References: AT&T Definity Vol. Implementation: Trunk Groups 5-92...
We can usually connect exchanges with each other with the help of trunks, but special tools, e.g. voice mail may connect to the exchange through the trunk:
+-----------+ trunk +-----------+
| exchange |===================| exchange |
+-----------+ +-----------+
+-----------+ spec. trunk +--------------+
| exchange |===================| spec. device |
+-----------+ +--------------+
Trunk is ended at one of the interfaces of the periphery
unit of the exchange. There is another case, when the two devices are connected to each
other indirectly through the interfaces (e.g. in case of a spec. device), but in general
exchanges are connected through their interfaces to a (digital)
transmission network, and connection of exchanges with a large distance from each other can be realized this way:
| |
+-----------+ | +---------------------+ | +-----------+
| exchange |===|transmission network |===| exchange |
+-----------+ | +---------------------+ | +-----------+
| |
interface interface
Forwarding extensions between devices and the exchange, or between exchanges happens in digital signalling systems (ISDN:DSS1, SS7; VoIP:SIP, H.323) in messages initiating connections (DSS1:SETUP, SS7:IAM, SIP:INVITE). Studying an information element of a message serving for forwarding (e.g Called Party Number ) we can see, that these strings have also attributes (type, dialing plan), that have an important role in route selection besides the number.
Egy központban a kapcsolatok felépítésére használt stringeket a központ számozási terve (Dialing Plan) specifikálja.
Implementáció: Definity, BCM50
An individual dialing plan of an exchange is based on some - mostly standardized - dialing schemes
Basic classes of dialing plans:
International public dialing plan is specified by the Recommendation E.164 of ITU-T.
Example: In case of dialing number 00 36 1 463 3111:
00 - access code of the service (international call)
36 - CC = country code
1 - NDC = National Destination Code
463 3111 - SN = Subscriber Number
This number is a standard international public network number.
It is also usual to use dialing plans in private networks, similarly to the public networks (access code + NDC + SN). Length of the SN is short int he private networks (3..5 digits), and in order not to have to use numbers with different lenghts between the sites of a firm, there was processed a dialing system (uniform, coordinated) in which all the devices of the private network can be called with the help of an equally short number. Implementations: BCM 50, Definity.Naturally, if the exchange of the private network connects to the public network, too, then there have to be a dialing plan for both networks (public/private). Automatic Route Selection - that can be programmed by the operator of the exchange - happens with the help of routing tables. Input of these tables is a partial string of the extension, which usually contains NDC. In case of a private network number, if it does not contain NDC, we usually assign a code for the routing - substitute for NDC - on the basis of the first characters of the extension.
Output of the routing table is a route identifier, which points to the route
description table which is at the proper direction for starting of building up the
call.
In addition to the basic route in the route description table the following possibilities could be programmed:
It is the private business of the two exchanges, which circuit of the trunk do they use for building up a call - the calling person cannot influence it.
In case of ISDN PRI trunks the time-slot number identifies a circuit of the trunk. Exchanges record numbers of free/engaged time-slots. Building up a new call they select from the free time-slots - according to the algorithm programmable by the operator. The most frequent algorithms are the following: ascend, descend, cyclical (the first free time-slot according to the ascending, descending, cyclical number). Number of the selected time-slot is sent by the calling exchange in CHANNEL_ID information element of SETUP message to the called exchange. Called exchange acknowledges it in a CALL_PROCEEDING message.
Making telephone calls through the trunk is custom-made in many cases, so the operators limit the availability of trunks. The simplest limitation classes are processed on the basis of types of outgoing calls:
(The signs applied here are consistent with signs applied in BCM-50 and DEFINITY exchanges)