tailieunhanh - Communication Systems for the Mobile Information Society phần 5

giữa các thiết bị người dùng cuối. Một chức năng điều khiển cổng đa phương tiện (MGCF) chỉ cần thiết nếu một trong những người sử dụng vẫn sử dụng điện thoại chuyển mạch kênh. Với các mạng UMTS truy cập vô tuyến có thể cho lần đầu tiên để thực hiện một giọng nói dựa trên IP-điện thoại di động và kiến trúc điện thoại video. | 128 Communication Systems for the Mobile Information Society between the end-user devices. A media gateway control function MGCF is only necessary if one of the users still uses a circuit-switched phone. With the UMTS radio access network it is possible for the first time to implement an IP-based mobile voice and video telephony architecture. This is not only due to the fact that UMTS offers enough bandwidth on the air interface for such applications but also due to a new way of handling cell changes for packet connections. With GPRS in the GSM access network the roaming from one cell to another mobility management for packet-switched connections is controlled by the mobile station. This results in an interruption of packet traffic of 2-3 seconds at every cell change. For voice or video calls this is not acceptable. With UMTS the mobility management for packet-switched connections can now also be controlled by the network. This ensures uninterrupted packet traffic even while the user is roaming from one cell to another. The overhead of an IP connection for voice telephony however remains a problem for the wireless world. As the delay must be as short as possible only a few bytes of voice data are put into a single IP packet. This means that the overhead for the header part of the IP packet is about 50 . Circuit-switched voice connections on the other hand do not need any header information and are transported very efficiently over the UMTS network today. While the IP overhead in a fixed-line network is still important it does not prevent the use of IP for voice calls over ADSL or cable due to the high bandwidth available on these services. However on the UMTS air interface with its limited bandwidth a connection which requires twice as much resource is not desirable as it de facto cuts the number of simultaneous calls per cell in half. It should be noted at this point that video telephony which is currently offered in UMTS Release 99 networks is not based on IP but