Pen MugenJiang Yong Wang Wenbo

（1. School of Telecommunication Engineering，Beijing University of Posts and Telecommunications， Beijing 100876， China；

2. China Potevio Co.， Ltd.， Beijing 100053，China）

AbstractAs an important branch of telecommunication technology，digital home networks have quickly developed in recent years.Although architecture of the digital home network based on telecommunication networks has been standardized in China，only wireless access is defined to be allowable.Therefore，it becomes the focus of further research how to establish a wireless digital home network.This article proposes the concept and shows the architecture of ubiquitous access for wireless digital home networks.Moreover，key technologies for ubiquitous access are discussed here，including cognitive radio，the concept of wireless Mesh，generic link transfer mechanism，Quality of Service（QoS）guarantee mechanism，Electromagnetic Compatibility（EMC）and coexistence of heterogeneous systems.

T he rapid development of information technology indicates we are moving away from the industrial society into the information society.The information society will greatly affect our lives.It is changing our behaviour and ways of thinking.American scholar Alvin Toffler predicted in his The Third Wave published in 198Os that the core of human lives would return from the society to families and individuals[1].With regard to the telecommunication，digital home networks will play an important role in the information society，and will become a hot technology.

A digital home network is established to offer certain services and applications in order to meet demands of certain users.These demands include communication among multiple devices，and between multiple devices and public networks in a limited range.Adigital home network is based on either wireless or wired architecture.It is noticeable that broadband wireless access is increasingly popular worldwide，and the number of broadband wireless users in China has been growing greatly in recent years.The focus on the research of broadband wireless access is for the reason that wireless digital home network has huge potentialmarket.In-Stat/MDR Company has predicted that the home network market would have grown up to 5.3 billion USdollars by 2OO7，and home multimedia wireless networks would take a share of 49%on this market，that is 2.6 billion USdollars[2].

The wireless access for digital home networks has a wide applications perspective.The continuous update and development of technologies made the seamless interaction popular among home appliances，mobile devices and computers.The consumer obtains，looks for and manages increasing number of digitalmedia contents coming with home appliances，mobile devices and computers.The consumer enjoys these different media contents and devices easily at various places of the home.It has been a development trend to fulfill wireless digitalhome networks and short range is one of the features.It pays more attention to the last-ten-meters access.It is using Ultra Wideband（UWB）as core technology for high-speed short-range wireless communication.It can guarantee communication rates of Consumer Electronic（CE），office equipment，peripheral devices of computers and mobile communication terminals in a home environment.However，only the high-speed rates between links fail to establish a high-speed wireless digital home network.It is necessary to define its network architecture and protocols for the media access layer.

1 Research and Standardization of Wireless Digital Home Networks

Considering the wireless air interface，the wireless digital home network is an access sub-network of the digital home network.As of the transmission distance of wireless communications，it belongs to short-range Wireless Personal Area Network（WPAN）.Presently，UWB is a leading technology for short-range WPAN to offer high-speed transmission in a short range，and other functions asdistance measurement and positioning.The UWBis a brand-new communication way relative to traditional narrow-band technology such as spread spectrum communication and broadband communication.As of the production signals，the UWB no longer uses sine waves to modulate base-band signals to channels，but uses base-band signals to directly excite antennas to transmit ultra-short impulses with time-domain narrow pulse as the message carrier.There are two leading UWB technologies.One is based on narrow pulse，advocated by some companies including Motorola.The other one is the pulse multi-wave UWB technology，similar to orthogonal frequency division multiple technology，mainly proposed by Intel.However，the standards and applications for these two technologies haven't grown mature yet.In addition，China has no Intellectual Property Rights（IPRs）on them.Contrarily，the impulse UWBtechnology features a high data rate，accurate positioning，and supporting more service types and China has certain IPRs on it.The IEEE 8O2.15 working group is responsible for the standardization of UWBtechnology.There are two UWB standards:the 8O2.15.3a protocol supporting super-short range and super-high data rates，and the 8O2.15.4a protocol supporting long range and low data rates.

