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DTMF - Dual Tone Multi Frequency Signalling

Posted On Tuesday, February 12, 2008 at at 11:12 PM by Murali Srinivasan  

Dual Tone Multi-Frequency, or DTMF is a signaling method for instructing a telephone switching system about the telephone number to be dialed.DTMF tones are also in caller ID systems to transfer the caller ID information.The DTMF system uses eight different frequency signals transmitted in pairs to represent sixteen different numbers, symbols and letters used are #, *, A, B, C, and D.

DTMF Keypad Frequencies.

The DTMF keypad is 4×4 matrix, each row representing a low frequency, and each column representing a high frequency. While pressing a key (say '1' ) will send a sinusoidal tone of the two frequencies (697 and 1209 hertz). The original keypads have levers inside, so each button activates two contacts. The multiple tones are generated by pressing a key.This is the reason for calling the system as Dual Tone Multifrequency. These tones are then decoded by the switching center to determine which key was pressed.





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E&M – Earth & Mouth Signalling

Posted On at at 10:28 PM by Murali Srinivasan  

E&M – Earth & Mouth is a type of line signaling that uses separate leads, called the "E" lead and "M" lead.There are NEAR & FAR ends.The near end sends a signal to the far end by applying -48 vdc to the "M" lead, which results in a ground being applied to the far end's "E" lead ie the near-end wanted to call the far end. When -48 vdc is applied to the far end "M" lead, the near-end "E" lead is grounded. When the near-end "E" lead is grounded, the far end is calling and wanted your ear.

8 Wires of E and M signaling


There are different E&M standards, with 2,4.6 & 8 wires.
  • 2 wire E&M is technically same with a normal CO trunk.2 wire E&M means 2 wires ie one pair for signaling E&M and 2 wires ie one pair for Voice.-Total of 2 pairs
  • In 4 wire E&M, 2 wires ie 1 pair is used for receiving voice and 2 wires ie 1 pair for transmitting voice and one pair for signalling.-Total 3 pairs

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What are Trunk Lines ?

Posted On Sunday, February 10, 2008 at at 11:39 PM by Murali Srinivasan  

Trunk lines are the phone lines coming into the PABX from the telephone provider.These incoming lines can be analog, T1, E1, ISDN or PRI and often carry multiple channels each capable of carrying its own phone conversation.These lines are provided to the respective cards of the PABX.The lines coming out from the PABX are known as EXTENSIONS.Trunk lines can be accessed from the extensions using trunk access codes provided by the PABX.



E1

E1 is the European format for digital transmission.E1 carries signals at 2 Mbps - 32 channels at 64Kbps (32*64 = 2Mbps), with 2 channels reserved for signaling and controlling.

T1

T1 is the North American standard.A phone connection supporting data rates of 1.544Mbits per second.A T1 line actually consists of 24 individual channels, each of which supports 64Kbits per second. Each 64Kbit/second channel can be configured to carry voice or data traffic.So T1 carries signals at 1.544 Mbps (24*64 = 1.54Mb). T1 lines are mostly used to connect internet.T1 lines are sometimes referred to as DS1 lines.

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EPABX - Electronic Private Automated Branch Exchange

Posted On Friday, February 8, 2008 at at 11:57 PM by Murali Srinivasan  

An EPABX / PABX / PBX is an electronic device which is mainly used in organizations.PBX's make connections among the internal telephones of a private organization and also connect them to the PSTN via trunk lines.An organization consists of many number of staffs.Each staff requires the extension as well as access to outgoing local,STD or ISD calls.According to the designation of the staff,we can provide the available features to them.A PABX take incoming trunk phone lines and route them to the appropriate extensions inside a company. The main reason a company buys a PBX is to allow them to purchase less trunk lines from their telecom provider than they would otherwise have to buy.PABX is able to route incoming or outgoing calls on available trunk lines if other lines are busy. It has many more features that are very much useful for a company to cut down its total cost on telephone services used by the company.The primary advantage of PABX's was cost savings on internal phone calls by the internal circuit switching by itself.Some of the features provided by PABX's are hunt groups, call forwarding, extension dialing etc.

OR

IP-PBX: Private Branch Exchange using VoIP Technologies

Private Branch Exchange (PBX) is a telephone switch used by enterprises and located at the premises of a company. The traditional PBX based on the TDM technology is reaching the end of its lifecycle due to the emergence of IP-PBX. The IP-PBX, based on the VoIP technologies, offers easier user administration and advanced applications. With an IP-PBX, the Local Area Network is the platform for connecting smart IP phones logically over a shared packet network to the call manager. This unifies the data applications and the voice network, but places demands on the packet prioritization aspects of the LAN infrastructure to ensure user satisfaction with the quality of audio. The key benefits of IP-PBXs are:

  • easy to administer users since all users are provisioned like a PC
  • support telephone user mobility with wireless LAN technologies
  • provide unified messaging services
  • lower total cost of ownership of the voice systems

The key component of IP-PBX are:

  • Call manager (or softswitch) is for call control, signaling and management
  • Analog Station Gateway allows plain old telephone service (POTS) phones and fax machines to connect to the IP PBX network
  • Analog Trunk Gateway allows the IP PBX to connect to the PSTN or PBX.
  • Digital Trunk Gateway supports both digital T1/E1 connectivity to the PSTN or transcoding and conferencing.
  • Converged Voice Gateway allows you to connect standard POTS phones with IP or any H.323-compliant telephony devices.
  • IP Phone is the end customer device replacing the traditional telephone set.

