducer)

The HART protocol is the oldest protocol and network for measurement purposes. HART supports simple star or point-to-point chain topology. It uses the 4- to 20-mA current loop for signal transfer; parameter propagation; status set-up; diagnostics and configuration by FSK modulation with 0.5 mA peak sine wave; logical 1 represented by 1200 Hz; and logical 0 represented by two cycles of 2200 Hz. The HART protocol is low cost, simple, and, because of the 4- to 20-mA physical interface, supported by many sensor producers. A low data transfer rate (10 measurements per second) can suffice for temperature, level and chemical quantity measurements, and processes control.

HART provides 13 compulsory commands and other commands are optional. Compulsory commands enable reading measured data, sensor number, measurable range, etc. Optional commands provide for calibration, setting physical values, writing a serial number, dialing one of four physical units, resetting the sensor, etc.


INDUSTRIAL PROTOCOL

Technical summary:

Topology: basically, point-to-point: one field node can be connected to two higher devices (supervisor devices), analog and digital transmission modes; alternatively: bus topology (multidrop) with a maximum of 15 nodes including two supervisor devices.Segment length: 3000 m in point–point and 300 m in multidrop topologiesMedium access control: master–slaveData transmission rate: 1.2 kb/s (standard) and 19.2 kb/s (high-speed mode)Response time: guaranteed; about 500 ms for one nodeMedium: twisted pair: 4 to 20 mAModulation: frequency shift keying (FSK)Power supply: via signal wiringEx mode: in special casesState of the art: wide range of sensors and actuators of many producers on the market: Rosemount–Emerson, Siemens, Yokogawa, Krohne, ABB Automation, Endress+Hauser, Ametek, Foxboro Eckardt, etc.Application area: temperature, pressure, flow, density, level, analytical sensors, actuatorswww: http://www.hartcomm.org


INDUSTRIAL PROTOCOL

ASI (Actuator Sensor Interface)

A simple sensor/actuator bus provides for use in automation of machines, production lines, and technologies.It is available predominantly for connection of binary sensors and binary actuators

Tree network topology is available. The segment length must not exceed 100 mwithout repeaters. Any combination of active and passive slaves up to 256 binary slaves and actuators is permitted on a segment. The network cycle period must not exceed 128 ms. Physical layer is based on reliable alternating pulse modulation(APM) methods. The physical medium is an untwisted pair in special mechanical shape.


INDUSTRIAL PROTOCOL

Technical summary:

Topology: bus, treeSegment length: 100 mMedium access control: master–slave (cycle polling) with 31 active or 124 passive binary slaves on a segment; alternatively, analog nodes translate analog signals with a maximum resolution of 18 bData transmission rate: 156 kb/sNetwork cycle: 5 ms (time for response of all active nodes)Medium: unshielded untwisted pairModulation: alternate pulse modulation APM (pulse width modulation), full duplexPower supply: by signal conduction (2 to 10 A) or by a separate two-wire connectionPhysical interface: ASIResponse time: guaranteedStandardized: EN 50 295, IEC 62026State of the art: more then 32 firms, e.g., Balluff, Pepperl & Fuchs, ifm electronic, Siemens, BernsteinApplication area: digital sensors, actuators, I/O moduleswww: http://www.as-interface.com


INDUSTRIAL PROTOCOL

Interbus

One of the oldest proprietary industrial sensor/actuator and device communication buses in use. Its topology is a double ring (main trunk) with short cross segments. Interbus is aimed at real-time data acquisition and control. Besides master and slave stations, there also are up to 64 data switchers (repeaters).The length of the main trunk is up to 13 km in copper wire and up to 100 km in optical fiber. The local, 10-m segments extending the ring can connect upto eight nodes each. The voltage level in the local bus segment is 0 to 5 V. The most common version supports a kind of express transmission of short process data blocks in combination with a slow message cycle for configuration, diagnostics, and other special functions.


INDUSTRIAL PROTOCOL

Technical summary:

Topology: double ring (main trunk) with short cross segmentsSegment length: 13 km for copper, 100 km for optical fiber, 10 m (local bus with a maximum of eight nodes with distances up to1.5 m)Medium access control: master–slave with 256 slaves; highly effective bus access methodData transmission rate: 500 kb/s (main trunk); 300 kb/s (local bus)Electrical interface: EIA RS 485Medium: unshielded twisted pair; optical fiberPower supply: localResponse time: guaranteedError coding: CRCStandardized: DIN E 19258, EN 50 254, EN 50 170


INDUSTRIAL PROTOCOL

Measurement Bus

The measurement bus, also known as the DIN mess bus, is designed primarilyfor measurement. The maximum length of the bus is 500 m and thedata transmission rate is 115.2 kb/s in free (rootless tree) topology. The control is master–slave with a maximum of 32 nodes or 961 and 4096 nodes, respectively, with extended address field and cascade sequencing. The physical medium is four line wire for full duplex; The master uses one twisted pair of messages; the other pair is used by slaves for responses in time-division multiplex mode with polling. The method preserves basic functions of the system even in case of alarms and network reconfiguration. The measurement bus is equipped with several safety mechanisms based on parity bit control, and time-out.


