3.1. T-1 Digital Telephony
This chapter provides a brief overview of T-1 and E-1 concepts and a description of how Dialogic hardware works in T-1 and E-1 environments.
A T-1 circuit is used to transfer digital information in a two-way, full duplex connection at a speed of 1.544 megabits per second (Mbps). In a T-1 environment, this rate is known as digital signal level 1 or DS-1. A T-1 circuit contains 24 voice channels, each operating at a rate of 64,000 bits per second (bps), a rate known as digital signal level 0 or DS-0. The formula used to calculate the DS-1 rate of 1.544 Mbps includes an extra 8,000 bits that are not part of the voice data but used to synchronize the data received and transmitted on the T-1 circuit.
64,000 bps (Voice Channel Rate, DS-0)
x 24 (Number of Voice Channels)
1,536,000 bps
+ 8,000 (Controlling Bits)
1,544,000 (T-1 Circuit Rate, DS-1)
The T-1 compatible Digital Network Interface boards de-multiplex the 24 voice channels on a T-1 circuit and pass them on to associated hardware (such as a voice board or other resource sharing module).
3.1.1. T-1 Frame FormatDigital data on a T-1 line is organized into D4 frames. A D4 frame consists of a single 8-bit sample from each of the 24 voice channels and one framing bit, for a total of 193 bits. Each 8-bit sample occupies what is known as a time slot within the frame. Figure 1 shows one D4 frame.
Figure 1. D4 Frame Format
The term time slot is derived from the method that is used to multiplex the 24 voice channels in a D4 frame. The channels are byte-interleaved in a frame. That is, each byte is a sample from a different voice channel and occurs in a fixed pattern within the frame (voice channel one in time slot one, voice channel two in time slot two, etc.). All D4 frames have the same pattern. This technique of interleaving is called time division multiplexing.
Twelve D4 frames make up what is known as a D4 superframe. Figure 2 shows a single D4 superframe, indicating the framing bit values of the individual D4 frames. The framing bits are used for frame synchronization, which is described in more detail in Section 3.1.2. T-1 Synchronization.
Figure 2. D4 Superframe Format
3.1.2. T-1 SynchronizationTo identify DS-0 voice channels for the receiver, the data being transferred must be synchronized. This capability is built into the D4 frame and superframe formats for T-1 systems. Each D4 frame in a superframe begins with a framing bit. The 12 framing bits in a D4 superframe are arranged in a predefined pattern: 100011011100. By searching for this pattern, the T-1 compatible Digital Network Interface hardware can determine the beginning and end of every D4 superframe, D4 frame, and time slot. When this pattern cannot be found, the resulting error is known as Receive Loss of Synchronization (RLOS). See Section 3.3.2. Loss of Synchronization Alarm Handling for information on T-1 alarm handling.
3.1.3. T-1 SignalingT-1 signaling information (on-hook and off-hook states) must be carried on a T-1 line. Signaling is accomplished using two bits called the A-bit and the B-bit. Each time slot in the sixth frame of the D4 superframe has the least significant bit replaced with signaling information. These are the A-bits. Similarly, each time slot in the 12th frame of the D4 superframe has the least significant bit replaced with signaling information. These are the B-bits. This strategy of replacing the least significant bit with signaling information is called robbed-bit signaling.
For example, in E&M (Ear and Mouth) protocol the signaling bits indicate whether the sending party's line is on-hook or off-hook. When the signaling bits are 0s, the line is on-hook, and when the signaling bits are 1s, the line is off-hook.
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