Moving from Mobile Devices to Mobile Networks Robin Kravets Department of Computer Science University of Illinois, Urbana-Champaign
[email protected]
Abstract As users collect multiple small computing devices, the amount of communication resources available to the user increases and so the demand for coordination of resources between these devices increases. The efficient use of these resources is enabled through the forming of a network of these MObile grouPEd Devices (MOPEDs) to support an individual. The support of MOPEDs in the Internet presents a shift from mobility support for an individual device with a single network interface to a network of devices with multiple network interfaces. This paper discusses the challenges involved in providing effective resource management for MOPEDs, focusing on managing bandwidth and energy. We discuss four major challenges: channel discovery, resource monitoring, resource allocation and mobility management.
1 Introduction Advances in processor technology, both in increased processing power and decreased energy consumption, have led to the creation of a new breed of small intelligent devices from tiny laptops, to PDAs, to smart phones, to wearable computers made from a collection of small independent devices. The collection of these devices to support a mobile individual demands the extension of the mobility paradigm from an individual device to a network of devices. As a user moves between different environments, these MObile grouPEd Devices (MOPEDs) cooperate as a coordinated local area network, interacting with and adapting to the current environment. The cooperation of the devices in a MOPED brings the potential for increased communication and computation power. The coordination necessary to support such cooperation may tax the available bandwidth and energy resources of the devices. The challenge lies in supporting such cooperation and coordination through intelligent communication management while emphasizing the efficient use of available resources. The introduction of MOPEDs changes the paradigm for mobile communications, defining communication with any of the devices in the MOPED to be successful communication to the local area network, and so to the individual. This shift is relevant to the mobile community who seek anytime/anywhere connectivity, requiring intelligent mobility management techniques. Many current solutions are focused on a single host and can specify only one host interface [1,2,3], while other research seeks the ability to address and locate a person and the device they are currently using [4,5]. We believe that the appropriate next step is mobility management for a MOPED, the network of devices that is associated with one person. In order to better support the needs of the user, the MOPED may interact with networks and services in the current environment surrounding the user in order to determine local connectivity and service availability. The design of a software network architecture for MOPEDs must enable the exploitation of knowledge about the concerns and requirements of users, the devices, the MOPED network topology, and available communication and routing. An appropriate solution must discover and monitor available communication resources, specify application requirements, determine acceptable communication configurations and evaluate the effectiveness of each configuration. We identify four major challenges in the management of communication resources for MOPEDs: channel discovery, resource monitoring, resource allocation, and mobility management.
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Channel discovery addresses the question of what communication devices can be used in the current environment. The challenge is the design of intelligent service discovery protocols based on the requirements of the user, focusing on the tradeoff between having current information about the availability of communication channels with the overhead in resources (i.e. bandwidth and power) used to acquire that information. Resource monitoring addresses the question of how to collect resource information and what information should be maintained at which devices. The challenge lies in balancing the cost of collecting and disseminating information with the benefit of having that information. The algorithms governing the collection and dissemination of resource information to devices in the MOPED must take into consideration the topology of the network, the energy and bandwidth availability of each device, and the priority at which each device actually needs the information. Resource allocation decisions guide the MOPED as to how to route a user’s data transmissions and receptions. The challenge is the design of intelligent transport and routing protocols able to use the collected resource information to determine which and how many of the MOPED’s external channels to use to complete a specific connection. Such protocols must have the ability to route different connections over different physical channels as well as the ability to multiplex one connection over multiple physical channels. The specific goals guiding such decisions are the efficient use of communication and energy resources in the MOPED. Mobility management provides the ability for external users to communicate with the user, and so the devices of the MOPED, wherever it is currently connected to the Internet. The challenge lies in expanding the current support for an individual device with a single interface to include multiple devices with multiple interfaces. We discuss the design of mobility management protocols based on the current technology for host mobility and investigate the role of proxies, specifically the viability of using a local proxy.
