The DDCMP module is a data link control procedure that ensures a reliable
data communication path between communication devices connected by data
links. DDCMP has been designed to operate over full-duplex and half-duplex
synchronous and asynchronous channels in both point-to-point and multipoint
modes. It can be used in a variety of applications such as distributed
computer networking, host/front end processing, remote terminal
concentration, and remote job entry-exit system operation.
The hierarchical relationship of the entities that constitute the DDCMP
module:
DDCMP
Port
Link
Logical station
The DDCMP entity is the top-level entity in the hierarchy of entities
belonging to the DDCMP module.
1 – character
DNA VERSION
Version number of the DDCMP architecture specification to which the
implementation conforms. You cannot modify this characteristic.
2 – ddcmp link
The DDCMP link entity defines the attributes of a link to a communications port that uses the DDCMP protocol.
2.1 – character
DEAD TIMER
Default: 10000 Value: 1-65535
Specifies the delay, in milliseconds, between polls of one of the set
of dead tributaries. This attribute is supported only if the
characteristic protocol is set to control.
DELAY TIMER
Default: 0 Value: 0-65535
Specifies the minimum delay, in milliseconds, between polls. You can
use this timer to limit the effect of a fast control station on slower
tributaries. A value of 0 means that no delay is added. This attribute
is supported only if the characteristic protocol is set to control.
PHYSICAL LINE
Default: No Name Value: local-entity-name
Name of the Physical layer entity that the link operates over. A value
for this characteristic must be set before the link entity is enabled.
You can modify this characteristic only when the entity is disabled.
PROTOCOL
Default: Point
Protocol mode used by the local station. You cannot modify this
characteristic. The value of this characteristic is a copy of the
protocol argument specified when the DDCMP link entity is created.
The default value is point and is the only permissible value.
RECEIVE BUFFERS
Default: 4 Value: 1-255
Number of receive buffers reserved for the link. You can modify this
characteristic only when the entity is disabled. Also, you can only
increase the characteristic value.
RETRANSMIT TIMER
Default: 3000 Value: 1-65535
Maximum time, in milliseconds, to wait between sending a message and
receiving a valid response. If this time expires, the local station
takes error recovery action. On full-duplex point-to-point links, the
timer is started immediately when a message is sent.
SCHEDULING TIMER
Default: 200 Value: 50-65535
Time, in milliseconds, between the recalculation of tributary polling
priorities. This attribute is supported only if the characteristic
protocol is set to control.
STREAM TIMER
Default: 6000 Value: 0-65535
Time, in milliseconds, for which a tributary or the remote station on
a half-duplex, point-to-point link can hold the line. This
characteristic is not supported if the characteristic protocol is
set to tributary.
2.2 – counters
NAKS RECEIVED INDICATING MESSAGE HEADER FORMAT ERROR
Number of NAK messages received that report errors in message headers
sent from the local station.
NAKS RECEIVED INDICATING RECEIVE OVERRUN
Number of NAK messages received that report a receive overrun at the
remote station.
PDUS RECEIVED WITH MESSAGE HEADER FORMAT ERROR
Number of times the local station detected an error in a message
header.
RECEIVE OVERRUNS
Number of times the local station detected a receive overrun.
SELECTION ADDRESS ERRORS
Number of times that the controller of a multipoint link received a
message with an address other than that of the currently selected
station. This counter is supported only if the characteristic protocol
is set to control.
STREAMING TRIBUTARIES
Number of times that the remote station or a tributary station
exceeded the maximum transmission interval without releasing the line,
or failed to release the line after sending a message with the select
flag set. This counter is present only on half-duplex, point-to-point
links, or when the local station is a controller of a multipoint link.
This counter is not supported if the protocol characteristic is set
to tributary.
TRANSMIT UNDERRUNS
Number of times the local station detected a transmit underrun.
2.3 – status
PHYSICAL PORT
Name of the port entity in the Physical layer returned when the port
is opened. If this is null, the port is not open.
