What are GMLC and SMLC in telecom?

Gateway Mobile Location Center(GMLC) and Serving Mobile Location Center(SMLC) are two GSM nodes for location-based services (LBS).  LBS opens new revenue streams for mobile operators and service providers. A basic GSM network for voice and data can deliver, location information, but that lacks accuracy. Accuracy means, the network operator can get the cell id of the GSM tower, which provides an approx location. But can not get the longitude and latitude of a mobile phone for exact location and other things e.f direction etc.

To overcome this, GSM uses GMLC and SMLC along with the basic network. With GMLC and SMLC, the home network operator can get geographical coordinates along with other useful details.  Other than accuracy, new network nodes also reduce the overhead on mobile operators for location queries and do not interfere with basic gsm services.

GMLC and SMSC telecom network
GMLC and SMSC network

GMLC and SMLC Interfaces:

Mobile operator deploys GMLC in-home network (HPLMN) along with the other network components e.g HLR, GGSNetc. While SMLC is located in the roaming network (VPLMN), having interfaces with MSC and BSC/RNC.  GMLC works as a location server to an external application, for location information. The external application is named LCS client. LCS client connects to the gateway to get the location of a mobile device.

Call Flow for GMLC and SMLC: 

Before any call to the location request,  for a device, the home network makes sure that the visiting network supports the location query. During the gsm map protocol location update procedure, VLR sends the supported LCS capability parameter in the update location message. If no LCS capability set is supported in VLR, then GMLC can not send the request to MSC for location information.

GMLC and SMLC call flow
GMLC and SMLC call flow
  1. LCS Client sends a location query to the GMLC with MSISDN. GMLC sends a location Send Routing Info for LCS, using gsm map protocol operation over the ss7 network.  
  2. In response, HLR sends the visiting MSC to the GMLC and IMSI.  GMLC sens Provide Subscriber Location, gsm map operation over SS7 to the Visiting MSC.  
  3. Visiting MSC starts location procedure towards SMLC.  SMLC sends the location to MSC or failure response. 
  4. MSC sends the location or failure response in Provide Subscriber Location Response.  At this stage, the LCS client gets the response for the location query.
  5. GMLC can use SGSN also in place of MSC.

LCS Client :

Till now mentioned about LCS client. Here we will discuss the LCS client and use case.  An LCS client can be any entity that can request the current location of a phone from the GMLC  server to provide a valuable service. As per the 3GPP spec, an LCS client may not need to be a regular GSM subscriber (no call, SMS, USSD, etc.) of a mobile network operator. E.g if a SIM card is placed in a car tracking device, only location information is required, and no other basic services are needed.  There can be the following types of LCS clients as per standard.

Value-Added Services, this type of client can add extra revenue to the mobile operator and useful service to a mobile subscriber.

PLMN LCS clients are the mobile operator entities. It may be used for operation and maintenance.

Emergency Services can help mobile subscribers in case of emergency service.

Lawful interception is useful for many legal agencies to track the mobile device to trace their activities.