Abstract:
A system and method for reducing call establishment delay in wireless network is provided, in which a network node establishes a call to a wireless terminal controlled by an AP via a server. The wireless terminal notifies the server of its listen interval. In the power saving mode, the wireless terminal wakes up every listen interval and listen the beacon to check whether any buffered packet for it. When the wireless terminal learns from the beacon that there are packets waiting, it communicates with the access point to retrieve them. The server records a listen time at which the wireless terminal will wake up and listen to the AP based on the listen interval. When a network node calls the wireless terminal, the server buffers the request for a time interval based on the listen time, and then sends the request to the wireless terminal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to the technical field of wireless networks and, more particularly, to a system and method for reducing call establishment delay in a wireless network.  
         [0003]     2. Description of Related Art  
         [0004]     With extensive implementation of wireless networks (such as 802.11/WLAN, 802.16/WiMAX) in a public network or enterprise network, a wireless network environment can provide a packet voice service, voice over IP (VoIP), or multimedia service to a personal communication as a cheap and convenient choice. Most Internet communication systems use a session initiation protocol (SIP) or similar signaling protocol as a communication protocol standard to establish a connection. As shown in  FIG. 1 , the configuration of a wireless communication system uses an IP network or the Internet as the core network  10 , and also uses an extension of wireless network access point/base stations (base stations)  11  to provide the wireless network access capability. One or more servers  12  are implemented in the core network  10  to assist in establishing a communication connection or session. The communication connection can be a voice over IP, video over IP, or other multimedia connection. With the help of the server  12 , the Internet or IP node  13  can establish a connection communication service through the signaling protocol and the wireless terminal  15  in the wireless network.  
         [0005]     However, the signaling protocol typically is based on a fixed network in design and does not consider the features of wireless network environments. As such, for a connection establishment, the signaling protocol typically supposes that the network is a wired network. When receiving a connection request from a caller in the wired network, the server  12  immediately forwards the call to the callee. In case of no immediate response, the server  12  further supposes that the wired network has a collision or the relative node is closed. In this case, the server  12  retransmit the call to the callee after starting a waiting mechanism for a span. Such a design of reducing network throughput and collision by prolonging the retransmission time is presented frequently in the signaling of the wired network. However, in a wireless network environment, a wireless node may not correctly receive partial downlink data due to power-saving design or temporary setup disconnection to the base station. Under the cited time-prolonging design, such a feature may cause the initial time prolonging on the call connection in the wireless network, and even with inappropriate parameters, no connection signaling protocol packet can be received by the wireless node.  
         [0006]     To clearly illustrate the cited problem, the SIP, wireless local area network (WLAN) and VoIP service are given as an example to describe the signaling protocol design, wireless network and communication service. As shown in  FIG. 2 , a network node  13  uses a server  12  (SIP proxy) to establish a session with a wireless terminal  15  (SIP user agent or WLAN SIP UA) covered by a base station  11  in the wireless network. The server  12  starts a timer A (Timer A) to count time t 1  (i.e., a duration from the time that the server  12  sends the request R 1  to the time that the sever  12  receives a response sent by the wireless terminal  15 ) after sending a connection request R 1  (SIP INVITE). If a response is not received in the time t 1 , the sever  12  re-sends the request R 1  and starts the timer A to count a period of time  2 ×t1. In the SIP specification, it recommends to use exponential retransmission mechanism (i.e., t1, 2×t1, 4×t1, 8×t1, 16×t1, 32×t1, etc.) to reduce the network load, until another timer B (Timer B) reaches a predetermined time.  
