Welcome to my bubble

Going for a ride? Then don’t forget your PAN

WELCOME TO MY

BUBBLE

by Theodore Zahariadis

[Image courtesy of Intel]

SO HOW FAR AWAY ARE WE FROM THE DAY WHEN EACH OF US WILL BE TAKING OUR OWN NETWORKS EVERYWHERE WE GO?

D

espite of the worldwide recession, especially in the telecommunications market, the vision of a forth-generation (4G) all-IP mobile/wireless network is emerging rapidly. In the 4G era, apart from the installed infrastructure cellular networks, powerful Personal Digital Assistance (PDA) devices and ad-hoc Personal Area Networks (PANs) are expected to impel towards the “simple, anywhere and anytime” ubiquitous services’ deployment. Advances in fabrication density and microprocessors’ design have shrunk devices’ size and weight, and enabled professionals to carry a number of electronic gadgets, including cellphones, earphones, PDAs, laptops, MP3 players, and electronic watches. These devices have

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IEE COMMUNICATIONS ENGINEER | JUNE/JULY 2003

impressive processing capabilities and multiple storage memories. A PAN is a human-centred network that connects such personal communications devices in a spontaneous architecture, within a short-range, “personal” or “body” space. The enhancement of small, yet powerful personal devices with intelligent communications features could drive the emergence of multiple, flexible service scenarios. In the near future, ordinary people will require forming “ad-hoc”, personal “PAN-bubbles” to enable simple and efficient communication between these devices and facilitate resource sharing (e.g., automatic transferring of phone numbers from a PDA to a cellphone, voice forwarding to/from a wireless earphone via a laptop, etcetera). The PAN-bubble may expand or shrink

Personal Area Networks

dynamically, depending on users’ needs. Devices that automatically recognise each other’s presence and share data as needed may provide enhanced functionality. For example, when users enter into a new location and aim to quickly sense/control the environment (gain connectivity, adjust the lighting), they could be recognised by environment sensors prompting welcome messages, personalised sales greetings and so on. The number of embedded processing devices is expected to grow rapidly in the forthcoming years. Moreover, new intelligent appliances are expected to evolve. A good example of this is the recent arrival of intelligent writing devices, such as the Anoto/Sony Ericsson ChatPet and the C-Technologies C-Pen. As shown in Figure 1, apart from an ink cartridge, the ChatPen incorporates a digital camera, a microprocessor and a Bluetooth transceiver. As the user writes, the camera records snapshots, the microprocessor recognises the characters and the information is transmitted through the Bluetooth interface to a computer or forwarded via a relay device (e.g., mobile phone). Similarly, the C-Pen scans documents, recognises the data and downloads them via an Infrared subsystem to a PC. However, there are a number of open issues that have to be faced before the wide deployment of Human Centred Networks.

A fundamental issue is how these new mobile personal devices will handle increasing intelligence levels. Systems on Chip, ASICs and miniaturised batteries have minimised the size and weight of devices. However, there are still important trends in the embedded microprocessors and in some cases the Digital Signal Processing (DSP) areas. Power Consumption, particularly for mobile applications based on small batteries, is a key differentiation factor. In order to reduce power consumption many techniques are used, including stopping a particular block if it is not in use and reducing the CPU core voltage to 1.8V or less. To lower the price/performance ratio, many vendors integrate various peripherals on a single chip. However, this is not always the cheapest solution. Peripherals increase chip complexity and pin-count, and require more testing, which may reduce the overall yield. Peripherals’ cores must be selected in a way that simplifies the design and shortens the development cycle of the complete system. Modern PDAs and cellular phones have to run voice encoding/decoding, video compression and multimedia applications. Thus, besides general-purpose CPU architectures with multimedia extensions, new high-end multimedia processors have specific code to handle MPEG-4/H.323/JPEG streams. Although the success of ➔

Fig. 1: Anoto/Sony Ericsson’s ChatPen and C-Technologies’ C-Pen Infrared subsystem Display Trigger button

