Wireless Network Technologies Supporting M2M Applications

1 Introduction
Greater evolvement in telecommunications will take place in the future. The focus may not be PC in the Internet or mobile phones in mobile networks, but other devices with communication capabilities. The devices or instruments that can be connected to networks will be far more than today, including TV sets, MP3 players, electronic papers or periodicals, intelligent buildings and even refrigerators. They will work in the same way as PC in the Internet and thus make Machine-to-Machine (M2M) communications. With the vast application of Radio Frequency Identification (RFID) and sensors as well as the employment of grid computing, the M2M applications, which are currently at its beginning, will gradually mature.

  M2M applications mean transmitting of telemetric information and remote control signals among the traditionally isolated equipments and instruments such as air conditioners, safety systems, elevators etc. M2M applications are drivers for not only parallel processing but also distributed computing; they can turn a home into a super computer by mounting embedded processors into all household appliances.
There are two elementary M2M applications in enterprises: surveillance and control. Surveillance includes assets tracking, stock management and supply chain automation. A manufacturer can use RFID to track the flow of product components in the factory, or locate boxes in the warehouse. The data transmission in this application is one-way and needs no response. Control is more complicated in which the decisions are made on the inputs of multi sources and then fed back.

  To promote the development of M2M applications, the wireless communication capability is required in all movable objects, any place where fixed lines are not applicable, and any devices with command execution, validation and control functions. M2M interconnection is possible anywhere wireless signal can cover. But it is difficult to decide what wireless network we should choose to establish M2M connection.

2 WiFi
WiFi is a wireless technology widely used today. Besides notebook PC, many network equipment has added the capability to process the 802.11 standard, just as convenient as to process Ethernet and IP protocols.

  Since WiFi is already quite popular in enterprises, it is natural to think about adopting it to provide M2M connection. The main advantages of applying WiFi to M2M connection are that it uses the existing networks and enjoys high bit rate (the Mega-bit level data rate of WiFi far surpasses the requirement of remote surveillance) and good compatibility. Another advantage is its capability to integrate voice, remote surveillance and RFID, such a versatile network makes it possible to simplify management, cut cost and reduce interference.

  The disadvantages of WiFi include the uneconomical power consumption, high cost, extensive protocol and the need of access point. Presently it costs at least $15 to upgrade a device for WiFi. The Bluesoft RFID is priced at $65. Though its cost will go down, it is only applicable to assets of high value nowadays. In a warehouse, Bluesoft RFID may be used for forklifts while not possible for boxes. WiFi is a network lack of relaying capability; devices can only be connected to Access Point (AP). The connection among APs and other networks is through normal wired Ethernet. Authentication is required or the wireless data packets must be separated, if the same wiring has been used in wired Ethernet services. Otherwise you’ll have to install new wires and switches.

  Many startup companies are now pushing WiFi mesh networks, where APs act as Routers, automatically detect each other, and transmit data through several hops. But the architecture of mesh networks is mainly designed for outdoor applications and APs are required to be in the range of visibility, thus making it suitable for service providers instead of enterprises. Currently there are companies that are making mesh network systems applied within buildings. They use proprietary AP connection protocols. There are manufacturers intending to provide WiFi mesh network systems, but it is impossible for their systems to be interoperable in a short term, since till now there is even no draft interoperability standard; it is estimated that the interoperability standard is not possible to be available before 2007.

3 Bluetooth
Bluetooth is better than WiFi in terms of cost and power consumption. Though it seems Bluetooth is not of much success, in fact the number of Bluetooth-embedded devices is almost double of that of WiFi. At present they are mostly applied to mobile phones and consume appliances, while the notebooks and PDAs equipped with both WiFi and Bluetooth are opening the door for new enterprise applications. Though initially Bluetooth was intended for voice services, it should be quite good to be applied to telemetric services, thanks to its low power consumption and considerable security performance. The market anticipation of Bluetooth sensors is brilliant.

  However, Bluetooth is not a network technology; it supports connections to at most 8 devices with limited communication distance. These limitations indicate that Bluetooth can’t be so useful unless it is combined with other wireless technologies. Bluetooth can transmit voice from headset, VPN service data from notebook PC or navigation data from automobile, or transmit telemeter data through WiFi in LANs.

