Telco slogan

IMEI : International Mobile Equipment Identity
IMSI : International Mobile Subscriber Identity
MSISDN : Mobile Station International Subscriber Directory Number = Country Code + operator destination code

GGSN/PGW => Radius message
CGN/FW => NAT message

 GGSN - Gateway GRPS Support Node

A Gateway GPRS Support Node (GGSN) is part of the core network that connects GSM-based 3G networks to the Internet. The GGSN, sometimes known as a wireless router, works in tandem with the Serving GPRS Support Node (SGSN) to keep mobile users connected to the Internet and IP-based applications.

The GGSN converts incoming data traffic from mobile users (via the SGSN) and forwards it to the relevant network, and vice versa. So, for instance, a Google query from a smartphone would be routed via the SGSN to the GGSN and sent on as a query to the Internet search engine, then the process would be reversed for the reply to the phone user.

The GGSN and the SGSN together form the GPRS support nodes (GSN). The GGSN is also linked into hosted services (such as voice and video) and to the billing, policy control, and user verification elements of the core network.

GGSN vendors include Alcatel-Lucent (NYSE: ALU), Cisco Systems Inc. (Nasdaq: CSCO), Ericsson AB (Nasdaq: ERIC), and Nokia Networks .

PGW - PDN Gateway
 
PGW is exactly doing the GGSN Functionalities in EPC (Evolved Packet Core) which is the Core Network of the LTE/4G Network.

in many commercial cases for 2G/3G/4G Converged Network, PGW replaces GGSN and acts as the Mobile Gateway for 2G/3G/4G radio access types.

CGN - Carrier-grade NAT

also known as large-scale NAT (LSN), is an approach to IPv4 network design in which end sites, in particular residential networks, are configured with private network addresses that are translated to public IPv4 addresses by middlebox network address translator devices embedded in the network operator's network, permitting the sharing of small pools of public addresses among many end sites. This shifts the NAT function and configuration thereof from the customer premises to the Internet service provider network.

Carrier-grade NAT has been proposed as an approach for mitigating IPv4 address exhaustion.^[1]

One use scenario of CGN has been labeled as NAT444,^[2] because some customer connections to Internet services on the public Internet would pass through three different IPv4 addressing domains: the customer's own private network, the carrier's private network and the public Internet.

Another CGN scenario is Dual-Stack Lite, in which the carrier's network uses IPv6 and thus only two IPv4 addressing domains are needed.