This chapter:
Note: This chapter is provided for informational use only. Your board's hardware perform all operations necessary to support the framing system used on the trunk. The TCPs perform all necessary signaling operations.
T1 and E1 are four-wire digital transmission links. T1 is used mainly in the United States, Canada, Hong Kong and Japan. E1 is used in Europe.
Data on a T1 or E1 trunk is transmitted in channels. Each channel carries information digitized at 64000 bits per second (bps). This transmission rate is called the digital signal level 0 (DS-0) rate.
T1 carries 24 channels. E1 carries 32 channels. The total throughput rate (called digital signal level 1 or DS-1) is:
Two types of information are carried on a trunk:
Signaling information can be conveyed using either channel-associated signaling (CAS) or common channel signaling (CCS). These signaling methods are described below.
With CAS, signaling information is sent for all channels at regular intervals, regardless of whether each channel's state changes or not. The information for each channel consists of a set of bits (called the ABCD bits). Whenever a channel's state changes, the ABCD bit pattern for that channel changes to convey the signaling bits.
On T1 trunks using a CAS protocol (such as wink-start), the signaling information for each channel is transmitted using a method called robbed-bit signaling. With this method, one of the bits in the voice information in each channel is changed at regular intervals to indicate the state of the channel. Since the intervals are widely spaced, sound quality in the channel is not compromised.
On E1 trunks using a CAS protocol, channel 16 carries the ABCD bits for all of the other channels. No robbed-bit signaling is used.
Different CAS protocols use the ABCD bits in different ways. For example, MFC-R2 protocols use only two bits to signal four separate states; the other bits are not used. Pulsed E&M protocols convey signaling using one bit only, by setting and resetting the bit at specific intervals to signal different states. (The specific bit patterns or intervals indicating different states differ from country to country. Refer to the appropriate Developer's Reference Manual for more information.)
To interpret the bits properly in a given country, your board must run a trunk control program (TCP) compatible with that country's protocol. For more information, see section 1.3.1.
With CCS, packets of signaling information for a channel are sent when the channel's state changes, instead of signaling bits. CCS information is sent in a dedicated channel, the data channel or D channel. Voice information is carried in bearer channels (B channels).
On T1 trunks using a CCS protocol (such as ISDN), the 24th channel is used as the D channel. It transmits packets of signaling information whenever the status of any of the other channels changes. No robbed-bit signaling is used.
In setups with multiple T1 ISDN trunks, a Network Facility Associated Signaling (NFAS) configuration is often used. In this configuration, the D channel on one of the ISDN trunks carries signaling for all channels on all other trunks. This leaves channel 24 free on all other trunks to be used as a B channel. (See Figure 16.)
Figure 16: Network Facility Associated Signaling (FAS)
On E1 trunks using ISDN, channel 16 is used as the D channel, carrying signaling packets instead of signaling bit patterns. NFAS configurations are not supported on E1 trunks.
On T1 and E1 trunks, the data in the channels is combined into a single continuous stream of data using time-division multiplexing (TDM). With TDM, the channels take turn "sharing" the trunk over and over again, each channel broadcasting 8 bits at a time. The time given a channel during a given round is called a timeslot. One round of timeslots is called a frame.
T1 and E1 delineate frames differently. The following sections describe T1 and E1 framing formats.
On T1 trunks, a frame consists of 24 timeslots, sent every 1/8000 sec. (See Figure 17.)
Two T1 framing formats are supported by the AG-T1: D3/D4 framing and Extended SuperFrame (ESF).
After each frame, the F bit is set or reset according to a pattern that repeats once every 12 frames: 100011011100. This makes the F bit recognizable even in the high-speed T1 bit stream. The 12 frames in this cycle together constitute an M24 superframe.
With CAS protocols, the least significant bit in each timeslot is "robbed" for signaling in the 6th and 12th frames in each M24 superframe. Since each bit has only two possible states (0 or 1), only four separate signaling conditions can be transmitted with CAS protocols. (Since CCS protocols do not use bit signaling at all, this limitation does not apply to CCS protocols.)
