Telephone Central System Services
A Telephone Call
To make a telephone call, one simply picks up the handset, enters a number, and waits for the system to perform its magic:
- Lifting the handset from its cradle releases a hook switch and causes a dc current to flow (20 - 120 ma). The central office monitors this loop current and interprets it as a request for service.
- The office acknowledges the request for service by sending dial tone. This normally occurs in less time than it takes to pickup the handset and place it to the ear. Once dial tone has been received, the subscriber starts to dial
- In the past, when dialing, a rotary dial switch opened and closed the loop in a predetermined manner. If one was very coordinated, it was possible to perform the same task by flashing the hook switch. To assure the customer that the system is responding, dial tone is removed once dialing starts. In most systems today DTMF (dual tone multi frequency) signaling is used How Telephones Work
- Depending upon the office type and digits received, a number of things might happen. In most cases, end-users are attached to what is called a class 5 or end-office. These are the most common types of telephone exchanges. Each class 5 office has one or more, three digit exchange numbers. These are the first three digits in an ordinary 7-digit telephone number.
- If the central office includes the customer dialed exchange number, it will know that the call is local and the other party is connected to the same office. The office will therefor control the entire call setup and takedown.
- If the first three digits do not correspond to an exchange handled by the end-office, it will have to find a trunk line to an office that can handle the call. This means that each office must know the exchange numbers of all the offices within its calling area, and how to get to them. The call setup and takedown will therefor be shared between the two exchanges. They must monitor the call in progress and inform each other of any change in call status.
- If the first digit dialed is a one, the office will recognize this as a long distance call, and will start looking for a spare toll trunk. A toll office has a greater knowledge base as to where distant exchanges are located and how to get to them.
- The telephone system must be intelligent enough to recognize that in a local call, only seven digits are usually required. Some very small exchanges however, allow local calls by omitting the exchange number and using only the last 4 digits or extension number. In large urban areas, it may be necessary to prefix local calls with a 3-digit area code. An international call may require up to 16 digits.
- In a touch-tone environment, the same procedure is followed, except that tones are used to convey numbers to the local office instead of interrupting loop current. Some calling features, generally known as CLASS (Custom Local Area Signaling Services) , are available in areas with touch-tone service.
But where do the telephone wires go
The telephone line goes to a terminal block in a service area interface. These are often located on a pole or small enclosure on the street. The service area interface bundles the subscriber drop cables into a single larger cable. These are in turn gathered together to form larger feeder cables. The entire wiring system somewhat resembles a huge tree.
cables pass through the vault and are terminated on the vertical side of the MDF (main distribution frame) . To protect the central office equipment from high voltage transients, such as Cables coming out of a central office may have hundreds or even thousands of pairs bundled together however by the time the cable gets to the end user, it is generally down to about 50 pairs. An individual subscriber consists of many cable sections spliced together. Bellcore claims that the average U.S. subscriber line has twenty-two splices. Feeder cables enter the central office in a large underground room called a cable vault. Each feeder may contain hundreds of pairs of wires, and be pressurized in order to prevent moisture or ground water from entering and affecting the transmission characteristics of the wire. A typical vault may contain tens of thousands of wire pairs The lightning strikes, which may travel down the wire, the lines are surge protected by carbon blocks or gas tubes.
The horizontal side of the MDF, connects the incoming telephone lines to the peripheral equipment. All that is required to connect a line appearance to a specific interface is to place a jumper between the vertical and horizontal sides of the MDF.Signals coming from an end-user are generally analog in nature. Consequently, the peripheral equipment converts the signals to digital form before passing them on to the rest of the network. Incoming trunks from other central offices are comprised of specialized carrier systems. They may be either analog or digital, but all new systems are strictly digital.
Most end-user voice & data interfaces are multiplexed on to high-speed paths, which pass through the internal switching, network before being routed to outgoing lines or trunks. Incoming digital carrier systems may be accepted directly into the switching network through a cross-connect or may be demultiplexed prior to switching.
Historically the telephone network was composed of a hierarchical structure consisting of 5 different office types. The most common of these is the class 5 end-office. An end-office connects directly to subscriber telephone sets and performs switching functions over a relatively small area. Telephone exchanges connect to subscribers by means of local loops or lines, generally one per customer. Telephone offices connect to each other by means of trunks. A class 5 or end-office interconnects telephones throughout a small service area. Each end-office may contain several three-digit exchange numbers and is aware of other local exchange numbers held by other offices. Calls between offices are routed over interoffice or tandem trunks. Long distance calls are routed to toll offices via toll trunks. The average class 5 office serves approximately 41,000 subscribers, and covers 30 square km in an urban environment. Some nodes may have no customers at all, and may be connected only to other nodes. These inter-node or trunk connections are usually made by FDM or TDM transmission links.
