WIRELESS ROUTING SELECTION SYSTEM AND METHOD
A technique involves untethered access points (UAPs) that can broadcast estimated transmission time (ETT) that represents an estimated time it would take for a packet to be transmitted from the first UAP to an AP that is wire coupled to a network. The proposed system can offer, among other advantages, accurate ETT values for use by UAPs of a wireless network.
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This application is a continuation of U.S. application Ser. No. 11/604,075, entitled “Wireless Routing Selection System And Method,” filed Nov. 22, 2006, which claims priority to and the benefit of Provisional Patent Application Ser. No. 60/812,403 entitled “Wireless Routing Selection System And Method,” filed Jun. 9, 2006, both of which are incorporated herein by reference in their entireties.
BACKGROUNDNext hop selection in a wireless protocol is made by selecting a least cost hop. Historically, cost has been determined by hop count, signal strength, error rate, utilization, and other factors. One technique for wireless routing selection involves defining cost based on expected transmission time (ETT) for some link (ETT1).
For example, link cost may be determined by measuring the transmission time to send a 1 Mbps stream of packets across the link and measuring its transmission time for some number of bytes. An algorithm may measure for each available bandwidth across the link, and the transmission time is defined as the time from when the packet is scheduled (specifically, sent to the radio) and the time that an acknowledgement is received.
The improvement of algorithms for next hop selection are the subject of research. Any improvements may have significant repercussions on the relevant technologies. Accordingly, any improvement in next hop selection would be advantageous.
These are but a subset of the problems and issues associated with wireless routing selection, and are intended to characterize weaknesses in the prior art by way of example. The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
SUMMARYThe following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools, and methods that are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.
A wireless network system is typically coupled to a wired network at some point. Such a point is sometimes referred to as an access point (AP). A plurality of untethered APs (UAPs) may be coupled to one another, and eventually to the AP, to allow a wireless network to grow to practically any size. However, as the network grows in size using UAPs, it becomes more difficult to figure out a best path from a mobile station, through the UAPs to the AP in an optimal fashion.
Advantageously, UAPs can broadcast estimated transmission time (ETT) that represents an estimated time it would take for a packet to be transmitted from the first UAP to the AP. Thus, a UAP that is right next to the AP should be able to give a low ETT to the AP. As the advertised ETTs percolate through the wireless network, UAPs can eventually settle on optimal paths to the AP. The better the estimate, the more likely the optimally chosen paths are actually optimal.
The proposed system can offer, among other advantages, accurate ETT values for use by UAPs of a wireless network. This and other advantages of the techniques described herein will become apparent to those skilled in the art upon a reading of the following descriptions and a study of the several figures of the drawings.
Embodiments of the invention are illustrated in the figures. However, the embodiments and figures are illustrative rather than limiting; they provide examples of the invention.
In the following description, several specific details are presented to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or in combination with other components, etc. In other instances, well-known implementations or operations are not shown or described in detail to avoid obscuring aspects of various embodiments, of the invention.
In the example of
In some embodiments, the ETTp calculation is for the time a packet is sent from a radio until the time an acknowledgement is received. This, however, does not include time spent on a queue waiting for the radio to become available. Advantageously, by including the time spent on the queue, the ETTp calculation can take into consideration the real time it takes to transmit a packet based on load and utilization.
In the example of
The AP 304 has a comparable queue 316, which is coupled to an ETT engine 318. The wireless device 302 may or may not have an ETT engine to determine how long a packet is enqueued on the queue 312, but in the example of
Advantageously, ETT can be used by a next hop selector to decide upon an optimal next hop. In an embodiment, each AP includes a next hop selector.
From “A Radio Aware Routing Protocol for Wireless Mesh Networks” by Kulkarni et al. defines cost based on ETT1, and how ETT1 can be aggregated to determine ETTp. However, the algorithm used by Kulkarni et al. can be improved in some specific cases. For example, the choice of 1 Mbps load rate for link cost calculation is arbitrary and may be significantly off. In an embodiment, expected load rate (ELR) is used instead. ELR is the load that a link would be subject to if it was selected as a next-hop.
