1. A method for performing a handover by a Mobile Subscriber Station (MSS) in a Broadband Wireless Access (BWA) communication system including a serving Base Station (BS) for providing a service to the MSS and a target BS adjacent to the serving BS, the method comprising the steps of: receiving a handover request message from the serving BS, the handover request message including a resource remain type field representing whether connection information having been set between the serving BS and the MSS is retained; and recognizing whether the serving BS retains the connection information according to a value of the resource remain type field.

2. The method as claimed in claim 1, further comprising the steps of: transmitting a handover indication message to the serving BS, the handover indication message including information reporting performance of the handover; and releasing connection with the serving BS upon transmitting the handover indication message recognizing that retaining of the connection information starts from a point in time at which the handover indication message is received by the serving BS.

3. The method as claimed in claim 1, wherein the handover request message notifies resource retain time information when the resource remain type field represents retaining of the connection information.

4. The method as claimed in claim 3, wherein the resource retain time information includes a resource retain time field representing a time duration value in the handover request message.

5. The method as claimed in claim 3, wherein the resource retain time information includes a system retaining time representing a pre-negotiated time when a registration is performed between the serving BS and the MSS.

6. The method as claimed in claim 1, further comprising the steps of: determining an reentry to the serving BS during a handover to the target BS; and reopening communication with the serving BS using the connection information, when the serving BS retains the connection information.

7. A method for performing a handover by a Mobile Subscriber Station (MSS) in a Broadband Wireless Access (BWA) communication system including a serving Base Station (BS) for providing a service to the MSS and a target BS adjacent to the serving BS, the method comprising the steps of: requesting the handover to the serving BS; receiving a handover response message from the serving BS, the handover response message including a resource remain type field representing whether connection information having been set between the serving BS and the MSS is retained; and recognizing whether the serving BS retains the connection information according to a value of the resource remain type field.

8. The method as claimed in claim 7, further comprising the steps of: transmitting a handover indication message to the serving BS, the handover indication message including information reporting performance of the handover; and releasing connection with the serving BS upon transmitting the handover indication message recognizing that retaining of the connection information starts from a point in time at which the handover indication message is received from the serving BS.

9. The method as claimed in claim 7, wherein the handover response message notifies resource retain time information when the resource remain type field represents retaining of the connection information

10. The method as claimed in claim 9, wherein the resource retain time information includes a resource retain time field representing a time duration value in the handover response message

11. The method as claimed in claim 9, wherein the resource retain time information includes a system retaining time representing a pre-negotiated time when a registration is performed between the serving BS and the MSS

12. The method as claimed in claim 7, further comprising the steps of: determining an reentry to the serving BS during a handover to the target BS; and reopening communication with the serving BS using the connection information, when the serving BS retains the connection information

13. A method for performing a handover by a serving Base Station (BS) for providing a service to a Mobile Subscriber Station (MSS) in a Broadband', 'Wireless Access (BWA) communication system including a target BS adjacent to the serving BS, the method comprising the steps of: transmitting a handover request message to the MSS, the handover request message including a resource remain type field representing whether connection information having been set between the serving BS and the MSS is retained; receiving a handover indication message from the MSS; and retaining the connection information according to a value of the resource remain type field

14. The method as claimed in claim 13, wherein retaining the connection information starts from a point in time at which the handover indication message is received form the MSS, when the value of the resource remain type field represents retaining of the connection information

15. The method as claimed in claim 14, wherein the handover indication message includes a handover indication type represents releasing the serving BS, that is HOJND Jype=00

16. The method as claimed in claim 13, further comprising a step of determining whether the connection information is retained according to the received handover indication message

17. The method as claimed in claim 13, wherein the handover request message notifies resource retain time information when the resource remain type field represents retaining of the connection information

18. The method as claimed in claim 17, wherein the resource retain time information includes a resource retain time field representing a time duration value in the handover request message

19. The method as claimed in claim 17, wherein the resource retain time information includes a system retaining time representing a pre-negotiated time when a registration is performed between the serving BS and the MSS.

20. The method as claimed in claim 17, further comprising a step of reopening communication with the MSS using the connection information, when the serving BS retains the connection information with the MSS in a reentry of the MSS.

21. The method as claimed in claim 17, wherein the connection information with the MSS is deleted when the value of the resource remain type field does not represent retaining of the connection information.

22. A method for performing a handover by a serving Base Station (BS) for providing a service to a Mobile Subscriber Station (MSS) in a Broadband Wireless Access (BWA) communication system including a target BS adjacent to the serving BS, the method comprising the steps of: receiving a handover request message from the MSS; transmitting a handover response message to the MSS, the handover response message including a resource remain type field representing whether connection information having been set with the MSS is retained; receiving a handover indication message from the MSS; and retaining the connection information according to a value of the resource remain type field.

23. The method as claimed in claim 22, wherein retaining the connection information starts from a point in time at which the handover indication message is received form the MSS, when the value of the resource remain type field represents retaining of the connection information.

24. The method as claimed in claim 23, wherein the handover indication message includes a handover indication type represents releasing the serving BS, that is HO JND Jype=00.

