ULTRA WIDE BAND POWER SAVE FIELD OF THE INVENTION [0001] This invention pertains generally to power-limited computing devices and, more particularly, to power management in power-limited networked wireless computing devices. BACKGROUND OF THE INVENTION [ 0002 ] The digital revolution ushered in by widely available computing devices is now well under way, and a secondary wave is now occurring. The secondary wave involves enhanced interconnectivity of the various available computing devices, as users insist upon a more mobile and/or less cluttered experience. For example, a traditional desktop PC can provide a great deal of utility. However the user is tethered in place by the size of the machine as well as its various wired connections. [0003 ] Today, many handheld and small devices provide substantial computing power to their users, and do so in a wireless manner, thus allowing freedom of movement. For example, cell phones, personal digital assistants, notebook computers and other devices can communicate wirelessly and are portable. For larger devices such as desktop computers, household appliances, and entertainment devices, when mobility is not a concern, wireless connectivity still allows freedom from wire clutter. [ 0004 ] However, wireless devices, by the their nature, often lack power cables or other provisions for connecting continuously to an external power source while in use, and thus must be supported by battery power alone. While battery technology has advanced recently and higher capacity batteries are becoming available (for example, Lithium Ion and Lithium Polymer batteries), there is still a continued substantial need to conserve and properly manage energy consumption in battery-powered wireless devices. BRIEF SUMMARY OF THE INVENTION [ 0005 ] Embodiments of the invention solve the shortcomings inherent in prior techniques by allowing ad hoc network devices to cooperatively conserve the battery power of member devices. In particular, long battery life is important for UWB devices. In an embodiment of the invention, devices in a network collaborate to conserve as much power as possible. Typically, devices will have disparate battery capacities or power reserves; some devices may have very small power capacity while others may be able to communicate for longer. [0006] In an embodiment of the invention, devices that have longer nominal battery life (or that are plugged into an external power source) bear more of the processing and communications burdens in order to help devices with more limited battery life to conserve power. In an embodiment of the invention, a network node selects a "sleep" level based on its current or projected battery capacity. Sleep levels define the types of communication to which the node in question will be a party and define respective communications radii within which the node of interest will communicate. Limiting the radius of communications is primarily directed to saving transmission power although processing power may also be saved. The node in question communicates its selected sleep level to the rest of the network so that the other network nodes can communicate accordingly with the node. [0007] Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments which proceeds with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS [ 0008 ] While the appended claims set forth the features of the invention with particularity, the invention and its advantages are best understood from the following detailed description taken in conjunction with the accompanying drawings, of which: [0009] Figure 1 is a schematic diagram of a wireless desktop device cluster within which embodiments of the invention may be implemented; [0010] Figure 2 is a schematic diagram of an ad hoc wireless consumer electronics network within which embodiments of the invention may be implemented; [ 0011 ] Figure 3 is a schematic diagram of a wireless home entertainment network within which embodiments of the invention may be implemented; [0012 ] Figure 4 is a schematic diagram illustrating an exemplary generalized computer networking environment suitable for incorporating embodiments of the invention; [ 0013 ] Figure 5 illustrates a flow chart corresponding to a process of selecting and broadcasting a sleep level within a network according to an embodiment of the invention; [ 0014 ] Figure 6 is a table illustrating sleep level numbers and their corresponding requirements in an embodiment of the invention; [0015 ] Figure 7 illustrates a flow chart corresponding to a process of selecting a sleep level based on remaining power capacity according to an embodiment of the invention; [ 0016 ] Figure 8 is a schematic timing diagram of a UWB frame illustrating transmission of a sleep level according to an embodiment of the invention; [0017 ] Figure 9 is a frequency spectrum diagram illustrating transmission of a sleep level according to an alternative embodiment of the invention; [0018] Figure 10 is a table illustrating sleep level numbers and their corresponding requirements, including communications radius restrictions, in an alternative embodiment of the invention; and [0019] Figure 11 is a schematic diagram generally illustrating an exemplary computer system usable to implement an embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION [0020] Embodiments of the invention will be described herein by