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Parameter Description
HTC Highest Two-way Channel.
LOC Lowest Outgoing-only Channel.
HOC Highest Outgoing-only Channel.
Each range (except PVC range) is defined by two parameters operating as the upper limit and
lower limit. The parameters are in the range of 1 to 4095 (including 1 and 4095), but they are
regarded correct only if they meet the following conditions:
{ In strict ascending order, 1 ≤ lic ≤ hic< ltc ≤ htc < loc ≤ hoc ≤ 4095.
{ If the upper limit (or lower limit) of a range is 0, then the lower limit (or upper limit) must also
be 0, (which indicates this range is disabled from use).
Finally, follow these guidelines:
{ At the two sides (DTE and DCE) of a physical connection, these six parameters of X.25
must be equal in a symmetric way, because different settings at the two sides are very likely
to result in an improper procedure and result in transmission failures.
{ Take the default of each parameter into consideration.
{ The new configuration cannot take effect immediately on a connection in use unless you
reset the interface by using the commands shutdown and undo shutdown.
• X.25 packet numbering modulo
The implementation of X.25 supports both modulo 8 and modulo 128 in packet numbering, with
modulo 8 being the default.
The X.25 protocol requires DTE and DCE have the same packet sequence numbering mode.
The new configuration is not effective unless you reset the interface by using the shutdown
command and the undo shutdown command.
Besides, the packet sequence numbering mode of X.25 layer 3 is different from the frame
sequence numbering mode of LAPB (X.25 layer 2). When modulo 128 numbering mode is
employed in the DTE/DCE interface with high throughput rate, for LAPB, only the efficiency of
local DTE/DCE interface is affected, and point-to-point efficiency increases. For X.25 layer 3,
the efficiency of end-to-end is affected, and the efficiency between the two DTEs increases.
• Traffic control parameters
X.25 protocol is a reliable transport protocol with powerful traffic control capability due to the
"window size" and "maximum packet size." However, it cannot perform traffic control effectively
and correctly unless correctly configured. Any inappropriate configuration will cause CLEAR
and RESET events of X.25.
Because most public X.25 packet networks use the default window size and maximum packet
size specified in the ITU-T X.25 Recommendation, the device also adopts the same default
values. You do not need to set the two parameters unless requested by the access service
providers.
After the default window size and the default maximum packet size are set, the SVC, which can
be established only through calling, will use these default values if related parameters are not
negotiated in the call process (parameter negotiation will be described in later sections). The
PVC, which can be established directly without calling, will also use these default values if no
window size or packet size option is appended when it is specified. (See later sections for PVC
configuration).
An X.25 sender will fragment the oversized data packets at the upper layer based on the
maximum packet size, and mark the final fragment packet (M bit not set). After the packets
reach the receiver, X.25 will reassemble the fragment packets, and determine whether a piece
of complete upper layer packet is received based on the M bit flag. Too small value of the
maximum packet size will consume too much router resources on message fragmenting and
reassembling, and lower efficiency.
{ The maximum packet size < MTU*8 < N1 of LAPB
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