Basic Concept of Signaling SS7

Message Transfer Part (MTP) is mainly responsible for transmitting "User Part" signaling messages across the SS No. 7 network. If a system or signaling network is faulty, MTP can take measures to prevent messages from losing, and repeatedly or disorderly transmitting. MTP consists of signaling data links, signaling link functions, and signaling network functions.

Here are the following exchange in SS7

Signaling point (SP):
A signaling point is a node of SS7. It can be identified by signaling point code (SPC). Source signaling point refers to a signaling point that generates signaling messages. Destination signaling point refers to a signaling point that receives signaling messages. In general, international SPC is 14-bit and national SPC is 24-bit.

Signaling link: 
A signaling link is a physical link that connects signaling points and transmits signaling messages.

Signaling network:
Logically, a signaling network is separated from a communication network. It is used to transmit signaling messages. A signaling network mainly consists of signaling points and signaling links.

Signaling link set:
A signaling link set is a set of parallel signaling links that directly connects two signaling points.

Signaling route:
A signaling route is a route that transmits signaling messages from source signaling point to destination signaling point.

Signaling transfer point (STP):
A signaling transfer point is neither a source signaling point, nor a destination signaling point. It is a signaling point that transfers signaling messages. If a signaling transfer point can serve as a signaling point, it is called an integrated STP. If a signaling transfer point cannot serve as a signaling point, it is called an individual STP.

There are the following two working modes:

Associated: 
Three signaling points (VMSC, GMSC, and VMSC) cannot serve as an STP. The signaling messages between VMSC and GMSC are transmitted through a directly connected signaling link. GMSC can serve as a voice transfer path; however, GMSC cannot serve as a signaling transfer point.If SS7 signaling messages are transmitted through two or more signaling links with one or more STPs, it is called quasi-direct connection. Figure 1-2 shows the quasi-direct connection.

Quasi-associated: 

There is a direct voice path between VMSC and VMSC; however, there is no direct signaling link between VMSC and VMSC. Signaling messages between VMSC and VMSC are transferred by GMSC. GMSC serves as an integrated STP.            

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Multiple Access Technology

There are 3 types of multiple access technology

1.       Time division multiple access (TDMA)

2.       Frequency division multiple access (FDMA)

3.       Code division multiple access (CDMA)

Time Division Multiple Access means that the wireless carrier of one bandwidth is divided into multiple time division channels in terms of time (or called timeslot). Each user occupies a timeslot and receives/transmits signals within this specified timeslot. Therefore, it is called time division multiple access. This multiple access mode is adopted in both a digital cellular system and a GSM. TDMA is a complex architecture and the simplest case is that a single channel carrier is divided into many different timeslots, each of which transmits one-way burst-oriented information. The key part in TDMA is the user part, in which each user is allocated with one timeslot (allocated when a call begins). The user communicates with a base station in a synchronous mode and counts the timeslot. When his own timeslot comes, the mobile station starts a receiving and demodulation circuit to decode the burst-oriented information sent from the base station. Likewise, when a user wants to send any information, he should first cache the information and waits for his timeslot to come. After a timeslot begins, the information is transmitted at a double rate and next burst-oriented transmission begins to be accumulated.

Frequency Division Multiple Access:  frequency division, sometimes called channelization, means dividing the whole available spectrum into many single radio channels (transmit/receive carrier pair). Each channel can transmit one-way voice or control information. Under the control of the system, any user can be accessed to any of these channels. Analog cellular system is a typical example of FDMA structure. Similarly, FDMA can also be used in a digital cellular system, except that pure frequency division is not adopted. For example, FDMA is adopted in GSM and CDMA.

CDMA is a multiple access mode implemented by Spread Spectrum Modulation. Unlike FDMA and TDMA, both of which separate the user information in terms of time and frequency, CDMA can transmit the information of multiple users on a channel at the same time. That is to say,mutual interference between users is permitted. The key is that every information before transmission should be modulated by different Spread Spectrum Code-Sequence to broadband signal, then all the signals should be mixed and send. The mixed signal would be demodulated by different Spread Spectrum Code-Sequence at the different receiver.Because all the Spread Spectrum Code-Sequence is orthogonal,only the information that was be demodulated by same Spread Spectrum Code-Sequence can be reverted in mixed signal.  

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GSM Location Update Flow

The procedure of location update is as follows:

• When a MS finds that better service is provided in a certain location area, this MS notifies the MSC of the MSC/VLR which this new location area belongs to.
• If the original location area and new location area belong to the same MSC/VLR, the MSC only changes the location information of this MS in the VLR. If the original location area and new location area belong to different MSCs/VLRs, the new MSC/VLR requests the data of this MS from the HLR.
• The HLR returns the information needed by the new MSC/VLR and at the same time notifies the original MSC/VLR to delete the original registration information of this MS and notifies the new MSC/VLR to register this MS.

Location Update When the MS Roams Inside the Same VLR Area

Location Update When the MS Roams In the Areas of Different VLRs  

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