Introduction

Channels and Transmission Rates

Signaling

Channel Associated Signaling (CAS)

Common Channel Signaling (CCS)

Framing

T1 Framing

E1 Framing

Voice Encoding

Companding

AMI, Ones Density, and Zero Code Suppression

Introduction

This chapter:

• Provides an overview of T1 and E1 channels, including voice encoding, framing, and signaling characteristics.

• Explains how CG boards interface with T1 or E1 trunks.
  Note: The following information is provided for informational use only. Your board's hardware performs all the operations necessary to support the framing system used on the trunk.

Channels and Transmission Rates

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:

• For T1, 24 channels, each carrying 64,000 bps, yield a throughput rate of 1,536,000 bps. An extra 8000 bps are used to carry framing and other information (as described in Section ). DS-1 for T1 is 1,544,000 bps.

• For E1, 32 channels, each carrying 64,000 bps, yield a rate of 2,048,000 bps.

Signaling

Two types of information are carried on a trunk:

• Voice information

• Signaling information (for example, indicating that a channel is on-hook or off-hook)

Signaling information can be conveyed using either channel associated signaling (CAS) or common channel signaling (CCS). These signaling methods are described in the following sections.

Channel Associated Signaling (CAS)

With CAS, signaling information is sent for all channels at regular intervals, regardless of whether each channel's state changes. 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, 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, some protocols use only two bits to signal four separate states; the other bits are not used. Other protocols convey signaling using one bit only, by setting and resetting the bit at specific intervals to signal different states. The specific patterns of bits used to indicate signaling states differ from country to country. Refer to the appropriate protocol reference manual for more information.

Common Channel Signaling (CCS)

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).

Different CCS protocols reserve different channels for transmitting data. For example, GR-303 occupies channels 24 and 12 on T1 trunks to run TMC and EOC protocols. GR-303 also uses robbed-bit signaling for call control on established connections. On E1 trunks using the V5 protocol signalling takes place on channels 15, 16, and 31.

Framing

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 turns "sharing" the trunk over and over again. Each channel broadcasts 8 bits at a time. The time given a channel during a given round is called a timeslot. One cycle of timeslots is called a frame.

T1 and E1 delineate frames differently. The following sections describe T1 and E1 framing formats.

When configuring a CG board, you specify which framing format to use. For more information about configuring the CG board, refer to the CG hardware documentation.

T1 Framing

On T1 trunks, a frame consists of 24 timeslots, sent every 125 µsec (1/8000 sec).

Figure 19. T1 Frame

CG boards support two T1 framing formats: D4 framing and Extended SuperFrame (ESF).

With D4 framing, a single framing bit (F bit) is sent after each frame, to mark the end of the frame and the beginning of the next one. Each frame consists of (24x8)+1 = 193 bits. The framing bits (8000 per second) take up extra bandwidth.

Figure 20. Framing Bits on a T1 Trunk

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 constitute one superframe.

With CAS protocols, the least significant bit in each timeslot is "robbed" for signaling in the 6^th and 12^th frames in each superframe. Since each bit has only two possible states (0 or 1), only four separate signaling conditions can be transmitted with CAS protocols.

Figure 21. Robbed-bit Signaling (D4 Framing Format)

With ESF framing, an extra bit appears after every frame, as in D4 framing. However, only every fourth extra bit is used for framing. This bit is set or reset in a pattern that repeats once every 24 frames, instead of the 12-frame repetition in D4 framing. The 24 frames in the cycle constitute one extended superframe.

All of the other extra bits (18 in all) are used alternately:

• Six of the bits are used for a cyclic redundancy check (CRC), to detect errors.

• The other 12 carry diagnostic data. This bandwidth is called the Facilities Data Link (FDL).

With CAS protocols, bits are robbed from each timeslot in the 6^th, 12^th, 18^th, and 24^th frame in the extended superframe (as shown in Figure 22). Thus instead of two signaling bits per superframe, ESF has 4 bits, allowing up to 16 separate signaling conditions to be transmitted.

.

Figure 22. Extended Super Frame

E1 Framing

On E1 trunks, a frame consists of 32 timeslots. A frame is sent every 125 µsec (1/8000 sec).

Figure 23. E1 Frame

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 data information.

Figure 24. CEPT E1 Timeslots

With CAS protocols, four ABCD bits are sent for each channel at a time. Since timeslot 16 can carry only 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. It takes 15 frames to cycle through the signaling for all channels.

After every 15 frames, an extra frame is sent to synchronize the receiver to the signaling channel. Therefore the full cycle contains 16 frames. A group of 16 such frames is called a multiframe.

Figure 25. E1 Multiframe

Voice Encoding

For CG boards, the information received is already Pulse Code Modulation (PCM) encoded.

Companding

Only 256 possible amplitude measurements can be represented 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, the values are assigned to amplitude values non-linearly, with many values 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 mu-law is used in the US, Canada, and Japan. Another method, called A-law, is used in the rest of the world.

When configuring a CG board, you select mu-law or A-law versions of the DSP files.

AMI, Ones Density, and Zero Code Suppression

To reduce cross talk on T1 and E1 trunks, and to eliminate DC bias, each 1 bit on the trunk is sent with the opposite electrical polarity of the preceding 1 bit. This transmission method is called alternate 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 maintain framing synchronization with the transmitting end. Various algorithms used in T1 and E1 transmissions get around this problem by ensuring that there are sufficient ones (enough ones density) to keep the transmitting and receiving ends in synchronization. These are called zero code suppression algorithms.

Algorithm	Description
B8ZS - binary 8-zero suppression	This is the algorithm used with ISDN protocols. To send an interval of successive zeroes, the sending end replaces the zeroes with a pattern of ones and zeroes in which bipolar violations occur; that is, one or more successive ones are sent with the same polarity, disrupting the AMI pattern. The pattern of bipolar violations is recognized at the receiving end and turned back into zeroes.
HDB3	High density bipolar 3 code - uses patterns of bipolar violations to replace sequences of 4 zero data bits in order to maintain ones density on clear channel transmission.
Jammed-bit 7 zero code suppression	In an interval of zeroes, the sending end jams every bit 7 high so the receiving end can recognize it. This method sacrifices data integrity, but quality is sufficient for voice transmissions.
No zero code suppression	Zero code suppression disabled.

CG boards configured as E1 boards can be set up to transmit without zero code suppression, or to use the high density bipolar 3 code (HDB3) algorithm. When configuring a CG board, you must specify which algorithm to use. For more information, refer to the CG board documentation.