PGW and SGW Selection procedures in LTE

Last time it was about MME Selection, and today I will cover the PGW and SGW selection as well.
All other points will cover in details in few next days. Also, I'm aware that good example is worth few thousands words, that's why will provide a iDNS configuration as a separate article with all those NAPTR and so on explained.

Possible selection scenarios

As last time, same list of selection scenarios in which EPS Nodes send the DNS Query message to the Internal DNS (iDNS) for the selection of EPS nodes.

1. As part of the UE Attach the eNB will query the iDNS Server using the TAI for the address of the MME it should provide to that UE.
2. During UE attach the MME query the iDNS Server to select the PDN-GW (Packet Data Network Gateway, PGW) where a requested (subscribed) PDN connectivity (APN) is located. Selection can be based on the information provided to the MME, when the UE attaches to the network.
3. Following the PGW selection, the MME query the DNS Server to select an available SGW to serve the UE using the TAC, which in most cases is based on network topology and the location of the UE within the network, so that the best SGW is selected.
4. SGSN will query the DNS Server to resolve the Old MME using LAC, and RAC (taken from Old GUTI received from UE) during LTE to 3G/2G Handover.
5. MME will query the DNS Server to resolve the Old SGSN using NRI, LAC, and RAC (taken from P-TMSI received from UE) during 3G/2G to LTE Handover.
6. During attach the MME is configured to support the HSS Peer Service and Interface Associations towards the Diameter proxy/edge agent. These are static configurations in the MME towards the Diameter Proxy and no DNS query is initiated by the MME to select the Diameter Proxy.

PDN-GW Selection

• Ref. to point 2, MME selects the PGW due to requested APN

MME selects the PDN-GW where a requested (subscribed) PDN connectivity (APN) is located. Selection can be based on the information provided to the MME, when the UE attaches to the network. The address of the PDN-GW could either be provided directly by the HSS, the use DNS to resolve the address during a bearer setup procedure or configured on the MME.
PGW selection is performed by the MME/SGSN at initial attach or PDN connection establishment.
PGW selection is performed by using the S-NAPTR procedure with:
"Service Parameters“ = {desired reference point, desired protocol}
“Application-Unique String” = the APN FQDN (per 3GPP TS 23.003)

Example, a small one

The MME performs PGW selection using the S-NAPTR procedure with:

"Service Parameters“ = {"x-3gpp-pgw:x-s5-gtp"}
“Application-Unique String” = <APN-NI>.apn.epc.mnc<MNC>.mcc<MCC>.3gppnetwork.org

 

SGW Selection 

• Ref. to point 3, after PGW Selection MME query iDNS to select the SGW based on TAC

MME selects an available SGW to serve the UE, based typically on network topology and the location of the UE within the network, so that the best SGW is selected, e.g. to reduce the probability that the SGW is changed.
SGW selection is performed by the MME/SGSN at initial attach or PDN connection establishment procedure. This occurs in the VPMN (Visited Public Mobile Network) or the HPMN (Home PMN) (non-roaming scenarios).
SGW selection is performed by using the S-NAPTR procedure with:
"Service Parameters“ = {desired reference point, desired protocol}
“Application-Unique String” = the TAI FQDN (per 3GPP TS 23.003 )

Example, short one

The MME performs SGW selection using the S-NAPTR procedure with:

"Service Parameters“ = {"x-3gpp-sgw:x-s5-gtp"}
“Application-Unique String” = tac-lb<TAC-low-byte>.tac-hb<TAC-high-byte>.tac.epc.mnc<MNC>.mcc<MCC>.3gppnetwork.org

The TAC is a 16-bit integer. The <TAC-high-byte> is the hexadecimal string of the most significant byte in the TAC and the <TAC-low-byte > is the hexadecimal string of the least significant byte. If there are less than 2 significant digits in <TAC-high-byte> or <TAC-low-byte >, "0" digit(s) shall be inserted at the left side to fill the 2 digit coding.
For example 100 (decimal), converted to Hexadecimal = 64, represented as 0064, so ther higher byte = 00 and lower byte is 64.

Source:
from my own experience