GPRS mobile data network-tlli and P-TMSI

GPRS mobile data network-tlli and P-TMSI

 

 

The GPRS mobile data network mentioned here refers to a 2g gprs network and a 3G (WCDMA/UMTS) Data Network (packet switching domain, PS domain ).

Because we often encounter tlli and P-TMSI, so here is a simple explanation.

The SIM card has an imsi (International Mobile Subscriber Identity written in 3GPP, and International Mobile is written in ITU

Station identity.

To enhance the privacy of communication, try to expose imsi as little as possible, that is, to include imsi in messages as little as possible during the communication process. Therefore, when the mobile phone is started into the network

After the network is connected, the core network device will assign a temporary ID to the mobile phone, that is, the corresponding ID of tmsi (temporary mobile subscriber identities). tmsi and imsi

The link is only known to mobile phones and core network devices. The MSC in the voice network (CS) and the sgsn in the Data Network (PS) will do this independently. Data Network

Allocated is called packet tmsi, that is, the P-TMSI. P-TMSI can be used to attach the phone attach when the success of the message to the phone, there are 4 bytes.

After this, the phone want to do what, do not need to bring imsi, as long as the P-TMSI to sgsn, sgsn can identify this phone.

 

2g gprs network is a little more complex, there are many network layers, so it is special, in addition to P-TMSI, there is a tlli (temporary logical link identity ).

 

When GRPS was designed at the time, the control plane and data plane were not separated between the base station controller BSC and the sgsn of the core network (WCDMA was explicitly defined separately), or

The author said that the design was not completely separated. At least from the underlying layer to the NS layer, the bssgp layer to the LLC layer are shared.

 

At the bottom of the complex network, a logical link control (LLC) is abstracted ). The LLC layer is addressing with tlli, and the logical link between the mobile phone (MS) and sgsn is identified with tlli.

 

Tlli is allocated by mobile phones, but in essence it is still allocated by sgsn. Because tlli is built based on the P-TMSI of the phone. If the mobile phone has been assigned by sgsn

P-TMSI, that would take 30 bits of the P-TMSI as the thirty bits of the tlli. Tlli also has the local, foreign and random, which are allocated based on the P-TMSI

Routing area and the current routing area. The remaining two bits can be used to differentiate the three types.

For P-TMSI, the standard does not give the definition of its format, then sgsn can be in-depth customization of it. The tlli standard defines its format, but since it is defined by the P-TMSI

So the sgsn can be customized in depth. Then the P-TMSI, or tlli, sgsn can be used as the internal cluster message distribution, routing

Selector, which can be freely used to design this system.

 

 

For standards, refer:
3GPP ts 23.060 General Packet Radio Service (GPRS); service description; Stage 2
3GPP ts 23.003 numbering, addressing and Identification

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