Telecommunications

Table of Content

Introduction 3

PBX and Centrex 4

Rotary and DTMF 6

WATS and Leased Lines 7

ISO and OSI 9

Conclusions 11

Reference List 12

Introduction

Networking and telephony is a part of our everyday environment, whether it be in our homes or in our place of business. We will somehow come into contact with one or the other in some shape or fashion. At Baker, we are no different from any other business. The infrastructure of Baker is not a simple pc and telephone setup. The network incorporates everything from a simple analog line to our data network backbone that is made of T-3 lines, ISND lines, and IPFR provided by the AT&T datacenter. Baker has 30 offices all over the country and a couple of offices in abroad in Europe. I will be discussing the setup of the Houston office, since I am most familiar with this office.

PBX and Centrex

The Houston office of Baker is comprised of three types of phones, well, at least until the company moves into one central location. Currently there are three offices in Houston that are all a block apart. The building that I support houses an old Merlin Legend telephone system. It’s funny, because I have to set the date on the voicemail to the year 1999 so that when users check their messages, it will give the correct month and date. The system is comprised of about 100 direct-inward dialed (DIDs), some used as analog and some use as digital lines. The digital lines are ISDN lines that allow for digital transmission of voice and data over ordinary telephone copper wires. Direct-inward dialing routes calls from the outside lines to the PBX, which allows telephone systems to bypass an operator. The PBX then routes the call to the desired extension and if there is no answer, the call goes to the users’ voicemail. PBXs are used to route calls between people within an organization and it routes calls to users from the organization to people that are not connected to the internal system. Centrex systems do not require onsite switching equipment like the PBXs. They usually handle outgoing calls made from inside an organization without the use or intervention of an operator. In the case of my office, a trunk or outside path is grounded by the PBX or central office, which is why users have to dial an access code (a nine in our case) before they can make any outside calls. I am not sure, but I know we have over 10 trunks that go out from our office. “The difference between PBXs and Centrex is in the location and ownership of the equipment that routes calls. Private branch exchanges are located on customers’ premises. Centrex is usually located at the telephone company and is part of the central exchange or central

office” (Dodd, 1999, chp2).

Rotary and DTMF

Going a little further into our telephony phone system structure, Baker’s telephones are all touch tone or tone dialing phones. Although the phones that are supplied to us have the ability to support pulse dialing for backward compatibility; I have not seen or know anyone that uses those features in today’s business world. The old name for pulse phones were rotary phones, and the name for touch tone phones are dual tone multi-frequency phone, or DTMF. “Pulse dialing or loop disconnect dialing, also called Rotary or Decadic dialing in the United Kingdom, and is pulsing in which a direct-current pulse train is produced by interrupting a steady signal according to a fixed or formatted code for each digit and at a standard pulse repetition rate” (Wikipedia). When a number is dialed on a rotary phone, the phone generates pulses that are making and breaking of the telephone connection. When dialing a number the user hears rapid clicks which represent when numbers are dialed, except for 0, which has ten clicks. Rotary phones are still in use, I personally couldn’t say where, but I know there out there. The successor of the rotary phone is the touch tone phone, which correctly is called the dual-tone multi-frequency (DTMF). “DTMF has generally replace loop disconnect, pulse dialing. The DTMF is used for telephone signaling over the line in the voice frequency band to the call switching center. DTMF