2 The Vision The development of short-range radio technology, such as Bluetooth [6], enables the cooperation of such devices over wireless links providing the basis for developing piconets or Personal Area Networks (PANs) from groups of devices. Although this supporting technology is still under development, many intelligent devices have been proposed and are being designed [7,8]. For example, an intelligent cell phone can share its phone directory with other devices connected via the PAN. It can use IP over the PAN to redirect a phone call to a laptop with external speakers, effectively creating a speakerphone on the fly. Such techniques exploit network topology and resources to allow cooperation between the devices based on information about the connectivity to and the presentation capabilities of each device. Consider a scenario where a user is carrying a cellular phone, a PDA with infrared (IR), and a laptop computer with wireless Ethernet (Figure 1), all of which are connected via a PAN. For external communication, the user has three choices: the cellular network, the IR network and the wireless LAN. If the user has an incoming phone call, a standard system automatically places the call through the cellular connection to the cell phone, costing the user the associated financial costs of such a call and providing a certain level of quality. If the cell phone is in roaming mode, the call might be expensive, while if it is within the service provider’s footprint, the costs may be negligible. Additionally, the quality of the cell phone connection may be good if the phone has a strong signal, but may be lossy in a location with poor service. If the call is first routed to a home agent, we claim that the user has a other options. If the laptop has access to a Wireless LAN and can send voice over IP, the call could be routed through the laptop and sent to the phone via the PAN, providing the user with an alternative for both cost and quality. Again, there may be associated financial costs, or the LAN connection could be free. The quality may depend on the location of the user and the utilization of the LAN. Additionally, if none of the channels to any of the devices has enough bandwidth to support a phone call, the call could be multiplexed over multiple channels. The type of connection to use must be based on the needs of the user as well as the availability of resources on each communication channel. Additionally, we must consider the effect of using various devices on the quality of the transmission. Routing a phone call through the laptop may introduce unacceptable delay and jitter for the phone call. Given that a MOPED is designed for a specific user, its decision and routing
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functions must have access to the needs of the user and the requirements of the application. Consider the above scenario, but with a PDA instead of a laptop. In this scenario, the user still has the option of receiving a call on the cell phone. If the priority of the phone call is low and the user only wants to be left a message, the contents of the call could be filtered out by an intelligent home agent, translated to text, and routed to the PDA. By doing so, we have used knowledge of the requirements of the user to reduce the cost in both bandwidth and power. Wireless LAN, In-Building Infrared Cellular Network
MOPED
Figure 1: Example MOPED For mobility support, we assume the use of the model of communication similar to MobileIP, where a home agent exists on the mobile host’s home network and provides forwarding and routing support for the mobile host. Without a home agent, the mobile host can make outgoing connections, but other users will not be able to locate it as it moves. With this design comes the assumption that all connections to the mobile host will be made through the home agent. In addition, we consider the use of a foreign agent when available. A foreign agent provides a stable local connection point for the mobile host. In this paper, we suggest enhancements to both home and foreign agents that enable support for MOPEDs.
3 The Challenges In an effort to design and implement effective communication resource management techniques for MOPEDs, we focus on challenges in four specific areas of research: channel discovery, resource monitoring, resource usage, and mobility management.
3.1 Channel Discovery The first step towards supporting the communication demands of MOPEDs lies in determining what communication channels are available through which communication mediums. The goal is the design of resource-aware channel discovery algorithms. Such algorithms must balance the tradeoff between timely discovery of communication channels and the amount of resources used to find such channels. Additionally, these tradeoffs must be considered in the context of the current needs of the applications running on the MOPED. We consider channel discovery techniques for two types of scenarios. The first scenario is when the MOPED is in an unfamiliar location with little or no resource management support. Determining what communication channels are available as well as the quality of those channels falls entirely to the MOPED. Such protocols are often based on probing techniques that can be very resource intensive. The second scenario is when the MOPED has access to certain information about its current location through the querying of a local service agent.
3.1.1
Probing Techniques for Communication Channel Discovery
A simple technique for channel discovery would be to have the devices periodically check the availability of communication channels. The devices in the MOPED could use this type of solution in environments about which they have no information. Unfortunately, constant monitoring of potential communication channels can be wasteful in both energy and bandwidth, making it necessary to design resource efficient discovery algorithms for such scenarios.
