The HDLC module implements one of the protocols in the Data Link layer.
The HDLC High-level Data Link Control protocol is intended to cover a
wide range of applications; for example, one-way, two-way alternate or
two way simultaneous data communication between data stations which are
usually buffered, including operations on different types of data
circuits; for example multipoint/point-to-point, duplex/half-duplex, and
switched/non-switched. This implementation uses HDLC to offer reliable
communication at the Data Link layer for point-to-point synchronous data
lines over a wide area network link. The HDLC module typically runs as a
Data Link module under the CLNS Network protocol.
The hierarchical relationship of the entities that constitute the HDLC
module:
HDLC
Port
Link
Logical Station
You must install the WAN device drivers before you can refer to these
entities.
The HDLC entity is the top-level entity in the hierarchy of entities
belonging to the HDLC module.
1 – character
VERSION
Version of the HDLC architecture specification to which the
implementation conforms. You cannot modify this characteristic.
2 – Link
An HDLC link entity is associated with a port of the supporting physical layer module. It contains attributes common to local HDLC operations for all logical stations on the line.
2.1 – character
ACKNOWLEDGE TIMER
Default: 3000 Value: 1-60000
Time, in milliseconds, to wait for an acknowledgment before using
error recovery procedures. The value of this characteristic
corresponds to the T1 parameter of HDLC. You can modify this
characteristic only when the entity is disabled.
HOLDBACK TIMER
Default: 0 Value: 1-60000
Maximum time to wait before sending an acknowledgment supervisory
frame if no other frames carry the acknowledgment. A value of zero
means that the frame will be sent immediately if no I-frame can be
sent. You can modify this characteristic only when the entity is
disabled.
LINK TYPE
Operational mode of the HDLC link. The value of this characteristic
is a copy of the link type argument specified when the
entity is created. You cannot modify this characteristic.
MAXIMUM DATA SIZE
Default: 1500 Value: 262-65532
Maximum number of bytes that users of the data link can specify.
This number applies to both transmit and receive frames. The value
of this characteristic must be greater than or equal to the value
of the minimum data size characteristic. You can modify
this characteristic only when the entity is disabled.
MAXIMUM UNSEQUENCED PDUS
Default: 1 Value: 1-127
Maximum number of unsequenced I-frames that a primary or secondary
station can send in a single transmission. This characteristic is
not used if the link type characteristic is balanced.
MINIMUM DATA SIZE
Default: 576 Value: 262-65532
Minimum number of bytes that users of the data link can specify.
This number applies to both transmit and receive frames. The value
of this characteristic must be less than or equal to the value of
the maximum data size characteristic. You can modify this
characteristic only when the entity is disabled.
PHYSICAL LINE
Default: None Value: Local-entity-name
Local entity name of the the Physical layer entity over which the
HDLC protocol is to operate. This characteristic must have a value
before the HDLC link is enabled. You can modify this characteristic
only when the entity is disabled.
PREFERRED CRC TYPE
Default: Either Value: 16-bit, 32-bit or Either
CRC types available for negotiation. If the value of this
characteristic is either, the station will try to use 32-bit but
will use 16-bit if that is all that the remote station supports.
PREFERRED LOCAL STATION ADDRESS
Default: 2 Value: 1-253
Address proposed for the local logical station during negotiation.
If there is no negotiation, the value of this characteristic is
the value that is used. If negotiation is necessary, the value of
this characteristic must be between 2 and 253. You can modify this
characteristic only when the entity is disabled.
PREFERRED MAXIMUM DATA SIZE
Default: 1500 Value: 262-65532
Default size, in octets, of frames that the station receives and
transmits. This value is used only if the link initialization does
not specify a buffer value. The value of this characteristic must
lie between those of the maximum data size and minimum data size
characteristics. You can modify this characteristic to a lower
value only when the entity is disabled.
PREFERRED WINDOW SIZE
Default: 2 Value: 1-127
Window size to be offered during negotiation for both receive and
transmit frames. The value of this characteristic must be compatible
with that for the sequence modulus characteristic.
PROFILE
Simple name that can be used when the HDLC protocol is dependent on
network subscription time commitments pertinent to the Data Link
layer. The value of this characteristic is a copy of the profile
argument specified when the entity is created. You cannot modify
this characteristic.
RECEIVE BUFFERS
Default: 4 Value: 1-128
Number of receive buffers reserved for the link. This characteristic
can only be set to a lower value when the entity is disabled.
RETRY MAXIMUM
Default: 10 Value: 1-255
Maximum number of times that a frame will be retransmitted before
the local station assumes that a fatal error has occurred. This
characteristic can be modified only when the entity is disabled.
SEQUENCE MODULUS
Default: 128 Value: 8 or 128
Whether modulo-8 or modulo-128 sequence numbering is allowed on the
HDLC link. The value 8 means that only normal sequence numbering is
allowed. The value 128 means that both extended and normal sequence
numbering are supported for negotiation.
2.2 – counters
BUFFER UNAVAILABLE ERRORS
Number of times that the Physical layer reported that no buffer was
available to hold a message. This counter is not supported in all
implementations.
CRC ERRORS RECEIVED
Number of frames received that had a bad CRC.
TIMES PDU RECEIVE OVERRUN
Number of times a physical line indicated an overrun condition to
the Data Link layer. This counter is not supported in all
implementations.
TIMES PDU TRANSMIT FAILED
Number of times that an attempt to send a frame failed.
UP TRANSITIONS
Number of times that the entity's status attribute state has changed
from off to on.
