Telecommunications - Configuration of X.25 Link Layer



4 X.25 Physical layer

4.1 V.24 Interface

4.2 V.35 Interface

4.3 V.11 Interface

4.4 Dial Backup

5 X.25 link level procedures

5.1 - Link Setup

5.2 L2 procedures - SABM - set asynchronous balanced mode

5.3 L2 procedures - UA - Unnumbered acknowledgment response

5.4 L2 procedures - DISC - Disconnect command

5.5 L2 procedures DM - Disconnected mode

5.6 - Information Exchange

5.7 Frame Types for Information Exchange

5.8 L2 procedures - I - Information frame

5.9 L2 procedures - RR - Receive Ready

5.10 L2 procedures RNR - Receive Not Ready

5.11 L2 procedures REJ - Reject

5.12 L2 procedures FRMR - frame reject response

5.13 L2 procedures SABME set async balanced mode extended

5.14 L2 procedures - XID Exchange ID

5.15 L2 procedures - SARM set asynchronous response mode

5.16 L2 procedures CMDR - Command reject

5.17 Level 2 Counters and Timers

6 link level subscription options

6.1 Extended frame Sequence Numbering

6.2 Multilink procedure

7 X.25 packet level procedures

7.1.1 Addressing

7.1.2 Virtual Circuits

7.1 CALL REQUEST PACKET

7.1 INCOMING CALL PACKET

7.2 CALL ACCEPTED PACKET

7.2 CALL CONNECTED PACKET

7.3 CLEAR REQUEST PACKET

7.3 CLEAR INDICATION PACKET

7.4 CLEAR CONFIRMATION PACKET

7.5 DATA PACKET

7.6 - RR PACKET...........................................57

7.7 RNR PACKET............................................57

7.8 REJ PACKET..............................................58

7.9 RESET REQUEST PACKET....................................59

7.9 RESET INDICATION PACKET.................................59

7.10 RESET CONFIRMATION PACKET..............................60

7.11 INTERRUPT PACKET.......................................61

7.12 INTERRUPT CONFIRMATION PACKET..........................61

7.13 RESTART REQUEST PACKET.................................62

7.13 RESTART INDICATION PACKET..............................62

7.14 RESTART CONFIRMATION PACKET............................62

7.15 on-line facility registration......................64

7.16 on-line facility confirmation......................64

7.17 Diagnostic Packet.................................65

7.18 Diagnostic Codes.......................................66

8 - CCITT X.2 Per-call facility options...................73

8.1 Flow control parameter negotiation.................73

8.2 Throughput class negotiation facility..............74

8.3 Closed User Group Selection facility...............75

8.4 Closed User Group with Outgoing Access Selection...76

8.5 Bilateral Closed user group facility

8.6 Reverse charging facility..........................78

8.7 Fast select facility...............................79

8.8 Network User Identification (NUI) facility.........80

8.9 Charging Information facilities

8.10 RPOA selection.....................................82

8.11 Call deflection selection facility.................83

8.12 Call redirection or deflection notification (CRN)..84

8.13 Called line address Modified Notification(CLAMN)...85

8.14 Transit Delay Selection and indication.............86

8.15 Abbreviated Address Calling........................87

9 - CCITT defined DTE facilities............................88

9.1 Calling Address extension facility.................89

9.2 Called Address extension facility..................90

9.3 Minimum Throughput Class............................91

9.4 end-to-end transit delay...........................92

9.5 priority facility..................................93

9.6 Protection facility................................94

9.7 Expedited data negotiation facility................95

.pa

10 - CCITT X.2 L3 Subscription options.....................96

10.1 On-line facility registration......................97

10.2 Extended Packet Sequence Numbering.................98

10.3 D-bit modification.................................100

10.4 Packet Retransmission..............................102

10.5 Incoming Calls Barred..............................104

10.6 Outgoing Calls Barred..............................104

10.6 Outgoing Calls Barred..............................105

10.7 One-way Logical Channel Outgoing...................108

10.8 One-way Logical Channel Incoming...................108

10.8 One-way Logical Channel Incoming...................109

10.9 Nonstandard Default Packet sizes...................111

10.10 Nonstandard Default Window sizes...................111

10.11 Default Throughput Classes Assignment..............111

10.10 Nonstandard Default Window sizes...................112

10.11 Default Throughput Classes Assignment..............113

