Telephone And Cell Phone Signals
Telephone signaling system link the variety of switching systems, transmission systems and subscriber equipments in a telecommunication network to enable the network as a while. The below signaling systems are involved in telephone network.
- Subscriber loop signaling
- Intraexchange or register signaling
- Interexchange or interregister signaling
In a telephone network, subscriber loop signaling depends upon the type of telephone instrument used. Multifrequency signaling has brought about new services like data-in-voice-answer, which fall in the class of user-to-user signaling facilities. Such facilities are more common in the context of data communication and integrated services.
The interexchange signaling is internal to the switching system is heavily dependent upon the type and design of a switching system. It varies from one model to another even with the same manufacturer. This signaling does not involve signaling systems of the type required on the switching network and hence is not discussed any further.
This section mainly deals with interexchange and network-wide signaling. When interexchange signaling takes place between exchanges with common control subsystems, it is called interregister signaling.
The main purpose of interregister signaling is the exchange of address digits which pass from exchange to exchange on a link-by-link basis. Network-wide signaling also involves end-to-end signaling between the originating exchange and the terminating exchange. Such a for of signaling is called line signaling.
Signaling techniques fall under two major classes:inchannel signaling and common channel signaling(CCS). Different types of signaling techniques exist in each of these two classes. Inchannel signaling, also known as per trunk signaling, uses the same channel which carries user voice or data to pass control signals related to the call or connection.
In contrast,CCS does not use the speech or the data path for signaling, also known as per trunk signaling,uses the same channel which carries user voice or data to pass control signals related to that call or connection. In contrast,CCS does not use the speech or the data path for signaling. It uses a separate common channel for passing control signals for a group of trunks or information paths.Inchannel signaling has the merit that no additional transmission facilities are required for signaling whereas this is not the case with the CCS.
Clearly, inchannel signaling is influenced by the various factors associated with the information path. This brings about different forms of in channel signaling. When as unamplified audio line plant which permits an individual metallic signaling path per speech circuit is used,d.c. signaling can be applied. The d.c. signaling is relatively simple, reliable and cheap.
However, where amplified audio circuits are in use, low frequency a.c. signaling may be adopted. Low frequency a.c. signaling and d.c. signaling cannot be provided when FDM transmission systems are used. In such cases, voice frequency signaling is used. Obviously,a digital signaling scheme is desired for TDM systems.
Voice frequency (VF) signaling may be inband or outband.Inband VF signaling uses the same frequency band as the coice.Inband VF signaling must be protected against false operation by speech. An interesting story prevails to illustrate the significance of this statement. A woman subscriber is supposed to have complained to a leading telephone company in the United States that her line gets disconnected often during a converstation.
The company being conscious of its reputation checked out thoroughly all the subsystems but found nothing abnormal and the problem continued to persist. The company then decided to analyse the lady’s voice and found that the lady has a tendency to generate a tone at around 2600 Hz lasting for duration of 100 ms or more which was being detected by the inband signaling system as the line disconnect signal.
Such occurrences preclude the use of inband signaling during speech phase.An important advantage of inband signaling is that the control signal’s can be sent to every transmission systems as the signals would be carried along the route like the speech signals. Even A-D and D-A conversion processed do not affect them. Inband VF signaling is the most widely applied signaling system presently.
Outland signaling uses frequencies above the voice band but below the upper limit of 4000 Hz of the nominal voice channel spacing. Signalling during the speech period is possible, thus allowing continuous supervision and control of a call. However, only a very narrow bandwidth is available for signaling. Extra electronics is required to handle the signaling band. As a result, this scheme is not used widely.
Both in band and out band signallling shcheme has limited information transfer capacity. Preclusion of use of inband signaling during speech phase limits its capability. The outbound signaling suffers from the very limited bandwidth. Both are capable of having only a small signal repertoire. The trend in modern networks is to provide enhanced signaling facilities for the subscriber, the switching system and the telephone administration.
Such a need is met by common channel signaling which may be implemented in two ways-channel associated mode and channel non-associated mode. In the former, the common signaling channel closely tracks the trunk groups along the entire length of a connection.IN the latter, there is no close or simple assignment of control channels to trunk groups.
A CCS network consists of two types of noded: signaling transfer points (STPs) and signaling pints(SPs).The signaling transfer points (STPs) usually have a connection with the switching centers’ although this is not essential. Since signaling originates from the control subsystems of the signaling viewpoint. Signalling point is capable of handling control messages directly addressed to it but is incapable of routing messages. Signalling transfer point is capable of routing messages and could also perform the functions of a SP.
In the non associated mode of operation,the signaling information may follow a route that is different from the one taken by the speech paths. The network topologies for the signaling and the speech networks are different.
A CSS network may use associated signaling in some constituent parts and nonassociated signaling in other parts. The term”quasiassociated signaling” is used to indicate such an operation.
The in channel and common channel signaling differ in many ways. A few of the differences need clear elaboration. Consider a situation when the called subscriber is busy. In the case of inchannel signaling, the busy tone originates from the terminating exchange and the switched path is held up as long as the calling subscriber does not go on hook.