▼Table 1. Features of 3 leading technologies for short-distance wireless communication

The Bluetooth is one of the popular technologies for wireless communications.It is a global，open specification for implementation of wireless data and voice communications.Essentially，it offers a universal short-distance wireless interface technology for the communication environment between fixed and mobile devices.Moreover，it combines telecommunication technology with computer technology，and fulfill mutual communication or operation among numerous different devices in a short range via wireless.The Bluetooth Special Interest Group is responsible for its standardization.The frequency for transmission is the 2.4 GHz Industry-science-medical（ISM）band，which is internationally open and license-free.The Bluetooth supports 1 Mbps transmission bit rate and 1Om transmission distance.

The Zig Bee is another technology for short-distance WPAN.It is a new short-distance wireless access technology with low power and low bit rate.It works at the license-free 2.4 GHz ISM band with a rate of 1Okbps to 25Okbps and transmission distance of 1Om to 75 m.It is similar to Bluetooth，but it is simpler and has lower rate and power than Bluetooth.Moreover，it supports the sleep mode mostly.Therefore，it is applicable to the cases that have no requirement of real-time transmission or continuous update，such as industrial control and sensor networks.The Zig Bee Alliance has promoted the ZigBee development and application.The alliance has more than 7Omembers，including seven founder members:Emer，Honeywell，Invensys，Mitsubishi，Motorola，Samsung and Philip.The Zig Bee is an extended set of the IEEE 8O2.15.4 standard.The 8O2.15.4 working group is responsible for protocols of the physical and Media Access Control（MAC）layers，while Zig Bee defines specifications for the application layer and security，supporting interaction between devices from different vendors.

Moreover，other potential short-distance WPAN standards include Near Field Communication（NFC），WiMedia，Global Positioning System（GPS），Digital Enhanced Cordless Telecommunications（DECT），wireless 1394，and private wireless systems.These standards meet demands of transmission rates，distances，and power consumption，and either or more focus on scalability of functions，meet demands of a single application scene，bring technology differentials.Table 1 shows the features of three leading technologies for short-distance wireless communication.

The IEEE 8O2.15 standard family is specialized for WPAN which is focused on a wireless short-distance wireless communication network.It has not been mature yet.Established in 1998，the IEEE 8O2.15 working group is dedicated to the standardization of Personal Area Network（PAN）and short-distance wireless network.According to different requirements on data rates，power consumption and Quality of Service（QoS），the IEEE 8O2.15 working group has defined three standards for different WPAN types.They are:the High Rate Wireless Personal Area Network（HR-WPAN）with a transmission rate higher than 2OMbps；the Medium Rate Wireless Personal Area Network（MR-WPAN）with a rate up to 1 Mbps；the Low Rate Wireless Personal Area Network（LR-WPAN）with a low rate.The IEEE 8O2.15 working group set up specialtask teams for standardization of HR-WPAN，MR-WPAN，and LR-WPAN.The IEEE 8O2.15.1 is the first WPAN standard defined by IEEE.It is based on Bluetooth which replaces the wire line connection and belongs to the medium-rate short-distance wireless communication network.The IEEE 8O2.15.2 was released in 1999，offering applications for the IEEE 8O2.15 WPAN.The devices based on it can coexist with other radio equipment based on open frequency bands（such as IEEE 8O2.11 equipment）.The IEEE 8O2.15.2 sends suggested modifications to other 8O2.15 standards to enhance the coexistence with wireless equipment working at open frequency bands.The purpose of IEEE 8O2.15.3 is to define WPAN standards calling for high data rate，low cost，low power consumption，and compatible with Bluetooth.Moreover，the IEEE 8O2.15 High Rate Alternative Physical Layer Task Group（TG3a）takes charge ofdeveloping optional standards for the physical layer of the IEEE 8O2.15.3 super-high-speed WPAN，in order to replace the physical layer of high-speed WPAN.The UWB technology is the main technology considered by the IEEE 8O2.15.3 working group.The IEEE 8O2.15.4 working group was specified for applications only requiring simple wireless access and having strict requirements on power consumption but less demands on data rates，such as industrialcontrol and home networks.Its study aims at application equipment with low data rate，offering comprehensive solutions to PAN.The Task Group 4（TG4）is responsible for developing protocols of the physical and MAC layers，while other protocols follow the existing corresponding standards.