2.3 – status
ACTUAL SEQUENCE MODULUS
Sequence number modulus in use on the link. Until negotiation is
complete, this attribute has the value 128. However, if the value of
the characteristic sequence modulus is 8, the modulus always appears
as the value 8.
INTERFRAME DELAY
Time required between frames to enable the local station to
successfully receive them.
LINE TYPE
Defines whether the underlying physical line is switched or
nonswitched. This status affects the subset of port states that are
related to a given link.
MAXIMUM PDU SIZE
Maximum frame size in octets that can be used on the link. Until
negotiation is complete, the value of this status attribute is the
same as the characteristic preferred maximum data size.
NEGOTIATED CRC TYPE
CRC mode that has been negotiated with the remote station. This mode
will not be used until the station is next enabled. Until
negotiation
is complete, the value of this is 16-bit.
PHYSICAL PORT
Name of the port in the Physical layer associated with the
communications link.
RESPONSE ADDRESS
Address that the remote station puts in response frames.
STATE
State of the HDLC link. This reflects the last enable or disable
command issued.
OFF - The link is disabled.
ON - The link is enabled.
WINDOW SIZE
Maximum number of I-frames that can be outstanding before an
acknowledgment must be received. Until negotiation is complete, the
value of this status attribute is the same as the characteristic
preferred window size.
3 – logical
The HDLC Link Logical Station entity controls the characteristics of an HDLC Logical Station. There is one station for each remote termination of a line associated with the HDLC link. The link name is the link entity within the HDLC module and the logical station name refers to the logical station.
3.1 – Counters
DATA OCTETS RECEIVED
Total number of octets received in the I-field of I-frames and
UI-frames. This total excludes protocol ID information in UI-frames
and frame retransmissions.
DATA OCTETS SENT
Total number of octets sent in the I-field of I-frames and
UI-frames. This total excludes protocol ID information in
UI-frames and frame retransmissions.
DATA PDUS RECEIVED
Total number of I-frames and UID-frames received. This number does
not include frames that had to be retransmitted.
DATA PDUS SENT
Total number of I-frames and UID-frames sent. This number does not
include frames that had to be retransmitted.
FRMRS GENERATED
Number of FRMRs frame rejects sent.
FRMRS RECEIVED
Number of FRMRs frame rejects received.
INVALID MODE COMMANDS
Number of command frames SABME, SABM, SNRME, SNRM,SARM, and SIM
received that are not applicable to this station.
NEGOTIATION FAILURES
Number of times that the XID negotiation with the remote station has
failed.
POLLS RECEIVED
Number of frames received with the poll bit set.
REJS RECEIVED
Number of REJ frames rejects received.
REJS SENT
Number of REJ frames rejects sent.
RNRS RECEIVED
Number of RNR receive not ready supervisory frames received from the
remote station.
RNRS SENT
Number of RNR receive not ready supervisory frames transmitted.
TIMES ACKNOWLEDGE TIMER EXPIRED
Number of times that the acknowledge timer has expired.
TIMES STATION HALTED
Number of times the station halt event has occurred.
TIMES STATION INITIALIZING
Number of times the station initialized event has occurred.
TIMES STATION INOPERATIVE
Number of times the entity's status attribute state became
inoperative.
TIMES STATION MAINTENANCE
Number of times the entity's status attribute state became
maintenance.
TIMES STATION RESETTING
Number of times that the station reset event has occurred.
TIMES STATION RUNNING
Number of times that the station running event has occurred.
TIMES STATION SETUP FAILED
Number of times the station setup failure event has occurred.
UNKNOWN UI PDUS RECEIVED
Number of UI-frames the local station received whose protocol ID
does not match that of any open port.
XIDS RECEIVED
Number of XID identification command or response frames received.
3.2 – status
COMMAND ADDRESS
Address the local station uses when sending command frames to the
remote station.
MAINTENANCE MODE
Whether the station is in maintenance mode. When set to true, the
station will be used exclusively for maintenance operations. When
set to false, the station operates in normal fashion.
The limit command sets the value of this attribute to true, and
the unlimit to false.
PROTOCOL STATE
State of the data link protocol.
ERROR - A protocol error occurred. For instance, the logical
station received an invalid frame.
HALTED - The protocol could not start. For example, there is
no client.
INITIALIZING - The protocol is being initialized.
INOPERATIVE - The protocol cannot be started because no contact has
been established with the remote station.
MAINTENANCE - The logical station is in maintenance mode.
RESETTING - The protocol is resetting after an error.
RUNNING - The protocol is running and capable of exchanging
frames with the remote station.
REMOTE VERSION
Version number of the HDLC protocol that the remote station is using.
This is received as part of an XID message from the remote station.
STATE
State of the logical station.
OFF - The logical station is off because of a disable command.
ON - The logical station is on because of an enable command.
4 – PORT
The HDLC Port entity represents one end of an HDLC connection. The entity maintains information about that link. Ports are created and deleted automatically when a client of HDLC uses the link.
4.1 – status
CLIENT
Name of the client using the port.
LOGICAL STATION
hdlc link logical station entity that the port is operating over.
PROTOCOL ID
Protocol ID that the port is using. For sequenced ports, this value
is decided during negotiation. For unsequenced ports, the value is
sent in every UI-frame.
STATE
State of the port.
OPEN - The port is assigned to a client. If the
communications line is unswitched, data transfer
can begin. For switched lines, an association must
be made with the line before data transfer can begin.
OPEN DISABLED - The port is associated with a client, but the link
or logical station associated with it is disabled.
TYPE
Type of port.
SEQUENCED - The port can send and receive sequenced and
unsequenced data.
UNSEQUENCED - The port can send and receive unsequenced data only.