         [0007]     The SIP specification recommends setting the timer B to a period of time  64 ×t1 as the predetermined time. However, such a design can cause some problems in the wireless network environment. As shown in  FIG. 2 , to reduce the power consumption, the wireless terminal  15  may enter a power saving mode (PSM). In the power saving mode, the wireless terminal  15  can listen the beacon sent by the base station  11  every t wakeup  interval, where the t wakeup  interval is referred to as a listen interval. When it is found that a packet temporarily stored in the base station  11  is indicated by the beacon of the base station  11 , the wireless terminal  15  switches to a normal operating mode in order to download data from the base station  11 . In the design of WLAN, the wireless terminal  15  notifies the base station  11  of its sleeping time when determining to enter the power saving mode. Accordingly, the base station  11  starts to temporarily store all packets for the wireless terminal  15  without discarding a packet. However, with limited memory space, the base station  11  can provide limited storage space for each wireless terminal  15  to temporarily store the packets. In this case, if the sleeping time of the wireless terminal  15  exceeds the time provided to temporarily store the packets by the base station  11  and also the network significantly congests, a packet lost may occur. As shown in  FIG. 2 , it is assumed that the base station  11  can temporarily store the packets for t q  interval and notify the wireless terminal  15  every t b interval of whether or not the packets are temporarily stored. Since the t wakeup  interval is much greater than the t q  interval, the SIP INVITE request (R 1 ) may be discarded by the base station  11  after it is sent to the base station  11  for a span. Thus, the wireless terminal  15  cannot receive the request R 1  and make the server  12  start a re-transmission mechanism. In this case, a t callest  interval is taken to complete a physical communication establishment, which heavily affects the time required for the communication establishment. In addition, upon the design of inappropriate parameters, the wireless terminal  15  may not receive any call message within 64×t1, thus the communication establishment will fail.  
         [0008]     European patent EP1033832 issued to Lucent technologies INC. for a “Wireless data communication system having power saving function” has disclosed a TIM (Traffic Indicator Message) to carry an information to a wireless terminal in order to indicate that a message is temporarily stored in a base station or not. Further, according to the detail of the information, the wireless terminal can determine when to enter a power saving mode. Such a way can effectively overcome the problem of power consumption on the wireless terminal. However, it is suitable for an environment of implementing numerous wireless network access point/base stations due to the custom-designed information fields. Furthermore, such a way cannot effectively overcome the seriously prolonged call setup time caused by the prolonged sleeping time in view of saving the power. Therefore, it is desirable to provide an improved system and method to mitigate and/or obviate the aforementioned problems.  
       SUMMARY OF THE INVENTION  
       [0009]     The object of the invention is to provide a system and method for reducing call establishment delay in a wireless network, which can effectively reduce the call establishment delay in the wireless network.  
         [0010]     According to a first feature of the invention, a method for reducing call establishment delay in a wireless network is provided, which is used in a network node to call a wireless terminal covered by a wireless network access point/base station through a server. The server sends a connection request to the wireless terminal through buffering of the wireless network access point/base station to thus establish a connection. The method includes: a listen interval notifying step, which uses the wireless terminal to notify the server of a listen interval; a listen time determining step, which uses the server to obtain a listen time according to the listen interval, so that the server is aware of when the wireless terminal subsequently wakes up and listens to the wireless network access point/base station; and a connection request sending step, which uses the server to dynamically determine a time to send a connection request to the wireless terminal when the network node calls the wireless terminal. The invention also discloses a system to implement this method.  
         [0011]     According to a second feature of the invention, a method for reducing call establishment delay in a wireless network is provided, which is used in a network node to call a wireless terminal covered by a wireless network access point/base station through a server. The method includes: a call establishing step, which uses the server to send a connection request to the wireless terminal through buffering of the wireless network access point/base station when the network node calls the wireless terminal to thus establish a connection, wherein the server is capable of locating the wireless terminal in a wireless network environment; and a connection request sending step, which uses the server to dynamically determine a time to send a connection request to the wireless terminal when the wireless terminal in the wireless network environment is located by the server. The invention also discloses a system to implement this method.  
         [0012]     According to a third feature of the invention, a method for reducing call establishment delay in a wireless network is provided, which is used in a network node to call a wireless terminal covered by a wireless network access point/base station through a server. The method includes: a wireless terminal inquiring step, which uses the wireless terminal to send an inquiry request to the server for inquiring retransmission information; a server responding step, which uses the server to send an inquiry response containing an inquiry result to the wireless terminal such that the wireless terminal determines a listen time according to the inquiry result for waking up to receive data; and a connection request sending step, which uses the server to dynamically determine a time to send a connection request to the wireless terminal. The invention also discloses a system to implement this method.  