10.12 Flow control Parameter Negotiation.................117

10.13 Throughput Class Negotiation.......................119

10.14 Closed user group..................................121

10.15 Closed user group with Outgoing Access.............121

10.16 Closed user group with Incoming Access.............121

10.17 Incoming Calls Barred within a CUG ................121

10.18 Outgoing Calls Barred within a CUG.................121

10.15 Closed user group with Outgoing Access.............122

10.16 Closed user group with Incoming Access.............123

10.17 Incoming Calls Barred within a CUG ................124

10.18 Outgoing Calls Barred within a CUG.................125

10.19 Bilateral Closed User Groups.......................131

10.20 Bilateral CUG with outgoing Access.................131

10.20 Bilateral CUG with outgoing Access.................132

10.21 Fast Select Acceptance.............................135

10.22 Reverse Charging Acceptance........................137

10.23 Local charging prevention..........................139

10.24 Network User Identification (NUI)..................141

10.25 NUI override.......................................141

10.25 NUI override.......................................142

10.26 Charging information...............................145

10.27 RPOA Subscription..................................147

10.28 Hunt Group.........................................149

10.29 Call redirection...................................153

10.30 Call deflection subscription.......................157

10.31 TOA/NPI address subscription.......................158

10.32 Direct Call........................................160

11 UK quality of service parameters........................161

11.1 Call request delay.....................................161

11.2 DATA packet transfer delay.............................162

11.3 Established Call Clear.................................162

11.4 Spontaneous Resets.....................................162

11.5 - Timers...............................................163


Glossary

BSI British Standards Institute
BT British Telecom plc
CCITT International Telephone and Telegraph Consultative Committee
CLAMN Called line address modified notification
CRN Call redirection notification
CUG Closed User Group
DCE Data Circuit-terminating Equipment (Network end of link)
DNIC Data Network Identification Code
DTE Data Terminal Equipment (Customers terminal equipment)
UK Global Network Service
UK(a) Existing UK X.25 service
UK(b) Enhanced Worldwide X.25 service
ICUG International Closed user group
ISO International Organisation for Standardisation
LCN Logical Channel Number
LCGN Logical Channel Group Number
NUA Network User Address
NUI Network User Identification
ONA Open network architecture
PAD Packet Assembler/Dissassembler
PVC Permanent Virtual Circuit
QOS Quality Of Service
RPOA Recognised Private Operating Agency
SVC Switched Virtual Circuit
X.2 CCITT Optional User Facilities
X.25 CCITT packet terminal interface recommendation
X.28 CCITT start-stop terminal interface recommendation
X.75 CCITT packet internetwork interface recommendation


Introduction

New Features:

I need to check if the following are dependent on LOGII:

Release 6

New Features:


New Features:

more powerful routing and validation options by means of:

Notes:




5 - X.25 link level procedures

The link level procedures provide an error correcting interface between customers equipment and the Packet Switching Exchange.

Normally there are no subscription options for the link level, UK will use the following default values.

UK(a) UK(b)
Polling from network:

-in disconnected phase

with DISC frame with SABM frame
-in Information transfer phase none RR(P) frame
Link setup: (see NET2) DTE initiated DCE initiated or DTE initiated
LAP: not recommended not recommended
LAPB: supported supported
Timer T1: Default 0.5 to 11 sec depending on line speed and packet length 3 seconds
Counter N2: Default 20 10

5.1 - Link Setup5.1 - Link Setup

When the link is DOWN the packet switching exchange 'polls' to request that the link be Setup.
UK(A) polls with DISCs

UK(B) polls with SABMs

The link is setup by a SABM, UA exchange as shown below.

Once the link is setup, Level 3 can be initialised and calls can be sent and received. The link normally remains setup unless either end wishes to suspend operation for some reason.
UK(A) interface conforms to 'DTE initiated - SABM Start' form of link setup as shown in NET2 Standard. UK(B) interface conforms to 'DCE initiated' form of link setup as shown in NET2 Standard.