Bluetooth Subsystem

Scanning Subsystem

Microprocessor

Battery

Force Sensor Ink cartridge System memory Microprocessor Camera subsystem


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TABLE 1: COMPARISON OF PAN TECHNOLOGIES

Medium

Electric Field

Magnetic Field

Infrared (IrDA)

Bluetooth

Frequency band

0.1-1MHz

5MHz

Optical

2.4GHz

Bandwidth

2.4 Kbit/s

250 Kbit/s

2.4 Kbit/s

1 Mbit/s

(up to 4 Mbit/s)

(up to 10 Mbit/s)

(up to 413 Kbit/s) Range

Human body

1-3m

Up to 10m

Up to 10m

Power

Very low

Low

Very low

Medium

Complexity

1x

1.5x

1.5x

2x

Security

Physical contact

No

Line-of-sight

Yes

Advantages

Controlled

Body coverage

Simple, low cost

Broadband, flexible

Disadvantages

Grounded objects

Antenna size

Line-of-sight

Power consumption

Promoters

< 10

< 10

5000+

2000+

Market penetration

Low

Very low

High

Very high

these new strategies is not guaranteed, they will certainly influence future processor design.

LINKING THEM ALL An important issue of PAN is the network connectivity technology, especially due to power consumption constraints. Four potential alternatives are envisaged: electric, magnetic, infrared and RF technologies. In the electric field technology, data are modulated over a tinny current (i.e., 1 nanoAmpere, 1000 times less than the charge created by hair combing) transmitted with the human body as conductor. Using the human body as medium, data can be exchanged between devices carried by the same person (e.g., phone, watch, PDA); between persons while in contact (during hand-shaking, electronic business cards may be exchanged); or between the user and the environment (cars may recognise their drivers when they push a button). The main drawback is that the earth ground needs to be electrically isolated from the body to prevent shorting of the communication circuit.

THE HUMAN BODY COULD BE USED AS A CONDUCTOR TO TRANSMIT DATA BETWEEN DEVICES

The magnetic field technology is based on magnetic inductance, optimised for PAN-bubbles. It enables PAN devices with lower power consumption, less interference and lower cost, but its fairly large antenna requirements can hardy be fulfilled by a miniature PDA. InfraRed (IR) is also a low-power, low-cost technology, but requires line-of-sight links. Line-of-sight can be an advantage from a security standpoint, but it lowers the communications robustness and overall system flexibility. RF is well positioned as the dominant access

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communication technology, whist Bluetooth is gaining momentum as an ad-hoc standard. Although power consumption is higher and the 2.4GHz band in which it operates is quite congested, its flexibility and coverage range turn it into the most suitable technology for wireless PANs (WPANs). As fabrication density and integration levels have increased, Bluetooth cores have shrunk into chipsets with very small footprint. A comparison of the alternative technologies is provided in Table 1. Another important challenge is the achievement of automatic network topology, resources and services discovery. PANs should enable the nodes to know about their presence in the bubble and provide for devices’ autoconfiguration. Among the major candidate protocols for automatic discovery are: 5 Service Discovery Protocol (SDP) is a Bluetooth built-in, link layer mechanism. It provides a robust solution for auto-configuration of any ad-hoc network built on top of Bluetooth devices, but cannot be easily used for PANs built by a diversity of devices. 5 Open Services Gateway Initiative (OSGi) proposes a comprehensive discovery protocol. However, OSGi assumes a central Gateway node, thus it is not directly applicable to PANs, unless one device is elected as master device. 5 SUN Jini is a distributed lookup service that provides for network resources and services discovery. Jini communications are based on Java Remote Method Invocations (RMI), which allow not only data, but also full objects to be exchanged over the network. In WPANs, Jini’s potential drawbacks are: a) the limited bandwidth resources; b) lack of QoS support and delay, as processing takes place in higher layers; and c) security concerns, since code mobility gives access to remote machines. 5 Universal Plug and Play (UPnP) is a collection of protocols based on IP. Discovery and advertisement messages of device resources are captured or retrieved by control points. IP addresses can be requested from a

Personal Area Networks

[Image courtesy of CSR]

DHCP server, or devices can use Auto-IP to get a nonroutable address. 5 Service Location Protocol (SLP) is an IETF lightweight protocol that provides for service discovery within an administrative domain. Services have to be registered to a central SLP server, so that clients may get all the network service information by invoking this server. The drawbacks include: a) the selection/election of a SLP server and b) the distribution of this knowledge to all devices.