STATE
State of the DDCMP link. The value of this attribute is determined by
the enable and disable commands.
OFF - The entity is disabled.
ON - The entity is enabled.
In addition the link is disabled and its state set to off if the
Physical layer port that the link uses is deleted.
3 – logical
The DDCMP Link Logical Station entity manages a link to a remote station. The link name is the DDCMP link associated with the logical station and the station name refers to the logical station.
3.1 – Character
ACTIVE BASE
Default: 255 Value: 0-255
Base priority to which an active tributary is reset after it has been
polled. This characteristic is supported only if the characteristic
protocol of the owning ddcmp link entity is set to control.
ACTIVE INCREMENT
Default: 0 Value: 0-255
Value to be added to the active tributary priority each time the
scheduling timer expires. This characteristic is supported only if the
characteristic protocol of the owning ddcmp link entity is set to
control.
ADDRESS
Default: 1 Value: 1-255
Data link address of the remote station or tributary. You can modify
this characteristic only when the entity is disabled.
BABBLE TIMER
Default: 6000 Value: 1-65535
Time, in milliseconds, for which a selected tributary or remote station
is allowed to transmit. This characteristic is not used on full-duplex
links. This characteristic is not supported if the characteristic
protocol of the owning ddcmp link entity is set to tributary.
BUFFER SOURCE
Default: Implementation_specific Value: see below
Source of the receive buffers.
CLIENT SUPPLIED - Buffers are provided by the client entity.
COMMON POOL - Buffers are assigned from the common buffer pool.
This characteristic is supported only if the characteristic protocol
of the owning ddcmp link entity is set to control. You can modify
this characteristic only when the entity is disabled.
DEAD THRESHOLD
Default: 8 Value: 0-255
Number of times that an active, inactive, or dying tributary is polled
before its status attribute polling substate is changed to dead
because of receive timeouts. This characteristic is supported only if
the characteristic protocol of the owning ddcmp link entity is set
to control.
DYING BASE
Default: 0 Value: 0-255
Base priority to which a dying tributary is reset after being polled.
This characteristic is supported only if the characteristic protocol
of the owning ddcmp link entity is set to control.
DYING INCREMENT
Default: 16 Value: 0-255
Value to be added to a dying tributary's priority each time the
scheduling timer expires. This characteristic is supported only if
the characteristic protocol of the owning ddcmp link entity is set
to control.
DYING THRESHOLD
Default: 2 Value: 0-255
Number of times an active or inactive tributary is polled before its
status attribute polling substate is changed to dying because of
receive timeouts. This characteristic is supported only if the
characteristic protocol of the owning ddcmp link entity is set to
control.
HOLDBACK TIMER
Default: 0 Value: 0-13000
Maximum time, in milliseconds, that the local station can delay
acknowledging a received message if there is no data to send. The value
of this characteristic is linked to the retransmit timer used on the
remote station. A suggested value is between 10% and 20% of that timer.
However, the actual values you can use may be limited by the
communications product. The default value indicates that no holdback
is used and the local station must acknowledge immediately.
INACTIVE BASE
Default: 0 Value: 0-255
Specifies the priority to which an inactive tributary is reset after it
has been polled. This characteristic is supported only if the
characteristic protocol of the owning ddcmp link entity is set to
control.
INACTIVE INCREMENT
Default: 64 Value: 0-255
Value to be added to an inactive tributary's priority each time the
scheduling timer expires. This characteristic is supported only if the
characteristic protocol of the owning ddcmp link entity is set to
control.
INACTIVE THRESHOLD
Default: 8 Value: 0-255
Number of times an active tributary is polled before its status
attribute polling substate is changed to inactive because of no
data response. This characteristic is supported only if the
characteristic protocol of the owning ddcmp link entity is set to
control.