The following diagrams illustrate the difference:
UK(A)

Network Customers equipment

DISC/P=0 ----------_

_ T1

DISC/P=1 ----------_

_ T1

DISC/P=1 ----------_

_ T1 equipment brought online

DISC/P=1 ----------_

_---------- UA/P=1

_---------- SABM must be sent before T3

UA ----------_ expires to bring link up

UK(B)

Network Customers equipment

SABM/P=0 ----------_

_ T1

SABM/P=1 ----------_

_ T1

SABM/P=1 ----------_

_ T1 equipment brought online

SABM/P=1 ----------_

_---------- UA/P=1

5.2 L2 procedures - SABM - set asynchronous balanced mode5.2 L2 procedures - SABM - set asynchronous balanced mode...#


+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| | 8 7 6 5 4 3 2 1 |

| | code = 0 0 1 P 1 1 1 1 = X'2F' or X'3F' |

_ +---------------------------------------------------------+

+------------------------------------------------+

| | Command | Command |

| | DCE to DTE| DTE to DCE|

|single link operation | A = 03 | B = 01 |

|multi link operation | C = 0F | D = 07 | _--- not used

+------------------------------------------------+

The SABM can be sent by either end of the link to place the link in asynchronous balanced mode. If the link is already in ABM a SABM-UA exchange can be used to reset the sequence numbers and to clear any RNR busy condition, any previously transmitted I frames that are unacknowledged remain unacknowledged.


According to CCITT either end could bring the link up, in practice Public Network always leaves it to the DTE to bring the link up, this is because Public Network does not know if the DTE wants to use ABM or ARM.

It could be argued that Public Network method of link establishment is not in accordance with CCITT 88.

Multilink not supported



5.3 L2 procedures -UA - Unnumbered acknowledgment

Level 2 procedures - Overview of Frame type

+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| | 8 7 6 5 4 3 2 1 |

| | coded = 0 1 1 F 0 0 1 1 = X'63' or X'73' |

_ +---------------------------------------------------------+

+-----------------------------------------------+

| | Response | Response |

| | DCE to DTE| DTE to DCE |

|single link operation | B = 01 | A = 03 |

|multi link operation | C = 07 | D = 0F |_--not used

+-----------------------------------------------+

The UA unnumbered response is used to acknowledge the receipt and acceptance of the SABM/SABME and DISC unnumbered commands. Received SABM/SABME and DISC unnumbered commands are not actioned until the UA response is transmitted.



5.4 L2 procedures - DISC - Disconnect command

+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| | 8 7 6 5 4 3 2 1 |

| | code = 0 1 0 P 0 0 1 1 = X'43' or X'53' |

_ +---------------------------------------------------------+

+------------------------------------------------+

| | Command | Command |

| | DCE to DTE| DTE to DCE|

|single link operation | A = 03 | B = 01 |

|multi link operation | C = 0F | D = 07 |_---not used

+------------------------------------------------+

The Disc command can be used by any end of the link to terminate the mode previously set and to put the link into the down state.

In Public Network the DISC frame is also used by the network to poll the DTE when the link is in the down state. This is not standard.

5.5 L2 procedures - DM - Disconnected mode Response

+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| | 8 7 6 5 4 3 2 1 |

| | coded = 0 0 0 F 1 1 1 1 = X'0F' or X'1F' |

_ +---------------------------------------------------------+

+-----------------------------------------------+

| | Response | Response |

| | DCE to DTE| DTE to DCE |

|single link operation | B = 01 | A = 03 |

|multi link operation | C = 07 | D = 0F |_---- not used

+-----------------------------------------------+


The DM response is used to report a status where the sender is logically disconnected from the link, and is in the disconnected phase. The DM response may be sent in this phase to request a set mode command, or, if sent in response to the reception of a set mode command, to inform the remote end that the sender is still in the disconnected phase and cannot action the set mode command.

Command With P bit set DM

Command without P bit set ignore

SABM UA or DM

If DM sent in response to polling discs, polling is stopped for T3=T1 * N2


5.6 - Information Exchange5.6 - Information Exchange..................................#

Once the link is established, level 3 Packets can be sent by either end, they are contained in the data field of an I (information) frame.

I-frames are acknowledged by either RR,RNR or REJ frames, see X.25 recommendation for the details of this protocol.