Noticeably，the wireless digital home network is a new wireless application field and the discovery of more wireless technologies that meet the home market is going on.The China Communications Standards Association（CCSA）has formally started the standardization of the digital home network based on telecommunication networks which involves the telecommunication services of home networks supports.For example，it includes the reference model，functional requirements，media structure，numbering and addresses，and the performance of home networks.In addition，it involves the EMC and environment protection requirements of home network devices.However，the standardization work of wireless digital home networks has not started yet.

2 Wireless Digital Home Access Technologies

In February 2OO6，CCSAapproved two industrial standard drafts:General Technical Requirements of Home Network Based on Telecommunication Networks and Technical Requirements on Devices of Home Network Based on Telecommunication Networks.The two standard drafts define in detail the architecture of the digital home network based on telecommunication networks，describe the functions of the digital home network，and discuss the possible services.The standards clearly define that access modes of the digital home network can be either the wireless or the wired[3].Considering the development of communication technologies in future，wireless broadband access of the digital home network would be an important development trend.The available standards for the digital home network don't involve wireless access technology.The wireless ubiquitous heterogeneous access architecture suitable for digital home networks is proposed and the key technologies are discussed here.

▲Figure 1. Heterogeneous aqrchitecture of ubiquitous wireless digital home access network.

The wireless access environment for digital home networks is called Piconet here.The wireless terminals can either be the single mode or multimode nodes，supporting the single or multiple access modes at the physical layer of the wireless digital home network.As shown in Figure 1，the wireless terminals on the homogenous Piconet access the Special Access Point（SAP），and the internal communication is fulfilled by the point-to-point model or SAPrelay.The wireless terminals on the heterogeneous Piconet access to the GAPof this Piconet，where the point-to-point model fulfills communication among homogenous nodes.Otherwise，this communication is fulfilled by a GAPrelay while the GAPprovides relay for communication among heterogeneous nodes.

The SAPimplements the internal control function on the homogenous wireless digital home network，while the GAP，fulfills the internal distributedcommunication function on the heterogeneous wireless digital home network.Heterogeneous interconnection and cooperative application of wireless digital home networks can be either wireless or wired.The SAPand GAPmay adaptively process time，frequency，code，and space domain signals.In addition，it may allocate the frequency resources，and offer the service of high-speed accessing to the external packet core networks via wired or wireless methods.Therefore，they and the other future mobile core networks，such as B3G/4G，mutually support each other，and use the General IPMultimedia Subsystem（GIMS）service together.

▲Figure2. Protocol stack's frame structure of the heterogeneous ubiquitous wireless digital home network.

The GIMSprovides users in any states with voice and data communication services by IP multimedia applications.Moreover，it allows inter-working with existing fixed/mobile voice and IPdata networks，such as Public Switched Telephone Network（PSTN）and Internet，forming an all-IP multimedia core network.The GIMS consists of the following functional modules to meet the demands of various services and external interfaces:Home Subscriber Server（HSS），Media Gateway Control Function（MGCF），Multimedia Resource Function Processor（MRFP），Media Gateway Function（MGF），security function，QoSfunction and Session Initiation Protocol（SIP）server.

According to the network architecture and the services supported，the physical-layer technology for terminals and the different communication demands，the possible applications include:

（1）Internal Communication on Homogenous Piconet

The SAPtakes charge of controlling signalling messages with terminals on the same Piconet.Moreover，with the support of different physical-layer technologies，it ensures that two terminals in the same mode directly conduct point-to-point communication.Otherwise it depends on the SAPto implement relay transmission of data.

