T1 Quick Reference

The following is a quick ref of the basic configuration options for T1s and what these configurations actually mean. It's here because it's hard to find something similar in any one particular reference. They all get way to complicated way to quickly or only give you the buzzwords.

Contents

Line Coding Method

AMI

(Bit 7) Zero Code Suppression

B8ZS

Bit Framing

D4

ESF

Errors

Red Alarm

Yellow Alarm

Alarm Indication Signal

References

Line Coding Method

• AMI (Alternate Mark Inversion)

  All T1 line coding is AMI. It represents a zero by absence of a voltage, and a one by a positive or negative pulse (alternating from the previous one). Positive voltages are +/- 3 or 3.5 volts (or 6 to 7 volts peak to peak) at the transmitting node/switch. Thus a set of 8 ones would look something like this:
  

  and a the string 10111001 would look something like this:

  

  T1 signaling requires that in every 15 bits there be at least one pulse (1) in order to maintain framing. This is referred to as the one's density requirement. This leads to the following (more common) implementations of AMI.

• (Bit 7) Zero Code Suppression/Jam Bit 7

  In every eight bit byte, if byte is all zeros, the 7th bit is forced to one. Unfortunately, the 7th bit of every byte becomes unreliable in this case. In T1 voice this is not important (the human ear makes up for it). In data, you lose 1/8th of your bandwidth (for a total of 56k). Also, if for some reason you need clear channel functionality (D-channel on a ISDN PRI), you cannot use Zero Code Suppression (by definition it is not a clear channel).

  With Zero Code Suppression 8 zeros (would never be transmitted but) would be converted to look something like:

  

• B8ZS (Bipolar Eight Zero Suppression/Substitution)

  An all zero byte recieves a B8ZS code (containing intentional bipolar violations to the AMI format). The intent is to guarantee that the far side stays in sync by guaranteeing that excessive 0s are not continuously sent (allowing the far end to slip gradually out of sync). B8ZS fulfills the one's density requirement, while providing a method to signal an all zero byte across the T1 link.
  

Bit Framing

This frame happens 8000 times a second leading to the formula:

which is the speed of a DS1/T1. In actuality, the framing/info bit is not usable, so the formula for 'data/voice-usable bits' becomes:

DS0s (one channel of a T1) are basically one byte position in each frame or:

Remember, when you use (Bit 7) Zero Code Suppression you lose one bit per 8, so the above formula (for B7ZS) becomes

There are two types of framing typically used today.

• D4/SF (Super Frame)

  A 12-frame format.

  The framing/signaling bit is used purely for frame alignment. The odd frames 'frame/signaling bit' is the F bit or Frame alignment signal (101010). The even frames 'frame/signaling bit' is the M bit or Multiframe alignment signal (001110).

  Additionally, each byte in the 6th and 12th frame have their Least Significant Bit used (stolen) for signaling (6th frame-A bits, 12 frame-B bits). An example of signaling via these bits is:
  
  

  E&M signaling

  AB = 00	on -hook (inactive)
  AB = 11	off-hook (active)

  When working with voice, the 6th and 12th frame signal bits are not crucial. The human ear 'glosses over' the lack of the bit. With data, however your channels (would) lose 1 bit out of every 6 bytes. You can assume that the channel is always off-hook, so use a clear channel instead.

• ESF (Extended Super Frame)

  A 24 frame format.

  The framing/signaling bit is used as follows:

  ESF Frame Number	F-Bit Assignment
  	FPS	FDL	CRC
  1		m
  2			CB1
  3		m
  4	0
  5		m
  6			CB2
  7		m
  8	0
  9		m
  10			CB3
  11		m
  12	1
  13		m
  14			CB4
  15		m
  16	0
  17		m
  18			CB5
  19		m
  20	1
  21		m
  22			CB6
  23		m
  24	1

  
  FDL - 4kbps facility data link (m bits), CRC - CRC6 (CB1 thru CB6)

  
  FDL can be used to report framing bit errors, CRC errors, out-of-frame (OOF) events, line code (bipolar) violations, and controlled slip events. Performance summaries can be reported including errored seconds (at least one CRC event), Bursty seconds (2-319 errored seconds), severely errored seconds (> 319 errored seconds, or OOF), and failed seconds state (10 consecutive severely errored seconds).

  Each byte in the 6th, 12th, 18th, and 24th frame has it's Least Significant bit used (stolen) for signaling (6th frame-A bits, 12th frame-B bits, 18th frame-C bits, 24th frame-D bits). An example of signaling via these bits is:
  
  

  E&M signaling

  ABCD = 0000	on -hook (inactive)
  ABCD = 1111	off-hook (active)

Errors

• Red Alarm

  Loss of (recognizable) signal.

  Out-of-frame condition on incoming DS1 signal for 2 to 3 seconds. Also referred to as "loss of signal" alarm.
  
  

• Yellow Alarm

  Yellow alarm sent to local site after a red alarm at remote site.

  An SF yellow alarm is when bit 2 is set to zero in all channels. An ESF yellow alarm is a data link (FDL) message of 0000000011111111.
  
  

• Alarm Indication Signal (AIS)

  All unframed 1s.

  Indicates that the signal was lost upstream. The all 1s are sent to indicate local network continuity.

References