Telecommunications Signaling and the SS7 Network
The telecommunications network was developed to provide circuit switched services. Circuit switched service is the procedure where network equipment and facilities are dedicated to a connection for the length of the session. The cost of using the service is based solely on the length of time of
Circuit switched services are separated into two main categories, voice and data. The reason for this separation is that are different requirements for each type of call. The telecommunications network was designed to process voice connections. A voice connection must therefore pass information which is in the normal range of human hearing, between 300-4000Hz. In telephone language this range is slightly narrowed and is referred to as Speech or 3.1KHz audio. Actually the 3.1KHz refers to sending "data" over a voice connection by using a modem.
The procedure used to initiate, monitor and release circuit switched connections is known as signaling. As stated earlier voice and data calls have different requirements. As the public network evolves more of the traffic on the network are in support of data applications. An example of data traffic would be a remote LAN access connection or a video conference session. The object of the various signaling techniques is to establish the desired connection quickly while providing the network with the necessary information for the desired service.
In-band signaling is the procedure that has been used on the telecommunications network since the inception of the automatic switch. In-band signaling is the procedure where the user hears dial tone and then enters the telephone number (address) of the desired connection. In this process the facilities (wire,fiber,etc.) are shared. The facilities are first used by the user to communicate the number to the network and then once the connection is made the same wires are used to send voice/data to the other end. When the user wishes to end the session the user hangs up, thus signaling to the network to release the connection. The procedures are as follows. Since the network is designed to handle such calls the assumption is that unless otherwise specified a voice connection is desired. The procedures are therefore simple.
1.) User lifts receiver, hears dial tone and dials digits.
2.) Voice/data is passed over the network connection.
3.) User concludes session by hanging up and thus releasing the circuit.
The connection requirements for a data call are such that much more information must be passed from the user to the network. This information contains parameters such as the speed of the connection, terminal configuration and the protocols in use. Indeed there are standard protocols required to make a data connection. For ISDN the protocol is called Q.931.
Out-of-band signaling is the procedure where the information associated with managing a connection are communicated to the network using facilities (wire, fiber, etc.) that are separate from those used to relay voice or data to the other end of the connection.
There are two types of out-of-band signaling in use on the public network. Both signaling procedures require that the user be in constant communication with the network. The D channel is used in ISDN to pass signaling information. In the SS7 network Links connect the network components.
Associated signaling is a procedure where information is passed from the user terminal to the network over facilities different from the voice/data circuit but along the same path. An example of this procedure is the Basic Rate Interface ISDN and the Primary Rate Interface ISDN in use on the MIT campus. The protocol used to relay connection information from the user to the network and from the network to the user is defined as Q.931. This is one of several ISDN protocols used but Q.931 is the layer 3 protocol used to manage end to end connections. An example of an ISDN connection using associated signaling follows.
1.) User initiates a connection, Q.931 packets are passed.
2.) Connection is established with the other end.
3.) User terminates the session by signaling network over the D channel.
Non-Associated signaling is a procedure where the connection information used to establish, monitor and release network components is relayed over facilities different than those used to pass voice or data. The signaling components are separate from the switching fabric. Indeed these components are a network within the public network for the purpose of managing connections. Management of connections is not only related to connecting endpoints but is also concerned with the health and well being of the entire network. Therefore much more information is relayed between network components such as facility failures, congestion or network component failures. This type of signaling is used to relay connection information between telephone offices on the public network. Non-Associated signaling is often called Common Channel Signaling and is used in conjunction with the SS7 protocol on the SS7 network.
The SS7 network refers to the interoffice signaling network in place on the public network. The SS7 portion of the name refers to the internationally recognized standard for interoffice signaling. This standard is a protocol procedure used to establish connections within and between networks.
The SS7 network contains a number of network components. A description of the main components follows.
* Service Switching Point SSP
A Service switching point is a telephone office which is directly connected to the SS7 network. In the telephone industry we refer to an SSP as an office capable of launching a query. Calls originating from non-SSP offices must be routed to an SSP for connection on the network.
* Signal Transfer Point STP
A Signal Transfer Point is a component on the SS7 network that passes signaling information between SSP's and Service Control Points for processing. Service Control points are discussed below. The STP is simply a special application packet switch. The US requirement for the RBOC's is that there must be a minimum of two STP's per LATA. This is to provide redundancy. The configuration is that each SSP is connected to two STP's. The messaging alternates between the two. The first message goes to one STP and the next message leaving the SSP goes to the other.
* Service Control Point SCP
A Service Control Point is a specials application computer that maintains an information database. This database provides information required by service requests from users on the network. Examples of SCP databases are a credit card database which is used to validate card numbers or an "800" database used to process calls to an 800 number.
Links are the facilities used to connect the components of the SS7 network. These links are usually 56Kb packet trunks. Depending on the location of the link within the network a letter identifier precedes the link to indicate the connection required. An example is that the links between an SSP and an STP are referred to as "A" links. Each SSP has two "A" links, one to each STP. As additional capacity is required to handle more messaging "A" links are added in pairs. For instance if another "A"link is required two will be added from the SSP, one to each STP.
The components that make up the SS7 network provide much more than the capability to manage individual connections. Digital switches and the addition of databases has added intelligence to the network. This intelligence provides the platform for the evolution of what is known as the Advanced Intelligent Network. The procedures described below will provide connection oriented switching well into the future.
The first example of an SS7 call demonstrates a regular voice call over the SS7 network. The second example will show the interoperability of the ISDN Q.931 protocol and the SS7 network protocol to make an end to end ISDN data connection. The third connection will show an "800" call type to indicate the operation of an SCP in a call scenario.
Analog Voice Call
1.) User lifts receiver and dials digits.
2.) Digits interpreted by the office which initiates the SS7 protocol procedure.
3.) Distant office notified of incoming call.
4.) Connection is established
5.) User ends the call, connections are released.
ISDN call over the SS7 Network
An ISDN call over the SS7 network requires that the protocol messaging from the ISDN terminal must be mapped into the information requirements of the SS7 protocol. Again the ISDN protocol is defined as Q.931 and the interoffice protocol is SS7.
1.) User initiates a session by dialing the digits. The office recognizes the SS7 call requirement.
2.) The distant end terminal has an idle call appearance and is alerted.
4.) Users terminate the session and the connection is immediately released.
800 NUMBER CALL
A call to an "800" number demonstrates some of the intelligence contained in the public network as a result of the SS7 network signaling techniques and the use of databases such as contained in an SCP. The call scenario is as follows.
I am a successful plumber with office locations in several surrounding towns. To make it easy for my customers to contact me I have an "800" number that can call for service. The number is 1-800 PLUMBER.
1.) User needing my service dials 1-800-PLUMBER
2.) The SSP realizing that this is an "800" type of call realizes that this call need s more information to be completed. It then initiates a query message to the SCP to find out the regular telephone number of the 1-800-PLUMBER location closest to the callers location. It does this by examining the callers telephone number which is also relayed to the SCP.
3.) Now the "regular" telephone number of the office closest to my customer has been obtained by the SSP. The SSP can now process the call to 555-2920.
4.) Call is answered. When the call is delivered to the plumbing company a software package on their PC displays the callers customer profile. This is possible because the callers telephone number is delivered when the phone rings.
5.) From here the call is released in the manner previously discussed.
This is one of a series of articles I have published in MIT TechInfo. Please refer any comments on the article to the writer.