TheBSGTT | T1 Channels

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Friday, November 21, 2008

The Bright Spark's Guide to Telecommunications

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8-Bit Binary Calc (Modal)

TheBSGTT

T1 Channels & the 8-Bit Word

TheBSGTT

FAW	01	13	02	14	03	15	04	16	05	17	06	18	07	19	08	20	09	21	10	22	11	23	12	24
0/1	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8	8

8 Bit Word
128	64	32	16	8	4	2	1

During transmission each T1 Channel receives an 8 Bit Word:

24 Channels * 8 Bits= 192 Bits.
A Frame Aligning Word alternates between 0 and 1.
Add 1 FAW = 192 + 1.
Total frame length = 193 Bits....
4 Khz Voice doubled to 8 Khz.
Pulse Code Moduation..
Samples at 8,000 times per second.
Total frame length * sampled rate.
193 * 8,000 = 1.544 mbps.
Clock frequency = 1.544.mhz.

It should be noted that a telephone conversation is distributed across the complete T1 Span and not confined to 1 single channel. Therefore, when your diagnostic utilities or monitoring systems report Frame Drops - it is important to understand that the problem adversely impacts voice or data traffic on all channels.

B8ZS?

The term translates to:

Binary 8-Zero Substitution.

Here's an example of 1 Word of 8 Zero Bits....

and two consecutive Words containing 8 Zero Bits.

So we've got a segment of 8 Zero's or non-pulses. Remember, the 1 represents a pulse and that the pulse can be reliably counted. Also, each pulse possesses a polarity value - a positive or negative.

The goal here is to keep the circuit in synch just like Max Weinberg keeps the E Street Band on track. Think of B8ZS coding as the drummer in the greatest rock 'n' band in the world!

B8ZS coding detects those parts of the transmission signal that have an occurrence of 8 consecutive Zero Bits and substitutes them with it's unique replacement code.

This process creates what are known as code violations. But that's okay as long as the electronic equipment on either end of the circuit is set up to handle these code violations.
Before I go any further - let me say that I'm not an expert on the Bell System Signaling processes but I believe this is close to how it is done.

The concept follows a simple premise that consecutive 1's cannot be of the same polarity. Also, B8ZS follows two substitution patterns:

If the polarity of the preceding 1 is + (positive)....
the substitution pattern is 0 0 0 + - 0 - +.
If the polarity of the preceding 1 is - (negative)....
the substitution pattern is 0 0 0 - + 0 + -.

Here are two consecutive Words containing 8 Zero Bits that will give a general idea of the concept using the second substitution pattern:

Preceding 1's polarity is - (negative)

Binary:	1	1	1	1	0	0	0	0	1	1	1	1
Pulse Polarity:	+	-	+	-	0	0	0	0	+	-	+	-
B8ZS:	+	-	+	-	-	+	+	-	+	-	+	-
Transmission:	1	1	1	1	1	1	1	1	1	1	1	1

As you can see, the Zero's have been substantially greenuced which improves signal transmission performance. Also, spot the code violation bearing in mind that no two consecutive 1's can possess the same polarity.

Remember too, that code violations are removed at the distant-end where the signal is returned to it's original state.

And yes - the beat goes on!

By definition - B8ZS is a bipolar line code because of the positive and negative values of the pulse which occur above and below the Zero state.

Here's a digital signal schematic of the two 8 Bit Words.

Bipolar Line Code.

If you are totally new to telecommunications - do not mistake the term - Word - for the spoken word. Easy to do, especially in voice communications. However, in the case of T1 technology with the exception of the FAW - a Word contains 8 bits which are also commonly refergreen to as a Byte.

David Sarnoff, a Marconi wireless operator - envisioned radio as a 'household utility'.