2-12 DS-1 configuration
297-2521-106 Standard 23.01 March 2001
•
errored seconds (ES)
• severe errored seconds (SES)
• unavailable seconds (UAS)
• states
Frame slip
A frame slip occurs when the transmit clock and receive clock on the DS-1
trunk are not synchronized. When this occurs, a frame must be inserted or
deleted. A counter is used to track the total number of frame slips. To
prevent frame slips, the clock is synchronized to the external clock.
Frame loss
The DS-1 frame format consists of a frame bit followed by 24 eight-bit
words that represent the 24 channels. If two-out-of-four to two-out-of-five
framing bits are received erroneously, receive framing is considered lost.
This means the receiving side cannot determine which data belongs to which
channel.
Frame loss is significant after 3 seconds; then the trunk is taken out of
service. The trunk is restored to service automatically when frame
synchronization is received continuously for 15 seconds.
BER monitoring
The BER is monitored using two indicators:
• BPV—In a bipolar pulse stream, pulses alternate in polarity. If two
pulses of the same polarity are received in succession, it is called a
bipolar violation (BPV). Each BPV implies that a data transmission error
has occurred. A count is maintained, in increments of 1024, for the
number of occurrences per day. There are four classes of BPV rates:
— class 1: 10
–3
error rate (10,240 BPVs in 6.6 s)
— class 2: 10
–4
error rate (10,240 BPVs in 65.96 s)
— class 3: 10
–5
error rate (10,240 BPVs in 659.6 s)
— class 4: 10
–6
error rate (10,240 BPVs in 6596 s)
• CRC—(applicable only with extended superframe format) an indication
of one or more bit errors (in a block of bits) from the received bit stream.
The cyclic redundancy check indicates performance on an end-to-end
span; BPV indicates performance on only the local span. With extended
superframe format, BPVs and CRCs are reported. If framing is not
extended superframe format, only BPVs are reported.
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