1. In a wireless communication system including a plurality of radio access networks (RANs) deployed under different radio access technologies (RATs) and a wireless transmit/receive unit (WTRU), a method of initiating a handoff between the RANs, the method comprising: a first RAN sending a list to the WTRU, the list including RANs co-located in a coverage area of the first RAN; the WTRU receiving the list from the first RAN and storing the list; the WTRU determining whether a handoff criteria is met by one of the co- located RANs and selecting one of the co-located RANs that meets the handoff criteria; and the WTRU initiating a handoff from the first RAN to the selected RAN.

2. The method of claim 1 further comprising: the WTRU requesting information regarding service area location of the co-located RANs from the first RAN, whereby the WTRU initiates the handoff to the selected RAN if the service area location of the selected RAN is within a predetermined range.

3. The method of claim 2 wherein the WTRU continuously determines whether the service area location of the selected RAN is within the predetermined range as the WTRU moves.

4. The method of claim 1 further comprising: the WTRU measuring a power level of signals received from the co-located RANs and the first RAN, whereby the WTRU initiates the handoff to the selected RAN if a power level of signals received from the selected RAN exceeds a power level of signals received from the first RAN.

5. The method of claim 1 further comprising: the WTRU requesting additional information including at least one of service locations, radio technologies, and frequencies and data rates of the co- located RANs in the list, whereby the WTRU triggers the handoff based on the additional information.

6. The method of claim 1 wherein the first RAN is of an evolved universal mobile telecommunication systems (UMTS) terrestrial radio access network (E-UTRAN) and the co-located RAN is at least one of a UMTS terrestrial radio access network (UTRAN), a GSM/EDGE radio access network (GERAN), a generic access network (GAN) and an IEEE-based inter-working wireless local area network (I-WLAN).

7. The method of claim 1 wherein the first RAN is an IEEE-based inter-working wireless local area network (I-WLAN) and the co-located RAN is one of an evolved universal mobile telecommunication systems (UMTS) terrestrial radio access network (E-UTRAN), a UMTS terrestrial radio access network (UTRAN), a GSM/EDGE radio access network (GERAN) and at least one of a generic access network (GAN).

8. The method of claim 1 wherein the first RAN is a universal mobile telecommunication systems (UMTS) terrestrial radio access network (UTRAN) and the co-located RAN is an IEEE-based inter-working wireless local area network (I-WLAN).

9. The method of claim 1 wherein the first RAN is an IEEE-based inter-working wireless local area network (I-WLAN) and the co-located RAN is a universal mobile telecommunication systems (UMTS) terrestrial radio access network (UTRAN)

10. A wireless communication system comprising: a plurality of radio access networks (RANs) deployed under different radio access technologies (RATs), a first one of the RANs configured to send a list of RANs co-located in a coverage area of the first RAN; and a wireless transmit/receive unit (WTRU) configured to: i) receive the list from the first RAN, ii) determine whether a handoff criteria is met by one of the co-located RANs, iii) select one of the co-located RANs that meet the handoff criteria, and iv) initiate a handoff from the first RAN to the selected RAN

11. The system of claim 10 wherein the WTRU is configured to request information regarding service area location of the co-located RANs from the first RAN, monitor the service area location of the WTRU and the selected RAN, and initiate the handoff to the selected RAN if the service area location of the selected RAN is within a predetermined range

12. The system of claim 10 wherein the WTRU is configured to measure a power level of signals received from the co-located RANs and the first RAN, whereby the WTRU initiates the handoff to the selected RAN based on the measured power level

13. The system of claim 10 wherein the WTRU is configured to request additional information including at least one of service area locations, radio technologies, and frequencies and data rates of the co-located RANs, whereby the WTRU triggers the handoff based on the additional information

14. The system of claim 10 wherein the first RAN is an evolved universal mobile telecommunication systems (UMTS) terrestrial radio access network (E-UTRAN) and the co-located RAN is at least one of a UMTS terrestrial radio access network (UTRAN), a GSM/EDGE radio access network (GERAN), a generic access network (GAN) and an IEEE-based inter-working wireless local area network (I-WLAN)