If no I-frames are being transmitted, RR/P=1 frames can be used for polling.

The network will poll as follows:
UK(A) - no polling with RRs

UK(B) - polling is a configuration option.

5.7 - Frame Types for Information Exchange5.7 Frame Types for Information Exchange....................#

5.8 L2 procedures - I - Information frame

+---------------------------------------------------+

Flag | address | control | Information | FCS | Flag

+---------------------------------------------------+

| _ _ _

| | +--------------------------++----------------------+

| | | packet level information || frame check sequence |

| | +--------------------------++----------------------+

| +---------------------------------------------------------+

| | bit: 8 7 6 5 4 3 2 1 |

| | value: N(R) P N(S) 0 |

| | |

| | Modulo 128 - bit: 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 |

| | value: <-- N(S) ---> 0 <--- N(R) --> P |

_ +---------------------------------------------------------+

+-------------------------------------------------------------------------+

| | Command | Command | Response | Response |

| | DCE to DTE| DTE to DCE| DCE to DTE| DTE to DCE |

|single link operation | A = 03 | B = 01 | B = 01 | A = 03 |

|multi link (not supp) | C = 0F | D = 07 | C = 07 | D = 0F |

+-------------------------------------------------------------------------+

I frames are numbered. The N(s) count provides for numbering the frame being sent and the N(r) provides acknowledgement for the I frames received. When duplex information exchange is in continual process, each station reports its current N(s) and/or N(r) counts in each I or S frame exchanged.

* Multilink not provided by release 4

* Minimum number of flags between frames - depends on packet/window size.

* Modulo 128 is subscription option.

Modulo 128 subscription option, no other changes known.

5.9 L2 procedures - RR - Receive Ready

+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| |Coding - bit: 8 7 6 5 4 3 2 1 |

| | value: N(R) P 0 0 0 1 |

| | |

| |modulo 128 - bit: 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 |

| | value: 0 0 0 0 0 0 0 1 <--- N(R) --> P |

_ +---------------------------------------------------------+

+-------------------------------------------------------------------------+

| | Command | Command | Response | Response |

| | DCE to DTE| DTE to DCE| DCE to DTE| DTE to DCE |

|single link operation | A = 03 | B = 01 | B = 01 | A = 03 |

|multi link (not supp) | C = 0F | D = 07 | C = 07 | D = 0F |

+-------------------------------------------------------------------------+


The RR frame confirms numbered I frames upto and including N(R)-1 and indicates that the originating station is ready to receive.

An RR can be sent as a response to an I frame.

An RR can also be sent as a command to 'poll' the other end of the link.

UK(a) - The network does not 'poll' with RRs when the link is established but no data is being transmitted.

UK(b) - The network can optionally poll with RRs if required.


Modulo 128 is new subscription option.

Multilink not supported

see ILRRC(t) poll with P-bit=1

see ILRR(t) poll with P-bit=0


5.10 L2 procedures - RNR - Receive Not Ready

+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| |Coding - bit: 8 7 6 5 4 3 2 1 |

| | value: N(R) P 0 1 0 1 |

| | |

| |modulo 128 - bit: 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 |

| | value: 0 0 0 0 0 1 0 1 <--- N(R) --> P |

_ +---------------------------------------------------------+

+-------------------------------------------------------------------------+

| | Command | Command | Response | Response |

| | DCE to DTE| DTE to DCE| DCE to DTE| DTE to DCE |

|single link operation | A = 03 | B = 01 | B = 01 | A = 03 |

|multi link (not supp) | C = 0F | D = 07 | C = 07 | D = 0F |

+-------------------------------------------------------------------------+

The RNR frame confirms numbered I frames upto and including N(R)-1 and it also indicates a temporarily busy condition due to buffering or other internal constraints.

&notes& = yTemporary notes for reviewers

We need to find out more about the action of the network under link level congestion conditions.

due to a bug in Public Network, if a busy condition is indicated with RNR (P-bit=0) it cannot be cancelled with P-bit=1.

*** does this still apply *********

This is involved with the Congestion mechanism, we don't fully understand the congestion mechanisms.

Modulo 128 is new subscription option.

