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Co-Link conguration : Using wireless diversity for more than just connectivity
1
1 Introduction
In the computer industry, the expression Plug-and-Play
has become mythical, a relentless quest in making things
easier and transparent for the user. Gone are the traditional
installation of drivers and conguration of I/O addresses and
interrupts, except when Plug-and-Play fails.
Networking has always been an hard area for normal
users : many things need to be congured, due to the
distributed nature of the problem. Wireless networking makes
this even more difcult.
This
paper
looks
at
the
challenge
of
wireless
autoconguration
and
proposes
a
solution,
Co-Link
Conguration, solving some conguration issues found in
typical wireless networking scenarios.
2 Connection Diversity
This work is part of the Connection Diversity project at
HP Labs, a project addressing multiple aspects of wireless
networking.
2.1 Motivation & assumptions
Connection Diversity explores how mobile information
devices can interact using the wide variety of wireless
technologies existing today, with a special emphasis on peer-
to-peer and ease of use. One goal is to bring the ease of use of
wireless technology to the same level as removable storage
(oppy, Compact Flash storage).
A principal underlying assumptions of Connection
Diversity is wireless diversity : the availability of multiple
wireless technologies with different characteristics in each
information device. Connection Diversity also assumes that
most wireless devices will perform user triggered transactions
with either direct peers or the infrastructure [10].
2.2 On-demand TCP and P-Handoff
The work presented in this paper builds on previous work
done with On-demand TCP [10] and P-Handoff [11].
On-demand TCP enables peer to peer TCP on a wide
variety of wireless links [10]. TCP/IP connections are
established and congured on-demand when applications
need them, between two peer devices, without the need for
infrastructure. We have implemented On-demand TCP over
IrDA and 802.11.
P-Handoff enables transparent migration of peer to peer
TCP connections between wireless links [11]. P-Handoff
doesnt require any infrastructure and is ne grained,
allowing exible use of available links. A Policy Manager
tries to optimally use those links for each connection based on
range, speed and cost.
3 The main challenge : conguration
Conguration is a traditional barrier to ease of use. With
wireless technology, conguration is usually even more
complex than with traditional wired technologies [12].
3.1 The lack of physical contact
A physical contact between two entities establishes a
context and a medium. With a wired link the physical action
of plugging a cable in the device binds this device to another
device or an infrastructure. With removable storage, inserting
the media in a slot binds the contained data to this device.
The main benet of wireless technology is that it removes
all physical contact between the device and other devices or
the infrastructure. The cost is extra conguration needed to
create a virtual context replacing the physical contact.
The most important class of parameters is administrative
conguration. Typically, a wireless identier (such as the
ESSID in 802.11 [4]) binds a wireless device to another
device or an infrastructure to create a virtual network.
In addition, some wireless technologies may require
physical layer conguration to interoperate properly [12].
These are some physical layer properties, such as modulation,
frequency or timing information (needed to tune the wireless
receiver and synchronise).
Lastly, security conguration may be needed [12].
Wireless links are perceived as less secure than a wired links,
so most wireless links require the conguration of an
authentication and encryption key to promote privacy.
Pervasive computing applications also need context or
location conguration to bind to a space [1]. The space
aggregates the local services and resources relevant to the
location [2] and often determines the administrative
Co-Link conguration :
Using wireless diversity for more than just connectivity
Jean Tourrilhes and Venky Krishnan
This paper describes a novel approach to setting up peer to peer wireless links, such as IrDA,
BlueTooth, and 802.11b. We explain how the lack of physical contact inherent in wireless creates new
conguration challenges and ease of use issues, and we discuss the limitations of current wireless
autoconguration techniques. We present Co-Link Conguration and Co-Link Activation, two techniques
using wireless diversity to improve the performance and minimise the power consumption of
autoconguration. We then describe two implementations of Co-Link, the simple IrDA Connection Point
and the peer-to-peer Co-Link implemented within the Connection Diversity framework.
jt@hpl.hp.com, venky@hpl.hp.com
Hewlett Packard Laboratories
1501 Page Mill Road, Palo Alto, CA 94304, USA. 2
conguration of the wireless link. Many resource discovery
protocols already exist, therefore this paper limits itself to the
separable issue of nding connectivity.
3.2 Typical wireless conguration
These conguration parameters are usually congured
manually in the device, either by the end user or his network
administrator, or are discovered over the wireless link.
An 802.11 link [4] requires that the ESSID, the mode and
the encryption key are congured before the device can join
the network. A typical 802.11 conguration :
wvlan0 IEEE 802.11-DS
ESSID:"Hewlett-Packard"
Mode:Managed
Frequency:2.447GHz
Access Point: 00:02:2D:0C:94:93
Bit Rate:11Mb/s
Tx-Power=15 dBm
Sensitivity:1/3
Retry limit:4 RTS thr:off Fragment thr:off
Encryption key:5736-6C0E-4365-C35D-4F1F-4BD9-0E70
Power Management period:100ms
A BlueTooth device [3] associates with a peer (and not a
network) and needs to know either its MAC address or a set
of service attributes (SDP) uniquely identifying this peer.
From this, it auto-discovers the frequency hopping pattern
and clock offset necessary to synchronise with the peer.
All those parameters are specic to each link technology,
therefore the manual conguration or wireless discovery is
done independantly for each link layer of the device.
3.3 Mobile Wireless networking
Wireless links enable mobility, therefore a wireless device
may want to interact with a wide variety of peers and
networks, each having a distinct conguration.
The simplest way to manage this mobility is to require the
user to manually enter the new wireless conguration each
time a change occurs. Unfortunately, the user may not have
the necessary information readily available, and will quickly
nd such operations tedious.
Most wireless systems allow users to store multiple
congurations and to either select one or to try them all. This
minimises user intervention, but is limited to a set of known
congurations. Those systems usually dont know when to
change conguration, the user must manually trigger it.
The third option is to use an autoconguration scheme
based on wireless discovery and have the user or the system
pick one of the discovered congurations.
3.4 Wireless discovery and its limitations
Wireless discovery uses a link level protocol on the
wireless link to probe the link, discovering the conguration
needed to connect to networks or peers available within range.
Wireless discovery may fail (wireless is unreliable) or take
a long time (several minutes). For some technologies, the set
of physical layer parameters may be so large that discovery is
not reasonable. Automatic wireless discovery consumes
additional battery power, so the user will often disable it.
Wireless discovery cannot autocongure the security
parameters. The main desired property of an encryption key
is that it cant be guessed or discovered. Some networks or
devices also wont answer discovery probes for security
reasons, making them impossible to discover.
In most cases, discovery of administrative conguration is
ambiguous, and doesnt resolve to a single choice. There may
be multiple peers or networks within range, so the device still
has to somehow choose among them. The device may rely on
the user to pick one, but the choices may be cryptic and
require user intervention. The other solution is to use
heuristics, prioritise already known networks (with user
preferences) or pick the network with the highest signal
strength, but those may not be the most appropriate choice.
The characteristics of discovery for some common
wireless links are detailed in Section 4.1 and in Table 1.
3.5 Usage model and conguration
The necessity of full autoconguration depends on the
usage model : many successful wireless products, such as
GSM phones and 802.11b cards. are sold today without it.
Most wireless connectivity falls in one of three main types of
usage, the net, my and this.
The rst type of usage, the net, is accessing remote peers
via the infrastructure, typically services and information in
the Internet or the phone system. In this case, the device may
pick any access point to the infrastructu