Rarintip Sirisukpoca และ Prasert Akkharaprathomphong. (2008) Improvement of GPRS international roaming process using Lean Six Sigma approach. Chulalongkorn University:ม.ป.ท.Rarintip Sirisukpoca และ Prasert Akkharaprathomphong. 2008. Improvement of GPRS international roaming process using Lean Six Sigma approach. ม.ป.ท.:Chulalongkorn University;Rarintip Sirisukpoca และ Prasert Akkharaprathomphong. Improvement of GPRS international roaming process using Lean Six Sigma approach. ม.ป.ท.:Chulalongkorn University, 2008. Print.