Multilink not supported

none known


5.11 L2 procedures REJ - Reject

Level 2 procedures - Overview of Frame type

+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| |Coding - bit: 8 7 6 5 4 3 2 1 |

| | value: N(R) P 1 0 0 1 |

| |modulo 128 bit: 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 |

| | value: 0 0 0 0 1 0 0 1 <--- N(R) --> P |

_ +---------------------------------------------------------+

+-------------------------------------------------------------------------+

| | Command | Command | Response | Response |

| | DCE to DTE| DTE to DCE| DCE to DTE| DTE to DCE |

|single link operation | A = 03 | B = 01 | B = 01 | A = 03 |

|multi link operation | C = 0F | D = 07 | C = 07 | D = 0F |

+-------------------------------------------------------------------------+

Used to recover from errors.

This command/response may be transmitted to request transmission or retransmission of numbered information frames. REJ confirms frames upto and including N(R)-1 and requests the retransmission of numbered information frames starting at the N(R) contained in the REJ frame.

Modulo 128 is new subscription option.

Multilink not supported



5.12 L2 procedures - FRMR - frame reject response

+------------------------------------------------------------------+

Flag | address | control | rej frame | 0 V(s) C/R V(r) | WXYZ0000 | FCS | Flag

| | | control fd| | | |

+------------------------------------------------------------------+

| _

| +---------------------------------------------------------+

| | 8 7 6 5 4 3 2 1 |

| | coded = 1 0 0 F 0 1 1 1 = X'87' or X'97' |

_ +---------------------------------------------------------+

+-------------------------------------------------------------------------+

| | Command | Command | Response | Response |

| | DCE to DTE| DTE to DCE| DCE to DTE| DTE to DCE |

|single link operation | A = 03 | B = 01 | B = 01 | A = 03 |

|multi link operation | C = 0F | D = 07 | C = 07 | D = 0F |

+-------------------------------------------------------------------------+

Level 2 procedures - Overview of Frame type

This response is transmitted when it receives an invalid frame other than bad FCS which are ignored or N(S) errors - REJ

A received frame may be invalid for several reasons:

its Control field is not implemented at the receiving station. This category includes unassigned commands.

the information field exceeds the locally agreed maximum.

The Control field in the received frame does not allow an I field to be received with the frame.

The N(r) that was received from the primary station is invalid.


5.13 L2 procedures - SABME - |

set asynchronous balanced mode extended

+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| | 8 7 6 5 4 3 2 1 |

| | code = 0 1 1 P 1 1 1 1 = X'6F' or X'7F' |

_ +---------------------------------------------------------+

+------------------------------------------------+

| | Command | Command |

| | DCE to DTE| DTE to DCE|

|single link operation | A = 03 | B = 01 |

|multi link operation | C = 0F | D = 07 |

+------------------------------------------------+

As for SABM but starts modulo 128 operation. Modulo 128 operation is not required on UK since it is only beneficial on links with a high loop delay (eg satellite links).

&notes& = yTemporary notes for reviewers

Although this can be supported by the network, it has not been tested by us and I can see no point in supporting it as this would be a maintenance overhead.

UK(a) supported

UK(b) non standard implementation.

5.14 L2 procedures - XID

Exchange ID

x.32 option

UK(a) not supported

UK(b) not supported. Use Packet level Registration instead.

&notes& = yTemporary notes for reviewers

This option is not supported by UK, so it might be better not to mention it. However I would like to include non-supported options for the following reasons,

* If we don't mention it, our competitors will.

* This gives us a chance to explain why we don't support it, and

why users probably don't need it.

* It makes this document more complete and authoritative.

The Following frame types are not used on UK

5.15 L2 procedures - SARM set asynchronous response mode

+-------------------------+

Flag | address | control | FCS | Flag

+-------------------------+

| _

| +---------------------------------------------------------+

| | 8 7 6 5 4 3 2 1 |

| | code = 0 0 0 P 1 1 1 1 X'0F' or X'1F' |

_ +---------------------------------------------------------+

+------------------------------------------------+

| | Command | Command |

| | DCE to DTE| DTE to DCE|

|single link operation | A = 03 | B = 01 |

+------------------------------------------------+

LAP (Asynchronous response mode - ARM) was used in the early days of X.25. It is now being phased out of the standards and is not supported by UK.