INDUSTRIAL PROTOCOL

Technical summary:

Topology: bus; root-free tree up to 115.2 kb/sSegment length: 500 m at the maximum transmission rate of 1 Mb/s and short node connection (maximum length of 5 m)Medium access control: master–slave with up to 32, 961, or 4096 slavesData transmission rate: 115.2 kb/sMedium: two twisted pairsElectrical interface: RS 485Modulation: NRZ base bandPower supply: by signal conductionStandardized: DIN 66348State of the art: emerging applicationsApplication area: measurement devices


INDUSTRIAL PROTOCOL

Controller Area Network (CAN)

The CAN is one of the most popular field-buses. Bosch and Intel developed it at the end of the 1980s for the automobile industry. It has been applied in cars but also in manufacturing. The topology is tree or bus with maximum communication speed of 1 Mb/s. CAN is a real-time protocol with multicasting; the medium access method is CSMA/CA (carrier sense multiple access with collision avoidance) for multimastermode. CAN is equipped with the following safety mechanisms: differential voltage for dominant and recessive levels; message frame checking with acknowledgments; and error counters with active, passive, and off-line modes.


INDUSTRIAL PROTOCOL

Technical summary:

Topology: bus; passive connection typeSegment length: 40 m up to 1 Mb/s; 1000 m up to 50 kb/sMedium access control: multi-master with CSMA/CAData transmission rate: 50 kb/s to 1 Mb/sMedium: shielded pair; optical fiberModulation: recessive and dominant differential levelsPower supply: localResponse time: guaranteedRobustness: high data safety grade Standardized: ISO 11898, open standard of physical and link layersExtras: different application layers: DeviceNet, CANopen and SDSState of the art: Bosch, Balluff, Baumer, Pepperl+Fuchs, Fraba Sensorsysteme, ifmelectronic, DruckApplication area: pressure, temperature, inclinometer, actuators, encoders


INDUSTRIAL PROTOCOL

LonWorks

LonWorks technology aims at completely distributed data acquisition and. Layer protocols are implemented by NEURON microcontroller. The set of transceivers corresponds to the set of communication media, including twisted pair, coaxial cable, radio, optical fiber, and power line. The related communication protocol, LonTalk, provides CSMA/CA medium access control. Priority slots in the protocol frame guarantee soft real-time properties. The NEURON chip consists of three 8-b microprocessors: the first implements medium access control; the second provides higher network layer protocols; and the third one supports the user application program. The technology was originally designed for building automation with special purpose address formats respecting domains and subdomains connected by routers and bridges. The total number of nodes is up to 32,385. Lon technology can connect simple sensors and actuators as well as high-efficiency devices. Besides building automation, the LonWorks technology is used in data acquisition and control systems.


INDUSTRIAL PROTOCOL

Technical summary:

Topology: treeSegment length: depends on network architectureMedium access control: peer-to-peer predictive-persistent CSMA/CDData transmission rate: 79 kb/s till 1.25 Mb/sMedium: twisted pair, coaxial cable, radio, power line, optical fiberElectrical interface: EIA RS 485 and othersModulation: base band with Manchester II or NRZPower supply: depending on physical mediaResponse time: soft real time, almost guaranteed for priority slotsStandardized: IEC 62026Extras: implements all seven layers of the ISO/OSI RMState of the art: Zellweger Analytics, Hubbell, Honeywell, Siemens


INDUSTRIAL PROTOCOL

Profibus

Profibus (process field-bus) is an industrial communication standard of German origin (DIN 19245, EN50 170). For sensor networking, the profibus DP (distributed periphery) and the profibus PA (process automation) variants can be employed.Thecombined token passing and master–slave medium access method can be used to adapt profi-bus to a concrete industrial application.Two classes of nodes include active stations, which can obtain a token to control the network for a preset time, and passive stations that play the role of slaves and send data on demand of active stations..


