 need the PDCH simultaneously, a spe- cial discipline that is handled by the MAC-protocol must be enforced. Basically, whenever some of the upper protocols need to send a PDU, the MAC must reserve capacity on the PDCH for sending the corresponding SDU and give it back when the SDU is sent. The RLC protocol provides normal link functions as segmentation, error control and re-sending, in order to make data transport over the radio interface reliable. The RLC is also the termination point for the TLLI on the MS-side. The LLC protocol discriminates between data flows that belong to different log- ical networks in GPRS: the GPRS short-message service logical network (SMS), the GPRS Process Control System logical network (GMM) and the GPRS Resource System logical network (SNDCP). The parameter used in the LLC protocol for discrimination is called SAPI and has 16 possible values. SAPI1 is used for GMM flows, etc. There are presently four SAPI values allowed for user-data flows in the resource system. One may wonder why there is a need for multiple SAPIs between LLC and SNDCP; a possible use is to sep- arate between several terminals of the same terminal system. If we compare the LLC protocol with LAPD in ISDN, the use of SAPI has some similarities. The difference from LAPD is the use of multiple SAPIs for the same logical network, as in case of SNDCP, where LAPD uses logical link identifiers instead (which is a better separation of concerns). The LLC also performs ciphering and other functions that make the service to upper protocols reliable and independent of the constraints of the RLC and MAC service. The SNDCP protocol discriminates between concurrent accesses to PDNs. The discrimination parameter is called NSAPI and is an identifier that is established by the GMM when an MS gets an access to a PDN. Each NSAPI value on a par- ticular TLLI represents a PDPtype (X.25 or IP) and a PDPaddress (an X.25 or an IP address). There are 11 possible values of NSAPI, which means that a terminal can use 11 PDN accesses in paralle