Microsoft KB Archive/217016

= Connection Problems Across T1 WAN Link =

Article ID: 217016

Article Last Modified on 1/24/2007

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APPLIES TO


 * Microsoft Windows 2000 Server
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Professional Edition
 * Microsoft Windows NT Server 4.0 Standard Edition
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows 98 Standard Edition

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This article was previously published under Q217016





SYMPTOMS
When two sites are connected by a T1 wide area network (WAN) link, you may be unable to map a drive or gain access to resources across the WAN. In some cases, Windows NT Server-based or Windows NT Workstation-based computers in the workgroup may be unable to join the domain.

Other symptoms that may indicate the existence of this problem include:
 * An attempt to ping across a T1 connection works (for example, server--T1--workstation).
 * A trace route (using Tracert.exe) across a T1 connection does not work. It is unsuccessful at the first router.
 * An attempt to ping by name works if name resolution is configured.
 * You can transfer small files across a T1 connection.
 * You cannot transfer large files across a T1 connection.
 * You can make a Telnet connection to a router across a T1 connection.



CAUSE
This behavior occurs because the telephone company has improperly configured the coding for the T1 circuit at the Channel Switching Unit (CSU).



RESOLUTION
To resolve this problem, contact the telephone company to test the T1 line and correct the configuration settings.



MORE INFORMATION
It is important to match the input signal line code with the line code characteristics of the transmission path.

When you configure the CSU for a T1 circuit, two modes are available:
 * Alternate Mark Inversion (AMI)
 * Bipolar with Eight Zeros Substitution (B8ZS)

Line coding for a T1 connection is either AMI or B8ZS. B8ZS is introduced on a circuit to ensure that the 12.5-percent-ones-density requirement is always met. B8ZS is actually an AMI-encoded circuit that includes additional unique code whenever the transmitted signal contains eight zeros in a row. A B8ZS circuit appears to be optioned as an AMI circuit unless the signal meets the eight-consecutive-zeros requirement.

Not all equipment in a B8ZS circuit injects a B8ZS code. For example, line repeaters typically do not enforce the B8ZS code and simply pass whatever signal they receive. Because of the difference in consecutive zeros, a pattern may not be successful if the test point is on the "span side" using AMI encoding, but is successful if transmitted from a central office or end point where B8ZS is used. This situation indicates that the circuit probably works for customer traffic as long as the B8ZS encoding is maintained. However, the circuit is marginal and is not suitable for AMI encoding.

B8ZS is bipolar AMI with a difference. In bipolar AMI, 0 indicates no line signal and 1 indicates a positive or negative level, alternating for successive ones. Note that a long string of zeros results in a constant signal. With B8ZS, if eight zeros occur in the message and the last pulse is positive, the eight zeros are encoded as 000+-0-+. If eight zeros occur in the message and the last pulse is negative, the eight zeros are encoded as 000-+0+-.

Normally, the difference between base-band coding prevents any application layer protocol from functioning properly. Problems arise because AMI and B8ZS are so similar. AMI code may lose synchronization during a long string of zeros. This behavior does not occur, using the new rules for B8ZS.

Ping.exe works across a T1 connection because it pads the Internet Control Message Protocol (ICMP) 32-byte data field with alphabetic characters. No packet sent across a T1 connection contains a string of eight zeros.

Tracert.exe pads the ICMP 64-byte data field with all zeros. Packets sent across a T1 connection do contain a string of eight zeros, which invokes the B8ZS code. If equipment is not configured to invoke the B8ZS code, the transmission is not successful.

Keywords: kbnetwork kbprb KB217016

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