Exchange Area Network
An exchange network consists of local and tandem exchanges connected by trunks. A tandem office interconnects class 5 offices by means of twisted pair, coax, microwave, or fiber optic carriers. Alternate routing paths between local exchanges are provided if the direct trunks are occupied. An exchange area includes all of the offices, which are aware of each other, but do not involve long distance charges. In very large urban areas, there is an overlap between exchange areas, which may also cross over area code boundaries
Long Haul Network
A long haul network consists of exchanges interconnected by toll offices. Toll offices keep track of long distance charges and are typically confined to national boundaries. These trunks consist of high capacity coax, microwave, or glass fiber. Messages used to control the call setup and takedown can be sent by two basic methods. Traditionally, inter-office messages are sent over the same channel that will carry the voice path, but in newer systems, common channel signaling is being employed. In this method, the offices have dedicated facilities, which are used to send inter-office messages. There are some advantages to this, perhaps the notable being the added degree of difficulty encountered if one wants to defraud the system. When in-band signaling was used, it was possible for people to dial long distance calls without being charged, if they created the tones used to disable the toll circuit.
Trunks are used to interconnect the various levels of telephone exchanges. It is necessary for these links to exchange on a wide range of information including:
- Call related signaling messages
- • Billing information
- • Routing and flow control signals
- Maintenance test signals
There are two ways for telephone offices to communicate with each other and pass on routing information. Information can be conveyed in the same channel that will be used to convey the voice signal, or it may be completely disassociated with it..
The CAS (Channel Associated Signaling) approach uses the voice channel to send information through a trunk. For example, a 2600 Hz tone is used in interoffice trunks to signal on-hook. A major disadvantage of this system is that subscribers can bypass toll centers by injecting the appropriate tones. One way to avoid this problem is by using out-of- band signals on toll trunks. Since the customer’s signal must pass through an audio anti-aliasing filter, it is not possible to inject the out-of-band signaling tone. A principle advantage of in-channel signaling is that the integrity of the voice path is checked each time a connection is established. Out-of-band signaling allows for continuous supervision of the connection throughout the call.
The CCIS (Common Channel Interoffice Signaling) approach has the signaling information conveyed on a facility completely separated from the customer’s voice path. This allows for a faster, more efficient control, however the reliability of the CCS network must be considerably greater than that of the individual voice paths. The signaling channel may follow the same route as the final connection path, or it may be completely disassociated with it. STPs (Switch Transfer Point) are need in the network if the signaling path is disassociated, thus effectively creating two networks: a speech network and a signaling network overlay. .
Virtually all calls requiring tandem or toll office routing are established and controlled by the SS7 signaling network. The SS7 signaling network is a packet switching facility comprised primarily of STPs (Signaling Transfer Point) and SCPs (Service Control Point) connected to the PSTN SSP (Signal Switching Point). STPs are deployed in pairs and are the brains of the system. They determine which trunks and offices should be used in establishing inter-office connections. The SCP is a database that keeps track of such things as: credit card authorization, virtual network subscriber listings, 800 number conversion tables, billing, and other special services. .
A Generic Communication Switch:
The internal architecture of a telecommunications switch is somewhat like the organization of the entire system. The internal structure is often illustrated by the traditional pyramid or hierarchical arrangement. The control or brains of the operation are shown at the top, and the peripheral units that connect to the outside world are placed at the bottom. Physically, the switch is simply a series of boxes, full of electronics: The MTBF (mean time between failure) for any PSTN switch must be very long, since business would soon grind to a halt if telephone traffic was interrupted for a prolonged period, but more importantly, emergency services would be severely curtailed. For these reasons, large public switches have a great deal of redundancy built in. Redundancy is provided in two basic ways; hot standby and load sharing. In the hot standby arrangement, two or more processors are fed with the identical information and are making decisions, however, only one of these processors is in charge and is executing decisions. In the event of a failure, the healthy unit assumes the full load. There is no degradation in performance, and no calls in progress should be lost. In a load-sharing configuration, all processors are actively working but not to their full capacity. In the event of a failure, the defective processor is isolated from the system, and the others pick up the slack. There may be degradation in performance, and calls in progress on the defective processor may be lost.
Central Control :
This contains the system intelligence and customer database. It knows who the customers are, what they want, and how to provide the service they require. In a step x step [step by step] switch, the intelligence is fully distributed and there is no central control, whereas in a crossbar facility, all of the intelligence is resident in a central controller or computer. In all modern systems the intelligence is somewhat distributed, with various functional blocks contributing to the decision making process. At onetime there was a sharp distinction between computers and telecom switches, but today this division is less clear, and central controllers may be regarded as a specialized computer.