&notes& = yTemporary notes for reviewers

Although its supported by the network, I think its best to phase it out as soon as possible.

UK(a) - Dynamically selected by user at link setup time.

UK(b) - Table build option.

Question: Is LAP (as opposed to LAPB) still supported at release 4.

Answer : Yes.


5.16 L2 procedures - CMDR - Command reject

Level 2 procedures - Overview of Frame type

+------------------------------------------------------------------+

Flag | address | control | rej frame | 0 V(s) 0 V(r) | WXYZ0000 | FCS | Flag

| | | control fd| | | |

+------------------------------------------------------------------+

| _

| +---------------------------------------------------------+

| | 8 7 6 5 4 3 2 1 |

| | coded = 0 1 1 F 0 0 1 1 = X'63' or X'73' |

_ +---------------------------------------------------------+

+-------------------------------------------------------------------------+

| | Command | Command | Response | Response |

| | DCE to DTE| DTE to DCE| DCE to DTE| DTE to DCE |

|single link operation | A = 03 | B = 01 | B = 01 | A = 03 |

|multi link operation | C = 0F | D = 07 | C = 07 | D = 0F |

+-------------------------------------------------------------------------+

Applies to LAP only which is not supported by UK.



5.17 - Level 2 Counters and Timers5.17 Level 2 Counters and Timers

Timer T1T1

The value of the DTE timer 1 system parameter may be different than the value of the DCE timer T1, the value of the DCE T1 timer is set at subscription time, UK values are normally as follows:

packet length 128 256 512 1024
2400 bps 2.4 3.0 6.0 11.0 values in
9600/19200 bps 1.0 1.0 2.0 3.0 seconds
48K / 64Kbps 0.5 0.5 0.5 1.0

The period of timer T1, at the end of which retransmission of a frame may be initiated, T1 is started at the beginning of the transmission of a frame.

The proper operation of the procedure requires that the transmitter's timer T1 be greater than the maximum time between transmission of frames and the reception of the corresponding frame returned as an answer to this frame. Therefore the receiver should not delay the response or acknowledging frame returned after the I frame by more than a value T2.

As far as I can work out the rules for Public Network are as follows:

T1 started - by sending an I frame

T1 restarted - if T1 is already running when a frame is sent then it is restarted from the beginning.

T1 cleared - when last frame sent has been acknowledged.

in the case of Public Network if the timer expires, Public Network will send a command RR and restart T1, if this happens N2 times without a response then polling with SABMs starts, after another N2 times Public Network then polls with DISCs.

T1(1-546) units: 0.1 sec default:30

see ILRRC(t) poll with P-bit=1

see ILRR(t) poll with P-bit=0

Timer T2T2

T2 is the amount of time available at the DTE or DCE before the acknowledging frame must be initiated in order to ensure its receipt by the other end, prior to timer T1 running out. ie (T2 < T1)

Timer T3T3

The period of Timer T3 will provide an adequate interval of time to justify considering the data link to be in a disconnected (out of service) state. (T3 >>> T4)

default: t1 * N2

Timer T4T4

Timer T4 is a DTE system parameter that may be used to identify an excessive period of no-activity on the data link, suggesting a possible faulty data or physical link condition.

The period of timer T4, which is a system parameter, represents the maximum time a DTE will allow without frames being exchanged on the data link. The value of T4 should be greater than T1 and may be very large for applications which are not concerned with early detection of faulty data link or physical link conditions. T4>>T1.

N1 - Maximum number of bits in an I frameN1

The value of the DTE N1 system parameter will indicate the maximum number of bits in an I frame (excluding flags and 0 bits inserted for transparency) that the DTE or DCE is willing to accept from the remote end. In order to support universal DCE operation, a DTE will support a value of DTE N1 which is not less than 1080 bits (135 octets).

N2 - Maximum number of transmissionsN2

The value of N2 will indicate the maximum number of attempts that will be made by the sender to complete the successful transmission of a frame.

UK default : 20

&notes& = yTemporary notes for reviewers

The UK2 default is 10, do we want to keep compatibility within the UK or with UK rest of world.

N2(1-255) default:10

k - Maximum number of outstanding I framesk

The frame window size - always 7 for UK

The value of the DTE k system parameter will be the same as the value of the DCE k system parameter.

