26.6 ADDRESSING AND SIGNALING UNITS

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Addressing: Each signaling point is assigned an address in a three-level hierarchy:

Network number, cluster number, member number

Each signaling point is assigned to a cluster of signaling points. Each cluster is part of a network. Each of these numbers is an 8-bit digit (0 to 255). The three-level address is known as the point code of signaling point. Network numbers are assigned by a national authority for each operator.

Each signaling point in the SS7 network has an address in the format network number, cluster number, and member number. Each of these numbers is between 0 and 255.

Signaling units: Signaling messages exchanged over the signaling links are called signaling units (SUs). In SS7 protocols, there are three types of SUs.

Message signal units (MSUs)
Link Status signal units (LSSUs)
Fill-in signal units (FISUs)

These SUs are transmitted continuously in both directions. FISUs are fill-in signal units—if a signaling point does not have MSUs or LSSUs to send, it will send FISUs continuously to facilitate link monitoring.

The three types of signaling units are message signaling units, link status signaling units and fill-in signaling units.

SU formats: The formats of the SUs are shown in Figure 26.5. The following fields used by MTP level 2 are common:

Flag: Marks the end of one SU and the beginning of the next SU. Flag will be of the form 01111110.
Checksum: Each signaling point calculates the checksum and asks for retransmission if the checksum does not match.
Length indicator: This is the number of bytes (octets) between itself and checksum. This is 0 for FISUs, 1 or 2 for LSSUs, and greater than 2 for MSUs. Hence, the type of SU is known from this field. If the length is greater than 63, the value 63 is placed in this field.
BSN/BIB and FSN/FIB: The backward sequence number (BSN), backward indication bit (BIN), forward sequence number (FSN), and forward indication bit (FIB) are used to confirm receipt of SUs and to ensure that they are received in order as well as for flow control.

click to expand
Figure 26.5: Signaling unit formats.

MSUs and LSSUs are assigned sequence numbers, which are put in the FSN of an outgoing SU. The SU is stored until acknowledgement is received by the signaling point. Since only seven bits are assigned, a signaling point can send only 128 unacknowledged SUs.

Signaling points acknowledge the SUs by placing the sequence number of the last correctly received SU in the BSN. The forward and backward indication bits are used to indicate sequencing or data corruption errors and to request retransmission.

Note

The fill-in signaling units are exchanged between signaling points continuously if there is no other data to transmit. These signaling units facilitate continuous monitoring of the status of the links.

26.6.1 Functions of SUs

FISUs: FISUs have no information payload. These SUs are exchanged when no LSSUs or MSUs are to be sent. These SUs undergo error checking, so constant monitoring of link quality is done.

Line status signaling units are exchanged between the nodes to exchange status of the link. The signaling unit need not contain the address because the links are point to point.

LSSUs: LSSUs are used to communicate information about signaling links between the nodes on either end of the link. The information is sent in the status field. This is to indicate initiation of link alignment, quality of received signal traffic, and the status of processors at either end. No addressing information is required because the SUs are exchanged between the two signaling points.

MSUs: In addition to the common fields indicated above, the MSU consists of the service information field, which contains three types of information:

Four bits to indicate the type of information in the signaling information field, called the service indicator. A value of 0 indicates signaling network management, 1 indicates signaling network testing and maintenance, 3 indicates SCCP, and 5 indicates ISUP.
Two bits to indicate whether the message is for national or international network (generally 2 for national network).
Two bits for priority. The priority value can be 0 to 3, with 3 indicating the highest priority. The priority field is used only when there is network congestion. The field indicates whether the message can be discarded or merits transmission in case of congestion.

The format of the MSU is based on the service indicator, with the first portion of the signaling information containing the routing label of seven octets. Three octets are for destination point code (address of the destination node), three octets for the message originator point code (address of the originating node), and one octet for signaling link selection to distribute load among the redundant nodes.

Message signaling units are used to exchange signaling information to set up and disconnect the calls.

The SS7 networks are now being deployed throughout the world by basic telecom operators as well as cellular operators to provide efficient signaling and value-added services.

Note

SS7 is used in mobile communication systems and ISDN. SS7 is also now being deployed in the PSTN.

Summary

Signaling System No. 7 is a data network that provides an efficient signaling mechanism in telephone networks. The network consists of signaling switching points (SSP), signaling transfer points (STPs) and signaling control points (SCPs), which are interconnected through high-speed links (56kbps or 64kbps). Using a separate dedicated data network for signaling results in fast signaling. To provide 100% availability of the signaling network, the STPs and SCPs and the links are provided with the necessary redundancy.

In addition to providing efficient signaling, SS7 allows value-added services such as toll-free numbers and calling card facilities to be provided. Due to these advantages, SS7 is used extensively in the ISDN and cellular mobile networks. SS7 signaling is also now being used in the PSTN.

References

Guy Redmill. An Introduction to SS7. Brooktrout Technology, July 2001.
http://www.openss7.org You can get information about open source software from Linux on this site.
http://www.cisco.com You can get excellent technical literature on SS7 and product information from this site.

Questions

Explain the drawbacks of the present PSTN signaling.
What are the advantages of signaling based on SS7?
Explain the SS7 protocol architecture.
Explain the procedure for call establishment using SS7 signaling.
Explain how a toll-free number can be accessed using SS7 signaling.

Exercises

1.	You can buy a prepaid telephone calling card to make telephone calls. The procedure is to dial a specific number given on the calling card, give your calling card number, and then dial the telephone number. SS7 signaling is used for verification of the calling card number and how much money is still left. Explain how the processing is done using a database query.
2.	SS7 signaling is used on the GSM network. Study the details of the signaling in mobile communication networks such as GSM and prepare a technical paper.
3.	SS7 is now being used on ATM networks. Write a technical paper on SS7 over ATM.
4.	Search the Internet for freely available source code for the SS7 protocol stack.

Answers

1.	When you buy a prepaid telephone calling card, the card contains a number and a telephone number you need to call. The procedure for verification is as follows: You dial the telephone number given on the calling card. You hear a message to dial the calling card number. You dial the calling card number. The server checks the database and finds out the value of the calling card, how much has already been spent, and how much is still left. You will hear this information, and then you can dial the telephone number of your choice. You can converse with the person. The total cost of the call will be deducted from the remaining amount, and the new amount will be stored in the database. The entire signaling is done through the SS7 network.
2.	In a GSM network, a separate data communication network is used to carry the signaling information. This network is based on SS7. Even for communication between mobile phone and the base station, there will be a separate time slot that carries the signaling information. The voice slots are only to carry the voice data.
3.	You can obtain the information on SS7 over ATM from http://www.cisco.com.
4.	SS7 protocol stack is available from http://www.openss7.org.

Projects

Obtain freely available source code for SS7 protocol stack, by searching the Internet. Run the stack on your system and study the source code to understand implementation aspects of it.
Implement an SS7 network over a LAN. You can implement a database query operation by implementing the STP and SCP over different nodes. Two nodes on the LAN can be considered subscriber telephones, one node as STP, and one node as SCP. When one subscriber makes a toll-free call, the number has to be passed to the STP. The STP has to send the information to the SCP, which has a database of the toll-free numbers and the corresponding actual telephone numbers. The SCP has to tell the STP the actual telephone number. The STP in turn has to tell the telephone subscriber (the designated node) the actual number, and then the message from the subscriber has to be sent to the other subscriber.

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