MAXIMUM BUFFERS
Default: 4 Value: Implementation specific
Maximum number of buffers that a tributary can use from the common
buffer pool. A value of 0 means that there is no limit to the number of
buffers that can be used. This characteristic is supported only if the
buffer source characteristic is set to common pool. This
characteristic is supported only if the characteristic protocol of
the owning ddcmp link entity is set to control. You can modify this
characteristic only when the entity is disabled. Also, you can only
increase the characteristic value.
MAXIMUM TRANSMIT
Default: 4 Value: 1-255
Maximum number of messages that a tributary or a remote half-duplex
station can send at one time. The value of this characteristic must be
greater than or equal to that of transmit window on the selected
station. This characteristic is not supported if either of the
following conditions is true:
o The characteristic protocol of the owning ddcmp link entity is
set to tributary.
o The communications link is full-duplex and point-to-point.
You cannot modify this characteristic.
POLLING STATE
Default: Automatic Value: see below
Specifies the effect of the local station's polling algorithm on the
state of a tributary. The value of this characteristic is reflected in
the value of the status attribute polling substate.
ACTIVE - The state is locked to active.
AUTOMATIC - The state varies according to the operation of the polling
algorithm.
DEAD - The state is locked to dead.
DYING - The state is locked to dying.
INACTIVE - The state is locked to inactive.
This characteristic is supported only if the characteristic protocol
of the owning DDCMP link entity is set to control.
TRANSMIT TIMER
Default: 0 Value: 0-65535
Time, in milliseconds, that the local station waits between data
transmissions. This characteristic is supported only if the
characteristic protocol of the owning ddcmp link entity is set
to control.
TRANSMIT WINDOW
Default: 1 Value: 1-255
Maximum number of data messages that the local station can send
without receiving an acknowledgment. This characteristic applies only
when the remote station is a control station or on a half-duplex,
point-to-point link. The value of this characteristic must be less
than or equal to the equivalent of the maximum transmit
characteristic on the control station or remote station.
3.2 – Counters
BUFFERS TEMPORARILY UNAVAILABLE
Number of times the local station could not service messages from the
remote station because there were no receive buffers available.
BUFFERS TOO SMALL
Number of times the local station could not service messages from the
remote station because the receive buffers were not large enough.
INCOMPLETE REPLIES TO SELECT
Number of selection intervals that were not properly terminated that
is, by a message with the Select bit set in the header, during which
a transmission was received or an attempt at transmission was detected.
This counter is supported only if the protocol characteristic of the
owning ddcmp link entity is set to control, or when the link is a
half-duplex, point-to-point link.
LOCAL REPLY TIMEOUTS
Number of times the local station failed to receive an acknowledgment
before the reply timer expired.
LOCALLY INITIATED STATE CHANGES
Number of times that the station protocol state changed through action
of the local station.
NAKS RECEIVED INDICATING BUFFER TOO SMALL
Number of times the remote station reported that it could not service
a message because the receive buffer was not large enough.
NAKS RECEIVED INDICATING BUFFERS TEMPORARILY UNAVAILABLE
Number of times the remote station reported that it could not service
a message because no receive buffer was available.
NAKS RECEIVED INDICATING DATA FIELD BLOCK CHECK ERROR
Number of times the remote station reported that a block check error
was detected in the data field of an incoming message.
NAKS RECEIVED INDICATING HEADER BLOCK CHECK ERROR
Number of times the remote station reported that a block check error
was detected in the header block of an incoming message.
NAKS RECEIVED INDICATING REP RESPONSE
Number of times the remote station reported that it did not receive
all the messages sent from the local station.
NAKS SENT WITH REP RESPONSE
Number of times that the local station detected that not all of the
messages sent from the remote station were received correctly.
NO REPLIES TO SELECT
Number of times that the select timer expired for any of the following
reasons:
o No valid control message was received.
o No valid header to a data message was received.
o No valid header to a maintenance message from the selected station
was received.
o No transmission from the remote station was received.
This counter is supported only if the protocol characteristic of
the owning ddcmp link entity is set to control, or when the link
is a half-duplex, point-to-point link.