Referring once again to the example of
In the example of
EDR is the rate determined by a rate selection algorithm. In general, the rate selection algorithm should meet the following goals: 1) To the extent possible, the selected rate should produce optimal throughput of packets transmitted to a client. This is not necessarily the same thing as minimizing retries. For instance, retransmitting one time a large packet at 54 Mbps may result in better throughput than transmitting the same large packet at a 1 Mbps with no retries. 2) To the extent possible, the algorithm should be computationally light. That is, it should not consume a lot of CPU time to determine a rate to use.
An example of a rate selection algorithm is as follows (though any applicable known or convenient rate selection algorithm could be used): The rate selection algorithm seeks to minimize retransmissions. For each client it maintains a ‘best rate’ value. The rate selection algorithm is a control system that lowers the best rate when the rate of retransmissions exceeds 50% and raises the best rate when the rate of retransmissions is less than 50%. For each transmitted packet, there are one of three possible outcomes. 1) The packet is successfully transmitted with no retransmissions, 2) the packet is successfully transmitted with one or more retransmissions, 3) the packet transmission is unsuccessful after all retransmission attempts.
For each client, a counter is maintained. When a packet is successfully transmitted with no retransmissions, this counter is incremented by 3. When a packet is successfully transmitted but with retransmissions, the counter is decremented by 6. When a packet is not successfully transmitted, the counter is not changed. When the counter reached a value of −50, then the next lower rate is made the best rate. When the counter reaches a value of 100, the next higher rate is used as the best rate; however, the best rate is not increased if it has been increased in the past 60 seconds. This prevents the best rate from increasing too fast.
For each packet, transmissions are attempted using up to four rates.
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- The best rate is tried 1 time. This is the initial transmission attempt, not a retransmission.
- The next best rate is tried for configured number of retransmissions minus 2. For example, the default value for the retry count is 5, and so by default the next best rate is tried 3 times.
- The next lower rate is tried 1 time.
- The lowest rate supported by the radio is tried 1 time.
This rate fall back schedule has the following properties. 1) If the best rate is successful, then there are no retries and the client's counter is increased. 2) If the best rate fails, then the next lower rate is used multiple times. The range of the next best rate is better than the best rate, and so the next best rate has a higher probability of success. The client's counter will be decremented in this case to reflect that the best rate was unsuccessful. 3) The radio's lowest rate has the best range, and so if it fails, then the client is not reachable or the failure is due to factors not related to distance. In this case, the client's counter is unchanged because the failure is not related to rate.
If the EDR is actually determined ELR, the algorithm further reduces the bandwidth required to compute ETT1, since the EDR need not be calculated through synthesized load. Notably, as shown in
It should be noted that sensing all data rates is less efficient than using the techniques described herein. Advantageously, by using EDR, all possible rates need not be tested, making this technique more efficient. Moreover, selected rates may not be the rate actually selected by a radio transmission module. For example, if data rate selection does not yield an answer that matches an algorithm such as Kulkarni's, the actual ETT1 will be different than the expected ETT1 and the algorithm will make suboptimal decisions. So using EDR can lead to performance improvements as well.
In an embodiment, the ETTp calculation can be improved by considering the amount of time a packet spends being processed in intermediate nodes. This is the time it takes to receive a packet on some interface and queue it on its egress interface. This time is referred to as node transit time (NTT). Therefore, in a non-limiting embodiment, ETTp=ETT1+ETTp_nh+NTT, where ETT1 is the link between a node and a next hop node, ETTp_nh is the ETTp advertised by the next hop node (e.g., the best advertised ETTp of potential next hop nodes), and NTT is the time a packet spends transiting a node. As was previously described, the ETT calculations include the time a packet spends in a queue waiting for a radio to become available. Conceptually, the NTT is the time a packet spends in a node waiting to be enqueued.
The techniques described herein work best when there are relatively few interesting destinations. Advantageously, this is exactly the case in most IP network environments. Most hosts are trying to communicate to their next hop IP router, which is typically eventually accessed over a wired network. Hence, the techniques described herein help answer the question “how do I get to the wired network?” Only a single destination need be evaluated and only a single value to ELR needs to be maintained.
In the example of
In the example of
At the UAP 502, the goal is to send traffic to the least expensive AP that is wired to a network. By least expensive, what is intended is that a weighted graph with edges that are ETT between nodes, would yield the smallest result possible (or practical). This AP may or may not be the AP closest to the UAP 502. The UAP 502, for illustrative purposes, is illustrated as a large circle with various components. However, the UAP 504, the UAPs 506, and/or the AP 508 may have similar components (not shown).