15. The system of claim 10 wherein the first RAN is an IEEE-based inter-working wireless local area network (I-WLAN) and the co-located RAN is at least one of an evolved universal mobile telecommunication systems (UMTS) terrestrial radio access network (E-UTRAN), a UMTS terrestrial radio access network (UTRAN), a GSM/EDGE radio access network (GERAN) and a generic access network (GAN)

16. The system of claim 10 wherein the first RAN is a universal mobile telecommunication systems (UMTS) terrestrial radio access network (UTRAN) and the co-located RAN is an IEEE-based inter-working wireless local area network (I-WLAN)

17. The system of claim 10 wherein the first RAN is an IEEE-based inter-working wireless local area network (I-WLAN) and the co-located RAN is a universal mobile telecommunication systems (UMTS) terrestrial radio access network (UTRAN)

18. In a wireless communication system including a plurality of radio access networks (RANs) deployed under different radio access technologies (RATs) and at least one wireless transmit/receive unit (WTRU), a method of initiating a handoff between the RANs, the method comprising: a first RAN sending a list to the WTRU, the list including co-located RANs in a coverage area of the first RAN; the WTRU receiving the list from the first RAN and storing the list; the WTRU measuring quality of signals received from the co-located RANs; the WTRU sending a signal quality measurement report to the first RAN; the first RAN determining whether a handoff criteria is met by the co- located RANs and selecting one of the co-located RANs that meet the handoff criteria; and the first RAN initiating a handoff from the first RAN to the selected RAN

19. The method of claim 18 further comprising: the WTRU requesting additional information including at least one of service locations, radio technologies and frequencies and data rates of the co- located RANs to the first RAN, whereby the WTRU uses the additional information in measuring the signal quality.

20. The method of claim 18 wherein the first RAN is an evolved universal mobile telecommunication systems (UMTS) terrestrial radio access network (E-UTRAN) and the co-located RAN is at least one of a UMTS terrestrial radio access network (UTRAN), a GSM/EDGE radio access network (GERAN), a generic access network (GAN) and an IEEE-based inter-working wireless local area network (I-WLAN).

21. The method of claim 18 wherein the first RAN is an IEEE-based inter-working wireless local area network (I-WLAN) and the co-located RAN is at least one of an evolved universal mobile telecommunication systems (UMTS) terrestrial radio access network (E-UTRAN), a UMTS terrestrial radio access network (UTRAN), a GSM/EDGE radio access network (GERAN) and a generic access network (GAN).

22. The method of claim 18 wherein the first RAN is a universal mobile telecommunication systems (UMTS) terrestrial radio access network (UTRAN) and the co-located RAN is an IEEE-based inter-working wireless local area network (I-WLAN).

23. The method of claim 18 wherein the first RAN is an IEEE-based inter-working wireless local area network (I-WLAN) and the co-located RAN is a universal mobile telecommunication systems (UMTS) terrestrial radio access network (UTRAN).

24. A wireless communication system comprising: a plurality of radio access networks (RANs) deployed under different radio access technologies (RATs), a first one of the RANs configured to send a list of RANs co-located in a coverage area of the first RAN and initiate a handoff to a selected RAN if handoff criteria is met by the selected RAN based on a measurement report received by the first RAN; and a wireless transmit/receive unit (WTRU) configured to measure quality of signals received from the co-located RANs and send a signal quality measurement report to the first RAN.

25. The system of claim 24 wherein the WTRU is configured to request additional information including at least one of service locations, radio technologies and frequencies and data rates of the co-located RANs to the first RAN, whereby the WTRU uses the additional information in measuring the signal quality.

26. The system of claim 24 wherein the first RAN is an evolved universal mobile telecommunication systems (UMTS) terrestrial radio access network (E-UTRAN) and the co-located RAN is at least one of a UMTS terrestrial radio access network (UTRAN), a GSM/EDGE radio access network (GERAN), a generic access network (GAN) and an IEEE-based inter-working wireless local area network (I-WLAN).

27. The system of claim 24 wherein the first RAN is an IEEE-based inter-working wireless local area network (I-WLAN) and the co-located RAN is one of an evolved universal mobile telecommunication systems (UMTS) terrestrial radio access network (E-UTRAN), a UMTS terrestrial radio access network (UTRAN), a GSM/EDGE radio access network (GERAN) and a generic access network (GAN).

