Understand the Telecom Basics

Q. What is SS7 (signaling System 7)?

Ans : SS7 (signaling System 7) - signaling is the most widely used signaling system where the signals are transmitted on a separate physical channel from the call channel. 

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

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

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

 

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 signaling messages 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. They are transfered in seperate channels 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. 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.

ISDN (Integrated Services Digital Network) is a set of CCITT/ITU standards for digital transmission over ordinary telephone copper wire as well as over other media.

There are two levels of service: The Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channels. Each B-channel carries data, voice, and other services. Each D-channel carries control and signaling information.

The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D- channel. Thus, a Basic Rate user can have up to 128 Kbps service.

Primary Rate consists of 23 B-channels and one 64 Kpbs D-channel in the United States or 30 B-channels and 1 D-channel in Europe.

ISDN in concept is the integration of both analog or voice data together with digital data over the same network. Although the ISDN you can install is integrating these on a medium designed for analog transmission, broadband ISDN (BISDN) is intended to extend the integration of both services throughout the rest of the end-to-end path using fiber optic.

The signaling part of the ISDN networks is the SS7 - ISUP signaling.

 

Signaling System = SS7

Signaling System 7 is a common channel signaling system. This means one channel (either analog and almost always digital) is used only for sending the signaling information whether the system has one bearer channel or multiple bearer channels.

In order to support this architecture, a new protocol was developed and is a variation of data packet switching. This means the signaling channel uses framing words, checksums , packets for address and information packets. The order of these packets are well defined and flexible for the user requirements.

Signaling System 7 on T1 circuits are typically split into what is called the A link and the B link. This provides link redundancy in case of equipment failure and link failure whereby the entire network does not fail. Normally, systems do not even include voice channels on the signaling links. (They normally put them on separate T1 circuits which are called "bearer" channels and they may be fixed mapped or connected through a digital crossconnect system (DACCS).

One timeslot on the signaling T1 link is used for transmission of signaling system 7. The applications have the flexibility to define any of the 24 timeslots as a signaling channel so the converter equipment must also have this flexibility.

There appears to be two modes of operation for the signaling channel. They are redundant load share and redundant standby.

Redundant load share splits traffic between the A and B signaling links.

Redundant standby links use a method where the signaling receivers receive the same information off the active link and update the memory of the standby link but the standby link is prohibited from transmitting on the link until the active link fails. (A protocol function in the SS7 determines link failure).

SS7 Structure

There are many "buzz" words thrown out about SS7, (i.e. TUP,ISUP,TCAP, etc.)

These are a simple description of the type of protocol used for the user functions but they omit the complete nature of the protocol which is as follows:

Physical Layer (1)

·       This is typically the requirement of the link (i.e. T1 or E1). The specification of the link applies to this layer.

·       T1 uses any timeslot of 24 channel link.

·       E1 typically uses timeslot 16 for the SS7 access.

·       T1 and E1 systems use 64KB/s data transfer on the timeslot for SS7.

 

Message Transfer Part (MTP) Layer (2) (Protocol Framing etc.)

·       The 64KB/s data from a timeslot in Layer 1 is processed here.

·       This layer forms the protocol framing, alarming, error correction and detection and provides a smooth data path to the next higher layer

 

Message Transfer Part (MTP) Layer (3) (Message Transfer Protocol functions)

·       The location of where to send messages and control of the link is performed here.

·       Management of the user part (Layer 4) messages is performed.

User Part (Layer 4) (TUP, ISUP, etc.)

·       This layer actually contains the information about the call

·       Typical information consists of called number, calling number, class of call, routing specifics, types of lines, etc.

TUP = Telephone User Part

·       TUP was an earlier implementation of SS7 and does not allow for data type applications (except in special applications).

ISUP = ISDN User Part

·       ISUP is turning out to be requested more of often then other protocols.

·       ISUP means Integrated Services User Part.

·       ISUP supports both voice and data applications.

·       ISUP provides a direct link to ISDN primary rate applications.

·       Other user parts - There are other user parts for mobile cellular applications, data only and specific functions which are not addressed at this time.

 

 

ISDN PRI

A PRI (Primary Rate Interface) is a telecommunication standard used in the Integrated Services Digital Network or ISDN, for carrying multiple DS0 voice and data transmissions between two physical locations. PRI was developed specifically for industrial or large quantity users. PRI is an industrial ISDN line while the Basic Rate Interface, or BRI, is used to cater to home and small enterprises.

Both Primary Rate Interface and Basic Rate Interface are made up of a number of B channels and D channels. B Channel or the Bearer Channel is used for data transmission, including voice, and D channel is meant for signaling and control. A PRI is made up of 23 B-channels and one 64 Kbps D-channel in a T-1 configuration and 30 B-channels and 1 D-channel using an E1 line.

 

 

The T-carrier system is standard in Japan, US and Canada, while E1 is popular across Europe and Australia. In other words, in North America and Japan PRI is represented as 23B+D with a total bit rate of 1.544 Mbit/s (T1) while it is 30B+D in Australia and Europe, which is equivalent to a bit rate of 2.048 Mbit/s (E1). The Primary Rate Interface makes use of the Q.931 protocol over the D channel.

A PRI is typically used to establish communication between a PBX, or a private branch exchange, which is a telephone exchange operated by the customer of a telephone company, and a Central Office of the telephone company or an Inter Exchange Carrier or IXC, a long distance telephone company. The advantage of primary rate interface or PRI is that the 23 or 30 B channels can be used in various combinations for specific data transmission needs, such as a videoconferencing, allowing bulk data transfer to be achieved more flexibly.

 

 

PRI is a standard which is used for transporting multiple data and voice transmissions between a source point and the receiving end. If you were wondering, PRI stands for Primary Rate Interface.

There are two different types of interfaces involved with ISDN (Integrated Services Digital Network), which is what PRI is associated with. There is the primary integrated services type, and then there is the Basic Rate Interface, which is also known as a BRI. A basic rate interface is used to cater to homes and smaller sized enterprises and businesses.

Between the two different interfaces, there are a different number of channels. There are B channels, and there are also what are known as D channels. The B channel, which is also known as the Bearer Channel, is used for transmitting data. This type of data may consist of voice data, for example. The D channel is used for signaling and control of the transmission. These channels combined make up the total PRI. In T-1 configurations, the PRI is made up of 23 B channels and only one D channel, which is 64 KBps. In E1 lines, it instead uses 30 B channels and again 1 D channel.

The T-1 system is widely used in areas such as the US, Canada, and Japan. However, in some places such as Europe and Australia, the E1 carrier system is more popular. North America and Japan can be said to be represented by the 23B+D system, with a total bit rate of 1.544 Mbs, which is what T1 was more specifically. European countries and Australia are represented by the bit rate of 2.048 Mbs, which is what E1 is more specifically.

PRI is used when establishing a connection between a PBX and a central office of any particular telephone company.  It may also be used to connect to an IXC (Inter Exchange Carrier), which is basically just a long distance telephone company. PRI has an advantage in that it can be used to combine certain data in it’s transmissions, which allows for more data transfer to be achieved overall.

What is a PBX?

PBX (private branch exchange) is a telephone exchange that serves just one company. This is a cheaper alternative to leasing a bunch of telephone lines from any one telephone company. PBXs, through the use of trunks, make connections through internal lines that are connected to each individual phone. The employees can then pick up the phone, and through the use of the PBX, make outside calls after dialing the outside access code.