Call flow describes the process by which a network routes a call to a mobile device. Even LTE devices still use 3G networks for most voice calls. 3G call flow is different in a GSM network than in a CDMA network. In the United States, AT&T and T-Mobile operate GSM (Global System for Mobile Communications) networks, while Verizon Wireless and Sprint Nextel operate CDMA (Code Division Multiple Access) networks.
GSM Call Flow
A GSM mobile phone uses a Random Access Channel (RACH) to request a dedicated channel from the base station. The base station responds with a Radio Resource Assignment on the Access Grant Channel (AGCH).
Then the device sends a service request to the base station on the Stand Alone Dedicated Control Channel. But the base station will not grant service until the device authenticates itself, so it sends an authentication request to the user’s SIM card in the form of a random 128-bit number (RAND). The mobile device uses the RAND to generate a 32-bit number, called the Signed Response. Once the base station gets this response, the user is authenticated.
Then the device sends to the base station the ISDN (integrated service digital network) number of the phone that it is trying to call, and the base station assigns a traffic channel. The base station then sends the call to the mobile switching center (MSC) via the base station controller (BSC).
Source: BSI
The mobile switching center (MSC) sends an initial address message to the network via the GSMC (Global System for Telecommunication), and the call is routed to the correct switching center, often via the landline network. The network generates an address complete message (ACM) when the correct switching center is found. When the MSC receives this message it alerts the base station, which in turn alerts the mobile phone of the caller to generate a ringing sound in the earpiece. If the called party answers, the MSC alerts the caller’s mobile device to transmit voice on the assigned traffic channel.
CDMA Call Flow
CDMA networks increase bandwidth by spreading code, and the codes are specific to each user. This means that many users can share the same frequency at the same time. Code spreading is accomplished through the use of PN (pseudo-noise) codes, pseudo-random numbers that will sum to zero over time.
The PN code modulates the pilot channel, the first channel a mobile device will acquire when it detects a CDMA network. Once it has acquired the pilot channel, the device identifies a specific carrier’s network and synchronizes its timing.
When initiating a call or data session, the device sends a signal first to the base station. In CDMA2000, the device initiates the session as either a voice session, a packet data session, or both.
Source: All About Telecommunications
The base station sends the signal to the base station controller, and RAN authentication occurs. Then the signal travels to either a packet-switched core network or to a circuit-switched core network.
In a circuit-switched core network, the mobile switching center (MSC) uses cellular radio-telecommunications intersystem operations (formerly known as IS-41) to send the call to the home location register and the authentication center. The authentication center (AC) shares a unique 64-bit key with the mobile device and uses this key to generate a shared secret data (SSD) code that is used to authenticate the device. Once the device is authenticated, the MSC routes the call through the cellular and landline network to complete the call.
In a packet-switched core network, data is “packed” in a certain order and will be “unpacked” in the reverse order by the receiver. The PSDN (packet data service node) facilitates this data “packing.” Data often travels first over a private IP network and then to the larger Internet.
