và nó không phải là bất thường MSC một là hơn 100 km từ nút BSS tiếp theo. Khi một MSC chỉ có một khả năng chuyển đổi giới hạn và sức mạnh xử lý, một PLMN thường bao gồm hàng chục hoặc thậm chí hàng trăm độc lập MSCs. Hệ thống Truyền thông di động toàn cầu (GSM) | General Packet Radio Service GPRS 89 Subscriber roams to a new location --------------------- GPRS tunnel is redirected to the new SGSN ------------------ Routing in the Internet remains unchanged Figure Subscriber changes location within the GPRS network GPRS Radio Resource Management As has been shown in Figure a GPRS timeslot can be assigned to several users at the same time. It is also possible to assign several timeslots to a single subscriber in order to increase his data transmission speed. In any case the smallest transmission unit that can be assigned to a user is one block which consists of four bursts on one timeslot on the air interface for GPRS and two bursts for EGPRS MCS 7-9. A block is also called a GPRS RLC MAC radio link control medium access control frame. Temporary Block Flows TBF in the Uplink Direction Every RLC MAC frame on the PDTCH or PACCH consists of an RLC MAC header and a user data field. When a user wants to send data on the uplink the terminal has to request resources from the network by sending a packet channel request message via the RACH or the PRACH as shown in Figure . The PCU then answers with a packet uplink assignment message on the AGCH. The message contains information in which timeslots the terminal is allowed to send data. As a timeslot in GPRS may not only be used exclusively by a single subscriber a mechanism is necessary to indicate to a terminal when it is allowed to send on the timeslot. Therefore the uplink assignment message contains a parameter called the uplink state flag USF . A different USF value is assigned to every subscriber that is allowed to send on the timeslot. The USF is linked to the so-called temporary flow identity TFI of a temporary block flow TBF . A TBF identifies data to or from a user for the time of the data transfer. Once the data transfer is finished the TFI is reused for another subscriber. In order to know when it can use the uplink timeslots the terminal has to listen to .