INDUSTRIAL PROTOCOL

Technical summary:

Topology: bus with passive node connectionSegment length: up to 9.6 km in copper and 90 km in optical fiber, up to 5 bus segments Medium access control: combined token passing and multi master–slave; pollingData transmission rate: wide range from 9.6 kb/s to 12 Mb/s (segment length up to 100 m)Electrical interface: EIA RS 485Medium: shielded twisted pairModulation: NRZNodes number: 31 or 128 (with repeaters)Power supply: externalResponse time: guaranteedStandardized: DIN 19 245, EN 50 170State of the art: FRABA, Hengstler, TWK Elektronik, Heidenhain, Siemens, AutomationX, Keller HCW, Brooks Instrument, Emerson, Barksdale Control, MettlerToledo, Pepperl+Fuchs, IVO, SICK, Max StegmannApplication area: flow, pressure, temperature, position, encoderwww: http://www.profibus.com


SN PROTOCOLS

MAC layer

The causes of energy waste at the MAC layer of a wireless network are the following: retransmissions of the corrupted packets during a collision; overhearing, that is, receiving packets destined for other nodes; control packet overhead.


ZIGBEE

ZigBee technology

ZigBee wireless technology is a short-range communication system for applicationswith relaxed throughput and latency requirements in wireless personal area networks.The key features of ZigBee wireless technology are low complexity, low cost, low power consumption, low data rate transmissions, supported by cheap fixed or moving devices. The main field of application of this technology is the implementation of WSNs. The IEEE 802.15.4 Working Group 1 focuses on the standardization of the bottom two layers of the ISO/OSI protocol stack. The other layers are normally specified by industrial consortia such as the ZigBee Alliance. In the following the main specifications related to both the physical layer and the MAC sub-layer as defined in the IEEE 802.15.4 and IEEE 802.15.4a standards will be reported.



ZIGBEE

COMPARISON AMONG TECHNOLOGIES

ZigBee

The nine promoter companies of the ZigBee Alliance include Philips, Honeywell,Mitsubishi Electric, Motorola, Samsung, BM Group, Chipcon, Freescale and Ember;then are more than 70 members.● Capacity of 250 Kbit/s at 2.4 GHz, 40 Kbit/s at 915 Mhz, and 20 Kbit/s at 868 Mhzwith a range of 10–100 metres.● Its purpose is to become a wireless standard for remote control in the industrial field.The ZigBee technology is targeting the control applications industry, which does notrequire high data rates, but must have low power, low cost and ease of use (remotecontrols, home automation, etc.).● Security was not considered in the initial development of the specification. Currently there are three levels of security.● It operates in the 3.2 – 10.2 GHz band.● ZigBee chips are low cost.



Ultra-wideband

● UWB is a revolutionary wireless technology for transmitting digital data over a widespectrum of frequency bands with very low power. It can transmit data at very highrates (for WPAN applications).● Ideally, it will have low power consumption, low price, high speed, use a wide swath of radio spectrum, carry signals through obstacles (doors, etc.) and apply to a wide range of applications (defense, industry, home, etc.).● Currently, there are two competing UWB standards for WPAN applications: theUWB Forum is promoting one standard based on direct sequence (DS-UWB), whilethe WiMedia Alliance is promoting another standard based on multiband OFDM.● Each standard allows for data rates from approximately 0 to 500 Mbps at a range of 2 metres and a data rate of approximately 110 Mbps at a range of up to 10 metres.● The Bluetooth SIG announced in May 2005 its intentions to work with both groupsbehind UWB to develop a high rate Bluetooth specification on the UWB radio.● For application to WSANs the standard proposed, which can be used as the PHY layer of the ZigBee protocol stack, is the IEEE 802.15.4a based on IR-UWB. It allows for typical data rate of 850 Kbit/s (up to 27 Mbit/s optional) with a range of 10 – 50 metres and location capability.



Bluetooth wireless technology

● Bluetooth wireless technology is geared towards voice and data applications.● It operates in the ISM unlicensed 2.4 GHz spectrum.● Typically, it can operate over a distance of few metres (up to 10 metres, depending on the device class also up to 100 metres). The peak data rate is 3 Mbps.● The Bluetooth specification allows for three modes of security.● The cost of Bluetooth chips is actually under 3USD.

Infrared (IrDA)

● IrDA is used to provide wireless connectivity for devices that would normally usecables to connect. IrDA is a point-to-point, narrow angle (30 cone), ad hoc data transmission standard designed to operate over a distance of 0 to 1 meter and at speeds of 9600 bps to 16 Mbps.● IrDA is not able to penetrate solid objects and has limited data exchange applicationscompared to other wireless technologies.● IrDA is mainly used in payment systems, in remote control scenarios or when synchronizing two PDAs with each other.



