Framing - T1

               Framing is the process of combining the information (bits or bytes) before transmitting it to the destination device. The generated collection of bits is called a frame. A frame can contain information of a single user or of multiple users. It has clearly defined boundaries to identify the start and end of a frame. In a TDM based system, a frame is a set of consecutive time slots in which the position of each bit or a time slot can be identified by reference to a frame-alignment signal. There are many framing formats (of different speed and size) defined worldwide and used across the telecommunication networks. Apart from the user information, a frame may carry other special bits (or bytes) like framing bits – to identify the start and end of a frame, address bits – to identify the source or destination devices/users, error correction or detection bits, timing or synchronization information and other maintenance information bits. The two basic TDM based framing techniques are T1 and E1.Also the basic unit of the T-carrier system is the DS0, which has a transmission rate of 64 Kbit/s, and is commonly used for one voice circuit.

T1 is the North American standard and is used in North America and Japan. T1 combines 24 separate voice channels onto a single link. This individual voice channels is called a DS0, which is the basic unit of any TDM based systems. The T1 data stream is broken into frames consisting of a single framing bit plus 24 channels of 8-bit bytes (1 framing bit per frame + 24 channels per frame * 8 bits per channel = 193 bits per frame). The frames must repeat 8,000 times per second in order to properly recreate the voice signal of individual users (24 such users). Thus, the required bit rate for T1 is 1.544 Mbps (8,000 frames per second * 193 bits per frame).

The T1 electrical interface consists of two pairs of wires - a transmit data pair and a receive data pair. Timing information is embedded in the data. T1 utilizes bipolar electrical pulses. Where most digital signals are either a ONE or a ZERO (unipolar operation), T1 signals can be one of three states. The ZERO voltage level is 0 volts, while the ONE voltage level can be either a positive or a negative voltage.

Encoding Methods 

There are a number of different encoding methods used on T1 lines. Alternate Mark Inversion (AMI), Bipolar With 8-Bit Substitution (B8ZS), and High Density Bipolar Three Code (HDB3) will be discussed here.

AMI encoding causes the line to alternate between positive and negative pulses for successive 1's. The 0's code is no pulse at all. Thus, a data pattern of 11001011 would cause the following pattern on an AMI line: - +,-, 0, 0, +, 0,-, +. With this encoding technique there is a problem with long strings of 0's in the user's data which produce no transitions on the line. The receiving equipment needs to see transitions in order to maintain synchronization. Because of this problem, DS-1 (the signal carried via a T-carrier) specifications require that users limit the number of consecutive 0's in their data steam to less than 15. With this scheme of encoding there should never be consecutive positive or negative pulses on the line (i.e., the following pattern should never occur: 0, +,-, +, +,-). If two successive positive or two successive negative pulses appear on the line, it is called a Bipolar Violation (BPV). Most T1 systems watch for this event and flag it as an error when it occurs.

B8ZS and HDB3 are both methods which permit the user to send any pattern of data without affecting the operation of the T1 line. Both of these encoding schemes make use of BPVs to indicate that the user’s data contains a long string of 0's. B8ZS looks for a sequence of eight successive 0's and substitutes a pattern of two successive BPVs. The receiving station watches for this particular pattern of BPVs and removes them to recreate the original user data stream.

 
HDB3 is the scheme recommended by the CCITT. This scheme watches for a string of four successive 0's and substitutes a single BPV on the line.

T1 Framing Techniques

SuperFrame (also called D4 Framing) 

In order to determine where each channel is located in the stream of data being received, each set of 24 channels is aligned in a frame. The frame is 192 bits long (8 * 24), and is terminated with a 193rd bit, the framing bit, which is used to find the end of the frame. In order for the framing bit to be located by receiving equipment, a pattern is sent on this bit.Equipment will search for a bit which has the correct pattern, and will align its framing based on that bit. The pattern sent is 12 bits long, so every group of 12 frames is called a Super Frame. The pattern used in the 193rd bit is 1000 1101 1100. In order to send supervisory information over a D4 link "bit robbing" is used. A voice signal is not significantly affected if the low-order bit in a byte is occasionally wrong. D4 framing makes use of this characteristic of voice and uses the least-significant bits in each channel of the 6th (A Bit) and 12th (B Bit) frames to send signalling information; on-hook, off-hook, dialing and busy status.

