This chapter provides an overview of how digits and signaling information are transmitted when a telephone call is made, and describes the T-1 robbed bit, E-1 CAS, R2 MF, and Direct Dialing In (DDI) signaling concepts.
2.1. Making Telephone Calls: Transmission of Digits and Signaling Information
Historically, making a telephone call started with taking your telephone handset out of its cradle. This action caused your telephone to go off-hook. For analog telephones, going off-hook closes a circuit (called the local loop) connected to the local Central Office (CO) and causes a loop current to flow through the local loop circuit created.
The CO reacts by generating dial tone (typically, a combination of 350 Hz and 440 Hz tones), which indicates that you can dial. Traditionally, you would dial your number using pulse dialing (also called rotary dialing). Pulse dialing sends digit information to the CO by momentarily opening and closing (or breaking) the local loop from the calling party to the CO. This local loop is broken once for the digit 1, twice for 2, etc., and 10 times for the digit 0. As each number is dialed, the loop current is switched on and off, resulting in a number of pulses being sent to your local CO.
Alternatively, you may dial a number using tone dialing, wherein sounds represent the digits dialed (0 through 9, # and * are dialing digits). Each digit is assigned a unique pair of frequencies called Dual Tone Multi Frequency (DTMF) digits (see Table 1. Signaling Used to Dial). Although DTMF signaling is designed for operation on international networks with 15 multifrequency combinations in each direction, in national networks it can be used with a reduced number of signaling frequencies (for example, 10 multifrequency combinations).
In addition to the DTMF digit standard, telcos also use a Multi Frequency (MF) digit standard (see Table 1. Signaling Used to Dial). MF digits are typically used for CO-to-CO signaling. The MF digit standard is similar to the DTMF digit standard except that different pairs of frequencies are assigned. Some MF digits use approximately the same frequencies as DTMF digits; for example, the digit 4 uses 770 and 1209 Hz for DTMF transmissions or 700 and 1300 Hz for MF transmissions. Because of this frequency overlap, MF digits could be mistaken for DTMF digits if the incorrect tone detection is enabled. The accuracy of digit detection depends on:
Table 1. Signaling Used to Dial (Hz)
|
Code |
Pulse |
DTMF |
MF |
R2 MF Forward |
R2 MF Backward |
1 |
1 |
697, 1209 |
700, 900 |
1380, 1500 |
1140, 1020 |
2 |
2 |
697, 1336 |
700, 1100 |
1380, 1620 |
1140, 900 |
3 |
3 |
697, 1477 |
900, 1100 |
1500, 1620 |
1020, 900 |
4 |
4 |
770, 1209 |
700, 1300 |
1380, 1740 |
1140, 780 |
5 |
5 |
770, 1336 |
900, 1300 |
1500, 1740 |
1020, 780 |
6 |
6 |
770, 1477 |
1100, 1300 |
1620, 1740 |
900, 780 |
7 |
7 |
852, 1209 |
700, 1500 |
1380, 1860 |
1140, 660 |
8 |
8 |
852, 1336 |
900, 1500 |
1500, 1860 |
1020, 660 |
9 |
9 |
852, 1477 |
1100, 1500 |
1620, 1860 |
900, 660 |
0 |
10 |
941, 1336 |
1300, 1500 |
1740, 1860 |
780, 660 |
* |
- |
941, 1209 |
1100, 1700 |
1380, 1980 |
1140, 540 |
# |
- |
941, 1477 |
1500, 1700 |
1500, 1980 |
1020, 540 |
For each call, signaling information (off-hook, number dialed) must be detected by the local CO and then sent to each successive CO until the destination CO is reached. The destination CO attempts to connect to the called party. Concurrently, the destination CO sends back signaling information (such as line busy, network busy signals, etc.) representing the condition or status of the called party's line. This signaling information passes through the network as audio tones or as signaling bits. The number of tones used and the frequency combinations used to convey this signaling information vary from country to country and from telco to telco. In addition, private networks may combine various signaling techniques.
After dialing, you listen to hear the progress and status of the call:
The CO typically indicates the progress of making a call by generating these various tones. When making long distance calls, the telco may make brief drops in loop current to indicate:
After a call is connected, a telco service may be requested by a flash-hook. A flash-hook puts the telephone on-hook briefly, long enough for the CO to detect the flash-hook, but not long enough to cause a disconnect. A flash-hook may signal a request for a second dial tone to allow 3-way conferencing or to transfer the call.
At the completion of the call, one or both parties hang up the telephone. Typically, the CO sends a disconnect signal. However, some telcos don't send a disconnect signal; therefore a local CO must use other methods to detect a remote disconnect.
2.1.1. Making Long Distance and Global Telephone CallsLong distance calls may involve transmitting dialing and other signaling information from the local CO, through several intermediate COs, to the distant called party's CO and then connecting to the called party. A mixture of signaling systems and protocols may be encountered especially when making global calls. Local call signaling must be translated into signaling that may pass over analog lines, T-1 digital trunks, E-1 digital trunks, optical fiber, satellite links, etc. All signaling sent over digital trunks must be converted to bits that can be transmitted or multiplexed with the digitized voice transmissions.
Each telco, country, or region tends to apply different signaling standards that must be observed to ensure that a call gets switched through to the called party. For example, some telcos may encode E&M (Ear and Mouth) signals onto the voice path using a single frequency (SF) tone. When present, this tone indicates an on-hook condition. Otherwise, the line is considered to be off-hook (absence of tone). Typically, when the same manufacturer's product is connected to both ends of a digital trunk, then the signaling technique used is transparent as long as all signaling is handled.
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