Telecommunications - Configuration of X.25 Flow control parameter negotiation

Flow control parameter negotiation

Description of Facility

The following facilities allow non-default flow control parameters to be selected. See throughput issues

Packet Size

Facility code packet size

from called DTE

packet size

to called DTE

43 hex

+---------------+ | 43 | 01000010 | packet size .............................#values are log to base 2 of

Window Size

Facility code window size window size
44 hex from called DTE to called DTE

| 43 | 01000010 | packet size .............................#


| X.2 LEVEL 3 PER CALL FACILITY | X.2 (1984) | UK(a) | UK(b) |interworking|

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


| Flow control parameter negotiation..| 2.5 E |



| Packet sizes 2048 and 4096..........| | . |

| |


The ability to negotiate packet and window size is a subscription option (see flow control parameter negotiation subscription option).

UK(a) - supported. In order to use this facility both ends of the call must subscribe to parameter negotiation, otherwise the values will be negotiated down to their default values.

UK(b) - supported. Packet and Window size is set independently at both ends of the call. The network will perform the appropriate concatenation and splitting of packets as required. If a packet has been split the M-bit will be set, except for the final packet.

Since UK(b) uses local significance and 128 byte packets internally within the network, an increase in Packet or window size may NOT increase the throughput, in fact this assembling and disassembling of packets may actually slow down throughput.

&notes& = yTemporary note for reviewers

UK(a) - If very high packet and window sizes are used and data is sent at a high rate, ie with only 1 or 2 flags between frames. This may cause the network node to crash. (Kevin Joseph has the figures).

UK(b) - High packet and window sizes should be discouraged since they do not improve throughput and they require a large amount of memory for the slot, this could reduce the number of ports that can be built on an engine, or cause the engine to go into congestion or crash.

&UKa& = yUKa support

UKa does not support packet sizes of 2048 and 4096.

Release 4 will negotiate packet sizes of 2048 and 4096 down to 1024, current release will clear the call if these values are requested.

Public Network always operates with the same packet and window sizes at each end of the network.

Action of network depends on whether the port is enhanced:

If enhanced:

* All incoming calls will contain packet and window facility fields

If not enhanced:

* All incoming calls will be negotiated to 2 and 128 by network, facility fields not passed to destination DTE.

* Call requests will be cleared if it contains flow control facility fields requesting values other than 2 and 128.

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

FR1 - Flow control parameter negotiation ... M = Mandatory

FS1 - Flow control parameter negotiation ... M = Mandatory

V9s - Packet sizes supported sending...


V9r - Packet sizes supported receiving ...


V10s - window sizes supported sending...

1-7 (1-127 if modulo 128 support)

V10r - window sizes supported receiving .

1-7 (1-127 if modulo 128 support)

Therefore release 4 is not compatible with ONA for packet size 2048 and 4096.

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