25. The method as claimed in claim 23, further comprising a step of determining retaining of the connection information when the received handover indication message includes information reporting performance of a handover by the MSS.

26. The method as claimed in claim 23, wherein the handover response message notifies resource retain time information when the resource remain type field represents retaining of the connection information.

27. The method as claimed in claim 26, wherein the resource retain time information includes a resource retain time field representing a time duration value in the handover request message.

28. The method as claimed in claim 26, wherein the resource retain time information includes a system retaining time representing a pre-negotiated time when a registration is performed between the serving BS and the MSS.

29. The method as claimed in claim 24, further comprising a step of reopening communication with the MSS using the connection information, when the serving BS retains the connection information with the MSS in a reentry of the MSS.

30. A handover method for minimizing a service delay due to a pingpong effect in a Broadband Wireless Access (BWA) communication system including a Mobile Subscriber Station (MSS), a serving Base Station (BS) communicating with the MSS, and a plurality of neighbor BSs, the method comprising the steps of: a) requesting to the serving BS a handover when it is detected that handover must be performed from the serving BS to a final target BS which is one of the neighbor BSs; b) detecting information for at least two candidate target BSs, on of which is selected as the final target BS, according to the handover request, the information being received from the serving BS; c) measuring CINR (Carrier to Interference and Noise Ratio) values transmitted from each of the candidate target BSs; d) comparing the measured CINR values with a first predetermined threshold value and excluding a first candidate target BS, which transmits a reference signal having a CINR value less than the first threshold value, from the candidate target BSs when CINR values less than the first threshold value exist; and e) determining the final target BS in the candidate target BSs excluding the first candidate target BS and notifying the serving BS of a handover to the final target BS.

31. The method as claimed in claim 30, wherein step a) comprises a step of detecting that the handover to the final target BS must be performed when a CINR value of the reference signal transmitted from the serving BS is less than a second threshold value, and the first threshold value is set to have a value exceeding the second threshold value.

32. The method as claimed in claim 30, wherein step e) comprises a step of determining a candidate target BS, which provides a service level or a bandwidth required for the communication from among the candidate target BSs excluding the first candidate target BS, as the final target BS.

33. The method as claimed in claim 30, further comprising a step of notifying the serving BS that a handover is not performed when the measured CINRs are less than the first threshold value.

34. The method as claimed in claim 30, further comprising the steps of: changing connection from the serving BS to the final target BS after notifying the serving BS of the handover to the final target BS; performing a network reentry procedure with the final target BS after changing the connection from the serving BS to the final target BS; and changing the connection from the final target BS to the serving BS when a CINR value of a reference signal transmitted from the final target BS is less than the first threshold value while the network reentry procedure is performed.

35. A handover method for minimizing a service delay due to a pingpong effect in a Broadband Wireless Access (BWA) communication system including a Mobile Subscriber Station (MSS), a serving Base Station (BS) communicating with the MSS, and a plurality of neighbor BSs, the method comprising the steps of: a) receiving a handover request including information for at least two candidate target BSs from the serving BS, one of the candidate target BSs being selected as a final target BS by the serving BS, the final target BS being one of the neighbor BSs, and detecting the information; b) measuring CINR (Carrier to Interference and Noise Ratio) values transmitted from each of the candidate target BSs; c) comparing the measured CINR values with a first predetermined threshold value and excluding a first candidate target BS, which transmits a reference signal having the CINR value less than the first threshold value, from the candidate target BSs when CINR values less than the first threshold value exist; and d) determining the final target BS in the candidate target BSs excluding the first candidate target BS and notifying the serving BS of handover to the final target BS.

36. The method as claimed in claim 35, wherein step d) comprises a step of determining a candidate target BS, which provides one of a service level and a bandwidth required for the communication from among the candidate target BSs excluding the first candidate target BS, as the final target BS.

37. The method as claimed in claim 35, further comprising a step of notifying the serving BS that a handover is not performed when the measured CINRs are less than the first threshold value.

38. The method as claimed in claim 35, further comprising the steps of: changing a connection from the serving BS to the final target BS after notifying the serving BS of the handover to the final target BS; performing a network reentry procedure with the final target BS after changing the connection from the serving BS to the final target BS; and changing the connection from the final target BS to the serving BS when a CINR value of a reference signal transmitted from the final target BS is less than the first threshold value while the network reentry procedure is performed.

39. A handover method for minimizing a service delay due to a pingpong effect in a Broadband Wireless Access (BWA) communication system including a Mobile Subscriber Station (MSS), a serving Base Station (BS) communicating with the MSS, and a plurality of neighbor BSs, the method comprising the steps of: a) one of deleting connection information of the MSS and retaining the connection information during a preset time, when the serving BS receives a notification representing that the MSS is to be handed over to a target BS which is a specific neighbor BS of the neighbor BSs; b) changing a connection to the serving BS when the MSS recognizes an occurrence of a pingpong effect during a network reentry procedure with the target BS; c) receiving one of a handover request message and a handover response message from the serving BS; d) detecting and recognizing if the connection information for the MSS is retained, which is contained in each message, according to reception of each message; and e) performing only an initial ranging procedure and reopening communication with the serving BS when the serving BS retains the connection information for the MSS.

