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Telecommunications - X.25 frame types


Frame Types

I Information frame

Supervisory frames

RR - Receive Ready

RNR - Receive Not Ready

REJ - Reject

Unumbered format frames

SARM set asynchronous response mode

SABM set asynchronous balanced mode

UA - Unnumbered acknowledgment response

DM - Disconnected mode Response

FRMR - frame reject response




Frame Types




I Information frame



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.


Supervisory frames

RR - Receive Ready



The RR frame confirms numbered I frames upto 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.

RR as a response

----------------

------ I -------->

<----- RR --------


Command RR

----------

------ RR P ------>

<----- RR P -------


RNR - Receive Not Ready

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

<----- I -------

------ RNR ---->

busy on

this end should not send any more I frames

------ RR ----->

busy off

this end free to send I frames again


due to a bug in Public Network, if busy is put on with RNR with P-bit=0 it

cannot be put off again with P-bit=1.

slide REJ


REJ - Reject


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

<--- I N(s)=5 ---
I frame 6 is lost perhaps due to noise
<--- I N(s)=7 ---
---- REJ N(R)=6 --> confirms frames upto 5 requests retransmission of 6
<--- I N(s)=6




Unumbered format frames

SARM set asynchronous response mode

asynchronous response mode (ARM) was used in the early days of X.25 now being phased out.

Sets asynchronous response mode - brings the link up in one direction. So to bring the link up fully requires an exchange in both directions as follows:

------ SARM ------>

<----- UA ---------

<----- SARM -------

------ UA -------->


SABM set asynchronous balanced mode


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.

assume link is down

----- SABM ------>

<---- UA ---------

link is now up

In theory 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.


SABME set asynchronous balanced mode extended

as for SABM but starts modulo 128 operation - not used on Public Network.


DISC - Disconnect command


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.

assume link up

<--- DISC ---------

---- UA ---------->

link is now down

In Public Network the DISC frame is also used by the network to poll the DTE when the link is in the down state.

DTE DCE
<--- DISC -----
<--- DISC P ---
<--- DISC P ---
<--- DISC P ---
---- UA P ----> the network will continue polling until the DTE responds with a UA the network will then wait for the DTE to bring the link up with a SARM or a SABM


UA - Unnumbered acknowledgment response

The UA unnumbered response is used by the DTE 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.


DM - Disconnected mode Response


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

In Public Network if DM sent in response to polling discs, polling is stopped for T3=T1 * N2


FRMR - frame reject response

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:


 


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