PDUS RECEIVED WITH DATA FIELD CHECK BLOCK ERROR
Number of messages received with a check error in the data field.
PDUS RECEIVED WITH HEADER BLOCK CHECK ERROR
Number of messages received with a check error in the header block.
RECEIVE ERROR THRESHOLDS REACHED
Number of times that the receive error threshold has been reached.
REMOTE REPLY TIMEOUTS
Number of times the local station received a REP message and sent an
acknowledgment in return. This sequence indicates that all messages
sent from the remote station have been correctly received.
REMOTELY INITIATED STATE CHANGES
Number of changes in the station protocol state caused by action of the
remote station.
SDU OCTETS RECEIVED
Number of data octets received from the remote station.
SDU OCTETS SENT
Number of data octets sent to the remote station.
SDUS RECEIVED
Number of data messages received from the remote station not including
retransmissions.
SDUS SENT
Number of data messages sent to the remote station not including
retransmissions.
SELECTION ERROR THRESHOLDS REACHED
Number of times that the selection error threshold has been reached.
This counter is not supported if the characteristic protocol of the
owning ddcmp link entity is set to tributary.
SELECTION INTERVALS
Number of times that the local station selected the remote or a
tributary station. The counter does not appear if the link uses the
tributary protocol. In addition, the counter appears only when the
local station is the control station for a number of tributaries, or is
operating over a half-duplex, point-to-point link. This counter is
supported only if the protocol characteristic of the owning ddcmp
link entity is set to control, or when the link is a half-duplex,
point-to-point link.
SEND ERROR THRESHOLDS REACHED
Number of times that the send error threshold has been reached.
STRTS RECEIVED WHILE IN MAINTENANCE
Number of times that the local station received a STRT protocol
message while in the Maintenance state.
3.3 – status
POLLING SUBSTATE
State of a tributary as determined by the polling algorithm. This
attribute applies only when the value of the link's protocol
characteristic is set to control. The value of this attribute is
affected by the value of the characteristic polling state. If the
characteristic polling state is set to automatic, the value of
this status attribute reflects the current state of the polling
algorithm. For all other values of the polling state characteristic,
the values of both attributes are the same.
ACTIVE - The tributary is active and responds with data when
selected.
DEAD - The tributary did not respond when selected within the
appropriate timeout period, when already in the Dying or
Inactive state. The tributary will be ignored until the
station reinitializes.
DYING - The tributary, currently in the Inactive or Active state,
has not responded within the appropriate timeout period
when selected.
INACTIVE - The tributary has not sent any data when selected by the
control station. However, the tributary has responded with
an appropriate message when selected.
PROTOCOL STATE
State of the data link protocol with the remote station.
HALTED - The protocol is stopped and no messages are being
exchanged with the remote station.
RUNNING - The protocol is on line and is exchanging messages
with the remote station.
STARTING - There is an attempt to initialize the protocol between
the local and remote stations. This uses the STRT and
STACK PDUs.
MAINTENANCE - The protocol is off line and dealing with maintenance
messages only.
STATE
Operational state of the local logical station.
OFF - The station is disabled.
ON - The station is enabled.
4 – port
A DDCMP port entity represents an access point to the Data Link layer service offered by ddcmp. Ports are created and deleted automatically when a client of DDCMP uses the link.
4.1 – status
CLIENT
Name of the client entity.
LINK
Name of the DDCMP link that the client is using.
LOGICAL STATION
Name of the DDCMP link logical station supplied by the client when the
port was opened.
STATE
State of the port.
OPEN - The port is assigned to a client.
OPEN DISABLED - The port is assigned to a client, but the link or
logical station entity used by the port is disabled.
CALL ATTACHED - The port is assigned to a client and the link is
associated with the current call on the line. Applies
only when the link operates over a switched line.
TYPE
Type of port.
NORMAL - For normal data communications.
SERVICE - For service operations. This is the value that modules