X25AM Programming Manual

Introduction
X25AM Programming Manual—527201-001
1-14
Frame-Layer Subscription Parameters
In the following discussion, subscription parameters are divided into two groups: frame-
layer parameters and packet-layer parameters. These parameters are specified during
system generation and are described in detail in the X25AM Configuration and
Management Manual.
Frame-Layer Subscription Parameters
Frame-layer subscription parameters include framing types, protocol used, timers,
frame size, send retries, maximum window size, and frame size.
Frame-layer Protocol
X25AM supports Link Access Procedure-Balanced (LAPB). The value for LAPB
depends on the network.
Frame Acknowledgement Receive Timer (T1)
T1 is a timer referring to the length of time allocated for a frame acknowledgment
before initiating recovery procedures. Two values are defined; one for each end of the
link.
The subscriber selects the value for their side. A value that is too small can cause
unnecessary retransmissions. HP recommends choosing a value based on line speed
and maximum packet size. A standard T1 value is 3 seconds.
Frame Acknowledgement Send Timer (T2)
T2 is a parameter referring to the period of time available before the acknowledging
frame must be sent to ensure its delivery before T1 runs out. T2 is always less than T1.
X25AM always sets T2 to zero.
Information-Frame Size Maximum (N1)
N1 refers to the maximum number of bits in an information frame. It depends on the
maximum packet size. X25AM calculates N1 based on the maximum (default) packet
size. The maximum packet size depends on two factors: the selection of the extended
format option, and the specification of the X.25 format (as 1980 or 1984).
Send Retry Maximum (N2)
N2 refers to the number of times a frame should be resent after T1 expires. A large
value, which increases the probability of correct transfer, must be balanced against a
small value, which provides faster detection of serious error conditions. HP
recommends choosing the value based on the subscriber’s needs and configuration.
The standard range is from 8 to 20.