6- link level subscription options
Extended Frame Sequence Numbering

6.1 - Subscription Option - Extended Frame Sequence Numbering

This is included in X.2 as an additional service. It is not required on UK since it is only beneficial on links with a high loop delay. (eg satellite links).

This option allows frames to contain sequence numbers operating modulo 128 (as opposed to modulo 8).

A link with extended frame sequence numbering allows the possibility of a greater throughput, because more frames can be transmitted without waiting for an acknowledgement. Since frame acknowledgement is local to each link, the delay caused by using modulo 8 may not be significant even on high speed lines. Therefore using this feature may not significantly improve link performance, except on links with relatively large inherent delays ex. satellite links.

This facility is indicated by bringing the link up with a SABME as opposed to a SABM frame . It requires an extra byte in the frame header of various frames to hold the sequence numbers.

+--------------------------------------------------------------------------------------------+

| X.2 LEVEL 2 PER USER SUBSCRIPTION | X.2 (1984) | UK(a) | UK(b) |interworking|

| OPTIONS | REF CLASS | now | now |R5a|R5b| R6 | |

+-------------------------------------+-------------+--------+-----+---+---+----+------------|

| Extended frame sequence numbering...| 1.30 A | T | | | | | | |

| |

P P P

+--------------------------------------------------------------------------------------------+
3

| Extended frame sequence numbering...| 1.30 A | T | | | | | | |

| | P P P

&notes& = yTemporary notes for reviewers

Although this can be supported by the network, it has not been tested by us and I can see no point in supporting it as this would be a maintenance overhead.

UK(a) supported

UK(b) non standard implementation.

CCITT compatibility

X.2 1984 ... A - additional service

X.2 1988 ... A - additional service

ISO 7776 compatibility

ONA compatibility

RD 0004 part 8:1.C (PICS proforma Public Network1984)

M8 - Modulo 8 ........... M = Mandatory

M128 - Modulo 128 ......... P = Prohibited

Therefore release 4 is compatible with ONA for this option.


If configured, only the extended frame sequence number set mode command (SABME) may be sent by the DTE. In other words Modulo 8/ Modulo 128 cannot be assigned dynamically.

table build questions:

TNDTED Q153(was q37 ) extended frame sequence numbering (YES/NO)

|TNDTED Q153| q37 | extended frame sequence numbering...# standard Public Network settings = NO

TNDTED Q154(was q38 ) extended link level window size (1-127)

|TNDTED Q154| q38 | extended link level window size.....# applies if Q153=yes

TNDTED Q155(was q39 ) frame window size value (1-7)

|TNDTED Q155| q39 | frame window size value (1-7).......# applies if Q153=no

standard Public Network settings = 7



Answers

Question: If a user subscribes to extended frame sequence numbering, can they still use a SABM and non-extended frames.

Answer : No. If the extended option is subscribed to the switch will try to format the frames on that line accordingly.

Question: What are the implications on buffers and TP resources of using extended frame sequence numbering.

Answer: TP uses more buffers.

Only the extended frame sequence number set mode command (SABME), may be sent by the DTE. A non-standard X.75 style extended frame sequence numbering scheme is employed.

Test plan and implementation issues

This feature uses a whole new set of frame formats, it will therefore be difficult to test and will require a new set of test scripts to be written.

*** check - if users who subscribe to SABME can still use SABM

6.2 - Subscription Option - Multilink procedures

Defined by X.2 as an additional service, not supported by UK.

Multilink allows data interchange over one or more physical links. It provides a very effective form of load sharing but at the cost of being very complicated to provide and maintain. If you require load sharing over several lines see packet level hunt group.

+--------------------------------------------------------------------------------------------+

| X.2 LEVEL 2 PER USER SUBSCRIPTION | X.2 (1984) | UK(a) | UK(b) |interworking|

| OPTIONS | REF CLASS | now | now |R5a|R5b| R6 | |

+-------------------------------------+-------------+--------+-----+---+---+----+------------|

| Multilink procedures................| 1.21 A | . | . | . | . | . | |

+--------------------------------------------------------------------------------------------+
3

| Multilink procedures................| 1.21 A | . | . | . | . | . | |

&notes& = yTemporary notes for reviewers

UK(a) - not supported

UK(b) - a non standard version is supported, this is an early X.75 version (French multilink). Both lines must be running on the same network software slot (on the same node).I suggest we dont offer it since it is non standard and it would be a big overhead to support.