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Hosts can determine connectivity through a specific communication device by activating the device and sending a probe message to any potential receivers or base stations. If the probe is unsuccessful, the host assumes no connectivity, sleeps for a period of time and tries again. If the probe is successfully received, an answer is returned, informing the host of an available connection. In order to maintain information about an available channel, the device must send periodic probes to insure accurate information about availability. The overhead of probing during the actual transfer of data can be removed, since an active transfer implies an active channel. Such probing techniques can be expensive in both bandwidth and energy. For transmission mediums that support passive discovery on the part of the mobile device, the overhead of the transmission of the probe message is reduced to the overhead of waiting for and processing a probe message from the base station. Channel discovery algorithms must include the use of both active and passive probing techniques based on the technology of the communication medium. Bandwidth is consumed by the actual transmission of any probe messages. When a channel is unavailable, no overhead is incurred, since no bandwidth actually exists. When a channel is available and not being used, the transmission of the probe messages consumes bandwidth for that channel. Although probes may be quite small, in current mobile wireless communication, bandwidth is considered a scarce commodity and must be carefully used. Although passive techniques can reduce the effect of probing on the individual device, the delay in waiting for the base station to send the probe may be unacceptable to the applications running on the MOPED. Energy consumption is a more complex. For an unavailable channel, energy is still consumed when the device is powered on for a period until it has been assumed that the probe will not be answered and for the transmission or reception of the probe message. For an idle available channel, the device is already powered on and the only overhead is for processing the probe. Although there is little or no additional energy consumed by the communication device to receive a probe response when the device is already active, that response may trigger activity on the device and so cause energy consumption from the CPU. Such energy consumption can be reduced using idle-time power management techniques, which allow the device to turn itself off during idle periods and wakeup periodically or on demand. These techniques must take into consideration the characteristics of the communication device, such as the amount of time it takes to power on and off the device. For both probing and idle-time power management techniques, the optimizations lie in trading resources (bandwidth and power) for the accuracy of channel information and timeliness of data transfers. If the probing periods are too short, too much energy will be consumed and too much bandwidth will be used. If the probing periods are too long, channel information will be stale and data transfers will be unacceptably delayed. Algorithms for such techniques must consider the transfer needs and traffic patterns of the application.
3.1.2
Querying Techniques for Communication Channel Discovery
If the MOPED is in an environment about which information is available, the devices could use this information to better determine how to do channel discovery. For example, when a user enters a building, a local agent may provide information about what types of services are available in the building. If it is known that there is no wireless LAN, then the devices with wireless LAN capability will know not to even try to use it. In many situations, a MOPED may have access to information such as the communication mediums supported in the area. The MOPED can use this information to guide its channel discovery algorithms. Probing may still be necessary, since the information provided only suggests potential not absolute availability. A MOPED using such information may find it incorrect or out-of-date since channel information may only be provided on a best-effort basis. The MOPED simply uses this information as a guide for determining how to do channel discovery.
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3.1.3
Communication Channel Discovery for MOPEDs
In the context of a MOPED, multiple hosts may have devices for the same type of communication medium. In this case, probing and querying by each device places an unnecessary burden on the resources of the MOPED. In this situation, a leader can be elected to maintain information about the availability of the specific communication medium. The leader is responsible for providing up-to-date information about that communication medium to the rest of the interested devices in the MOPED. In the next section, we will discuss the role of monitoring, maintenance and dissemination of resource information throughout the MOPED.
3.2 Resource Monitoring The next challenge is resource monitoring through the collection and maintenance of communication resource information, including expected availability and quality of communication channels. Due to the limited resources of a MOPED, it may not be able to collect and maintain sufficient resource information in all nodes. Although the collection of some statistics may be performed by the MOPED, any extensive resource monitoring is expected to be performed by an external service agent. Once the information is collected, it must be made readily available to the appropriate nodes. Such dissemination may be costly in both power and bandwidth, especially if the information is never used or is out-of-date by the time it is used. We address two components of resource monitoring: resource information collection and resource information dissemination. For resource information collection, we explore issues of what resource information to collect, how the MOPED can collect such information and how the MOPEDs information can be enhanced by external information sources. For resource information dissemination, we explore techniques for resource information dissemination based on available resources, such as bandwidth and power, and the interest and needs of the individual devices for such information.
