PS Core Network Concepts

Most of the educational documents related to PS Core Network start with Call Flows. Attach Call Flow, PDP Context, Paging, etc. Basically that was my problem when I started working in PS Core because the Call Flows include a lot of messages which in turn include a lot of parameters and Information Elements so starting with the Call Flows without knowing at least the Identifiers included in these messages is not the best approach to understand PS Core principles.

This is why this article will be all about the MBB terms that are commonly presented in all Call flows and in most of the MBB talks in general. Once one is comfortable with that, the Call flows will be easy to interpret.

I am bringing some for clarification. 

International Mobile Subscriber Identity (IMSI)IMSI

IMSI is a unique Identifier that is allocated to each MS in GSM/UMTS System and stored in SIM Card. (Conforming to ITU E.212 numbering standard)  


Temporary Mobile Subscriber Identity (TMSI)

In order to support the subscriber identity confidentiality service the VLRs and SGSNs may allocate Temporary Mobile Subscriber Identities (TMSI) to visiting mobile subscribers.

Below is an MS providing P-TMSI Identity to Network


International Mobile Station Equipment Identity IMEI identifies the Handset and not the user. It consists of the following elements

– Type Allocation Code (TAC). Its length is 8 digits.

– Serial Number (SNR) is an individual SN uniquely identifying each equipment within the TAC.

– Spare digit: this digit shall be zero, when transmitted by the MS.

Try typing *#06# to know your IMEI!  


Access Point Name (APN)

In PS Core, an Access Point Name (APN) is a reference to GGSN where in general, it can be seen as a reference to the Service.

According to the APN set on Handset and provisioned in Network, user will get a specific IP Address that is routable to a certain service.


This is my phone where the provisioned APNs are set and configured. most probably if I choose MMS then i will get an IP address routable to MMS PDN.

let’s see now from a high level perspective how the session is established (user gets internet access) in a 3G domain.  


  1. UE sends Attach Request to Core Network including the MS Identity
  2. SGSN initiates the authentication procedure (AKA) for the UE in coordination with HLR.
  3. Once authenticated, SGSN updated Location in HLR and retrieves the user subscription profile.
  4. If there are no restrictions, SGSN accepts the Attach Request.
  5. At this stage, UE is capable to send request for Services in the Form of “Activate PDP Context Request” message.
  6. SGSN Conveys the message to GGSN via GTP message “Create PDP Context Request” enriched by parameters from Subscriber Subscription profile.
  7. GGSN validates the request and verifies that UE has no restriction with respect to Charging & Policies. Allocates an IP Address for the UE and then respond with GTP message Create PDP Context Accept.
  8. SGSN replies to UE with Activate PDP Context Response message carrying the IP address allocated via GGSN. (Prior to this message Radio Resources are being set via SGSN and RNC)
At this stage, UE can browse the Internet making use of the routable IP Address allocated via GGSN where the bearer is divided into Radio Access Bearer (RAB) between UE & SGSN and GTP Tunnel between SGSN & GGSN. 

GGSN is performing the Tunneling for the DL traffic to UE and the de-tunneling for the UL Traffic to Internet.

With the above Call flow, I close the 3G talk and starting from the next article, I will discuss the 4G Evolved Packet Core Setup.

I hope that was beneficial.. see you in next article.

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Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

Today, with his carefully selected instructors, OrhanErgun.Net is providing IT courses to tens of thousands of IT engineers. 

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