The standard telecommunications model is comprised of four telecommunication layers. Here we will look at and discuss the role and working of each component layer.
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LATA, or "local access transport area," in United States telephone system terminology, refers to the specific geographical coverage areas created from the breakup of AT&T into the seven regional "Baby Bells" in 1984.
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As its name suggests, the Intra-LATA layer takes care of telecommunication between two parties residing in a single LATA. In the U.S., a LATA is the geographical area where a Regional Bell Operating Gompany (RBOC) controls all the exchange of telecommunications and other access services. As the communication within the LATA is controlled by a regional company, it makes the process relatively simpler. Intra-LATA calls are confined within the single LATA so only the local exchange carrier is used for Intra-LATA calls, which are of two types: local calls and local toll (distant) calls.
Telephone sets convert audio signals to electrical signals that are transmitted to the central exchange office with the help of cables. If the calling party and receiving party both are residing in the same city coming under one LATA, the central office can connect the call to the receiver with the help of switch, which can physically connect the two parties by completing their circuit.
In case the call is made to another city that falls under the same LATA as that of caller, these calls are "local toll calls." The call is passed on to local company "trunks" using a local exchange carrier that further connects it to switch. The switch will complete the circuit and provide an electrical signal to the receiver telephone, and the telephone will further convert the electrical signal to an audio signal for the receiver.
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The Inter-LATA Layer
One Regional bell operating company can provide services in only one LATA and can't provide its services in another LATA. So communication between two LATAs cannot be controlled by a single RBOC. This is what differentiates inter-LATA layer from intra-LATA layer in the telecommunications model. Calls originated from one LATA and terminating in another LATA are controlled by the inter-LATA layer of the telecommunications model. When the caller initiates a call to the distant receiver, the caller's “line card" automatically transmits the carrier represented as a four digit code.
This four digit code called PIC (primary interexchange carrier) code is used by the switch at the central office to route the call to another LATA through an interexchange carrier, which is usually known as IXC. The interexchange carrier is used for distant calls among two LATAs. The call is handed off to its destination, which can be identified with the help of the PIC code, by the carrier. The local telephone company, in the receiving LATA, takes care of the call and routes it to the called individual through a switch. Thus this telecommunication layer of telecommunications model comes into the picture to provide telecommunication among two LATAs.
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The Signaling Layer
When the caller initiates a call, the call is governed through the intra-LATA layer or inter-LATA layer, depending on the receiver's LATA. Then the call is established through physical circuit using switches. As the call is routed via physical or IP (internet protocol) pathways, the switch identifies the called number, with signaling switches serving the call receivers and notifying them of an incoming call. Earlier, distant calls used to take a minute or so to establish because of the involved hand-offs among different carriers. Recently with the advent of "out of band signaling," distant calls can be made in a fraction of a second. This telecommunications layer plays a key role in making call processing easier.
Crucial information like the telephone number of the caller, PIC (carrier identification code), and mode of delivery is exchanged between the switches during signaling. Advanced features like caller identification, call tracing, repeat dial, etc., provided by different telephone companies, have been made possible by signaling. Throughout the signaling process, information concerning the call is moved between two switches which streamline the phone call process.
Technology has brought drastic changes from wire-dependent communication to wireless communication. With this winning more favor among the public, the signaling layers work in the same ways as wire signaling layers, but they can be modified to accommodate wireless systems.
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The Billing Layer
In addition to streamlining call setup and enabling advanced features, signaling also makes billing substantially simpler. Once the call is initiated, signaling records the call setup, its route, carriers used during the call exchange (in case of inter-LATA layer), and duration of the call. All this information is saved and stored automatically. The billing layer makes use of this information to provide revenue to the telephone company. Additional chargeable services used by the caller can be tracked by the signaling layer and used by the billing layer to add to the bill of the caller.
With advances in the telecommunication model, different methods have been implemented to cope with different forms of billing that are required to be prepared for different users. The usage of SMS and MMS, for example, are among the many other services provided under the communication sector for which there must be different billing methods.
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Because of the competitive market, telephone companies are offering cheaper inter-LATA calls than intra-LATA calls. This leads to confusion among consumers regarding inter-LATA and intra-LATA calls. Thus it is good for a customer to be aware of the telecommunications model and the working of the above mentioned telecommunication layers in order to make an informed decision.