Editing GPRS core network (section)

===Interfaces in the GPRS network===

;Ga: The interface serves the CDRs (accounting-records) which are written in the GSN and sent to the charging gateway (CG).  This interface uses a GTP-based protocol, with modifications that supports CDRs (Called ''GTP' '' and ''GTP prime'').
;Gb: Interface between the [[base station subsystem]] and the SGSN the transmission protocol could be Frame Relay or IP.
;Iu: Interface between the Radio Network Controller and the SGSN. The interface exchanges signaling and payload. 
;Gc: Interface between the [[GGSN]] and [[Home Location Register|HLR]] so that the GGSN can get the location details of a [[mobile station]]. To avoid implementing MAP/SS7 in the GGSN, this interface is optional. When not present, the GGSN routes inquires to the HLR via an [[SGSN]].
;Gd: Interface between the SGSN and the SMS Gateway. Can use MAP1, MAP2 or MAP3.
;Ge: The interface between the SGSN and the [[Service Control Point|service control point]] (SCP); uses the [[CAMEL Application Part|CAP]] protocol.
;Gf: The interface between the SGSN and the Equipment Identity Register (EIR), used for checking the mobile's equipment identity number ([[IMEI]]) against a list of reported stolen mobile phones.
;Gi: IP-based interface between the GGSN and a public data network (PDN) either directly to the [[Internet]] or through a [[WAP gateway]].
;Gmb: The interface between the GGSN and the Broadcast-Multicast Service Center (BM-SC), used for controlling MBMS bearers.
;Gn: IP-based interface between SGSN and other SGSNs and (internal) GGSNs. [[Domain Name System|DNS]] also shares this interface. Uses the GTP Protocol.
;Gp: IP-based interface between internal SGSN and external GGSNs. Between the SGSN and the external GGSN, there is the border gateway (which is essentially a [[firewall (networking)|firewall]]). Also uses the GTP Protocol.
;Gr: Interface between the SGSN and the HLR. Messages going through this interface uses the MAP3 protocol.
;Gs: Interface between the SGSN and the MSC (VLR). Uses the [[BSSAP|BSSAP+]] protocol. This interface allows paging and station availability when it performs data transfer. When the station is attached to the GPRS network, the SGSN keeps track of which routing area (RA) the station is attached to. An RA is a part of a larger location area (LA). When a station is paged this information is used to conserve network resources. When the station performs a PDP context, the SGSN has the exact BTS the station is using.
;Gx: The online policy interface between the GGSN and the charging rules function (CRF). It is used for provisioning service data flow based on charging rules. Uses the diameter protocol.
;Gy: The online charging interface between the GGSN and the [[online charging system]] (OCS). Uses the diameter protocol ([[Diameter Credit-Control Application|DCCA application]]).
;Gz: The off-line ([[Call detail record|CDR]]-based) charging interface between the GGSN and the Charging system Uses GTP'.
;Lg: The interface between the SGSN and the Gateway Mobile Location Center ([[GMLC]]), used for location-based services.
;S3: This interface is defined between the SGSN and the MME and it is used to exchange user and bearer information in inter-system mobility procedures. This interface is implemented using [[GPRS Tunnelling Protocol#GTP-C - GTP control|eGTP-C]].
;S4: This interface is defined between the SGSN and the SGW (Serving Gateway). The term "S4-SGSN" refers to a Release-8 SGSN that has at least one set of S4/S3/S16 interfaces enabled.<ref>3GPP TS 29.303 version 10.1.0 Release 10, p.7</ref>
;S6d: The interface between SGSN and Home Subscriber Server (HSS). It is a [[Diameter (protocol)|Diameter]] based interface that is used for transferring subscription and authentication data of the user to HSS for authenticating and authorizing user access.
;S13': This interface is defined between the SGSN and the EIR (Equipment Identity Register). The S13' interface applies only to the S4-based SGSN.<ref>SAE and the Evolved Packet Core: Driving the Mobile Broadband Revolution, p. 365</ref> The interface is based on Diameter protocol.
;S16: This interface is defined between two SGSNs and exclusively uses GTPv2-C.<ref>SAE and the Evolved Packet Core: Driving the Mobile Broadband Revolution, p. 237-238</ref>