SS7 (signalingSystem 7) or "out-of-band" signaling is the most widely used signaling system where the signals are transmistted on a seperate physical channel from the call channel.

Because of that SS7 network can examine all parts of a call route for free lines an allocate them at once. This leads to a more efficient network and fast call setup times in generally in and about 3 seconds where as in early CAS systems this might take about 20 seconds.

Also it has many more advantages compared to the early signaling systems such as CAS (Channel Associated Signaling) or "in-band" signaling where signals and the call are transmitted on the same channel.

SS7 allows telecommunications networks to offer wide ranges of services such as telephony, fax transmission, data transfer, ISDN and IN (Intelligent Networks). SS7 is popular because :

- Flexible and expandible message types allow transmission of more info per signal

- Reliability

- Capacity up to 64Kbps.

In SS7 networks, the signals are transmitted on dedicated channels seperate from the call channel and the signal data are encapsulated in a packet (also called frame or signal unit) that is encoded digitally before transmission. For this, the switching office must be equipped with the electronic processors to encode the incoming and to decode the outgoing messages.

Each SS7 message is at least made up of 5 fields.

- Address of the destination office : This is the address of the destination office also named as DPC (Destination Point Code).

- Address of the originating office : This is the address of the originating office also named as OPC (Originating Point Code). Not to cause any misunderstanding, this OPC does not identify the caller. Instead, it's the address of the office where the caller is attached.

- Message packet number is to synchronize the messages at the destination in case they arrive out of order in which they are sent.

- Data field cantains messages, all the information, circuit identification number (CIC) to identifiy the call which the signal refers.

- Error checking field is to check if there is any error occured during the transmission.

SS7 Signaling Types :

SS7 supports 2 types of signaling:Circuit related signaling and non-circuit related signaling.

Circuit related signaling is used to setup, maintain and release a call's circuit. Every circuit related signal contains a CIC (Circuit Identification Code) in the data field of the message body. With this code the switch can identify which call circuit refers which signal.

Non circuit related signalingmessages don't refer to a particular circuit so they don't carry CIC. They are used to exchange special information between the switching offices.

SS7 Signaling Transfer Modes :

The signaling and the call are transfered on different channels in 3 types of modes.

- Associated Signaling Mode : In this mode the messages follow the same path as the related call. i.e. they go in a seperate channel but follows the same route with the call channel.

- Non-Associated Signaling Mode : In this mode the messages are not required to travel the same path as the call to which they refer. Also each message can travel in different paths too. This mode is more efficient but cause problems if the messages arrive out of the order in which they are sent so, it's generally not used.

- Quasi-Associated Signaling Mode : This signaling mode is a special case of non-associated signaling mode. In this mode signals follow a different path relative to the call but the same path relative to each other to make sure the messages arrive in the same order in which they are sent.

Steps of a Basic SS7 Call:

1) The caller takes the phone "off-hook", dial the destination number. The subscriber signaling pass this information to the local calling office.

2) The local originating office which use SS7, encapsulates the dialed number and the CPC (calling Party Category) information in to the first signal IAM (Initial Address Message) to setup the call to the destination office. In some cases IAM can be replaced with IAI (Initial Address Message with Additional Information) to pass more information.

3) On the route to the destination each receiving office checks the DPC (Destination Point Code) with its own Point Code to see if the message is destined to itself. If not it transfers the message to the next office in the route. When the destination office finally receives the IAM or IAI, it checks the subscriber number to see if it's free. If free then sends back the ACM (Address Complete Message).

4) At this point, the voice circuit is opened, ring back tone is put on the circuit back to the caller and ringing current is sent to the dialled number's phone.

5) When the called subscriber answers, the destination switching office sends back ANC (Address Charge Message) to the first office to begin call charging.

6) When the conversation is over, to release the call circuit, the originating switching office sends CLF (Clear Forward) and the destination switching office sends back the RLG (Release Guard) signals.

SS7 and ISUP:

SS7 signaling system is used both in PSTN (Public Switched Telephone Network) and ISDN (Integrated Services Digital Network). ISUP is the ISDN User Part used for SS7 signalling. It's the most widely used signaling.

ANI service might be an example of a ISDN service where ISUP signaling is involved. ISDN Network offers both voice and non-voice services to ISDN customer such as ANI (automatic Number Identification) also known as CLIP (calling Line Identification and Presentation) or caller ID. The ISUP signal IAM includes the calling lines number. ANI service allows callers number to be displayed at the dialled numbers ISDN terminal by reading the calling number from the IAM message.

Another example of a ISUP in ISDN network is the User-to-User text messaging between ISDN customers. These messages can be sent with USR (User-to-User) message which is read by the called numbers ISDN terminal.