How does IP-PBX work? An IP phone connects to a LAN either through a hub port or a switch port. The phone can talk with CallManager and registers itself. CallManager stores the IP-address-to-phone-number mapping (and vice versa) in its tables. When a user wants to call another user, the user keys in the called party's phone number. The CallManager translates the phone number to an IP address and generates an IP packet version of ring tone to the called IP phone through the TCP connection. When the called IP phone receives the packet, it generates a ring tone. When the user picks up the phone, CallManager instructs the called IP phone to start talking with the calling party and removes itself from the loop. From this point on, the call goes between the two IP phones. When any change occurs during the call due to a feature being pressed on one of the phones, or one of the users hanging up or pressing the flash button, the information goes to CallManager through the control channel.

If a call is made to a number outside of the IP PBX network, CallManager routes the call to an analog or digital trunk gateway which in turn routes it to the PSTN.

Over years of development and improvement, many technology hurdles of VoIP for the deployment of IP-PBX, such as QoS and inter-operability between different vendor products, have been resolved. IP-PBX is ready for prime time deployment.



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Integrated Services Digital Network

Posted On Thursday, February 7, 2008 at at 11:06 PM by Murali Srinivasan  

Integrated Services Digital Network

ISDN is a circuit-switched telephone network system, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than an analog phone. It offers circuit-switched connections (for either voice or data) in increments of 64 kbit/s. One of the major use cases is Internet access, where ISDN typically provides a maximum of 128 kbit/s.By combining your two B-channels you have access to up to 128 kbps more than four times as fast as a 28.8 kbps modem on a standard phone line. ISDN is a set of protocols for establishing and breaking circuit switched connections, and for advanced call features for the user.It is used for real-time videoconferencing. ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group videoconferencing systems.

The Basics

There are two types of ISDN channels or communication paths:

  • B-channel
    The Bearer ("B") channel is a 64 kbps channel which can be used for voice, video, data, or multimedia calls. B-channels can be aggregated together for even higher bandwidth applications.
  • D-channel
The Delta ("D") channel can be either a 16 kbps or 64 kbps channel used primarily for communications ie signaling between switching equipment in the ISDN network and the ISDN equipment at your site.

ISDN Configurations

The user will get the ISDN connection in 2 configurations. They are,

  • Basic Rate Interface (BRI)
    BRI is the ISDN service most people use to connect to the Internet. An ISDN BRI connection supports two 64 kbps B-channels and one 16 kbps D-channel over a standard phone line. BRI is often called "2B+D" referring to its two B-channels and one D-channel. The D-channel on a BRI line can even support low-speed (9.6 kbps) X.25 data.    BRI is the most common ISDN service for Internet access. A single BRI line can support up to three calls at the same time because it is comprised of three channels (2B+D). Two voice, fax or data "conversations," and one packet switched data "conversation" can take place at the same time.
  • Primary Rate Interface (PRI)
    ISDN PRI service is used primarily by large organizations with intensive communications needs. An ISDN PRI connection supports 23 64 kbps B-channels and one 64 kbps D-channel (or 23B+D) over a high speed DS1 (or T-1) circuit. The European PRI configuration supports 30B+D.
ISDN Applications
  • Internet access
  • Telecommuting/remote access to corporate computing
  • Real-time Video conferencing
  • Small and home office data networking

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PSTN - Public Switched Telephone Network

Posted On at at 2:22 AM by Murali Srinivasan  

Public Switched Telephone Network, is the public circuit-switched telephone network system based on copper wires carrying analog voice data. This is in contrast to newer telephone networks base on digital technologies, such as ISDN and FDDI. Telephone services carried by the PSTN is often called PLAIN OLD TELEPHONE SERVICE (POTS).The PSTN is largely governed by technical standards created by the ITU-T, and uses E.163/E.164 addresses (known more commonly as telephone numbers) for addressing.

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Want To Know More About Telecom ?

Posted On Wednesday, February 6, 2008 at at 4:00 PM by Murali Srinivasan  










This site opens the window to the world of TELECOM. It gives the basic knowledge about Telecommunication. You can find out the meaning and brief description about the technologies, signaling and other engineering terms used in telecom field.

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