reference to the Ultra Wideband (UWB) wireless communication technology, although it will be appreciated that the techniques described herein are useable with respect to devices implementing other communications technologies as well. UWB is sometimes alternatively referred to as impulse, baseband or zero-carrier technology. UWB is a wireless communications technology that transmits very short ultra-low power radio signals across a wide frequency spectrum. UWB receivers can translate the received burst by recognizing a particular pulse sequence sent by the transmitter. The FCC has defined UWB as including any signal that occupies more than 500 MHz or having more than 20% fractional bandwidth, in the 3.1GHz to 10.6 GHz band. The bandwidth of a UWB signal is typically around 25% of the center frequency. For example, a "2 GHz" UWB signal may have a bandwidth of 500 MHz. One benefit of UWB is the ability to detect the range to other ultra wideband devices using round-trip delay time. [0021] The spectrum allowed for UWB is 7500 MHz. This is substantially greater than the spectrum for other technologies in the United States. For example, ISM at 2.4 GHz encompasses an 83.5 MHz spectrum, while U-NI at 5 GHZ takes up 300 MHz (to be increased to 555 MHz). The broad spectrum of UWB allows it to be useful in PC cluster and home cluster scenarios where broadband connectivity is required. For example, a PC cluster may comprise a PC and a storage device and/or an IO device, such as a docking station, and/or a printer or other peripheral, all wirelessly interconnected. In a home cluster, a computer such as a PC or laptop may be wirelessly connected to consumer electronics devices such as a digital camera, video camera, MP3 player, projector, TV, etc., allowing high-speed content transfers. Another potential home cluster environment is within an automobile or other vehicle. Typical bandwidth requirements for consumer electronic and entertainment applications is as follows: HDTV, 19 Mbps; DVD player, 10 Mbps; MPEG2, 1-8 Mbps; MPEG1, 1.5 Mbps; Broadband access, 1-10 Mbps; Video Conference, 1-2 Mbps; TV terminal, 2-5 Mbps; Stereo CD player, 1.4 Mbps; Computer Network, 1-10 Mbps; and Telephone, 8-64 kbps. [0022 ] The power of a UWB signal is typically very low. For example, a UWB signal may be on the order of 1000 times lower than that currently used for Wi-Fi RF transmissions. The low power requirement is enabled by the ease of detectability of UWB signals, i.e. the ease with which the signal may be extracted from background noise. [ 0023 ] The modulation technique used for UWB is typically binary phase-shift keying (BPSK). In BPSK, each pulse is sent at zero or 180 degrees, i.e., right side up or inverted. Thus, BPSK modulation is efficient in its use of the spectrum, requiring about half the bandwidth of a comparable pulse position modulation system. [ 0024 ] The current work regarding UWB technology is focused on development of a standard for wireless personal area networks (PANs), i.e., IEEE 802.15.3a. The purpose of this task group is to provide a specification for low complexity, low cost, low power consumption and high data rate wireless connectivity between wireless devices within or entering the personal operating space (PO). The data rate must be sufficiently high (greater than 110 Mbps) to satisfy a set of consumer multimedia industry needs for wide personal area network (WPAN) communications. The present requirements are as follows: data rate at 10 meters—110 Mbps; data rate at 4 meters—200 Mbps; optional data rate at 1 meter—430 Mbps; power consumption—100-250 mW; bit error rate—10"5; co-located uncoordinated piconets—4; interference capability—robust to IEEE systems; and co-existence capability— reduced interference to IEEE systems. [0025] A number of device environments within which embodiments of the invention may be used are illustrated in Figures 1-3. Figure 1 shows an exemplary wireless desktop device cluster. Exemplary devices shown include a printer 101, monitor 103, camera 105, game controller 107, video camera 109, mouse 111, keyboard 113, and tablet 115. The various devices communicate wirelessly via a personal computer 117. The wireless protocol of each device need not be the same. For example, a number of protocols are shown including IEEE 1394, USB 2.0, USB 1.0, and Bluetooth. [0026] Figure 2 illustrates an ad hoc wireless consumer electronics network. The network contains exemplary devices including a flat screen television 201, video camera 203, modem 205, personal video player 207, digital camera 209, and printer 211, as well as a personal computer 213. The various devices are interconnected by a suitable wireless protocol such as IEEE 1394 and/or USB 2.0. [0027 ] Finally, Figure 3 illustrates an exemplary wireless home entertainment network. The network includes a number of devices including a television 301, speakers 303, gaming devices 305, and multimedia stack 307. The multimedia stack 307 includes an HDTV receiver, cable box, Tivo box, hard drive, DVD player, and home theatre module. [0028] There are two approaches for UWB currently proposed, namely a single band approach and a multi-band approach. The single band approach is less desirable in that it proposes to use the whole 7.5 GHz as one carrier. The multi-band approach segments the 7.5 GHz into equal channels. The basic premise is to use multiple