  From a long-term point of view, Ultra Wide Band (UWB), a new wireless technology with high bit rate and low power consumption, may replace Bluetooth. While in the short term, UWB encounters many problems in techniques and policies. In Feb. 2004, the UWB standardization work of 802.15 Work Group ran into a stalemate: it’s difficult to make a decision between different recommendations coming from Intel and Motorola. However, Motorola went on its own way and established a MultiBand OFDM Alliance (MBOA) to continue the work on its own standard, which will then be submitted to IEEE for approval. It is estimated that, this will become a de facto standard at the end of 2004, and its first product will come out in the middle of 2005.

  Even if UWB standard published now, it would be impossible for it to replace Bluetooth immediately, because Bluetooth has already existed for many years and UWB can’t rival it from cost point of view. Therefore, UWB may mainly apply to be a 480 Mb/s wireless USB to connect PC and its peripheral equipment (such as printer or monitor) initially. It may take 5 to 10 years for UWB to replace Bluetooth.

4 ZigBee
Though both Bluetooth and WiFi can be applied to surveillance and control, they are originally designed for other applications. Formerly in 802 standards, the researches always focused on improving bit rate, and there are no technology specially designed for telemetry. Therefore, 802.15 Work Group specially designed for telemetry a wireless technology numbered 802.15.4. It aims at low power consumption and low cost (several dollars each), which doesn’t provide the bit rate as high as WiFi, nor the QoS of Bluetooth. Like Bluetooth, 802.15.4 also adopts the frequency-hopping technology that has good anti-interference performance. What’s different from Bluetooth is that, it can form full mesh network and access point is not needed, because it adopts ZigBee lightweight routing protocol. The manufacturers who develop 802.15.4 product established the ZigBee league, an organization whose function is similar to WiFi. So now, there are more people who know about ZigBee than 802.15.4.

  802.11 standard is infrastructure oriented, Bluetooth is mobile phone oriented, and ZigBee is network oriented. ZigBee may become the most important one in 802.15 technologies, but how the mesh network it forms would be adopted to existing networks is still a problem. The first devices of ZigBee are estimated to be remote controllable fluorescent lamps and came out in late 2004. The switch of the lamp is movable and people can switch on the lamp anywhere in darkness. Consumers may welcome this kind of applications, while what enterprises may be more interested in are sensors. Enterprise applications require high security. ZigBee has not yet got a complete standard, which would define the security in several different levels aiming at different applications.

  There are some other problems besides security. One of them is the difference among the definition on the physical layer. IEEE defined two kinds of 802.15.4 to provide different data-rate (40 kb/s and 250 kb/s) in 900 MHz and 2.4 GHz band respectively. ZigBee protocol shall run on the two physical layers. In order to get smaller and cheaper ZigBee chips, manufacturers and users must choose one of the two physical layers. The third possible physical layer for ZigBee may be UWB, which is not yet standardized and still remains to be a long-term objective. The main benefit of UWB is that, its power consumption is even lower than that of 802.15.4 and allows devices to work without batteries. There is a forecast saying that the UWB mesh networks will eventually comprise of "intelligent dusts", a kind of "fine" wireless device powered by nano-windmill or photovoltaic cell.

  Now cost is the major issue, a UWB chipset of low data rate and low power consumption is currently priced at least 20 dollars while the objective of ZigBee is several cents only, which is an object hard to reach at present. So it’s impossible for ZigBee to be arbitrarily embedded into ordinary objects in the near future. This is why the first remote controllable lamps are fluorescent lamps of high power efficiency, whose lifetime is 10 times longer than incandescent lamps. ZigBee league believes that the ZigBee chips will be ubiquitous like microprocessors and its applications are far more beyond telemetry and remote control.

  Figure 1 illustrates the architecture of 3 different wireless networks: WiFi, Bluetooth and ZigBee. In WiFi, each WiFi device connects to AP, and APs connect to enterprise’s intranet through wired Ethernet. In Bluetooth, devices connect to other wireless devices for relay, which generally are mobile phones acting as hubs. In ZigBee, all devices can transmit each other’s service and need no wired networks.

5 Passive RFID
Passive RFID is an existing wireless technology implemented by smaller and cheaper products than ZigBee. The RFID has no power supplies or processors themselves and hence can’t startup transmission or relay services. They will be activated only when they enter the electromagnetic field of RFID readers.