&internal& = yCCITT compatibility

X.2 1984 ... A - additional service

X.2 1988 ... A - additional service

ISO 7776 compatibility

ONA compatibility

RD 0004 part 8:1.C (PICS proforma Public Network1984)

Ls - Single Link ........ M = Mandatory

Lm - Multi Link ......... P = Prohibited


DX1: no

DX2: no



Test plan and implementation issues

none since it is not supported.

If it were, its complexity would make it very difficult to test,administer and maintain.


5 - UK1 specific options - Level 2

The following L2 facilities are not defined by CCITT X2 but are possible per-user options allowed by UK -.


Currently Public Network polls with DISC frames. The DTE needs to UA the DISC and then bring the Link up with a SABM or SARM (SABME or SARME for mod 128).

X.25 (1988) says:

"Either the DTE or the DCE may initiate datalink set-up. Prior to initiation of datalink set-up, either the DCE or the DTE may initiate datalink disconnection for the purpose of insuring that the DCE and the DTE are in the same phase. The DCE may also transmit an unsolicited DM response to request the DTE to initiate datalink set-up."

It is arguable that Public Network is contrary to the letter of the standard, it is certainly contrary to the spirit.

TNDTED Q137(was q20 ) hdlc/bsc framing (YES/NO) default NO
TNDTED Q137| q20 | hdlc/bsc framing.....#standard

Public Network settings = NO

NO=Bi-sync framing
YES=HDLC framing
bi-sync framing is not used on Public Network

TNDTED Q138(was q21 ) ASCIIor EBCDIC for bsc framing
TNDTED Q138| q21 | ASCII or EBCDIC

for bsc framing.....#(ASCII/EBCDIC)

standard Public Network settings = <not answered>
used for bi-sync framing only - not used on Public Network

TNDTED Q139(was q22 ) high speed hdlc line (Yes/No)
TNDTED Q139| q22 | high speed hdlc

line................#standard Public Network settings = yes for 48K,64K otherwise no.

high speed is defined as a bit transfer rate greater than 9.6

kbps. When configuring X.25 lines at

a bit transfer rate greater than 9.6

kbps, allow for an extra adjacent

port when configuring topological

PORT statements through CONFIG program.

TNDTED Q164(was q47 ) internal clocking speed
TNDTED Q164| q47 | internal clocking

speed.............#(2.4,4.8,9.6,EXT,NO)

standard Public Network settings = EXT
This question either sets or disables

the UK1 line clocking speeds. Line clocking is available only for lower line speeds. This feature is disabled when external clocking from the modem or other source is used. Enter EXT or NO if external clocking is used.

TNDTED Q175(was q60 ) DCE link level addressing (Yes/NO)
TNDTED Q175| q60 | DCE link level addressing..........#default YES
standard Public Network settings = YES

NO = normal link level addressing

YES = DTE uses DCE addressing
this might be used when the other end of the link is connected to another network (for example VASSCOM). this must be used with great care since X.25 protocol is not symmetrical (eg link establishment)

TNDTED Q195(was q83 ) dte link active/passive (ACTIVE/PASSIVE)
TNDTED Q195| q83 | dte link active/passive.............#standard Public Network settings = PASSIVE
The response to this question determines whether the DTE is to actively bring up the link when it is down.

PASSIVE = allows the DTE to initiate the link level communication using LAP or LAPB

ACTIVE = network will initiate the link level

ONA compatibility

RD 0004 part 8:1.C (PICS proforma Public Network1984)

LE - When in the Disconnected mode the DTE initiates

M = Mandatory

Link setup by transmitting UA_RSP or DM_RSP in

response to the incoming DISC_CMD, followed by

a SABM

Therefore release 4 is compatible with ONA for this option.

Test plan and implementation issues

no changes expected here.

Check if LAP is still supported on DX2s

See:

subaddress routing: ADRLIN

ADRPOS


 


metadata block
see also:

 

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