frequency bands to efficiently utilize the UWB spectrum by transmitting multiple UWB signals at the same time. The signals don't interfere with each other because they operate at different frequencies within the UWB spectrum. Each of these signals can be transmitted simultaneously to achieve a very high data rate or can be used as a means of multiple access to allow multiple users to communicate at the same time. Several standard digital modulation techniques can be on each individual UWB signal. The output of the modulated UWB signals can be added together before transmission. [ 0029 ] A multi-band UWB system design has a number of advantages including: more scalable and adaptive than single band designs; better co-existence characteristics with systems such as 802.1 la; and lower risk implementations because it leverages more traditional radio design techniques. These advantages can be retained while maintaining similar complexity and power consumption levels as single band designs. [0030] [0031] Wi th respect to being scalable and adaptive, an advantage of the multi-band approach is that, for example, low bit rate systems can use few bands, high bit rate systems can use many bands. Another advantage is to be potentially adaptive to different radio regulations worldwide, in the event that they do not have the same harmonized spectrum allocations, as happened for the 2.4GHz and 5GHz bands used by WiFi and Bluetooth. [0032] With respect to co-existence, another advantage of the multi-band approach is to increase the level of coexistence with other services such as IEEE 802.1 la. A receiver can dynamically adjust the in-band interference by removing the affected band, or a transmitter can avoid transmitting in a band already used by another service in close proximity. [0033] Finally, since the multi-band technique is based on well known wireless communications scheme, modified for use with the UWB spectrum, the technology presents lower implementation risk. This makes multi-band the best candidate for commercial applications that require standards technology and multiple vendors for high volume adoption. [ 0034 ] Multi-band systems permit adaptive selection of the bands to provide good interference robustness and co-existence properties. When the system detects the presence of an 802.1 la system, for example, it can avoid the use of the bands centered at 5.35GHz or 5.85GHz. This same feature can also be utilized to provision for different spectrum allocations outside of the United States; the bands that share the spectrum with extremely sensitive systems can be avoided. [0035] A single band UWB system would need to employ notch filters to achieve the same result. Notch filters are not an ideal solution because they either increase the receiver's noise figure or require higher performance Low Noise Amplifiers. The problem with notch filters is that they are not adaptive and need to be realized with off-chip dedicated hardware. In addition, notch filters in most cases distort the receive pulse and require additional complexity to compensate for this effect. [0036] The UWB MAC then consists of several components shown in Figure 4. The basic component is the device or DEV 401. One DEV 403 will typically assume the role of the piconet coordinator (PNC) of the piconet 400. [ 0037 ] The PNC 403 performs the following functionality: provides basic timing for the piconet 400 with the beacon; manages the quality of service (QoS) requirements; manages power save levels; and implements security and access control for the piconet 400. Because a piconet 400 is formed with no pre-planning and for as long as the piconet 400 is needed, this type of operation is referred to as an ad hoc network. [0038] The MAC allows a DEV 401 to request the formation of a subsidiary piconet. The original piconet 400 is referred to as the parent piconet. The subsidiary piconet is referred to as either a child or neighbor piconet, depending on the method the DEV 401 used to associate with the parent PNC 403. Child and neighbor piconets are referred to collectively as dependent piconets since they rely on the parent PNC 403 to allocate channel time for the operation of the dependent piconet. An independent piconet is a piconet that does not have any dependent piconets. [0039] Since very long battery life is considered one key feature for UWB devices (PNC or DEV), it is important these devices collaborate in a piconet setting to conserve as much power as possible. For a given piconet, devices will have unequal battery life. That is, some will have very short life while others may be able to communicate for longer. Traditionally, wireless devices go into sleep mode based on a broadcast channel and amount of activity addressed to that particular client. [ 0040 ] As noted above, low power consumption and long battery life are important attributes of UWB devices. In an embodiment of the invention, network members coordinate among themselves to collectively conserve power. For example, devices that have longer nominal battery life (or that are plugged into an external power source) can bear more of the processing and communications burdens, thus helping devices with more limited battery life to conserve power. [0041] For example, in an embodiment of the invention, a network node can select a "sleep" level to enter based on its current or projected