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Q9. What is the difference between AMI, B8ZS, and HDB3?

A9. AMI, B8ZS, and HDB3 are different types of line coding used in T1 and E1 communications systems. AMI (alternate

mark inversion) is used in both T1 and T1 systems. B8ZS (bipolar with 8 zeros substitution) is used in T1 systems. HDB3

(high-density bipolar 3) is used in E1 systems.

AMI is the most basic encoding scheme, in which ones are represented by voltage pulses and zeros are represented by

the lack of a voltage pulse. In AMI, the polarity of each pulse is the opposite of the previous pulse. The problem with

using AMI is that if not enough voltage pulses are sent down the line, the receiver will not correctly decode the data

stream.

To solve this problem, two different encoding schemes were developed to replace a sequence of zeros with a special code

word of voltage pulses. The B8ZS encoding scheme replaces each sequence of 8 zeros with the code of 000VB0VB, where

the V pulse (bipolar violation) has the same polarity as the previously pulse and the B pulse (correct bipolar pulse) has

the opposite polarity as the previous pulse. HDB3 encoding is slightly more complicated because, unlike the B8ZS

encoding that has a balance of positive and negative voltage pulses, the HDB3 encoding must select a code word to

maintain the balance. In HDB3, each sequence of 4 zeros is replaced by 000V or B00V. The decision of which code word

to use is made so that the number of B pulses between consecutive V pulses is odd. This maintains the balance of positive

and negative pulses when using HDB3 encoding. The table below shows examples of the different encoding types.

*The previous V bit is directly before the beginning of the data sequence.

Q10. What are the names of the regulatory bodies that govern the requirements for network

interface line protection?

A10. There are many different regulatory bodies that govern how the T1 and E1 line interfaces must operate under surge

and power cross conditions. The list below contains the commonly referenced regulatory bodies and the specifications that

apply in each case.

FCC Part 68: Effective July 23, 2001, the Administrative Council for Terminal Attachments (ACTA) assumed operational

responsibility for Part 68. ACTA is the newly-formed industry council for Part 68 certification requirements and

administration. This followed the FCC's decision to privatize its Part 68 responsibilities and select TIA and ATIS to serve as

joint sponsors of the Council. Because of this, FCC Part 68 has been renamed to be TIA/EIA-IS-968.

Old Part 68 URL:

www.fcc.gov/wcb/iatd/part_68.html

New Part 68 URL:

www.part68.org

UL 1950: Underwriter Laboratories has made changes to the naming of the over-voltage specifications known as UL 1950,

3rd edition. This specification is now referred to as UL 60950, 3rd edition.

Old UL1950 URL:

www.ul.com/pag1950

New UL60950 URL:

data.ul.com/pagos

Q11. What is the difference between jitter and wander?

A11. Jitter is defined as the magnitude of phase variation (with respect to a reference clock or data signal) whose

frequency of variation is greater than 10Hz. Jitter is measured in unit intervals (UI), where 1 UI is equal to one data bit-

width. For an E1 signal, 1 UI is equal to 488ns, and for a DS1 signal, 1 UI is equal to 648ns. However, if the rate of

change in phase is less than 10Hz, then this phenomenon is known as wander and is measured in nanoseconds.

Q12. What standards bodies govern telecommunications equipment compliance?

A12. The following is a list of the largest telecommunications standards bodies:

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