         [0013]     According to a fourth feature of the invention, a method for reducing call establishment delay in a wireless network is provided, which is used in a network node to call a wireless terminal covered by a wireless network access point/base station through a server. The method includes: a call establishing step, which uses the server to send a connection request to the wireless terminal through buffering of the wireless network access point/base station when the network node calls the wireless terminal, so as to thus establish a connection; a packet discarding and recording step, which uses the wireless network access point/base station to discard and record the packet when the wireless network access point/base station is not able to buffer a packet to be sent to the wireless terminal in a power-saving mode; and a wireless terminal listening and receiving step, which keeps the wireless terminal awake for a span in order to receive the packet when the wireless terminal wakes up from the power-saving mode and listens to the wireless network access point/base station to thus obtain a record associated with the packet discarded and recorded,. The invention also discloses a system to implement this method.  
         [0014]     According to a fifth feature of the invention, a method for reducing call establishment delay in a wireless network is provided, which is used in a network node to call a wireless terminal covered by a wireless network access point/base station through a server. The method includes: a packet buffering step, which uses the wireless network access point/base station capable of recognizing a packet used for call establishment to maintain the packet in buffering without discarding. The invention also discloses a system to implement this method.  
         [0015]     Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a configuration diagram of a typical wireless network communication system;  
         [0017]      FIG. 2  is a flowchart of a typical connection establishment of wireless network phones using the session initiation protocol (SIP);  
         [0018]      FIG. 3  is a flowchart of a system connection establishment according to a first embodiment of the invention;  
         [0019]      FIG. 4  is a flowchart of a system connection establishment according to a second embodiment of the invention;  
         [0020]      FIG. 5  is a flowchart of a system connection establishment according to a third embodiment of the invention;  
         [0021]      FIG. 6  is a flowchart of a system connection establishment according to a fourth embodiment of the invention;  
         [0022]      FIG. 7  is a flowchart of a system connection establishment according to a fifth embodiment of the invention; and  
         [0023]     FIGS.  8 (A) and (B) show graphs of required time respectively for a typical SIP calling and an invention calling. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]     To illustrate the system and method for reducing call establishment delay in a wireless network in accordance with the present invention, the session initiation protocol (SIP), the wireless local area network (WLAN), and the voice over Internet protocol (VoIP) are given as an example to describe signaling protocol design, wireless network, and communication service. However, a similar VoIP signaling protocol such as H.323, another wireless network communication protocol such as WiMAX, and other communication services such as video over IP can be used. In addition, a server used in the invention indicates a network node with a server function, such as a typical SIP or H.323 server, or a node capable of servicing other nodes or devices in a peer-to-peer network.  
         [0025]      FIG. 3  is a flowchart of a system connection establishment according to a first embodiment of the invention. As shown in  FIG. 3 , a network node  13  calls a wireless terminal  15  covered by a wireless network access point/base station  11  through a server  12 . The server  12  receives a connection request R 1  sent by the network node  13  and sends it to the wireless terminal  15  to thus establish a connection. The connection request R 1  (SIP INVITE) is buffered by the wireless network access point/base station  11  before it is sent to the wireless terminal  15 . In order to reduce the call establishment delay, the wireless terminal  15  in this embodiment requires first informing the server  12  of its listen interval t wakeup . Accordingly, the server  12  can be aware of a listen time t listen  for the wireless terminal  15  to listen to a beacon of the base station  11  (step S 31 ). The wireless terminal  15  in the power-saving mode can wake up every listen interval for listening to the beacon sent by the base station  11  (step S 32 ) and switches to a normal operating mode to receive a packet buffered by the base station  11 .  