battery capacity. The node in question communicates its selected sleep level directly or indirectly to the rest of the network so that the other network nodes can communicate accordingly with the node. This arrangement according to an embodiment of the invention will be described in greater detail below. [0042 ] Figure 5 illustrates a process for forming an ad hoc network from the standpoint of a potential node in the network. For clarity, Figure 5 focuses on the aspects of an embodiment of the invention and omits networking details that are familiar to those of skill in the art. The description of Figure 5 will proceed in conjunction with a description of Figure 12, which illustrates a node schematically with respect to the portions generally involved in embodiments of the invention. [ 0043 ] In step 501 of the flow chart 500, a node, such as node 401 of Figure 4, detects that one or more other UWB devices are within transmission range of the node. This detection can be performed by a connection module such as module 1201 of node 1200, or other entity. In step 503, the node of interest establishes a connection via connection module 1201 with the one or more other detected nodes in a manner that will be known to those of skill in the art. Once a connection is established, the node of interest determines an appropriate sleep level via a power management module 1203 in step 505 and transmits its sleep level to the one or more other nodes in step 507 using the connection module 1201. Applications such as application 1205 that use the wireless connection facilities of the node 1200 need not be aware of or involved in the management of the connection and sleep levels as discussed above. [0044 ] As discussed above, the node of interest chooses an appropriate sleep level and transmits an identification of the selected level to other network nodes. Figure 6 is a table setting forth exemplary sleep levels. The node chooses a sleep level based on energy conservation criteria according to an embodiment of the invention. In a further embodiment of the invention, the remaining capacity of the node's battery is used as a criterion to select a sleep level. [0045] In the illustrated embodiment of the invention, there are five sleep levels, although a greater or lesser number of levels may be made available depending upon user preferences. The table 600 illustrates the five sleep levels in column 601 and the consequences of each level in column 603. The first level, labeled "1," is selected when the battery power of the node is above a highest threshold. In this level, the node is to be treated as an ordinary node with no special considerations. The second level, labeled "2," is selected when the battery power of the node is between a second highest threshold and the highest threshold. In this level, the node follows conventional sleep level (i.e. waking up periodically to transmit and receive) and is to be avoided as a mesh node except for control messages. For example, messages pertaining to the status of the network, particularly as it affects that node, are passed through in this level, while user data (pictures, audio, application data, etc.) are not. [ 0046 ] The third level, labeled "3," is selected when the battery power of the node is between a third highest threshold and the second threshold. In this level, the node is to be avoided as a mesh node entirely. Thus, the node may wake up periodically and transmit any data it needs to, but will not receive control messages or any other data from the other nodes. The wake up interval can be standardized, such as every 1/20 of the full transmit interval. In this sleep level, certain transmissions such as beacons are still received (i.e. the node will awake for such even if the sleep interval has not expired). [0047] The fourth level, labeled "4," is selected when the battery power of the node is between a fourth highest threshold and the third threshold. In this level, the node is in deep sleep of x frames. Thus, the node will wake up every x" frame to transmit any data it needs to, but will not receive control messages or any other data from the other nodes. [0048] The fifth level, labeled "5," is selected when the battery power of the node is between a lowest threshold and the fourth threshold. In this level, the node is in deep sleep of y frames. Thus, the node will wake up every yth frame (where y>x) to transmit any data it needs to, but will not receive control messages or any other data from the other nodes. With respect to the fourth and fifth levels, it is preferable that the node in question specify the number of frames for which it will sleep. However, in an embodiment of the invention, the number of frames is standardized and specification of the level serves to specify the number of frames. [ 0049 ] The thresholds are a matter of user preference. However, in an embodiment of the invention, the thresholds correspond to approximately 20% increments (or similar equal increments for other numbers of sleep levels). Thus, level 1 corresponds to greater than 80% remaining battery capacity, level 2 corresponds to greater than 60% remaining battery capacity, level 3 corresponds to greater than 40% remaining battery capacity, level 4 corresponds to greater than 20% remaining battery capacity, and level 5 corresponds to 20% or less remaining battery capacity. [0050] It will