Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 86

Consider Figure 3.5 illustrated earlier. As shown in that figure, the GSM BSC and the SGSN can exchange the Gb‐interface protocol stack messages using either the Frame Relay network protocol or IP network. The Sub‐Network control protocol takes care of the data link layer protocol (Frame Relay or IP) to be used by the Gb‐interface. The higher‐layer applications are independent of the sub‐Network control protocol. In case the Frame Relay protocol and its physical E1 interface go down, the IP Transport/Ethernet interface can be brought into services immediately so that the Gb‐interface is not affected and GPRS services are not down permanently.

Similarly, between the UMTS UTRAN and its CN, the Iu‐interface can be configured to use either the ATM over STM or IP transport network physical layer; refer to TS 25.412 [53].

 Reason for Multiple Data Link Layer Provisions

Typical reasons to support multiple data links by a network element are as follows:

 Make the higher layer(s), e.g. RNL, independent of the TNL being used for the introduction of a new transport network without affecting the application or RNL;

 Create a backup transport network; and

 System and application requirements, e.g. use ATM to transport multimedia contents, whereas frame relay can transport data only.