         [0026]     When the network node  13  sends the connection request R 1  for establishing the connection with the wireless terminal  15  (step S 33 ), the server  12  forwards the connection request R 1  to the wireless terminal  15  (step S 34 ). If no response is received in a predetermined interval, the server  12  temporarily stores the connection request R 1  for a span of t proxyqueue  based on the listen time t listen , which is next time for the wireless terminal  15  to listen to a beacon of the base station  11 . When the listen time t listen  is about running out, the request R 1  is transferred to the wireless terminal  15  (step S 35 ). In this embodiment, it is considered that certain network delivery time between the wireless terminal  15  and the base station  11  is required, and a span for the base station  11  to temporarily store a packet of the wireless terminal  15  is also required. Therefore, for a reasonable duration (Tx Window), the connection request R 1  sent by the server  12  can be received by the wireless terminal  15 . In this case, the server  12  only requires sending the SIP INVITE signal (R 1 ) within Tx Window before the time t listen , and the wireless terminal  15  can receive the request R 1 . Therefore, the listen time for the wireless terminal  15  to wake up and listen in can be computed appropriately through synchronization of the server  12  and the wireless terminal  15 , so that unnecessary connection requests can be significantly reduced to thus avoid connection delay from connection request retransmission.  
         [0027]      FIG. 4  is a flowchart of a system connection establishment according to a second embodiment of the invention. As shown in  FIG. 4 , the network node  13  calls the wireless terminal  15  covered by a wireless network access point/base station  11  through the server  12 . The server  12  receives a connection request R 1  sent by the network node  13  and sends it to the wireless terminal  15  to thus establish a connection. The connection request R 1  (SIP INVITE) is sent to the wireless terminal  15  through buffering of the wireless network access point/base station  11  in order to establish the connection. In order to reduce the call establishment delay, the server  12  in this embodiment must aware that the wireless terminal  15  is located in a wireless network environment, and this can be achieved by assigning a dedicated name to the wireless terminal  15  or sending the message from the wireless terminal  15  to the server  12  (step S 41 ) in order to close the exponential retransmission way used for calling the wireless terminal  15 . Accordingly, when the network node  13  calls the wireless terminal  15  (step S 42 ), the server  12  can transfer the connection request R 1  to the wireless terminal  15 . If no response is received in a predetermined interval, the server  12  periodically re-sends the connection request R 1  to the wireless terminal  15  within the time t 1  (step S 44 ).  
         [0028]     The wireless terminal  15  in the power-saving mode can wake up every listen interval t wakeup  for listening to a beacon sent by the base station  11  (step S 45 ) and switches to a normal operating mode to receive a packet buffered by the base station  11 . The time t 1  is smaller than listen interval t wakeup , and this can assure that the wireless terminal  15  after waked up can receive the connection request without missing. Thus, the connection delay is improved. This embodiment can allow the server  12  to have different retransmission time in a wired network device and a wireless terminal to further improve the connection delay.  
         [0029]      FIG. 5  is a flowchart of a system connection establishment according to a third embodiment of the invention. As shown in  FIG. 5 , the network node  13  calls the wireless terminal  15  covered by a wireless network access point/base station  11  through the server  12 . The server  12  receives a connection request R 1  sent by the network node  13  and sends it to the wireless terminal  15  to thus establish a connection. The connection request R 1  (SIP INVITE) is sent to the wireless terminal  15  through buffering of the wireless network access point/base station  11  in order to establish the connection. If the connection request R 1  cannot be buffered by the base station  11 , its corresponding packet is discarded.  
         [0030]     When the network  13  sends the connection request R 1  to establish the connection with the wireless terminal  15 , the server  12  re-sends the connection request R 1  using exponential retransmission mechanism (step S 57 ) until the timer B (Timer B) times out.  
         [0031]     The wireless terminal  15  in the power-saving mode can wake up every listen interval t wakeup  for listening to a beacon sent by the base station  11  (step S 53 ) and switches to a normal operating mode to receive a packet buffered by the base station  11 . In order to reduce the call establishment delay, the wireless terminal  15  in this embodiment sends an inquiry request in a predetermined time t inquiry  (e.g., 64×t1) to the server  12  in order to inquire whether or not a connection request is waiting to be sent to the wireless terminal  15  (step S 54 ). In addition, the wireless terminal  15  keeps awake in order to receive an inquiry response. After the inquiry request is received, the server  12  sends the inquiry response back to the wireless terminal  15  to notify an inquiry result (step S 55 ). If the inquiry result shows that the connection request R 1  to be sent to the wireless terminal  15  exists, the server  12  sends the connection request R 1  to the wireless terminal  15  (step S 56 ) to thus reduce the initial time delay.  