（2）Internal Communication on Heterogeneous Piconet

The GAPis responsible for controlling signalling messages with terminals on the same Piconet.Moreover，with the support of different physical-layer technologies，it ensures that two terminals in the same mode directly conduct point-to-point communication.Otherwise，it depends on the SAPto implement relay transmission of data just as it does in heterogeneous modes.

（3）Communication among Homogenous Piconets

The SAPis responsible for communication among homogenous terminals on different Piconets.Its functions include Mesh routing，resource assignment，resource management，and distributed power control.

（4）Communication among Heterogeneous Piconets

The GAPtakes charge of communication among terminals in different access modes on different heterogeneous Piconets.The wireless and wired means of access are both allowed.

The inter-working architecture and distributed control mechanism of wireless digital home networks provide users with general service environment to meet the users'diversified service demands whenever and wherever.The integration of wireless mobile communication technologies and computer technologies is making the cooperation of networks with different modes and structures possible.This cooperative service is not a simple integration of heterogeneous networks，but an effective integration of resources making use of network differentials and complementarities.It helps make full use of strengths of local networks to offer ubiquitous customer-based services.Figure 2 shows the frame structure of the heterogeneous ubiquitous wireless digital home network.

The network layer uses distributed Mesh routing technology，based on the characteristics of the heterogeneous wireless digitalhome network.Comparing with traditional point-to-point communication technology，this technology have advantages as energy saving，self-configuration，self-healing，and easy scalability.

The Generic Link Layer（GLL）is above the data link layer.It plays the following important roles.

·As the convergence layer of multiple access technologies，it provides a generic upper layer with the unified interface and masking the multiple access environments at the lower layer.

·It controls and complements MAC-layer functions of various access technologies，that is to say，it makes effective use of wireless resources by multiple systems.

The control part utilizes the distributed wireless resource among multiple systems to improve the spectrum efficiencing and to guarantee QoSon the heterogeneous network.Moreover，it manages the power consumption and the network security information.Key technologies are necessary to further investigate，such as Cognitive Radio（CR）and wireless Mesh.

2.1 Cognitive Radio

The CRtechnology is inherited from Software Defined Radio（SDR）.It is a new intelligent wireless communication technology.With intelligence，it can let wireless devices and their antennas to sense the RFenvironment where they are.It is able to adjust the bands used to avoid interference when necessary，and further to guide SDRto set the optimum operation and parameters.Considering the basic working principles，the CRfirst analyzes the wireless transmission scene when transmitting signals.The excitement produced at the transmitting end under the wireless environment is generally the non-stable space-time signal and fulfilled by the space-time processing.The receiving end detects usable frequency bands with little or no interference by judging interference intensities of different frequency bands，and makes decisions to adjust some transmission parameters.The receiving end then sends all these messages back to the transmitting end for controlling the power and frequency spectrum used of the signal transmitted.Moreover，due to the interchange of wireless channels，adaptive antenna wave beam may be used to suppress the interference in the time division multiplexing mode.

On wireless digital home network，the UWBequipment is generally deployed at the physical layer，which occupies a wide broadband.The CRtechnology helps to allocated the frequency spectrums dynamically and adjust the transmitting power at the RFend to avoid strong interference and further improve system performance of the ubiquitous network.

2.2 Wireless Mesh Network

Various terminals of a wireless digital home network make up a distributed wireless network.In the current research of distributed networks，the wireless Mesh technology is expected to bring an important evolution to the wireless broadband field.In the wireless Mesh topology，every node has the routing function，and it only communicates with its neighbouring nodes.Therefore，the wireless Mesh network is a self-organized and self-managed network.In fact，it is more as a wireless version of the Internet，with data packets transmitted from one route to another route to the destination.These features make the wireless Mesh network have advantages of energy saving，self-configuration and easy-to-enlarge compared with the traditional point-to-multipoint communication networks.Companies such as Nokia and Nortelstarted to apply wireless Mesh technologies to broadband network access（the related standard is IEEE 8O2.11s and IEEE8O2.16j），and related products（for example，wireless routers have been put into commercial use）[4].

