DATA PACKET
distinguishes packet type, in the case of data packets is coded as follows:
more data bit - used when packet is full to say next packet is continuation
of this one
Note: an even number packet type always represents a DATA packet.
rr packet
rnr packet
REJ packet
RESET REQUEST PACKET
| RESET INDICATION PACKET |
| RESET CONFIRMATION PACKET |
An SVC can be reset at any time by either DTE or by the network.
Reset by DTE
| DTE | DCE DCE | DTE |
| <-- RESET INDICATION 00dd | <-- RESET REQUEST 00dd | |
| RESET CONFIRMATION | RESET CONFIRMATION |
Reset by Network
| DTE | DCE DCE | DTE |
| RESET INDICATION ccdd | RESET INDICATION ccdd | |
| RESET CONFIRMATION | RESET CONFIRMATION |
5.7 INTERRUPT PACKET
| INTERRUPT CONFIRMATION PACKET |
15 - Data transfer - DATA Packet, RR, RNR, REJ
Data packets are sent with sequence numbers and are acknowledged with RR,RNR,REJ
or DATA packets using a similar system to I-frames at the link level.
However the purpose is different to link level, at link level the purpose
is error correction, at packet level the purpose is flow control.
In the case of multiple calls over a link the flow control on a call is independent
of the flow control on all the other calls.
Flow Control - Packet Sequence Numbers
Without flow control a high speed terminal sending data to a low speed terminal
could flood the network. We need a mechanism to tell the high speed terminal
when to back off.
The Packet Sequence numbers provide such a flow control mechanism.
Terminology:
| P(S) | Is the send sequence number, used in data packets. P(S) is incremented by one in each sucessive data packet |
| P(R) | Is the receive sequence number, used in RR, RNR, REJ and data packets to acknowlage data packets from the distant end P(R) is set to the next P(S) that end is expecting to receive. |
| Modulo 8 | Sequence numbers P(S) & P(R) are modulo 8, that is they count from 0 to 7 and then back to 0. These numbers can therefore be represented by 3 bits |
Example:
First packet is sent with P(S)=0
Following data packets have P(S) incremented by 1 each time. So the next packet is sent with P(S)=1.
Two packets have now been sent without acknowlagement so, assuming the maximum
window size is 2. This means that the window is now closed. No more data packets
can be sent until an acknowlagement is received.
Distant end sends an RR to acknowlage both data packets, P(R) is set to the
next P(S) it is expecting to receive, in this case 2.
The window has now been opened and more data packets can be sent continuing
to increment P(S)
The following model shows how the sequence numbers flow round.
Local Significance
see D-bit
Subscription Option - D-bit modification
Modulo 128
Subscription Option - Extended Packet
Sequence Numbering
Recovery from errors
Subscription Option - Packet Retransmission
19 - Timers
T10 Public Network value: 60 secs
started: DCE sends Restart
stopped: DTE sends Restart conf
action: send Restart local procedure error
T11 Public Network value: 180 secs
started: DCE sends Incoming call
stopped: call accept or clear
action: send clear local procedure error
and remote procedure error to distant end
T12 Public Network value: 90 secs
started: DCE sends Reset indication
action: SVC sends clear indication to local and remote
DTEs with clearing cause of local and remote procedure error
respectively
T13 Public Network value: 180 secs
non CCITT timers
Public Network value: 90 secs
started: DCE receives restart request
Public Network value: 180 secs
started: DCE receives call request
Public Network value: 90 secs
started: DCE receives Reset request
Public Network value: 90 secs
started: DCE receives Clear request
T20 Public Network value 180 secs
started when DTE sends a Restart request
terminated when: DTE receives restart conf or restart
action on expirey: DTE retransmits the restart request and
restarts T20 up to maximum or R20 times
T21 Public Network value 200 secs
started when DTE sends a Call request
terminated when: DTE receives call connected,incoming
call,
clear indication or clear
action on expirey: DTE transmits a clear request packet
T22 Public Network value 180 secs
started when DTE sends a Restart request
terminated when: DTE receives reset conf or reset ind.
action on expirey: DTE transmits the reset request and
restarts T22 up to a maximum of R22 times.
T23 Public Network value 180 secs
started when DTE sends a Clear request
terminated when: DTE receives clear confirmation or clear
ind action on expirey: DTE transmits the clear request and re-
starts T23 up to a maximum of R23 times.
T24 optional use
default value: 60 secs
window status transmission timer
started when DTE sends a packet with a P(R).
terminated when:
action on expiry: DTE transmits a RR or RNR packet re-
flecting the current window condition and restarts T24
T25 optional use
default value: 150 secs
window rotation timer
started when DTE sends a DATA packet.
terminated when: there are no outstanding DATA packets in
the window action on expirey: DTE retransmits all DATA packets in
the window and restarts T25 up to a maximum of R25 times
T26 default value: 180 secs
interrupt response timer
started when DTE sends a INTERRUPT packet.
terminated when: DTE receives an INTERRUPT CONFIRMATION
packet action on expirey: DTE transmits a reset request
T27 default value: 60 secs
reject response timer
started when DTE sends a REJECT packet.
terminated when: DTE receives the first retransmitted DATA
packet. action on expirey: DTE retransmits the REJECT packet and
restarts T27 upto a maximum of R27 times.
T28 CCITT 1984 ONLY
default value: 300 secs
registration request response timer
started when DTE sends a REGISTRATION REQUEST packet.
terminated when: DTE receives a REGISTRATION CONFIRMATION
packet. action on expirey: DTE retransmits the REGISTRATION
REQUEST packet and restarts T28 upto a maximum of R28 times.