3.2.1
Resource Information Collection
In order to understand how to collect resource information, we first define the information we are interested in for MOPEDs. We break resource information in two categories: internal and local. Internal resources are the individual devices and connection that make up the MOPED and are under the complete control of the MOPED. For each device, we must track its CPU, power and presentation capabilities and current usage. Although this may be more straightforward for CPU and power, it becomes more complex when considering the presentation of data. For example, a phone is only capable of presenting one audio conversation at a time, and so any other incoming audio connections cannot use that presentation format for the phone. For each connection in the MOPED, we must track bandwidth availability and usage since it is not assumed that all connections will have the same bandwidth capabilities, allowing for PANs connected via multiple communication techniques (i.e. Bluetooth and IR). Local resources are the link layer technologies available at the site that the MOPED is currently located. Information about the availability of these resources was discussed in Section 3.1. Information about the quality of these resources can be obtained in similar ways: independently by the MOPED or with the help of an external service. The MOPED can collect transmission statistics for active channels relatively cheaply by monitoring the data transmissions for those channels. For non-active channels, probing for available bandwidth and current delay can be expensive. Due to the bandwidth and power limitations of a MOPED, it is not very likely that the MOPED itself will do any non-active communication channel monitoring. For more detailed local information, the MOPED can query a local service agent for information about expected levels of service for given communication channels. It may be too complex to expect the agent to have information about all possible connections, but it may be able to provide hints based on some amount of collected data.
3.2.2
Resource Information Dissemination
In a small MOPED, it is possible that one node could collect all of the data and act as a central agent. It is important to consider the communication overhead when querying a resource information repository. If the node requesting the information is multiple hops away from where it is being stored, the network channels
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and the available power of the MOPED may not be being used efficiently. Decisions for resource information dissemination must be based on information about the resources available at each node as well as the underlying connectivity of the PAN. It will also be necessary to insure that the appropriate nodes receive accurate and timely data. In order to use any collected resource information, it must be available to the appropriate nodes in a timely fashion. The simplest centralized solution propagates information to interested nodes. The simplicity of this solution fits well with the limited resources of a MOPED since there are limited synchronization costs. Although a centralized solution is easy to maintain, it may place an unacceptable burden on the managing node. This leads us to consider distributed algorithms. It may not be viable to expect small nodes to store any information. Under such circumstance, the dissemination algorithm should favor larger nodes with larger capacities, especially power. For both centralized and distributed solutions, it is important to understand the underlying network topology of the MOPED. If the PAN is implemented using Bluetooth, the communication paradigm used is master-slave. Groups of nodes communicate with one master and the masters communicate amongst themselves via what is called a scatternet. Understanding this hierarchy could provide valuable guidance as to how to organize the resource information. Due to the dynamic communication demands on the MOPED, we believe that an adaptive solution needs to be developed; incorporating both centralized and distributed solutions as best fits the current communication needs.
3.3 Resource Allocation The third challenge is resource allocation, the decision making process about what and how many communication channels to use as well as what protocols to use to transport the data. Once information about available communication channels and the resources associated with them is made available, the MOPED must decide how to route and transmit the user’s data to and from the MOPED based on knowledge of the needs of the application and the resource information. Resource adaptation techniques are based on the idea that applications are willing to accept a lower quality for one resource parameter if they can get an improved quality for a currently more important parameter. The choices such adaptation algorithms have are based on application-level specification of requirements and availability of networking resources. The space of application-level specification is constrained by the presentation options for the specific data (e.g. frame rate or frame size for video). Adaptation decisions can be based on the use of utility functions [10], which enable the specification of value to particular resource parameters and the evaluation of the effect of changes in such parameters. Such utility functions have been used to guide the configuration of real-time and multimedia applications [11,12,13].
3.3.1
Resource Allocation Algorithms
We assume a simple model of best-effort delivery coupled with monitoring and policing of data channels. Although this eliminates the possibility of providing service guarantees, we believe that the expected usage of the MOPED does not warrant the overhead associated with guaranteed service. Additionally, since the MOPED handles data from only one user, coordination of that user’s data transmission can be more closely controlled. Determining an acceptable connection must be based on the needs of the application. We assume that the user specifies a set of options and their priorities for each type of data transfer. Additionally, the user has specified preferred devices for each type of data. For example, if the user has an incoming audio transfer, they might specify that audio should first go to the phone, next the laptop and finally the PDA. They might also specify that the quality of the audio should be high, low, or translated into text. Determining how to route a user’s connection must be based on knowledge of the transmission and routing options and the effect of choosing one of those options on the performance and lifetime of the MOPED. Finding an optimal solution in such a large search space is an NP-complete problem and beyond the capabilities and time requirements of the MOPED. In this context, the potential gain of lower overhead
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from using resource evaluation algorithms that are able to reduce the search space should pay off despite the fact that they may not find an optimal solution.
