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Mar 6, 2013

How MME is selected - MME Selection procedure

It's beed a while from last time, but here we go. Many times I saw people accesing this site looking for MME seletcion, or how LTE pick MME/SGW/PGW nodes. Hope the below clarifies.

In each LTE network the Internet Domain Name System (DNS) is widely used to handle the Dynamic Peer selection of the PDN-GW, SGW, MME, SGSN and HSS within the network. We could use statistic assingments but, hey! belive me, DNS is much easier if handled properly.

In a few days I will try to cover the details of DNS config for points mentioned below - stay tunned.

EPS Nodes sends the DNS Query message to the Internal DNS (iDNS) for the selection of EPS nodes for the following uses cases.

  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.

Today it will about two of them referring to MME Selection, and the list above will be copied few times more when covering the SGW, PGW, SGSN and HSS selection.

DNS Interface

DNS interface is not via a 3GPP standardised interface, the eNBs, MMEs, SGSN and Diameter Proxy are connected (IP connectivity) to DNS servers in the EPS. Typically within the EPC network the EPS nodes would access the DNS servers via the Gn interface or O&M interface.

MME Selection

  • Ref. to point 1, MME Selection during the Attach to LTE Network:
eNodeB selects an available MME to serve the UE, based typically on network topology and the location of the UE within the network, so that the best MME is selected, e.g. to reduce the probability that the MME is changed.
As part of the UE Attach the eNB will query the DNS Server using the TAI for the address of the MME. Typically the DNS will have a pool of MMEs to select from in response to the query.
The format of the TAI FQDN is constructed as follows:
tac-lb<TAC-low-byte>.tac-hb<TAC-high- byte>.tac.epc.mnc<MNC>.mcc<MCC>.3gppnetwork.org
The DNS may provide more than one MME address (candidate set) from which the eNB can select a MME.
  • Ref. to point 4, SGSN selection durring LTE->2/3G handover:
SGSN query the DNS Server to identify the Old MME during LTE to 3G/2G Handover. It uses the old GUTI received from UE.
There are procedures where the old MME must be contacted by a Release-8 SGSN supporting only Gn/Gp. The primary use case is context transfer during Handover.  A UE moving from eUTRAN to pre-Release-8 UTRAN/GERAN the UE will provide a derived P-TMSI based on a GUTI. As a result the source MME looks like a pre-Release-8 SGSN to a pre-Release-8 target SGSN node. 
For a Release 8 Gn/Gp-SGSN to find all Gn and Gp interfaces of an MME based on the old GUTI and it would use a "Service Parameter" of " x-3gpp-mme:x-gn", " x-3gpp-mme:x-gp".
For pre-Release 8 compatibility operators would continue to provision A/AAAA records for the corresponding Gn/Gp interfaces regardless of whether the source SGSN is pre-Release-8 or not. Gn/Gp interfaces are provisioned redundantly for both ".gprs" and ".3gppnetwork.org" top level domains.

from my own experience