In the example of
Some time later (or concurrently) the station 510 sends packets to the UAP 502, which are received at the ingress interface 512. The NTT module 522 receives an indication, such as a first timestamp, that a first packet has been received. As much as is practical, it would probably be valuable to have the timestamp represent the exact time the first packet was received at the ingress queue 512, though an estimate may be used. At this point, the ETTp engine 514 knows only ETTp values for the UAP 504 and UAPs 506, but has no link information. It should be noted that in practice there will typically be link information as described later. Nevertheless, assuming for a moment that no link information is available, the ETTp engine 514 can provide the advertised ETTp values to the next hop selector 516, which picks an appropriate optimal path to the destination based on the advertised ETTp values. Specifically, the next hop selector 516 chooses the shortest (e.g., lowest weight) path to the destination.
The first packet is enqueued at the egress interface 518, as appropriate. It may be noted that the first packet may or may not need to be enqueued in a case where the relevant link is underutilized (or saturated but not congested). In any case, when the first packet is received at the egress interface 518, the NTT module 522 receives an indication, such as a second timestamp, that the first packet has been received at the egress interface 518. At this point, the NTT module 522, by comparing, for example, a first timestamp and a second timestamp, can calculate the amount of time that the first packet spent at the UAP 502. This information is useful for purposes that are described below.
The first packet is sent from the egress interface 518 to the UAP 504. For illustrative purposes, it is assumed that the UAP 504 is the next hop in an optimal path. In a non-limiting embodiment, the UAP 504 sends an acknowledgement, as soon as the first packet is received, that the first packet was received. The acknowledgement is received at an acknowledgement interface 526. It should be noted that the acknowledgement interface 526 may be part of a radio interface that includes the ingress interface 512 (or even the egress interface 518). In any case, the acknowledgement interface 526 provides the ETT1 module 524 with an indication, such as a timestamp, that an acknowledgement was received from the next hop node. The ETT1 module 524 uses the indication (e.g., second timestamp) that was generated when the first packet was enqueued on the egress interface 518 and the indication (e.g., third timestamp) that was generated upon receipt of the acknowledgement to provide an ETT1 value.
At this point, the ETTp engine 514 has enough information to know ETTp from the UAP 502 to the destination. Specifically, ETT1+NTT+ETTp_nh=ETTp from the UAP 502 to the destination. This ETTp value can be provided to an ETTp broadcast engine 528. In the example of
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As used herein, access point (AP) refers to receiving points for any known or convenient wireless access technology. Specifically, the term AP is not intended to be limited to 802.11 APs.
Some portions of the detailed description are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of operations leading to a desired result. The operations are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.
It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.
The algorithms and techniques described herein also relate to apparatus for pertaining the algorithms and techniques. This apparatus may be specially constructed for the required purposes, or it may comprise a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus.
As used herein, the term “embodiment” means an embodiment that serves to illustrate by way of example but not limitation.
It will be appreciated to those skilled in the art that the preceding examples and embodiments are exemplary and not limiting to the scope of the present invention. It is intended that all permutations, enhancements, equivalents, and improvements thereto that are apparent to those skilled in the art upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present invention. It is therefore intended that the following appended claims include all such modifications, permutations and equivalents as fall within the true spirit and scope of the present invention.
Claims
1. A method comprising:
- receiving, at a first wireless node, an estimated transmission time (ETT) from each wireless node from a plurality of wireless nodes that are within a range of the first wireless node, the ETT for each wireless node from the plurality of wireless nodes is a next-hop-to-destination-path ETT (ETTp) associated with one remaining wireless node from the plurality of wireless nodes;
- at the first wireless node, adding for each wireless node from the plurality of wireless nodes a link ETT (ETT1) to a corresponding ETTp to determine a node-specific path metric for that wireless node; and
- at the first wireless node, selecting a next hop based on the determined node-specific path metrics.
2. The method of claim 1, wherein each ETTp is a function of an ETT1, another ETTp, and a node transmission time of a second wireless node from the plurality of wireless nodes.
3. The method of claim 1, further including, at the first wireless node, calculating an advertised ETTp for the first wireless node, the advertised ETTp based on at least one ETT1, the ETTp of the first wireless node, and a node transmission time of the first wireless node.