Radio frequency identifi cation (RFID)

● There are over 140 different ISO standards for RFID for a broad range of applications.● With RFID, a passive or unpowered tag can be powered at a distance by a readerdevice. The receiver, which must be within a few feet, pulls information off the tag,and then looks up more information from a database. Alternatively, some tags areself-powered, active tags that can be read from a greater distance.● RFID can operate in low frequency (less than 100 MHz), high frequency (more than100 MHz), and UHF (868 to 954 MHz).● Uses include tracking inventory both in shipment and on retail shelves.



According to the mechanism for collision avoidance, MAC protocols can be divided into two groups: scheduled based and contention based. Many MAC layers in WSNs adopt scheduled protocols. Since channels are pre-allocated to individual nodes, there is no energy wasted on collisions due to channel contention. Among protocols belonging to this group, time division multiple access (TDMA) seems particularly suitable for WSNs, for many reasons: for example, it can support low-duty-cycle operation; it can avoid overhearing by turning off the radio during the slots of other nodes.

MAC PROTOCOLS: SCHEDULED BASED (1/3)


Generally, TDMA is based on a cluster structure: nodes form clusters; one of the nodeswithin the cluster is selected as the cluster head, and it acts as a base station; peer-to-peer communications are not supported, so each node within a cluster has to communicatethrough its cluster head; communications between different clusters and interferencebetween them is managed thanks to other approaches, such as frequency division multipleaccess (FDMA) or code division multiple access (CDMA).



In general, scheduled protocols can provide good energy efficiency, but they also present some disadvantages. For example, TDMA protocols are not very scalable; when new nodes join or old nodes leave a cluster, the base station must adjust the slot allocation. Frame length and static slot allocation can limit the throughput. Slotted structure needs a precise synchronization.Protocols presented in the following introduce some variations into classic scheduledprotocols, in order to meet specific wireless network requirements and to reducethe mentioned disadvantages. For example, the base station may dynamically allocateslot assignments on a frame-by-frame basis; the role of the cluster head may be rotatedamong the nodes of a cluster in order to balance energy consumption.



Contention protocols present some advantages compared to scheduled protocols. They are more flexible with respect to changes in traffic load, network topology, and node density, since resources allocation occurs in an on-demand fashion. There is no need of clusters creation since they support peer-to-peer communications.Finally, they do not require fine-grained time synchronizations as in TDMA protocols. Nevertheless, contention based protocols often do not achieve the same energy efficiency as the scheduled schemes; this is due especially because nodes waste energy listening to the channel during the contention for the media and by retransmitting the packets that have experienced collisions on the media.

MAC PROTOCOLS: CONTENTION BASED (1/2)

MAC PROTOCOLS: CONTENTION BASED (2/2)


Each frame is divided into two parts: the first one is a listen period, followed by a sleepperiod. The listen period is needed to allow coordination among nodes that have data tosend. During the sleep period, nodes that have no data to send can turn off while nodeswith data to send remain asleep to communicate.

Each node independently chooses its own listen/sleep schedule and shares it with itsneighbours: it periodically broadcasts its schedule in a SYNC packet, which providessimple clock synchronization; these operations require a period of time called Synchronization period. In this way communication between nodes is possible: a node schedules its transmission during the listen time of its intended destination.

FLOODING GOSSIP ALGORITHMS (1/2)

Flooding and gossiping are two classical data-relay mechanisms that do not require any routing algorithms and topology maintenance.In flooding, each sensor receiving a data packet broadcasts it to all of its neighbours,regardless of whether or not a neighbour has already received the data from anothernode; this process continues until the packet arrives at the destination.Gossiping is a slightly enhanced version of flooding: when a node receives a packet it randomly chooses one of its neighbours to which it forwards the packet. The mechanism of packet forwarding for both the two protocols is shown in Figure.


FLOODING GOSSIP ALGORITHMS (2/2)

Flooding is very easy to implement, but it has several drawbacks: implosion, caused by duplicated messages sent to same node; overlap, when sensor nodes cover overlapping geographic areas and therefore they collect overlapping pieces of data; resource blindness, since in classic flooding nodes do not modify their activities based on the amount of energy available to them at a given time.Gossiping does not solve the problem of overlap but it can avoid implosion by justselecting a random next node rather than broadcasting; however, this cause delays inpropagation of data.


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Sensor Networks: Lessons 17 - RIS · One of the oldest proprietary industrial sensor/actuator and device communication buses in use. Its topology is a double ring (main trunk) with