3g GGSN and SGSN

[summasumma]

Hi all,

I am new to the cellular communication domain.
I am learning the basic of 3G and have the following doubts:
can anyone clarify on these?

1. Why do we need SGSN and GGSN as seperate boxes in the Core N/w?
Can't we have juz one box so that we can avoid doing GTP tunneling frm
sgsn to ggsn?

2. What is APN(Access point name) means? does it represent the domain
name of the IP data network the UE is trying to access?

3. When the UE(User Equipment) wants to access the data network, how
it identifies which SGSN it needs to communicate with?

4. And in-turn, How SGSN identifies which GGSN it needs to communicate
with, for the particular user request it received?

5. Who is responsible for assigning the IP address to the UE after the
attach procedure? GGSN or SGSN or the IP n/w which UE is trying
to access?

Thanks in advance.

Regards.
summa

[Techguru01]

Access point name is basically identifies an IP ‘PDN’, that a mobile data user desires to communicate with. In accumulation to identifying a PDN, an APN may also be used to describe the type of service, that is provided by the PDN. APN is useful in 3GPP data access networks, eg general packet radio service (GPRS), evolved packet core (EPC).

[Jamilet]

when a UE tries to access the data network, first it communicates with the BTS (NodeB for UMTS) and the bts exchanges information with the RNC which is connected. Each NodeB is connected with one specific RNC, and each RNC will be connected to one specific SGSN according to the design of the network. The UE sends information through some physical and logical channels to the NodeB.

In LTE (4G) we have a NodeB directly connected to the GGSN, so we avoid many things. Let´s check something about 4G to understand more

[Jamilet]

- SGSN pick up the information from de HLR and the VLR (remember that the mobile sends information first to the NodeB about its location). Then, SGSN knows which GSGN.
- GSGN gives IP address to the mobile station (2G) or User Equipment (3G)