CS62180B datasheet(48/50 Pages) CIRRUS

48 / 50 page

Alarms

Figure A10 shows a useful overview of the alarm

operation in a PCM link. When an intermediate

monitoring system (or central office repeater) de-

tects a loss of signal, it transmits an all "1’s"

signal (Blue alarm, or Alarm Indication Signal)

on the line to maintain clock recovery operation

in the subsequent digital repeaters and the desti-

nation’s receiver. The same Blue alarm may be

used by the source transmitter if, for some rea-

son, it cannot maintain normal functionality

(such as during loopback).

When the loss of signal is detected at the inter-

mediate monitor, an internal Red alarm (also

known as a Service Alarm Indication, or Prompt

Maintenance Alarm) is generated. While in a

Red alarm mode, the monitor transmits a Yellow

alarm back to the source’s receiver, indicating a

remote loss of alignment. This Yellow alarm in-

forms the source that there’s a problem farther

down the line and it’s transmission is not being

received at the destination.

Zero Substitution

As was mentioned in the T1 overview, data is

transmitted over dual twisted-pair cable with

digital repeaters at 6000 ft. intervals. It is en-

coded in a bipolar AMI (Alternate Mark

Inversion) format. Successive "1’s" are encoded

alternately as positive and negative voltage

pulses. A zero is simply an absence of pulses.

This means that a long stream of "0’s" is indis-

tinguishable from a dead line. Clock recovery

circuits in the network maintain clock synchroni-

zation by syncing to the "1’s" pulses in the

transmission stream. Synchronization may be

lost if there are too many consecutive zero’s,

hence there is a general requirement that there be

at least 12.5 % "1’s" density in the transmission

stream. Furthermore, no more than 15 consecu-

tive "0’s" are allowable. Various zero substitution

schemes have been developed to meet these re-

quirements. The CS62180B supports B7

and B8ZS zero suppression formats.

B7 Zero Substitution

B7 zero substitution guarantees at least one "1"

in all DS0 channels. This satasfies the 12.5 %

ones density, and guarantees that more than 15

consecutive zeros will never occur. In B7 substi-

tution systems, the 7

th bit (2nd LSB) of an all

zero channel is forced to a "1". This strategy

maintains 1’s density in voice grade transmis-

sion, with negligible audible interference. The

drawback with the B7 format is that it’s impossi-

ble for the receiving end to detect and remove

the changed bits. This makes B7 zero suppres-

sion unacceptable for clear channel transmission,

in which the integrity of the data must be main-

tained.

B8ZS Zero Substitution

B8ZS (Bipolar Eight Zero Substitution) satisfies

the one’s density requirement without corrupting

transmission data. Instead of operating on indi-

vidual channels, the B8ZS format looks at the

entire transmission stream. Any eight consecutive

zeros are replaced with an 8 bit code. This code

uses specific bipolar violations of the AMI for-

mat to distinguish it from the ordinary data. If

the last "1" transmitted before a string of zeros

was encoded as a positive pulse, then the B8ZS

code for the next eight bits will be 000+-0-+.

Similarly, if the last "1" was a negative pulse,

then the code will be 000-+0+-. In either case,

bipolar violations occur in the fourth and seventh

bits. These violations are decoded as a string of

Blue Alarm

Monitor

Loss of

Signal = Red

Alarm

Yellow Alarm

Source

Transmit

Receive

Transmit

Receive

Destination

Digital

Repeater

Yellow

Alarm

Yellow

Alarm

Blue

Alarm

Figure A10. Alarm Operation on a T1 Link.

CS62180B

DS225PP2

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