4. The method of claim 1, further including broadcasting an advertised ETTp from the first wireless node to a second wireless node from the plurality of wireless nodes.
5. The method of claim 1, further including measuring an ETT1 from the first wireless node to each wireless node from the plurality of wireless nodes.
6. The method of claim 1, further including:
- placing a packet on an egress queue of the first wireless node at a first time;
- receiving an acknowledgement that the packet was received by a second wireless node from the plurality of wireless nodes at a second time; and
- measuring an ETT1 between the first wireless node and the second wireless node based on the first time and the second time.
7. The method of claim 1, further including:
- receiving a packet on an ingress interface of a wireless node from the plurality of wireless nodes at a first time;
- placing the packet on an egress interface of that wireless node at a second time; and
- calculating the ETTp of that wireless node based on the first time and the second time.
8. An apparatus comprising:
- a first access point configured to be wirelessly coupled to a second access point;
- the first access point is configured to broadcast an estimated transmission time (ETT) for a packet to be transmitted from the first access point to the second access point, the ETT including an estimated node transition time (NTT) at the first access point and an ETT over a link to the next hop that is based on current load.
9. The apparatus of claim 8, the first access point configured to transmit the packet to a network via a station.
10. The apparatus of claim 8, the first access point is configured to compare a next-hop-to-destination-path ETT (ETTp) of the second access point to an ETTp advertised by a third access point, and to send the first packet to the third access point when the advertised ETTp of the third access point is lower than the ETT of the second access point.
11. The apparatus of claim 8, the first access point is configured to calculate an ETT between the first access point and the second access point by queuing the first packet to be sent to the second access point and by measuring the amount of time the first packet is queued.
12. The apparatus of claims 8, the first access point is configured to calculate the ETT between the first access point and the second access point by sending the first packet to the second access point, receiving an acknowledgment of receipt from the second access point, and measuring the amount of time between sending the packet and receiving the acknowledgment.
13. The apparatus of claim 8, wherein the first access point is untethered.
14. The apparatus of claim 8, wherein the first access point is untethered and the second access point is tethered.
15. An apparatus, comprising:
- an estimated transmission time (ETT) engine configured to receive an estimated path transmission time (ETTp) advertised for each wireless node from a plurality of wireless nodes linked to an access point (AP), the ETTp for each wireless node from the plurality of wireless nodes being a function of a link ETT (ETT1), the ETTp for a remaining wireless node from the plurality of wireless nodes, and a node transition time (NTT) of that wireless node; the ETT engine configured to measure, for each wireless node from the plurality of wireless nodes, the ETT1 between the AP and that wireless node; and the ETT engine configured to determine a node-specific path metric for each wireless node from the plurality of wireless nodes, based on the measured ETT1 and the ETTp for that wireless node; and
- a next hop selector coupled to the ETT engine and configured to select a transmission path for a message received at the AP, based on the node-specific path metrics.
16. The apparatus of claim 15, wherein the ETT engine is configured to calculate an advertised ETTp for the AP, based on the measured ETT1s and the received advertised ETTps.
17. The apparatus of claim 15, wherein the ETT engine is configured toe to calculate the advertised ETTp for the AP as a function of one of (1) the measured ETT1's, (2) a received advertised ETTps or (3) a computed NTT of the AP.
18. The apparatus of claim 15, wherein the AP computes an NTT as a function of an amount of time to be enqueue the message within the AP for transmission to another node.
19. The apparatus of claim 15, wherein the ETT engine is configured to calculate each ETT1 as an indication of an amount of time between a packet being enqueued at an egress interface of the AP and an acknowledgement of the packet being received by the AP.
20. The apparatus of claim 15, further including:
- an ETTp broadcast engine coupled to the ETT engine and configured to broadcast the advertised ETTp for the access point, from the access point to a wireless node from the plurality of wireless nodes.
Type: Application
Filed: Mar 7, 2011
Publication Date: Jun 30, 2011
Applicant: TRAPEZE NETWORKS, INC. (Pleasanton, CA)
Inventors: James Murphy (Pleasanton, CA), Gary Morain (San Jose, CA)
Application Number: 13/042,080
International Classification: H04W 40/04 (20090101); H04L 12/26 (20060101);