System information

Troubleshooting Serial Line Problems 15-329
Troubleshooting Clocking Problems
Figure 15-7 All-Ones 1500-Byte ping Test
Troubleshooting Clocking Problems
Clocking conflicts in serial connections can lead either to chronic loss of connection service or to
degraded performance. This section discusses the important aspects of clocking problems: clocking
problem causes, detecting clocking problems, isolating clocking problems, and clocking problem
solutions.
Clocking Overview
The CSU/DSU derives the data clock from the data that passes through it. In order to recover the
clock, the CSU/DSU hardware must receive at least one 1-bit value for every 8 bits of data that pass
through it; this is known as ones density. Maintaining ones density allows the hardware to recover
the data clock reliably.
Newer T1 implementations commonly use Extended Superframe Format (ESF) framing with binary
eight-zero substitution (B8ZS) coding. B8ZS provides a scheme by which a special code is
substituted whenever eight consecutive zeros are sent through the serial link. This code is then
interpreted at the remote end of the connection. This technique guarantees ones density independent
of the data stream.
Older T1 implementations use D4 (also known as Superframe Format [SF]) framing and Alternate
Mark Inversion (AMI) coding. AMI does not utilize a coding scheme like B8ZS. This restricts the
type of data that can be transmitted because ones density is not maintained independent of the data
stream.
Another important element in serial communications is serial clock transmit external (SCTE)
terminal timing. SCTE is the clock echoed back from the data terminal equipment (DTE) device (for
example, a router) to the data communications equipment (DCE) device (for example, the
CSU/DSU).
zounds#ping
Protocol [ip]:
Target IP address: 192.169.51.22
Repeat count [5]: 100
Datagram size [100]: 1500
Timeout in seconds [2]:
Extended commands [n]: y
Source address: 192.169.51.14
Type of service [0]:
Set DF bit in IP header? [no]:
Validate reply data? [no]:
Data pattern [0xABCD]: 0xffff
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 100, 1500-byte ICMP Echos to 192.169.51.22, timeout is 2 seconds:
Packet has data pattern 0xFFFF
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 100 percent (100/100), round-trip min/avg/max = 4/6/8 ms
zounds#
1500 byte
packet size
All ones
ping