Has been some time since I have to review MME traces with Wireshark. I have to tell you that the most difficult part of this task was to remember the interfaces and protocols. And this is why I have ended with this post. Part of it is to have a place where I can store this information, and of course, share with colleagues that might end up in need of this data.

LTE Architecture

EPC Interfaces

I have follow a common way in this diagrams to show in full lines the user plane and in dash lines the control plane.

Elements in the EPC

Mobility Management Entity (MME): It is responsible of all the control plane links. It might have either IPV6 or IPV4 addressing and it communicate with the eNodeBs using S1 Application Protocol (S1-AP defined in the TS 36.413). The interface towards the Serving Gateway (S-GW) is the intergace S11 which is addressed with IPv4.

Home Subscriber Server (HSS): Is basically the main subscriber database. Other entities like the IP Multimedia Subsystem or IMS (missing from the picture above) will query this database for details on the subscribers and their capabilities. HSS is connected to the MME through an interface called S6a which is IPv4 or IPv6 addressed.

Equipment Identity Register (EIR): Although is similar to the HSS it provides a database of user equipment. This allow to have black or white list to grant or denied access to the network to certain UE. EIR is connected to the MME through an interface called S13 which is IPv4 or IPv6 addressed.

Serving Gateway and PDN Gateway (S-GW & P-GW): The main function of the Serving Gateway is routing and forwarding of user data packets. It also participates in hand-over packets (during the handover) to different eNB. The P-GW on the other hand connects the UE to external networks (in the user plane)

Cheers!

Diego Goncalves Kovadloff

References:

MME Administration Guide, StarOS Release 21.24 – Mobility Management Entity Configuration [Cisco ASR 5000 Series]. (2021, 11 octubre). Cisco. https://www.cisco.com/c/en/us/td/docs/wireless/asr_5000/21-24/MME-Admin/21-24-mme-admin/21-17-MME-Admin_chapter_010.html

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