articles for telecom engineers

Signalling

In any network, the definition of signalling is the exchange of information. In a telecommunication network, signalling is the exchange of information that relates to the establishment and control of connections, including management.

Today most transmission between telephone exchanges is digital, but in some cases signalling operates on specifications developed for analogue exchanges. The exchanges in these networks use Channel Associated Signalling (CAS). The CAS restricts signalling to the PCM link in which the telephone connections take place.

This means that the number of connections that the CAS signalling controls is equal to the capacity of the PCM link. That is, 30 in 2 Mbit/s systems and 24 in 1.5 Mbit/s systems. The CAS signalling also uses one time slot for signalling purposes in 2 Mbit/s systems on its own. CCS signalling has better features than CAS.

A number of features differentiate CCS signalling from CAS signalling:

.? Signalling and speech can be sent on separate PCM links. This gives network designers the possibility to design powerful networks that can withstand the failure of one or more PCM links.

?It has built-in error detection in the signalling, therefore enabling error correction. The CAS, however, only has the option, if there are errors, of dropping the connection, leaving reestablishment to the user.

?It can carry signalling for a large number of connections, i.e. more than 1000, in a single time slot, therefore freeing time slots for connections.

?Using data network techniques, it can carry other service information. For example, requests for number information, either from an operator information or through computers in the network.

Thus, the designers of Signalling System No. 7 have given different user groups their sets of messages, depending on the needs of these user groups. The architecture of Signalling System No. 7 makes it easy to implement new messages for a new user group without affecting existing user groups in the system.

Trunk Connections and Kind of Trunks

Trunks are the systems which enables calling from an automatic station to another one. For this, the trunk selects a voice channel between the stations. Then it does the connection and sends the specific number of the telephone address. During the conversation it holds the connection. At the end it cuts the connection of the lines.

In every PCM-30 there are 32 trunks. One of them is synchronization trunk and the other is the signalling trunk. The old system of Turk Telecom was R1. In that system in every PCM-30 2 trunks was separated for the use of the system. In R1 every PCM-30 has a specific synchronisation and signalling trunk. If the signalling trunk of a PCM-30 was cut, the other 30 conversation trunk in that PCM-30 can not be used. But now Turk Telecom uses the new system named as NO 7. In this system, all -signalling trunks of PCM-30 can be shared.

Synchronization

For understanding synchronization we must first understand SS7. SS7 is a data protocol which is between the stations. In this system data is sent by data packets. Synchronisation is a trunk which enables sending these packets in fixed intervals. There is a device named atomic clock in Ankara Turk Telecom Administrative Provincial Directorate. This device gives clock pulses to all stations in Turkey. So, all stations can work at the same interval. If they do not work at the same time interval, we can not reach the correct person we are trying to call.

In the second week of my summer practice, I have generally dealt with the signalling system SS7. Since this is the base for all the systems that are used in Turk Telecom I have examine this in detail. To do this, I again read some documents from the internet and some written documents which I have found from the office. Then I have learnt some practical things for construction of a SS7 system. For example, I have learnt how to make a cross and normal CAT 5 cable. After that I have learnt how to lay these cables and where to connect these cables.