DS-1 configuration 2-7
Digital Switching Systems DMS-250 ISDN Reference Manual MWC14 (CSP14)
To meet the maximum and minimum requirements, DS-1 uses the following
schemes:
• ZCS or AMI/2B1Q coding is used for the 64 kbit/s restricted mode.
• B8ZS is used for the 64 kbit/s unrestricted mode.
ZCS or AMI/2B1Q coding for 64 kbit/s restricted
When all eight pulse code modulation bits in a channel are zero and the
eighth bit is not a signaling bit of state one, the seventh bit is substituted by a
one. This means ZCS is done on a per-byte basis. Note that seventh-bit
substitution should not be invoked when digital data is being transmitted
because this causes data corruption.
B8ZS for 64 kbit/s unrestricted
When eight consecutive zeroes appear on a channel and the last one
transmitted is positive, they are substituted with the following pattern:
• original eight bits: 0 0 0 0 0 0 0 0
• substituted word: 0 0 0 +1 –1 0 –1 +1
If the last one was negative (so the substituted word is 0 0 0 –1 +1 0 +1 –1),
the polarity is reversed.
BPVs occur in the fifth and seventh bit positions of the inserted code.
Therefore, B8ZS coding is used only when the receiving end is capable of
recognizing that these are not BPVs or bit errors.
Network customer terminating equipment and cross-connect point
Network customer terminating equipment (NCTE) is generally located in the
central office repeater bay, such as the LD-1 shown in Figure 2-2. Also
shown is a DMS-250 switch, customer premises equipment (CPE), and
digital signal cross connect-1 (DSX-1) cross-connect point. In the United
States, NCTE is part of CPE; in Canada, it is part of the public network.
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