SGSN or Serving Gprs supported node 

When a mobile subscriber roams from one location to another.  The vlr and sgsn in roaming network keep changing. At what distance the roaming nodes will change, depends on the area covered by each node. The VLR is used for voice and sms. The SGSN is used for mobile internet services.  In 2G network, mobile device connects to BTS over radio channel, BTS connects to BSC and BSC connects to SGSN.  Protocol between BSC and SGSN is BSSMAP.  In 3G network, mobile device connects to NodeB over radio channel, NodeB connects to RNC and RNC connects to SGSN. Protocol between SGSN and RNC is RANAP, for location and registration. For data tunnel management protocol is GTP.








SGSN Interfaces :

A node in telecom never works standalone. It connects to the other nodes over various interfaces. Each interface have its own protocol stack. Following are the interface connects to the SGSN with other nodes in mobile network.

Gn interface : The Roaming-SGSN connects to the GGSN over Gn interface. Gn interface have GTP protocol.  GTP uses services of UDP protocol.

Gr Interface : Interface between Roaming-SGSN and HLR. It uses Ss7 protocol stack.

S6d Interface : Its between HSS and SGSN. HSS is subscribe database in home network for LTE network. . Diameter is the protocol for s6d interface. Diameter protocol uses TCP or SCTP.

Iu Interface: Interface between SGSN and RNC. RANAP is the protocol. RANAP runs over Connection oriented SCCP protocol.

Mobile phone registration for data:

To use data services , first a mobile phone has to register itself with the network for GPRS. For registration the SGSN sends authentication  and update gprs location request to the HLR , located in home network. Protocol between SGSN and HLR is ss7.  GSM MAP protocol have messages for GPRS authentication and update location. GSM MAP uses SS7 layers as transport.  Once a subscriber is successfully authenticated and registered with home network, mobile device start displaying data sign. Most of the case in E for 2G and H for 3G.

Data Access from mobile phone:

After successful registration of phone for data, subscriber can browse the internet.  This requires to setup GTP tunnel with the home GGSN.  SGSN does signaling for tunnel setup with the GGSN. This requires setup of PDP context between the two nodes.

PDP Context setup :

  • SGSN sends the Create PDP Context request to the GGSN.
  • Upon receiving PDP context request , GGSN verifies if a subscriber is allowed to data with the requested Quality of service.
  • If GGSN decides to allows the data, then sends PDP context response as accepted. Else rejects the PDP Context. 
  • GGSN allocates an ip address to the device.

PDP Context management
PDP Context management

After successful PDP context setup, actual user data flows between mobile device and external packet network


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