         [0032]      FIG. 6  is a flowchart of a system connection establishment according to a fourth embodiment of the invention. As shown in  FIG. 6 , the network node  13  calls the wireless terminal  15  covered by a wireless network access point/base station  11  through the server  12 . The server  12  receives a connection request R 1  sent by the network node  13  and sends it to the wireless terminal  15  to thus establish a connection. The connection request R 1  (SIP INVITE) is sent to the wireless terminal  15  through buffering of the wireless network access point/base station  11  in order to establish the connection. If the connection request R 1  cannot be buffered by the base station  11 , its corresponding packet is discarded.  
         [0033]     When the network  13  sends the connection request R 1  to establish the connection with the wireless terminal  15  (step S 61 ), the server  12  re-sends the connection request R 1  using exponential retransmission mechanism (step S 68 ). The wireless terminal  15  in the power-saving mode can wake up every listen interval t wakeup  (step S 63 ) for listening to a beacon sent by the base station  11  (step S 45 ) and switches to a normal operating mode to receive a packet buffered by the base station  11 . In order to reduce the call establishment delay, this embodiment discards a packet to be sent to the wireless terminal  15  in sleeping when the base station  11  cannot buffer the packet, and keeps this in a record (step S 64 ). As such, the wireless terminal  15  can be notified to receive the packet by a subsequent beacon (step S 65 ). After being waked up and receiving this beacon, the wireless terminal  15  enters in the power saving mode for a span in order to receive its packet (step S 66 ). Thus, a next connection request R 1  sent by the server  12  can be received correctly (step S 67 ). This embodiment can reduce the initial time delay through the base station  11  operated with the wireless terminal  15 , without modifying the server  12 .  
         [0034]      FIG. 7  is a flowchart of a system connection establishment according to a fifth embodiment of the invention. As shown in  FIG. 7 , the network node  13  calls the wireless terminal  15  covered by a wireless network access point/base station  11  through the server  12 . The server  12  receives a connection request R 1  sent by the network node  13  and sends it to the wireless terminal  15  to thus establish a connection. The connection request R 1  (SIP INVITE) is sent to the wireless terminal  15  through buffering of the wireless network access point/base station  11  in order to establish the connection.  
         [0035]     When the network  13  sends the connection request R 1  to establish the connection with the wireless terminal  15  (step S 71 ), the server  12  re-sends the connection request R 1  using exponential retransmission mechanism (step S 72 ). The wireless terminal  15  in the power-saving mode can wake up every listen interval t wakeup  for listening to a beacon sent by the base station  11  (step S 73 ) and switches to a normal operating mode to receive a packet buffered by the base station  11 . In order to reduce the call establishment delay, the base station  11  in this embodiment can identify the connection request R 1  associated with the call establishment and buffer a packet corresponding to the connection request R 1  for the wireless terminal  15  in the power saving mode, other than discarding the packet (step S 74 ). Thus, the wireless terminal  15  after waked up can receive the connection request R 1  buffered in the base station  11  (step S 75 ), thereby reducing the delay of call establishment.  
         [0036]     For verifying the performance, the operation of a typical SIP in a wireless network is analyzed.  FIG. 8 (A) illustrates the simulation results and the relationship between the listen interval and the call establishment latency under different AP buffer sizes, where the x-coordinate indicates the listen interval for the wireless terminal and the y-coordinate indicates the averaged call establishment latency. As shown in  FIG. 8 (A), it is found that the delay of communication establishment initiation (i.e., the averaged call establishment latency) can be as high as 15-25 seconds when the base station  11  can only buffer 100 ms packets for the the wireless terminal. Such a communication establishment delay cannot be accepted by a real-time VoIP call, and even for 500 ms, it has the latency of 5 to 10 seconds.  FIG. 8 (B) shows an analysis graph resulting from the operations of the five embodiments according to the invention. As shown in  FIG. 8 (B), it is found that all embodiments can relatively reduce the call establishment time to 0.1 to 3 seconds. Such a call establishment time can be accepted by a typical VoIP.  
         [0037]     Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.