pɹˈaɪɚ tə ðɪ ɐtˈæk ˌɔn pˈɜːl hˈɑːɹbɚ, ðɪ ɐmˈaʊnt ʌv ɐvˈeɪləbəl tɹˈæfɪk wʌz lˈoʊ, ænd lˈɪɾəl pɹˈɑːɡɹɛs hɐdbɪn mˌeɪd ɔnðə mˈoʊst ɪmpˈoːɹtənt dʒˌæpənˈiːz nˈeɪvi sˈɪstəm, kˈɔːld baɪ jˌuːˈɛs ˈænɐlˌɪsts dʒˌeɪˈɛn twˈɛnti fˈaɪv. dʒˌeɪˈɛn twˈɛnti fˈaɪv wʌz jˈuːzd fɔːɹ hˈaɪ lˈɛvəl ˌɑːpɚɹˈeɪʃənz: mˈuːvmənt ænd plˈænɪŋ kəmˈændz, fɔːɹ ˈɪnstəns.
Prior to the attack on Pearl Harbor, the amount of available traffic was low, and little progress had been made on the most important Japanese Navy system, called by US analysts JN-25. JN-25 was used for high level operations: movement and planning commands, for instance.