28. The system of claim 24 wherein the first RAN is a universal mobile telecommunication systems (UMTS) terrestrial radio access network (UTRAN) and the co-located RAN is an IEEE-based inter-working wireless local area network (I-WLAN).

29. The system of claim 24 wherein the first RAN is an IEEE-based inter-working wireless local area network (I-WLAN) and the co-located RAN is a universal mobile telecommunication systems (UMTS) terrestrial radio access network (UTRAN).

30. In a wireless communication system including an evolved universal mobile telecommunication system (UMTS) terrestrial radio access network (E- UTRAN) and a wireless transmit/receive unit (WTRU), a method for initiating a handoff from the E-UTRAN to another radio access network (RAN), the method comprising: the E-UTRAN sending a list to the WTRU, the list including RANs co- located in a coverage area of the E-UTRAN; the WTRU receiving the list from the E-UTRAN and storing the list; the WTRU determining whether a handoff criteria is met by one of the co- located RANs and selecting one of the co-located RANs that meets the handoff criteria; and the WTRU initiating a handoff from the E-UTRAN to the selected RAN.

31. The method of claim 30 further comprising: the WTRU requesting information regarding service area location of the co-located RANs from the E-UTRAN, whereby the WTRU initiates the handoff to the selected RAN if the service area location of the selected RAN is within a predetermined range.

32. The method of claim 30 further comprising: the WTRU measuring a power level of signals received from the co-located RANs and the E-UTRAN, whereby the WTRU initiates the handoff to the selected RAN if a power level of signals received from the selected RAN exceeds a power level of signals received from the E-UTRAN.

33. The method of claim 30 further comprising: the WTRU requesting additional information including at least one of service locations, radio technologies, and frequencies and data rates of the co- located RANs in the list, whereby the WTRU triggers the handoff based on the additional information.

34. A wireless communication system comprising: an evolved universal mobile telecommunication system (UMTS) terrestrial radio access notwork (E-UTRAN) configured to send a list of radio access networks (RANs) co-located in a coverage area of the E-UTRAN; at least one co-located RAN deployed under different radio access technology (RAT); and a wireless transmit/receive unit (WTRU) configured to: i) receive the list from the E-UTRAN, ii) determine whether a handoff criteria is met by one of the co-located RANs, iii) select one of the co-located RANs that meet the handoff criteria, and iv) initiate a handoff from the E-UTRAN to the selected RAN.

35. The system of claim 34 wherein the WTRU is configured to request information regarding service area location of the co-located RANs from the E- UTRAN, monitor the service area location of the WTRU and the selected RAN, and initiate the handoff to the selected RAN if the service area location of the selected RAN is within a predetermined range.

36. The system of claim 34 wherein the WTRU is configured to measure a power level of signals received from the co-located RANs and the E-UTRAN, whereby the WTRU initiates the handoff to the selected RAN based on the measured power level.

37. The system of claim 34 wherein the WTRU is configured to request additional information including at least one of service area locations, radio technologies, and frequencies and data rates of the co-located RANs, whereby the WTRU triggers the handoff based on the additional information.

38. In a wireless communication system including an evolved universal mobile telecommunication system (UMTS) terrestrial radio access network (E- UTRAN) and a wireless transmit/receive unit (WTRU), a method for initiating a handoff from a second radio access network (RAN) to the E-UTRAN, the method comprising: the second RAN sending a list to the WTRU, the list including E-UTRAN co-located in a coverage area of the E-UTRAN; the WTRU receiving the list from the second RAN and storing the list; the WTRU determining whether a handoff criteria is met by the E- UTRAN; and the WTRU initiating a handoff from the second RAN to the E-UTRAN if the handoff criteria is met.

39. The method of claim 38 further comprising: the WTRU requesting additional information including at least one of service locations, radio technologies, and frequencies and data rates of the E- UTRAN, whereby the WTRU triggers the handoff based on the additional information.

40. A wireless communication system comprising: an evolved universal mobile telecommunication system (UMTS) terrestrial radio access network (E-UTRAN) at least one alternative radio access network (RAN) co-located in a coverage area of the E-UTRAN, the alternative RAN configured to send a list of E-UTRAN available in a coverage area of the alternative RAN; and a wireless transmit/receive unit (WTRU) configured to: i) receive the list from the alternative RAN, ii) determine whether a handoff criteria is met by the E-UTRAN, and iii) initiate a handoff from the alternative RAN to the E-UTRAN if the handoff criteria is met.