INTELLIGENT ROUTING WPANs will require sophisticated algorithmic solutions to handle high-speed, cost-effective, energy-efficient and reliable intra-PAN and inter-cellular packet forwarding/routing. Intelligent routing protocols for WPANs have to adapt to changing network conditions, offer multi-hop paths and forward packets using the optimum hop. The selection of the next hop should be based on the minimisation of a cost function. Some cost metrics are: 5 Shortest hop routing, the most common metric in ad-hoc networks. The cost metric is the shortest distance in terms of path legs or traverse delay. 5 Best Link quality routing may lead to longer distances, but less retransmissions. 5 Various protocols in Power-aware routing aim at designing a network that consumes the minimum overall energy, or using a maximum residual energy path routing algorithm to maximise the time until any node fails. 5 Other cost metrics include the IPv6 Ad-hoc On-demand Distance Vector (AODV), which minimises the number of required broadcasts by creating routes on demand, and the Multicast AODV, which uses multicast messages to discover the routing information. Different service types (e.g., voice, live video or document transfer) have significantly different requirements for delay, bandwidth and packet loss. Provisioning of Quality of Service (QoS) routing in WPANs may be the most demanding challenge, as the network topology and the QoS capabilities of the candidate links may be timevarying parameters. Potential WPAN QoS mechanisms may be categorised in the following groups: 5 Computational Complexity. Delay is important, especially in streaming applications. Thus, the computational complexity of the route selection criteria must also be considered. As the QoS routing problem is in many cases NP-complete, due to the effective QoS constraints, sub-optimal algorithms may be used. For example, an optimal path based on a single primary metric such as bandwidth may be elected first, and then optimised over a secondary metric (e.g., delay) until most requirements are fulfilled. 5 Local vs. Global Routing Information. QoS aware routing may be based on local state information only, or

Bluetooth chipsets are getting increasingly smaller

5

5

assume knowledge of global states, even if it is inaccurate or outdated. Most of the local state algorithms use probe packets with appropriate nodal identity and QoS information, while algorithms based on global state use the notion of ticket-based probing for identifying a feasible route. Complexity is again an important factor. Distributed vs Hop-by-hop. Other routing algorithms assume a hop-by-hop approach and others establish endto-end links based on distributed information. Detect and Repair vs Reroute. Multiple mechanisms are considered for preserving QoS routing by detecting broken routes and either repairing them or rerouting the flow on an alternate route that can fulfil the QoS requirements. Redundant routes of various kinds may further reduce the likelihood of QoS violation.

SECURITY AND AAA Generally, WPANs have a rapidly changing multi-hop topology. This creates new requirements and challenges for security and Authentication, Authorisation and Accounting (AAA) protocols. Authentication is required as a PAN may carry important business or personal data. As mobile hosts may join or leave the network at any time, there is no way to know which ones are trusted. Dealing with physical security may be achieved by a distributed key management service with multiple Certificate Authorities, where authentication is provided if at least n nodes authenticate the new device. In general, authorisation is based on one central server that keeps the access control lists (ACL). In PANs however, either a device has to be elected as ACL server or a distributed approach has to be implemented. And although the need of charging in PANs is debatable, accounting is required when traffic is exchanged between the PAN and the access network, or when external network tunnels interconnect PAN-bubbles. Theodore Zahariadis is a technical director at Bell Labs/Ellemedia


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