SGSN or Serving GPRS supported node 

SGSN’s full form is Serving GPRS Support Node. A mobile phone connects to SGSN for the internet while roaming. When a subscriber roams from one location to another, VLR (for voice) and SGSN for data keep changing. After how much distance travel the roaming nodes will change, depending on the area covered by each node.

In a 2G network, the mobile device connects to BTS over a radio channel, BTS connects to BSC and BSC connects to SGSN ( and VLR too for voice).  The protocol between BSC and SGSN is BSSMAP.

In the 3G network, the mobile device connects to NodeB over a radio channel, NodeB connects to RNC and RNC connects to SGSN. The 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 has its own protocol stack. The following are the interfaces that connect to the SGSN with other nodes in the mobile network.

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

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

S6d Interface: It is between HSS and SGSN. HSS is the subscription database in-home network for the LTE network.  Diameter is the protocol for the s6d interface. The diameter protocol uses TCP or SCTP.

Iu Interface: Interface between SGSN and RNC. RANAP is the protocol. RANAP runs over a 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 requests to the HLR, located in-home network. The protocol between SGSN and HLR is ss7.  GSM MAP protocol has messages for GPRS authentication and an update location procedure. GSM MAP uses SS7 layers as transport.  Once a subscriber is successfully authenticated and registered with the home network, a mobile device starts displaying data signs. In most cases, it is E for 2G and H for 3G.

Data Access from a mobile phone:

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

PDP Context setup :

  • SGSN sends the Create PDP Context Request to the GGSN.
  • Upon receiving the PDP context request, GGSN verifies if a subscriber is allowed to data with the requested Quality of service.
  • If GGSN decides to allow the data, then it sends the 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 a successful PDP context setup, actual user data flow between a mobile device and an external packet network

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