41. The system of claim 40 wherein the WTRU is configured to request additional information including at least one of service area locations, radio technologies, and frequencies and data rates of the E-UTRAN, whereby the WTRU triggers the handoff based on the additional information.

42. In a wireless communication system including an evolved universal mobile telecommunication system (UMTS) terrestrial radio access network (E- UTRAN), an interworking wireless local area network (I-WLAN) and a wireless transmit/receive unit (WTRU), a method for initiating a handoff between the E- UTRAN and the I-WLAN, the method comprising: the WTRU monitoring I-WLAN channels while the WTRU is attached to the E-UTRAN; the WTRU locking onto a detected I-WLAN channel; and the WTRU initiating a handoff procedure if criteria for a handoff to the I-', 'WLAN is met.

43. A wireless communication system comprising: an evolved universal mobile telecommunication system (UMTS) terrestrial radio access network (E-UTRAN); an IEEE-based interworking wireless local area network (I-WLAN); a wireless transmit/receive unit (WTRU) comprising: an E-UTRAN radio unit; and an I-WLAN radio unit; and a handoff controller configured to i) monitor I-WLAN channels while the WTRU is attached to the E-UTRAN, ii) lock onto a detected WLAN channel, and iii) initiate a handoff procedure if criteria for a handoff to the I-WLAN is met.

44. In a wireless communication system including an evolved universal mobile telecommunication system (UMTS) terrestrial radio access network (E- UTRAN), an interworking wireless local area network (I-WLAN) and a wireless transmit/receive unit (WTRU), a method for initiating a handoff between the E- UTRAN and the I-WLAN, the method comprising: the WTRU monitoring E-UTRAN channels while the WTRU is attached to the I-WLAN; the WTRU locking onto a detected E-UTRAN channel; and the WTRU initiating a handoff procedure if criteria for a handoff from the', 'I-WLAN to the E-UTRAN is met.

45. A wireless communication system comprising: an evolved universal mobile telecommunication system (UMTS) terrestrial radio access network (E-UTRAN); an IEEE-based interworking wireless local area network (I-WLAN); a wireless transmit/receive unit (WTRU) comprising: an E-UTRAN radio unit; and an I-WLAN radio unit; and a handoff controller configured to i) monitor E-UTRAN channels while the WTRU is attached to the I-WLAN, ii) lock onto a detected E-UTRAN channel, and iii) initiate a handoff procedure if criteria for a handoff to the E- UTRAN is met.

46. In a wireless communication system including a plurality of radio access networks (RANs) deployed under different radio access technologies (RATs) and a wireless transmit/receive unit (WTRU), a method of initiating a handoff between the RANs, the method comprising: a first RAN sending an inter-access network information to a WTRU pertaining to locally available third generation partnership project (3GPP) and non-3GPP access technologies; the WTRU receiving the inter-access network information and storing the information; and the WTRU initiating a handoff based on the inter-access network information.

47. The method of claim 46 wherein the inter-access network information is provided upon request from the WTRU.

48. The method of claim 46 wherein operator preferences are included in the inter-access network information based on locally available 3GPP and non- 3GPP access technologies.

49. The method of claim 46 wherein the inter-access network information is restricted to access technologies and networks that the WTRU may use based on at least one of a capability the WTRU or a subscription by the WTRU.

50. A wireless communication system including a plurality of radio access networks (RANs) deployed under different radio access technologies (RATs) and a wireless transmit/receive unit (WTRU), the system comprising: a first RAN configured to send an inter-access network information to a WTRU pertaining to locally available third generation partnership project (3GPP) and non-3GPP access technologies; and the WTRU configured to receive and store the inter-access network information ; whereby the WTRU initiates a handoff based on the inter-access network information.

51. The system of claim 50 wherein the inter-access network information is provided upon request from the WTRU.

52. The system of claim 50 wherein operator preferences are included in the inter-access network information based on locally available 3GPP and non- 3GPP access technologies.

53. The system of claim 50 wherein the inter-access network information is restricted to access technologies and networks that the WTRU may use based on at least one of a capability the WTRU or a subscription by the WTRU.']