  Passive RFID itself is not something new, there are thousands of buildings using RFID access cards, and millions of cars using RFID attached to their windshield to pay for highway toll. However, two things are new: one is the cost of RFID that is low enough to be attached anywhere; another is the new standard being published. A $0.1 passive RFID using Electronic Product Code (EPC) can store the full description of an object. Here EPC is a standard based on the Extended Markup Language (XML) to descript objects.

  However, few enterprises are now eager to attach the passive RFID labels on their goods. Though the labels are cheap, the infrastructure system to read the label and process the data is expensive. The coverage range of a passive RFID reader is the same to that of Bluetooth, therefore the objects being tracked must near the reader. One way is putting readers at each entrance or exit of buildings and then connecting them together through ZigBee; another way is using mobile readers, who transmit the data through WiFi or save data into memory.

  For the moment there is a phenomenon that the use and potential of passive RFID is over-estimated. Wal-Mart is the biggest passive RFID user in America, who requires its suppliers to attach RFID labels to all supplied goods. However, only its first 100 suppliers are affected and only cartons and containers are labeled. It’s impossible that all goods on the shelf will have RFID label in the foreseeable future.

  Table 1 is a brief comparison among the 4 wireless technologies mentioned above.
6 The 802.15 Standards Short distance wireless technologies were initially developed to support Personal Area Network (PAN), which is made up of portable devices. IEEE as well as other industry organizations believes that their potential is far more than that. IEEE 802.15 Work Group is now working on defining several different short distance technologies listed as below, some of which are beyond the area of personal applications.

  802.15.1 is essentially a formal standardized version of Bluetooth low layer protocol. Bluetooth Special Interest Group takes on most standardization work; the outcome will be submitted to IEEE for approving. Original 802.15.1 standard is based on Bluetooth 1.1; presently most Bluetooth devices are based on this version. The new version 802.15.1a will be correspondent to Bluetooth 1.2; it is fully backward compatible, with some enhanced QoS functions.

  802.15.2 is a variation of Bluetooth and 802.15.1 in order to reduce the interference with 802.11b and 802.11g networks. These networks all work on 2.4 GHz band, so 802.15.2 or other special techniques are required if Bluetooth and WiFi are required to be coexisted.

  802.15.3, also called WiMedia, is targeting at high data rate. Primitively, it is designed for consumer devices such as TV sets, digital cameras, etc. The data rate defined in the original version is as high as 55 Mb/s, applying physical layer which is the 802.11 based but not compatible. Later, more manufacturers are prone to 802.15.3a, the UWB physical layer of MBOA is applied and the data rate hits 480 Mb/s. The manufacturers that intend to produce 802.15.3a products established the WiMedia league to work on the testing and labeling of equipment to ensure the standard conformance.

  802.15.4 as described above is aiming at long battery life and low device cost. The data rate is as low as 9.6 kb/s and no support on voice. ZigBee league is now drafting high layer protocols running on 802.15.4 to allow mesh networks of unlimited scale.

802.15.5 is under study. Its ultimate goal is to form mesh networks in the MAC layer, thus no need to use ZigBee or IP routing.

  802.15.6 has not yet been listed as 802.15 standard, its wireless network will be based on the trillion Hertz radiation (T Radiation).

  T radiation holds both characteristics of light and wireless and its theoretical data rate can be up to thousands of Mega bits per second.

7 Conclusions
Various wireless networks that can be applied to M2M applications in the future are discussed above. Today, most devices with these technologies claimed to be targeting at ubiquitous wireless networks are still too large and too expensive to be embed in everything. However, people are now working hard to make them smaller and cheaper. It is really possible to produce wireless routers costing $0.5 that can be installed inside a wristwatch. What remains unclear now is whether and how to connect these new network devices to existing enterprise networks.

  It is easy to create a network device while quite difficult to build a reliable network. The mesh networks comprising of mobile nodes need redundant routing and the mesh network technology must be implemented in vast devices. It isn’t a problem technically, while it’s hard to say whether it is practical and in accordance with people’s demands.

  Passive RFID label and sensor have already had great impact on some industry, Which makes supply chain clearer and asset management easier. The active RFID with memory and processor can carry out more tasks. Frankly speaking, the risk of RFID is quite low considering its potential and the cost saved. However, their market outlook is not clear yet today and it’s difficult to tell what will be eventually the choice of market, whether it’s passive RFID, active mesh network or both of them. There are many wireless standards under study, some of which may end in a failure. We have to fully understand this.

Manuscript received: 2004-12-26