Extended Superframe (ESF) Framing

The Extended Superframe Format (ESF) extends the D4 superframe from 12 frames to 24 frames. ESF also redefines the 193rd bit location in order to add additional functionality. In ESF the 193rd bit location serves three different purposes: 

• Frame synchronization 
• Error detection 
• Maintenance communications (Facilities Data Link - FDL)

Within an ESF superframe, 24 bits are available for these functions. Six are used for synchronization, six are used for error detection, and twelve are used for maintenance communications. In D4 framing, 12 bits are used per superframe for synchronization. In ESF framing, 6 bits are used per superframe for synchronization. There is no link-level error checking available with D4 framing (except for bipolar violations). ESF framing utilizes a 6-bit Cyclic Redundancy Check (CRC) sequence to verify that the frame has been received without any bit errors. As a superframe is transmitted, a 6-bit CRC character is calculated for the frame. This character is then sent in the six CRC bit locations of the next superframe. The receiving equipment uses the same algorithm to calculate the CRC on the received superframe and then compares the CRC value that it calculated with the CRC received in the next superframe. If the two compare, then there is a very high probability that there were no bit errors in transmission. As was stated earlier, 12 bits are used for maintenance communications. These 12 bits give the maintenance communications channel a capacity of 4,000 bits per second. This function enables the operators at the network control center to interrogate the remote equipment for information on the performance of the link. As with D4 framing ESF utilizes "robbed bits" for in-band signalling. ESF utilizes 4 frames per superframe for this signalling. The 6th (A bit), 12th (B bit), 18th (C bit), and 24th (D bit) frames are used for the robbed bits. The function of the robbed bits is the same as in D4 framing.

T1 Alarms

An alarm is a response to an error on the E1 line or framing. Three of the conditions that cause alarms are loss of frame alignment (LFA), loss of multi-frame alignment (LFMA), and loss of signal (LOS). The LFA condition, also called an out-of-frame (OOF) condition, and LFMA condition occur when there are errors in the incoming framing pattern. The number of bit errors that provokes the condition depends on the framing format. The LOS condition occurs when no pulses have been detected on the line for between 100 to 250 bit times. This is the highest state alarm where nothing is detected on the line. The LOS may occur when a cable is not plugged in or the far end equipment, which is the source of the signal, is out of service. The alarm indication signal (AIS) and remote alarm indication (RAI) alarms are responses to the LOS, LFA, and LFMA conditions. The RAI alarm is transmitted on LFA, LFMA, or LOS. RAI will be transmitted back to the far end that is transmitting frames in error. The AIS condition is a response to error conditions also. The AIS response is an unframed all 1's pattern on the line to the remote host. It is used to tell the far end it is still alive. AIS is the blue alarm, RAI is the yellow alarm. A red alarm that can occur after a LFA has existed for 2.5 seconds. It is cleared after the LFA has been clear for at least one second.

Red alarm indicates the alarming equipment is unable to recover the framing reliably. Corruption or loss of the signal will produce “red alarm.” Connectivity has been lost toward the alarming equipment. There is no knowledge of connectivity toward the far end.

Yellow alarm, also known as remote alarm indication (RAI), indicates reception of a data or framing pattern that reports the far end is in “red alarm.” The alarm is carried differently in SF (D4) and ESF (D5) framing. For SF framed signals, the user bandwidth is manipulated and "bit two in every DS0 channel shall be a zero. The resulting loss of payload data while transmitting a yellow alarm is undesirable, and was resolved in ESF framed signals by using the data link layer. "A repeating 16-bit pattern consisting of eight 'ones' followed by eight 'zeros' shall be transmitted continuously on the ESF data link, but may be interrupted for a period not to exceed 100-ms per interruption. Both types of alarms are transmitted for the duration of the alarm condition, but for at least one second.

Blue alarm, also known as alarm indication signal (AIS) indicates a disruption in the communication path between the terminal equipment and line repeaters or DCS. If no signal is received by the intermediary equipment, it produces an unframed, all-ones signal. The receiving equipment displays a “red alarm” and sends the signal for “yellow alarm” to the far end because it has no framing, but at intermediary interfaces the equipment will report “AIS” or Alarm Indication Signal. AIS is also called “all ones” because of the data and framing pattern.