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distributed System	not good for problem solving	Difficult to solve feature interaction problem cooperatively
amateur feature designers	create feature interaction problems	業餘設計家容易忽略 FI 問題，且只有片面資訊，無法預知 FI 問題，
some features are impossible	lack of in-band signal	公用電話投幣信號之偵聽與處理，必須在通話中處理，但通話的traffic並未經過 SG
end system may control state	end system may be out of control	911/119 緊急電話功能要求主叫方不能掛斷電話，如果 End System 擁有控制權時，網路 operator 將無法保證此種功能需求。
too easy to get IP	incoming call screening may fail	騷擾者將很容易獲得新 ID 擺脫 call screen 的過濾。
new trust model	phone number no longer implies house ID	經不能經由電話作身份的認定。