3.3.2
Multiple Communication Channels
An important tool for adaptive resource management is the ability to reconfigure communication protocols and communication protocol parameters during active data transfer. By permitting applications to configure a communication channel during its operation, changing service requirements may be accommodated, and changes may be made concerning the types of data communicated via that channel. Traditional Internet connectivity solutions have focused on hosts with one primary physical network connection. This assumption breaks down when dealing with mobile hosts with the potential of multiple network connections. Current techniques have forced mobile hosts into the standard fixed host solutions and focused on one of the mobile hosts connections at a time. Recent work has dealt with how to use multiple interfaces in a parallel fashion, by using vertical handoffs [4]. Still only one interface is actively transmitting packets at any time. We believe that there will be circumstance where the bandwidth from one device will not sufficiently support a single application. In this context, it is possible to multiplex communication across multiple physical channels, exploiting all the bandwidth potentially available to the mobile host. A MOPED with multiple limited bandwidth connections may be better served by using multiple communication channels for a single data transmission and its effect on the performance. The decision to use multiple channels must be based on knowledge of the energy usage of such a transmission. We believe that, although having more than one active interface may consume more power, with intelligent management of the devices, total power usage can be minimized over the lifetime of the device.
3.4 Mobility Management The final challenge lies in the fact that the MOPED is an autonomous network that has complete mobility. Since we are no longer dealing with an individual device as the mobile unit, but now considering a network of devices as a unit, we need to extend current mobility management techniques. For devices, mobility management can be provided by protocols such as MobileIP. We consider mobility management protocols able to manage such a network of devices using and extending existing techniques where possible. A home proxy and local proxy are a logical extension of the home agent and foreign agent in MobileIP. The functionality of the home agent is extended to include more information about the topology, connectivity and current usage of the MOPED. By incorporating this information into the home proxy, intelligent routing of connections to the appropriate device in the MOPED is enabled in a similar manor to MobileIP. Essentially, the home proxy must be able to run the same resource allocation algorithms that are run locally on the MOPED in order to determine how to correctly route the incoming connection. The use of the home proxy also enables communication with non-mobile-aware users and supports the ability to hide information about the MOPED from other users. If the MOPED is far away from the home proxy, the coordination between the MOPED and the home proxy may be too expensive and potentially contain outof-date information. For this reason, a local proxy is introduced that has the same resource allocation capabilities as the home proxy and resides topologically local to the MOPED. Although non-mobile-aware users have the overhead of routing through the home proxy and the local proxy, mobile-aware users can bypass the home proxy. When considering that a MOPED represents the user it supports, we must integrate the use of current techniques for addressing people. The basic assumption is that an external user wants to locate the person not an interface on a machine. Intelligent location management to find people is also aware of what device the person is currently using. In the context of MOPEDs, the user is represented by multiple devices with multiple interfaces simultaneously. As discussed in the previous section, the decision about which interface to route a connection to is more complex due to the consideration of changing resource needs and availability. Although this blurs the lines between resource allocation and location management decisions, we still need support for location management.
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4 Conclusions In our current networked society, the evolution from personal computers to connected personal computers seems in retrospect inevitable. The benefits of being connected are so great that the whole revolution of the personal computer seems fated to have taken us to the Internet. We believe that the same effect will be felt on the small personal devices that we all carry. The benefits of having an integrated collection of phones, PDAs, laptops, intelligent screens, web-browser-writing-tablets and other new devices that are on the horizon are so great compared to the stand-alone devices that in a few years this development will also be seen as inevitable. In this paper, we have presented four major challenges in the design of a network infrastructure to support MOPEDs. We believe that creating a low-level infrastructure and the services necessary for the actual existence of MOPEDs will accelerate the transition from a disconnected collection of devices to a cooperating whole. On creating this communication infrastructure, we also take the next logical step in allowing communication to flow simultaneously through all available communication channels. This creates a very robust environment, where it becomes easier to guarantee complete communication coverage through the efficient use of the available communication resources of the MOPED.
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