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