Figure 8.14 illustrates a scenario for IP packet sending from MS to GGSN. The IP packet is received by the SNDCP layer on the MS side as an N-PDU. If the N-PDU size is longer than a given size N201, the SNDCP layer segments the N-PDU into several SN-PDUs. It then sends each SN-PDU to the LLC layer, which formats the PDU received into an LLC PDU frame either in UI frame for unacknowledged frame transfer or in I for acknowledged information transfer, depending on the reliability class for QoS. Next the LLC PDU is sent from the MS to the SGSN via the BSS by being encapsulated in RLC PDU on air interface and in BSSGP PDUs on Gb interface. The LLC layer within SGSN forwards to the SNDCP layer each LLC frame received into an SN-PDU. This layer reassembles SN-PDUs into an N-PDU if segmentation was performed on the MS side, and next forwards the N-PDU to relay entity. The role of the latter is to adapt N-PDU format in T-PDU format and vice versa. G-PDUs are tunneled in GTP tunnels across the Gn or Gp interface between SGSN and GGSN. Next, the GTP layer in GGSN extracts the T-PDU from the G-PDU in order to send the T-PDU (IP PDU) to the IP layer.
N201 is the maximum length of the information field in LLC frame.
IP fragmentation is processed on the Gn interface (or Gp interface) when the T-PDU size is larger than the maximum transmission unit (MTU) value. The latter represents the maximum datagram size that can be transmitted through the GPRS backbone network via Gn or Gp interface.
Figure 8.15 illustrates a scenario for IP packet sending from a GGSN to an MS. This scenario follows the same scenario as IP sending from MS to GGSN but in the opposite direction.