Figure 19: Robbed-bit Signaling (D3/D4 Framing Format)
The other extra bits (18 in all) are used alternately as follows (see Figure 20):
With CAS protocols, bits are robbed from each timeslot in the 6th, 12th, 18th and 24th frame in the extended superframe (as shown in Figure 20). Thus instead of two signaling bits per superframe, ESF has 4 bits, allowing up to 16 separate signaling conditions to be transmitted. Since CCS protocols do not use bit signaling at all, this limitation does not apply to CCS protocols.
Figure 20: Extended Superframe
In your AG configuration file, you can specify which framing format to use with the FrameType= statement. For more information, see Chapter 3.
On E1 trunks, a frame consists of 32 timeslots, sent every 1/8000 sec. (See Figure 21.)
In each frame, channels are numbered 0 through 31. Half of the first channel (channel 0) is used for frame synchronization. The other half can be used as a facilities data link (FDL).
With CAS protocols, signaling information for each channel is carried in channel 16. This eliminates the need for robbed-bit signaling. Channels 1 through 15 and 17 through 31 (30 channels in all) carry voice information. (See Figure 22.)
With CAS protocols, four ABCD bits are sent for each channel at a time. Since timeslot 16 can only carry 8 bits of information per frame, it is not possible to send the signaling for all 30 channels in each frame. Therefore, channels take turns using channel 16, two at a time. Thus it takes 15 frames to cycle through the signaling for all channels. Since CCS protocols do not use bit signaling at all, this limitation does not apply to CCS protocols.
After every 15 frames, an extra frame is sent to synchronize the receiver to the signaling channel. Thus the full cycle contains 16 frames. A group of 16 such frames is called a multiframe. (See Figure 23.)
In a T1 or E1 channel, voice information is typically converted from analog to digital signals (and vice versa) using the Pulse Code Modulation (PCM) digital encoding method. The device used to perform this conversion is called a codec (COder-DECoder). First, the incoming analog signal is sampled 8000 times per second. For each sample, the amplitude is measured, and represented by an 8-bit digital value. This value is placed in a timeslot for the channel. The receiving device reverses this process to produce the analog signal again.
There are only 256 possible unique amplitude measurements with 8 bits. 256 digital values are not enough to represent the entire amplitude range of the human voice at a usable quality level. However, most of the characteristics of a voice signal that make it understandable to the human ear exist at the lower end of the amplitude range. Therefore, to get around the digital sound representation problem, the values are assigned to amplitude values non-linearly, with many values made available to represent various amplitudes in the low end of the range, and few values to measure the high end. This compression method is called companding.
Different companding algorithms are used in different geographic regions. A companding method called µ-law is used in T1 transmissions, in the US, Canada and Japan. Another method, called A-law, is used in E1 transmissions in the rest of the world.
You select the companding algorithm to use when you install your driver software. In your AG configuration file, you also select µ-law or A-law versions of DSP files, etc. For more information, see the Developer's Manual that came with your protocol software.
To reduce crosstalk on T1 and E1 lines, and to keep energy low on a trunk line, each "1" bit on the trunk is sent with the opposite electrical polarity of the preceding "1" bit. Between two successive "1" bits, the voltage returns to zero and pauses. This transmission method is called alternative mark inversion (AMI).
"0" bits are sent as intervals of zero voltage. Multiple zeros in a row appear at the receiving end as one long interval of no voltage. If these gaps are too long, it is difficult for the receiving end to keep in framing sync with the transmitting end. There are various algorithms used in T1 and E1 transmissions to get around this problem, by insuring that there are sufficient "1"s (enough ones density) to keep the transmitting and receiving ends in sync. These are called zero code suppression algorithms.
The following zero code suppression algorithms are supported by the AG-T1:
The AG-E1 can be configured to transmit without zero code suppression, or to use the high density bipolar 3 code (HDB3) algorithm. In HDB3, sequences of 4 zero data bits are replaced by patterns of bipolar violations.
In the AG configuration file, you can specify which algorithm to use, using the LineCode statement. For more information, see Chapter 3.
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