Fig4 CONVERSION TABLE BETWEEN CCITT CODES No 2 & 5
Fig5 FRAME LEVEL PROCEDURE
Fig6 PACKET LEVEL PROCEDURE
Fig7 COMMAND FORMATS FOR CALL SET-UP AND CLEAR DOWN
Fig8 SIGNALLING STATES OF A CUSTOMER'S CHANNEL

TELETYPE COMPATIBLE PROCEDURE

Commands <x>
"A [ + \| - ] <callee's add> <caller's add complement >]"	: call request
(+ means COLLECT CALL)
"L"	: clear down request
"F"	: end of transmission request
"I"	: initialization request
""	: ineffective
Messages <y>
"% APP [ + \| - ] <callee's add> [,<caller's add>]"	: call request confirmation
"% COM"	: connection confirmation
"% COM [ + \| - ] <callee's add>, <caller's add> "	: incoming call indication
"% LIB"	: clear down confirmation
"% LIB [R < cause1 > \| C <cause2 >]"	: clear down indication
[R means NETWORK, C means OTHER CUSTOMER]
"% FIN"	: end of transmission confirmation
"% INI"	: initialization
"% ERR"	: command in error
"% PER"	: data loss due to the lock of flow control
End of message markers (TTY to RCP)
"ETX"	: end of message
Transparency (TTY to RCP)
"DLE DLE"	: DLE to other customer
"DLE ETX"	: ETX to other customer
Flow control
1) From RCP "XOFF"	: stop sending
"XON"	: resume sending
2) To RCP "DLE"	: stop sending
"CR"	: resume sending

CONVERSION TABLE BETWEEN CCITT CODES No 2 & 5

(1) - The CCITT no 2 combinations H 0, H 1, ...H P are translated into the corresponding CCITT no 5 control characters SOH, STX,... US, but these control characters are not translated into CCITT no 5.

(2) - FS, i.e. figure shift, and LS,i.e. letter shift are translated into SYN and DEL respectively only if the next CCITT no 2 character is one of: blank, LS, FS, SP, CR or LF.

(3) - Combinations H H and H / modify the rule for translating all the subsequent letters from CCITT no 2 to no 5. They become lower case and upper case respectively.

Figure 5

FRAME LEVEL PROCEDURE

	synchronization procedure
	variable length block 0 ≤ L ≤ 255 may contain several packets
	service fields
	cyclic redundancy field
	variable length padding

Service Field detail

TD = type of the frame:
0 = initialization
(L,TA,TT,SA,ST,TR=0)
1 = empty ( L=0)
2 = ordinary (L≥0)

ST = sequence number

TT = frame number

SA = expected sequence number

TA = expected frame number

TR = frame number for which there
is no buffer available

B. SERVICE FIELD PROCESSING

+1 if SA ≠ SEI

+ after transmission
of a non empty block

TA if SAI ≠ SAI

if PEI ≠ PRC then all
transmitted blocks
must be empty

+ 1 if ST = SAI and
TT ≠ PAI

+1 if ST = SAI and
TT = PAI and the block
received is not empty

+1 when a new buffer
is available

Figure 6

PACKET LEVEL PROCEDURE

A. PACKET FORMATS

	V logical channel number TP Type of packet
	0 service packet; cf CV field 1 PR or "ready to receive" packet 2 unused 3 D or "data packet"
	K number of additional bytes which can be accepted
	N number of data bytes which follow
	CV subtype of a service packet 0 I or "initialization" packet 1 M or "end of message" packet 2 P or "previous M processed" packet >2 unused

B. EXAMPLE OF UTILIZATION

Sender :
Receiver :

Figure 7

COMMAND FORMATS FOR CALL SET-UP AND CLEAR DOWN

bytes which are transmitted sequentially on channel 0

bytes 10101010 (=command separator) are duplicated when transmitted

A. CALL REQUEST
	command separator command code logical channel number TE=1 if collect call; CS = class of service
	callee's identification A = address of a subscriber, an internal network service or another network CA = address complement

	caller's identification

B. CLEAR DOWN
	command separator command code logical channel number origin of clear down cause of clear down

OL =

0 : customer in the direction
the command comes from (CL is given by the customer)

1 : the network

2 : the customer who receives
the command (CL is unused)

OL = 1 then CL=

0 : wrong callee's address

1 : callee not operating

2 : callee busy on all its incoming channels

3 : callee refusing collect calls

4 : network overloaded

5 : network failure

6 : the other customer has a passive terminal
and recieved an F command

C. CONNECTION CONFIRMATION AND END OF TRANSMISSION

command separator
CC = 1 : connection confirmation, CC = 3 : end of transmission
logical channel number

SIGNALLING STATES OF A CUSTOMER'S CHANNEL

Two minor changes have been made to Figure 8: Outgoing channel: line between states AC & ACL with label C has benn given an arrowhead. Incoming channel: Labelling of line from state ACL' to state A has been slightly changed to make it obvious that either message L or message A can be received in state ACL'.