40. The method as claimed in claim 39, wherein step e) comprises the steps of: synchronizing with the serving BS; transmitting a ranging request message, which corresponds to the initial ranging procedure including a Basic Connection Identifier (CID) used to communicate with the serving BS and, and receiving a ranging response message as a response for the ranging request message; and reopening the communication by means of the connection information of the MSS retained by the serving BS.

41. The method as claimed in claim 39, further comprising a step of reopening the communication with the serving BS by performing an existing network reentry procedure when the serving BS has deleted the connection information of the MSS.

42. The method as claimed in claim 39, further comprising a step of detecting information for preset connection information-retaining time when the MSS recognizes that the serving BS retains the connection information for the MSS.

43. The method as claimed in claim 42, wherein the serving BS retains the connection information for the MSS according to the information for the connection information-retaining time.

44. A handover system for minimizing a service delay due to a pingpong effect in a Broadband Wireless Access (BWA) communication system including an MSS (Mobile Subscriber Station), a serving BS (Base Station) for communicating with the MSS, and a plurality of neighbor BSs, the system comprising: a MSS for requesting handover to a serving BS when it is detected that handover must be performed from the serving BS to a final target BS which is one of the neighbor BSs, detecting information for at least two candidate target BSs, which are selected as the final target BS, according to the handover request, the information being received from the serving BS, measuring CINR (Carrier to Interference and Noise Ratio) values transmitted from each of the candidate target BSs, comparing the measured CINR values with a first predetermined threshold value and excluding a first candidate target BS, which transmits a reference signal having a CINR value less than the first threshold value, from the candidate target BSs when CINR values less than the first threshold value exist, and determining the final target BS in the candidate target BSs excluding the first candidate target BS and notifying the serving BS of handover to the final target BS; and a serving BS for receiving the request of the MSS, transmitting the information for at least two candidate target BSs, which are selected as the final target BS by the MSS, and receiving a notification representing that the MSS is to be handed over to the final target BS from the MSS.

45. The system as claimed in claim 44, wherein, in detecting that the MSS must be handed over to the final target BS, the handover to the final target BS is detected when a CINR value of the reference signal transmitted from the serving BS is less than a second threshold value, and the first threshold value is set to have a value exceeding a second threshold value.

46. The system as claimed in claim 44, wherein, in determining the final target BS by the MSS, a candidate target BS, which provides a service level or a bandwidth required for the communication from among the candidate target BSs excluding the first candidate target BS, is determined as the final target BS.

47. The system as claimed in claim 44, wherein a notification representing that the handover is not performed is transmitted to the serving BS when the measured CINRs are less than the first threshold value.

48. A handover system for minimizing a service delay due to a pingpong effect in a Broadband Wireless Access (BWA) communication system including an MSS (Mobile Subscriber Station), a serving BS (Base Station) for communicating with the MSS, and a plurality of neighbor BSs, the system comprising: a MSS for receiving a handover request including information for at least two candidate target BSs from a serving BS, the candidate target BSs being selected as a final target BS by the serving BS, the final target BS being one of the neighbor BSs, and detecting the information, measuring CINR (Carrier to Interference and Noise Ratio) values transmitted from each of the candidate target BSs, comparing the measured CINR values with a first predetermined threshold value and excluding a first candidate target BS, which transmits a reference signal having the CINR value less than the first threshold value, from the candidate target BSs when CINR values less than the first threshold value exist, and determining the final target BS in the candidate target BSs excluding the first candidate target BS and notifying the serving BS of handover to the final target BS; and a serving BS for requesting the handover of the MSS and receiving information for the final target BS from the MSS.

49. A system for performing a handover in a Broadband Wireless Access (BWA) communication system including a Mobile Subscriber Station (MSS), a serving Base Station (BS) for providing a service to the MSS, and a target BS adjacent to the serving BS, the system comprising: a MSS for requesting the handover to the serving BS, receiving a handover response message from the serving BS, the handover response message including a resource remain type field representing whether preset connection information is retained, and recognizing whether the serving BS retains the connection information according to a value of the resource remain type field; and a serving BS for receiving a handover request message from the MSS, transmitting the handover response message to the MSS, the handover response message including the resource remain type field representing whether the connection information having been set with the MSS is retained, receiving a handover indication message from the MSS, and retaining the connection information according to the value of the resource remain type field.

50. A system for performing a handover in a Broadband Wireless Access', '(BWA) communication system including a Mobile Subscriber Station (MSS), a serving Base Station (BS) for providing a service to the MSS, and a target BS adjacent to the serving BS, the system comprising: a MSS for receiving a handover request message from the serving BS, the handover request message including a resource remain type field representing whether connection information having been set with the serving BS is retained, and recognizing whether the serving BS retains the connection information according to a value of the resource remain type field; and a serving BS for transmitting the handover request message to the MSS, the handover request message including the resource remain type field representing whether the connection information having been set with the MSS is retained, receiving a handover indication message from the MSS, and retaining the connection information according to the value of the resource remain type field.']