he Message Transfer Part (MTP) protocol, which is familiar from the SS7 standard (refer to Figure 2.60, where BTSM is the base transceiver station management and BSSAP is the BS Subsystem Application Protocol). Most functions of the linking protocol fall in the LAPDm and LAPD, from which it is derived. The first function of the linking layer is to construct frames out of bits. On the LAPD and MTP, frames are delimited with start and end flags, whereas in the LAPDm, a frame length indicator allows frames to be distinguished. On several link interfaces, there is a limitation on the number of frames to be transmitted due to buffer size constraints. For this reason, segmenting and reassembling at the receiver end are other necessary functions of the linking layer. In addition, the layer 2 main functions can be summarized as: frames construction; segmentation and assembly; error detection and correction; multiplexing of different flows at the same time (e.g. idle mode signalling and text short message); Radio Network Optimisation Transfer functions Transfer functions BSSAP BSSAP LAPDm LAPDm Layer 1 Layer 1 Layer 1 Layer 1 Layer 1 Layer 1 Air Interface Abis Interface A Interface Figure 2.60 Architecture of the signalling protocols up to layer 3 flow control, which is a kind of monitoring of throughput bottlenecks and reporting to the source sender to slow down the flow. Data in the linking layer are transported on the air interface using the FACCH or SACCH (see the previous subsection on the physical layer). Signalling in the GSM between different network elements follows common channel signalling system 7. For more details on the link layer, the reader can refer to the GSM recommendations. Network Layer Networking allows end-to-end connection of two elements not necessarily physically interconnected. Its functions can be declined as: routing, multipath channelling, addressing. The layer 3 protocol provides the functions for establishing, maintaining and terminating point-to-point (p-2-p) connections. A p-2-p connectio