tailieunhanh - Wireless Data Technologies reference phần 3

Đây là lý do tại sao hầu hết các ứng dụng giọng nói packetised sử dụng UDP để tránh khôi phục lại bất kỳ gói tin bị mất hoặc lỗi. ITU-T xem xét sự chậm trễ mạng cho các ứng dụng thoại trong Khuyến nghị . Khuyến nghị này định nghĩa ba ban nhạc một cách chậm trễ như thể hiện trong Bảng | 30 MULTIPLE ACCESS WIRELESS COMMUNICATIONS set which included authentication calling-number ID a message-waiting indicator MWI and voice privacy. IS-54B was superseded in 1994 with the introduction of IS-136 followed closely by revisions A and B. IS-136 was backward compatible to IS-54B and included a DCCH and advanced features. IS-136A upbanded IS-136 for seamless cellular service between the 800 MHz and 1900 MHz frequency bands. In addition it introduced over-the-air activation and programming services. IS-136B includes a new range of services including broadcast SMS packet data and others. 4 GSM Introduction It may be hard to realize this but it really was not all that long ago when just a plain old telephone was a luxury item. Nevertheless as we all know technology s only constant is change. Currently many folks need to be accessible everywhere whether they are at work or play in the office or at home. To meet this demand the GSM standard Global System for Mobile Communications for mobile telephony was introduced in the mid-1980s. Today GSM is the most popular mobile radio standard in the world. A boom is underway and this is demonstrated by the fact that many GSM users find life without their phone practically inconceivable. Nowadays when we speak of GSM we usually mean original GSM also known as GSM900 since 900 MHz was the original frequency band. To provide additional capacity and enable higher subscriber densities two other systems were added later Gsm1800 also DCS1800 and GSM1900 also PCS 900 . Compared to GSM 900 GSM1800 and GSM1900 differ primarily in the air interface. Besides using another frequency band they use a microcellular structure . a smaller coverage region for each radio cell . This makes it possible to reuse frequencies at closer distances enabling an increase in subscriber density. The disadvantage is the higher attenuation of the air interface due to the higher frequency. Where now A few years ago Michael Jackson sang . just call .