be appreciated that in a network such as an ad hoc network, transmissions between nodes can be direct or can utilize intermediate nodes. Moreover, although the illustrated process transmits an indication of a sleep level at the time of connection, it will be appreciated that sleep level can be periodically transmitted or may be transmitted only when it changes. [0051] Figure 7 illustrates a flow chart 700 for the process of a node changing its sleep level during operation of the network, such as in response to a decrease in remaining battery life. In step 701, the node evaluates its current power level (e.g., remaining battery life). In step 703, the node compares the current power level to sleep level thresholds. In step 705, the node identifies its current sleep level based on which thresholds the power level lies between. [0052 ] In step 707, the node transmits an identification of the selected sleep level to the network nodes with which it is currently in communication. There may be other nodes with which the node of interest is not directly in communication, but the sleep level indication is preferably forwarded to such nodes by one or more of the direct recipients. In step 709, the network nodes other than the node of interest record the sleep level for the node of interest, such that future communications are made only in accordance with the selected sleep level. [0053 ] The way in which the sleep level indication is transmitted is not critical to the above described embodiments of the invention. However, in further embodiments of the invention, two primary transmission mechanisms are used respectively. In summary, the first mechanism provides for transmission of the sleep level indicator in a fractional time domain allocation while the second mechanisms provides for transmission of the sleep level indicator in a fractional frequency domain allocation. [0054 ] Figure 8 illustrates a mechanism for transmission of the sleep level indicator in a fractional time domain allocation, although it will be appreciated that other mechanisms for transmitting the sleep level indicator can be used. The schematic illustration of Figure 8 shows a frame 800 in a UWB transmission. The frame comprises generally control or administrative data and user data. The frame begins with a beacon allocation 801. The beacon allocation 801 is typically used to send synchronization information as well, potentially, as other administrative information if necessary. In an embodiment of the invention, the sleep level indication is placed in the beacon allocation 801. [0055] After the beacon allocation 801, a contention access allocation 803 is provided. The contention access allocation 803 is usable by the transmitting node to send data on a contention basis. Following the contention access allocation 803, the remainder 805 of the frame is contention free. [0056] The contention free portion 805 of the frame comprises management channel time allocations (MCTAs) 807, as well as channel time allocations (CTAs) 809. The MCTAs 807 are usable for sending management and control information on a non-contention basis. The CTAs are used to transmit user data (e.g., video, audio, etc.) on a non-contention basis. In a further embodiment of the invention, the sleep level indicator is transmitted in one of the MCTAs 807. This is desirable for example when there is no excess capacity in the beacon allocation 801. [ 0057 ] In an alternative embodiment of the invention, the sleep level indicator is transmitted in a fractional frequency domain allocation. An exemplary manner in which this is implemented is shown in Figure 9. In particular, Figure 9 is a schematic frequency diagram showing an allocation of the frequency space occupied by a UWB signal according to an embodiment of the invention. In the OFDM proposal for UWB, the UWB signal 901 is 530 MHz wide, and is divided into many channels 903. A selected one 905 of these channels 903 is used to transmit the sleep level information in this embodiment of the invention. [ 0058 ] As described above, a node of interest selects a sleep level based on its power level and transmits that sleep level to other nodes in an embodiment of the invention. In this manner, the network devices cooperate to conserve the power of network devices in a manner dictated by those devices. The sleep levels described above differentiate data based on type but not on data source or target. [ 0059 ] In an alternative embodiment of the invention, one or more sleep levels differentiate data based on a source or target of the data. For example, while a first sleep level may permit all types of communications to all devices, a lower sleep level may restrict the sources or targets to closer devices. A reduction in communication range (and hence RF power) aids a device in conserving power. [0060] Figure 10 illustrates a sleep level chart associating relative power levels with sleep levels and their requirements. In increasing levels, the radius of communication decreases, i.e., the physical radii associated with successive sleep levels increment monotonically, and thus the RF power required to communicate decreases. Thus, in sleep level 1, the radius is not restricted. In sleep level 2, communications are restricted to all devices within radius r1. Similarly, in sleep level 3, communications are restricted to all devices within radius r2