In 1975, the Signal System Number 7 was recognized by the International Telecommunication Union as the standard in communication around the world.  The SS7 particularly replaced the SS6, which was the previous version of the protocol and was restricted to 28 bits in signaling messages.  This particular system provides the Public Switched Telephone Network and wireless communication network controllers.   The basic functions of SS7 are controlling the call from start to finish, such as setting up the call by transporting the signaling messages, managing the call, and terminating the call.  Other functions of SS7 include managing numbers, and charging the customers; call forwarding, conference calls, and secured voiced and data messaging.  SS7 provides unique roaming services and the customer with easy accessibility.  It operates in two modes:  Associated Mode and Quasi-Associated Mode.  These modes are the backbone of how SS7 transports its signal through network devices.  For example in the Associated Mode, SS7 signal must be transported through switches via the Public Switched Telephone Network.  On the other hand, Quasi-Associated Mode only requires a path from one switch to another without going through a third party.  The SS7 protocol has been widely used as the global standard network for telecommunication.  This paper will present a brief summary of the functions, and usage of the protocol.

       The SS7 protocol stack is similar to the Open System Interconnection or known as the OSI model.  However, there are only four levels are considered to be similar with the OSI layers.  The SS7 protocol stacks are identified at the Message Transfer Part (MTP) level 1, level 2, level 3, and the top level.  These are the same as the physical layer, data link layer, network layer, and the application layer.  The top level consists of TUP, TCAP, ISUP, and SSCP; each section of this level is important to the protocol stack.  The Message Transfer Part (MTP) level 1 is defined in the physical and electrical characteristics in providing the signaling; which include the E-1 network which has 2048 kb/s with providing 30 voice channels with a synchronizing channel and a common signal channel, and the DS-1 network has 1544 kb/s with providing 24 voice channels, or 23 voice channels and one common signaling channel.  The message Transfer Part (MTP) level 2 provides the accurate data transmission during the call including the call initiation and termination.  In this transmission, the layer implements sequence validation, error protection, and flow control.  If the error is detected during the signal transmission, the message will be retransmitted.  The MTP level 3 offers the routing function between the signal points, which avoids failure connection transmission when the signals are congested.  The ISDN User Part distinct the protocol is initiating the call, managing the call, and terminating the circuits to end the call.  Perhaps, if the call is made within the switch system network, it does not require using ISUP signaling.  The Telephone User Part is simpler than ISUP, example is no caller ID; however, TUP is used more in handling analog circuits.  Certain parts of the world still use TUP for setting up the call and terminating the call processes.  The majority networks in the globe replaced TUP by the ISUP, especially in the European Union Nations and North America.  The Signaling Connection Control Part (SSCP); is a connection-oriented and connectionless network.  Its functions are controlling the toll free number, calling card number, and customer identification number; therefore, it is easy to track the billing usage.  The SSCP would translate these numbers into the subsystem number to identify the destination signal to the Transaction Capabilities Application Part.  The TCAP provides information between non-circuits data across the SS7 by using SSCP services.  Its functions are authentication, personal identification number validation, equipment identification, and roaming services.

       The Signaling Links is known to transmit both directions between 56 to 64 kilobits per section.  These signals are transmitted on an out-of-band channel, separate from voice channel because they are Intelligent Network to separate these two types of signals.  It also prevents fraudulent network service usage, such as stealing network for commercial purposes and reselling the calling cards.  The Signaling Point are unique because the point codes.  Point codes are addresses in the SS7 network and identify the destination and source.  The Signaling points use routing table rule for the path of individual messages.  There are three types of Signaling Point in SS7:  Service Switching Point (SSP), Signal Transfer Point (STP), and Service Control Point (SCP).  The SSP’s are performing as switches that initiating and terminating the call.  The Signaling messages are being setup, manage and terminate in the voice circuits for every single call.  The STP provides signaling points by routing new message for every incoming message to the specific SSP.  It also acts as a firewall to ensure exchanged manages are secured.  The SCPs provide the configuration service in the event of signaling failure, or for special services like “800” numbers.  If the transmission failed, the SCP re-route the signaling messages to a different base links, or remove the call to a different destination for 800 services.

     There are six different Signaling Link types; these are from A to F, in respect to the SS7 protocol.  The A link is connecting between the signaling end points for message from or to the destination sources.  The B link is connecting between STP’s to provide signaling message transmission.  The C link is connecting between STP’s; however, it pairs up the signal point for reliability if a link fails.  The D link provides secondary connection in STP’s unless there is a problem with the primary, and then the D link temporarily replaces the B link connection.  The E link is connecting between SSP and STP, it is an alternating path in the event that the A link is not reachable.  The F link is connecting between the two end points likes the A link.  Normally, it is not used in STPs, unless there is a problem with STP’s, and not being used; therefore, the F link is replaced for straight connection.

      The typical call process in the SS7 involves two phone switches; one is as a sender “A” and the other one is a recipient “B.”   During the initial calling process, the sender signaling message goes through a switch “A” to determine the destination number and linking itself to the switch “B.”   The trunk voice routing would be setup in response to address message to define the destination source.  Therefore, from the switch “A” routing the message to the switch “B,” and determine whether the call is signaling or busy due to the recipient engaged in calls.  Once the setup is done, the switch “B” would start to formulate the answer message to identify the connection.  The answer message indicates if the destination reason has gone “off-hook.”  This leads to a voice channel connection.  If the sender hangs up the phone first, then the switch “A” would generate a release message (REL) to the recipient or switch “B.”  The opposite happens when a user hangs up.  In either this case, the other party receives a release complete message (RLC).

         In brief the SS7 is a global standard protocol defined by the ITU for telecommunication, and especially for wireless networks.  It consists of 4 different levels and similar to the Open System Interconnection Model.  It has three different signaling Point types and sign different link types.

References:

. Bates, Regis J. “Broadband Telecommunications Handbook.” McGraw-Hill Companies Inc, United States. 2002

. Heine, Gunnar.  “GSM Networks: Protocols, Terminology, and Implementation.” Artech House Inc., Norwood, MA. 1999.

. “SS7 Tutorial” International Engineering Consortium.  November 27, 2008.  <http://www.iec.org/online/tutorials/ss7>