The Gs interface is the interface between the SGSN, and the MSC/VLR.  In other words, it is the interface between the PS domain and the CS domain.

When you switch the MS on, it makes an attach to the CS domain, and this is the 'IMSI attach'.

when you enable data on the MS, it makes attach to the PS domain, this is the 'GPRS attach'.

If the Gs interface exists between the SGSN and the MSC/VLR, and the MS supports working in both CS and PS domains. In this case, the MS can attach to both PS and CS domains in the same transaction, this is the 'Combined attach procedure'.

In this case, the MS will make one transaction instead of two transactions. Therefore, it will save the radio resources of the network.

As a result, this will make an association between the SGSN and the MSC/VLR. this means the VLR stores the SGSN number, and the SGSN stores the VLR number.

according to the 3GPP standard, the SGSN – MSC/VLR association will exist in following occasions,

– Combined GPRS / IMSI attach.
– GPRS attach when the MS is already IMSI-attached.
– Combined RA / LA update when the MS performs IMSI attach and is already GPRS-attached.
– Combined RA / LA update when an IMSI and GPRS-attached MS changes from an area of ‘network operation mode 2’ to an area of ‘network operation mode 1’.

3GPP standard 23.060

• network operation mode 1, means the Gs interface exists.
• network operation mode 2, means that this interface does not exist.

The Gs interface protocol stack is as following,