SGSN:

The SGSN or Serving GPRS Support Node is one of the new network elements that need to be introduced for GPRS. The SGSN is part of the GPRS core network. Together with the GGSN, the SGSN forms the GPRS core network (figure 1). Note that the standard allows for the physical integration of SGSN and GGSN.

 

Figure 1: The SGSN controls a number of BSS's

The SGSN is basically a packet switch that can route incoming data packets in uplink and downlink direction. In uplink direction, the SGSN is connected to the GGSN via the IP-based intra-PLMN-backbone. In downlink direction, the SGSN interconnects a number of BSS's to the GPRS core network. Therefore, each SGSN serves a limited geographic area as illustrated in figure 2, comparable to the MSC in plain GSM.

An-08_04.gif (14645 Byte)

 

Figure 2: The geographic dedication of the SGSN

Important Functions of the SGSN:

  • Peer Function: The SGSN is the main peer of the mobile station for all GPRS related transactions. With the optional Gs-interface being implemented, the SGSN will even forward circuit-switched pagings towards the mobile station (see also Network Operation Mode).

  • Mobility Management: Like the MSC/VLR in circuit-switched GSM, the SGSN is in charge for mobility management in GPRS. This relates in particular to functions like GPRS Attach / Detach, Cell Updates and Routing Area Updates. In that respect, the SGSN assigns P-TMSI's and keeps track of a subscriber's state and location.

  • Ciphering and Compression: These two optional functions are also tasks of the SGSN. Accordingly, there is a big difference to GSM where ciphering is a function of the BTS and compression is a function of the TRAU. Note that the TRAU does speech compression while the SGSN does data compression.

  • Charging: Together with the GGSN the SGSN collects CDR's (Call Data Records). Opposed to the GGSN, the SGSN collects CDR's for the use of the own network resources. These CDR's are forwarded to the Charging Gateway (CG) via the IP-based intra-PLMN backbone.

Parameter Description Type (Mandatory / Optional - Fixed (Unchangeable) / Variable
IMSI Subscriber Identity M/F
MM State The respective GPRS Mobility Management State (e.g. IDLE, STANDBY, READY) M/V
P-TMSI The GPRS Counterpart for the GSM TMSI. Temporary Subscriber Identity M/V
P-TMSI Signature   M/V
IMEI Equipment Identity Number O/F
MSISDN Telephone Number M/F
Routeing Area Current routeing Area M/V
Cell Identity Current Cell in READY State M/V
Cell Identity Age The time that elapsed since the last received LLC PDU from the Mobile Station at the SGSN M/V
VLR Number The number of the current VLR/MSC M/V
New SGSN Address IP-Address of the SGSN to which not sent N-PDUs should be forwarded to O/V
Authentication Triplets Authentication and ciphering Parameters M/V
Kc The currently used Ciphering Key M/V
CKSN Ciphering Key Sequence Number of the Kc M/V
Ciphering Algorithm Currently selected Ciphering Algorithm M/V
MS Radio Access Capability

The Radio access capabilities of the Mobile Station

M/V
MS Network Capability The Network Capabilities of the Mobile Station M/V
DRX Parameters The Parameters for the Discontinuous reception M/V
MNRG Shall activity from the MS be reported to the HLR or not M/V
NGAF Shall activity from the MS be reported to the VLR/MSC or not M/V
PPF PS or CS services M/V
Subscribed Charging Characteristics The Charging Characteristics of the Subscriber M/V
OMC Identity Contains Identity of the OMC that shall receive the recorded trace O/V
SMS Parameters The SMS related Parameters O/V
Recovery Indicates if HLR or VLR is performing database recovery M/V
Radio Priority SMS Parameters for SMS tramnsmission O/V
PDP Context Identifier The Index of the PDP context O/V
PDP State PDP state ACTIVE or INACTIVE O/V
PDP Type IP or PPP O/V
PDP Address The fixed PDP-address of the mobile station (empty if dynamic IP-addressing is used)
The mnemonic address of the serving GGSN (e.g. www.abcd.d2.gprs) applying to the DNS-rules.
O/V
APN Subscribed The APN received from the HLR O/V
APN in Use Indicates the APN Network Identifier O/V
NSAPI Network Service Access Point Identifier O/F
TI Transaction Identifier O/F
GGSN Address in Use The IP Address of the currently serving GGSN O/V
VPLMN allowed May the MS use only the HPLMN or the VPLMN too? O/V
QoS Profile Subscribed Quality of Service Profile that a subscriber is paying for (one per PDP-context) O/F

Table 1: Information Storage within the SGSN

[2GTS 02.60 / 3GTS 23.060 / 3GTS 29.060]