The wireless Mesh is still under the research of its key technologies and primary application.All wireless access standardization groups of IEEE 8O2 are involved in the research of wireless Mesh.The IEEE 8O2.15 has established the WPAN Mesh working group（TG5 WPAN Mesh Networking）.The wireless Mesh technology is applied to heterogeneous wireless access networks based on UWB，Bluetooth or other access technologies to fulfill the inter-working.The following is a list of potential strengths and possible technical breakthroughs of the wireless Mesh technologies.

·Coverage of the wireless digital home network could be expanded through the relay.

·Frequency utilization and system capacity could be improved since usable bandwidth grows along with the increase of nodes.

·There is no complex establishment of relay infrastructure and the initial construction cost is low.

·The network is easy to enlarge.

·The network is distributed via self-configuration and self-organization

·There are multiple routes for selection，enhancing the flexibility and self-healing of the system.

2.3 Link Transfer Mechanism of Generic Access Point

The GAPon a heterogeneous wireless home network is responsible for data forwarding and transfer among wireless home networks in different modes.Therefore，it has to be able to communicate with various wireless home network nodes and external networks（by wired or wireless）at the same time.It should be designed as a multi-mode device that allows software configuration to support adaptive communication with multiple wireless home networks.The available solution is to add a Generic Link Layer（GLL）above the physical and MAC layers，as shown in Figure 3.

The GLL implements the following functions:

·Providing upper-layer protocols（such as IPv6）with a unified interface.

·Mapping MAC Protocol Data Units（PDUs）among different wireless home communication networks to support communication of the devices on different wireless home networks.

·Supporting dynamic access and selection mechanism of wireless home networks.

·Having a modularized structure to support new standards for wireless home networks.

There is a significant difference between MAC standards of different networks.First，the physical frame structures are different，which makes the scheduling time levels that greatly depend on the frame length and micro time slots difference.Second，due to different frame structures，MACPDUs of different networks have different sizes.Moreover，special networks adopt the different frequencies，duplex modes，modeling，encoding models，and network structures that lead to a significant difference in rates of services supported by different special networks.Therefore，it is necessary to study and design GLLfunctions with excellent adaptation for meeting demands of good packet formats，rate matching，QoSand so on.

▲Figure 3. Generic link layer.

In addition，a heterogeneous wireless home network needs to use an access point to communicate with external networks such as the Internet，cellular communication networks，and Metropolitan Area Networks（MANs）.Multiple solutions are available to an Access Point（AP）that supports high-speed throughput.One is based on the future concept of all-IPnetwork，in which the APof a heterogeneous wireless home network may directly access the edge router of the corresponding IPnetwork.Another solution integrates the Wireless Metropolitan Area Network（WMAN）technology and considers the GAPas a user node in WiMAXsystem，which is connected to a bone network through the air interface of WiMAXsystem.

2.4 The QoS Guarantee Mechanism

The future heterogeneous wireless home networks must support various multimedia services.Different services have different requirements on QoS.For example，real-time services such as video stream are sensitive to time delay and require high rates，while the transmission of some files may tolerate a long time delay.The future heterogeneous wireless home networks should have effective QoSguarantee mechanism，offering various acceptable services with different QoSon time delay，delay jitter，bit rates，and loss tolerance.

According to the architecture of the heterogeneous wireless home network，QoSguarantee is shown in two aspects.In one aspect，if the communication between homogenous nodes must go through the SAP，the SAPshould guarantee real-time communication of nodes and the transmission bit rate，while restrict the error ratio within the controlled range.Moreover，if multiple nodes directly communicate with each other，effective mechanism is necessary for communication coordination and control，in order to improve communication efficiency and resource utilization.In the other aspect，considering the heterogeneous communication entities，the GAPshould guarantee the communication quality by effective resource management of different services and nodes.For example，the time delay of data processing among heterogeneous networks should be supressed as short as possible.

