Instructor: Sam Nanavaty The Telephone Network. Instructor: Sam Nanavaty Local telephony services provided by a Local Exchange Carrier (LEC). The LEC

Instructor: Sam Nanavaty

The Telephone Network


• Local telephony services provided by a Local Exchange Carrier (LEC).

• The LEC is however restricted in its operation to within its Local Access Transport Area (LATA).

• Calls between LATAs have to be handled by an Inter Exchange Carrier (IEC or IXC).


• The Central Office (CO) switches provide the service for the Public Switched Telephone Network (PSTN)

• Businesses can also have their own switch called a Private Branch Exchange (PBX) which is a smaller version of the CO switch.

• The connections between the CO switches are called Interoffice Trunks and carry all the calls.

• Circuits that connect the CO switch to the business Private Branch Exchange (PBX) are called CO Trunks

• Trunks between PBXs are called Tie Trunks.



• The PSTN is circuit-switched and guarantees end-to-end connection during the call.

• The resources associated with that call are tied up for the duration of the call.


Basic PBX services• Lowest Cost Path Routing - having the preferred paths, based on

cost, line quality, reliability and billing information. • Automatic Call Distribution (ACD) - finding available telephones

in a pool, often called Hunt Groups. • Voice Mail • Call Forwarding - automatically forwarding of calls when a

telephone is unavailable. • Calling Line Identification (CLI) - that maps the caller's number to

a name in a database. • Calling Number Blocking - blocking of unwanted numbers. • Voice Conferencing - allowing a conversation to occur between

more than two people


Calls made outside local area are handled by Toll centers

Higher level of switching depends on the destination of call and the traffic volumes at any given time

The end office is the lowest level of switching entity, there may be one or more end offices (Central offices) based on the number of subscribers in the area

The PSTN is a hierarchical network consisting of 5 levels of switching facilities


FIGURE 12-1 Interconnection of switching exchanges in North America.

Warren HiokiTelecommunications, Fourth Edition

Copyright ©2001 by Prentice-Hall, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.


FIGURE 12-2 Long-distance switching hierarchy within the PSTN depicting possible routes to complete a call between parties A and B.





FIGURE 12-3 Long-haul network depicting various trunk media. (Courtesy of Bell Laboratories.)





FIGURE 12-4 Switching centers throughout the United States (Courtesy of Bell Laboratories.)





FIGURE 12-26 Hierarchy of the Bell System FDM groups.





FIGURE 12-27 (a) Formation of the Bell System’s channel group; (b) frequency distribution of the channel group.





FIGURE 12-28 Frequency distribution of the Bell System supergroup.





FIGURE 12-29 Simplified diagram illustrating TDM.




Instructor: Sam NanavatyWarren HiokiTelecommunications, Fourth Edition




FIGURE 12-30 Block diagram of the North American digital multiplexing hierarchy.





FIGURE 12-31 Coding for the T1 carrier PCM signal.





FIGURE 12-32 T1 carrier frame format.





FIGURE 12-33 Frame structure for a T1 carrier frame and superframe. (From Bell System Technical Reference Publication 62411, High Capacity Digital Service Channel Specification, Sept. 1983.)





HW Assignment #1

• What is a PBX and what are the PBX interface standards?

• Identify various PBX manufacturers• What are the typical functional features offered by a

PBX?• Identify the companies that make the Central Office

switches• What are the key functional features offered by a C.O.

switch?

• Here is a URL to get you started:

• http://en.wikipedia.org/wiki/PBX#Overview

http://en.wikipedia.org/wiki/PBX#Overview

Documents

Instructor: Sam Nanavaty The Telephone Network. Instructor: Sam Nanavaty Local telephony services provided by a Local Exchange Carrier (LEC). The LEC