Signaling System No. 7 (SS7) is a robust and foundational signaling protocol suite that underpins much of the world's Public Switched Telephone Network (PSTN) and mobile networks. While it doesn't directly "see" or process the dialed digits of a phone number in the same way a routing table does in an IP network, SS7 plays a critical role in facilitating call routing based on these numbers through a series of interconnected processes and network elements.
Here's how SS7 enables call routing using phone numbers:
1. Call Setup Initiation (Originating Switch - SSP):
When a caller in Dhaka dials a phone number, the originating Service Switching Point (SSP), which is the local exchange or mobile switching center serving the caller, recognizes the dialed digits.
The SSP analyzes the dialed number to determine if it's within its local serving area or requires routing to another network or exchange.
2. Forming the Initial Address Message (IAM):
If the call needs to be routed, the originating SSP constructs an Initial Address Message (IAM). This is a crucial SS7 message that contains all the necessary information to set up the call.
Key information within the IAM includes:
Calling Party Number: The phone number of the person making the call.
Called Party Number: The phone number that was dialed. This is the afghanistan mobile phone number list primary piece of information used for routing.
Circuit Identification Code (CIC): Identifies the specific voice trunk circuit that the SSP proposes to use for the call.
Other Parameters: Information about the call type, requested services, etc.
3. Routing the IAM Through the SS7 Network (STPs):
The IAM is then sent over the dedicated SS7 signaling network. In many network architectures, especially larger ones like those in Dhaka involving multiple operators or long-distance calls, the IAM will likely pass through one or more Signal Transfer Points (STPs).
STPs are essentially routers for SS7 signaling messages. They don't carry the voice traffic but examine the destination information within the IAM (primarily the Called Party Number) and, based on their routing tables, determine the next signaling point in the path towards the destination.
SS7 routing can be based on:
Point Codes (PCs): Unique addresses assigned to each node (SSP, STP, SCP) in the SS7 network. STPs route messages based on the destination PC.
Global Title Translation (GTT): Phone numbers are often treated as "global titles." STPs or dedicated Service Control Points (SCPs) perform GTT, which involves looking up the Called Party Number in a database to determine the actual network address (Point Code and Subsystem Number) of the destination exchange. This is crucial for number portability, toll-free numbers, and other advanced services.
4. Reaching the Terminating Switch (SSP):
The IAM traverses the SS7 network, hopping from one STP to another (if necessary), until it reaches the terminating SSP, which is the local exchange or mobile switching center serving the called party in Dhaka or elsewhere.
5. Call Establishment by the Terminating SSP:
The terminating SSP receives the IAM and analyzes the Called Party Number. It checks if the number is valid and if the subscriber is available.
If the called party is available, the terminating SSP allocates a voice circuit to the called party's line and sends a signaling message back through the SS7 network (e.g., an Address Complete Message - ACM) to indicate that the call is progressing.
When the called party answers, the terminating SSP sends an Answer Message (ANM) back to the originating SSP, at which point the voice path is fully established over the reserved circuit, and billing begins.
In essence, SS7 facilitates call routing based on phone numbers by:
Encapsulating the dialed number (Called Party Number) within signaling messages (like the IAM).
Using a dedicated signaling network to transport these messages efficiently and reliably.
Employing STPs to route these messages based on network addresses (Point Codes) and through Global Title Translation of the phone number to find the destination network.
Coordinating the actions of different network elements (SSPs, STPs, SCPs) to establish and manage the voice path for the call.
Therefore, while SS7 doesn't directly route based on the digits of the phone number like an IP router uses an IP address, it provides the signaling framework and mechanisms (message formats, routing protocols, database lookups) that enable the telecommunications network to interpret phone numbers and efficiently route calls to their intended destinations, both within Dhaka and internationally. The speed and efficiency of SS7 are critical for the seamless call connections we experience daily.
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