The QoSis not only considered in wireless access networks，but also in end-to-end communications.The holistic home network supports three types of QoSguarantee functions that form three individual but interrelated plains:the control，the data and the management plains.

The control plain includes a series of control functions related to traffic paths.The QoSfunctions on the control plain include admission control and resource reservation.The admission control function is to control traffic access at the entry of the gateway node.The access standard is generally a decision process driven by the admission control strategy.The admission control of traffic access depends on the priority levels of customer service made by negotiation in advance.The admission control strategy depends on the quantity of usable resources and whether an acceptance of traffic access would influence the QoSof existing traffic.The admission control mechanism ensures the performance parameters negotiated by a service provider and its users during their business deal.Besides，it can offer the service usability and reliability expected by users（also negotiated by the service provider and users in advance）.The access acceptance mechanism divides the traffic into four priority levels:emergency service（generally reserved for emergency traffic），high priority，general priority，and best effort.The resource reservation mechanism supports resource reservation for services with high priority levels，and the Authentication，Authorization，and Accounting（AAA）control messages.In addition，it makes arbitration when more than one service providers compete for resources.

The data plain has the control functions that are directly related to traffic.The QoSfunctions on the transmission plain of a home network include traffic classification，packet labelling，traffic shaping and control，queuing and scheduling，congestion avoidance，and cache management.

The management plain involves network operation and management.The QoSfunctions on the management plain of a home network include traffic measurement and strategy management.

The gateway of a home network had better support Policy Enforcement Point（PEP）and cooperate with Policy Decision Point（PDP）on the access network/WAN.Strategy messages exchanged betweenPEPand PDPinclude packet labels，acceptance judgment，user priority，explicit route，packet dropping，array bandwidth distribution，value-added services on demand，and network congestion responses.

2.5 Electromagnetic Compatibility and Coexistence of Heterogeneous Systems

Frequency resources allocation and interference avoidance is an important factor for network coexistences，which therefore involves resource coordination and control between the different networks.The EM interference relatively little among various electronic devices on the home access network based on CR mechanism.However，it is still necessary to give the theory of EMC and system coexistence mechanism of practical heterogeneous ubiquitous wireless home networks，ensuring EM interference to match the related levels.

The usable frequency spectrum for wireless home networks is still uncertain.If the UWB technology is used in the physical layer of wireless home networks，the UWB system must ensure the workability of other systems and avoid interference with them.That is，because the UWB equipment will coexist with other wireless communication systems in a very broad frequency band，which is an important factor for further promotion of UWBtechnology.Even if UWB equipment may satisfy the restriction of transmitting power regulated by frequency monitoring departments，it still needs to avoid certain sensitive frequency bands[5].

There willbe so many problems to be further researched specified for theory of EMC and system coexistence of UWB-based wireless home networks.For example，the optional frequency band is uncertain；restrictions on transmitting power are not defined；the compatibility research and testing technology for frequency band selection are not mature；the technical solutions are undetermined.

3 Conclusions

As an extension of broadband networks and broadband value-added services，the digital home networks are offering new opportunity for telecom operators.They are located on the home part of traditional telecommunication networks，and the bridgehead of telecommunication multi-service operation and management.They can be used to extend end-to-end services of traditional telecom operators and ensure the operation of new services.In China，the standardization work of digital home networks has just started，and is with an expectation of primary achievements in 2OO9.However，the investigation of wireless access mechanism of digital home networks has not started yet.The traditionaltypical access mechanism with centralized control is also suitable for digital home networks.Traditional CEproducts and restriction of bandwidth will make it popular for various terminals to use peer-to-peer communication models.Meanwhile，the wireless Mesh mechanism will play an important role in the future digital home access networks.Considering the ubiquitous heterogeneity of future networks，a novel access network architecture is proposed here，and the corresponding key techniques are discussed as well.Possible solutions to these key technologies are the research challenge.