; only the differences between two consecutive headers are transmitted) Data compression (V.42 bis) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Mobile Station to SGSN Interface Chapter 8 Figure 8-14 IP Header IP Data SNDCP compression and segmentation. Compressed Header Compressed IP Data SNDCP SNDCP SNDCP Header Header Header N-201 The segmentation and reassembly procedures are different for acknowl- edged and unacknowledged mode of operation. Any (possibly compressed) combined N-PDU and SNDCP header is segmented by SNDCP if it is longer than the maximum number of bytes in the information field of an LLC frame: Maximum number of N201-I (1,520 bytes) is used for acknowledged data transfer mode (SN DATA segmentation). Maximum number of N201-U (500 bytes) is used for unacknowledged data transfer mode (SN UNIT DATA segmentation). The MORE (M) bit is used to indicate the last segment. Two types of PDU formats are used by SNDCP, depending on the trans- fer mode. These are shown in Figure 8-15. In acknowledged mode, the SNDCP header is two bytes long. In unacknowledged mode, the SNDCP header is five bytes long. Using the interfaces defined in this chapter has covered the various ways the Layer 3 protocols work with the upper-layer primitives of the Layer 2 protocols to perform the necessary truncation of the data packets SO that they will work in the LLC format. The use of SNDCP NSAPIs passed down to the Layer 2 protocols helps to unambiguously identify the mobile station using the GPRS network. Through this interface, the GPRS network nodes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Mobile Station to SGSN Interface Mobile Station to SGSN Interface Figure 8-15 N-SAP 