T200 and N200 are timers related to the LAPD protocol used for communication between the BTS and MS. T200 is started when a frame is transmitted and defines the time between retransmissions if an acknowledgment is not received. The frame will be retransmitted up to N200 times before a link failure is detected. If the T200 timer expires after N200 retransmissions without an acknowledgment, the connection will be dropped due to a data link failure between the BTS and MS. Setting the T200 and N200 values too low can cause increased call drops, while too high may cause unnecessary retransmissions.

1. GSM Timers
Timer Name Description Value
T100 RADIO-LINK-TIMEOUT
Detects the presence of the radio link by detecting
SACCH frames every 480 ms.
4 SACCH
multiframes. That
is 1.92 seconds if
the SACCH is
completely
absent.
T200 Data linktimer
Used for re-transmission on the data link. The value
varies depending on the message type.
155 ms for
FACCH
T301 Alerting (ringing) timer
Timer used to limit the amount of time a user has
to answer a call.
20 seconds
T303 Mobility Management
connection timer
Time the network waits after sending a CM SERVICE
REQUEST until receiving a response. This occurs before
initiating call clearing procedures towards the MS.
10 seconds
T305 Release timer
Time the network waits after transmitting a DISCONNECT
message until receiving a RELEASE message.
10 seconds
T306 In-band tones release
timer
Time the network waits after transmitting a DISCONNECT
message while in-band tones/announcements are
provided, until receiving a RELEASE message.
10 seconds
T308 Release timer
Time the network waits after sending a RELEASE
message until receiving a RELEASE COMPLETE
message. This occurs before re-transmitting the
RELEASE or releasing the Mobility Management
connection.
10 seconds
T310 Call proceeding timer
Time the network waits after receiving a CALL
CONFIRMED message until receiving a
ALERTING, CONNECT, or DISCONNECT message
before initiating clearing procedures towards the MS.
10 seconds
T313 Connectacknowledge
timer
Time the network waits after transmitting
a CONNECT message until receiving
theCONNECT ACKNOWLEDGE message before
performing clearing procedures with the MS.
10 seconds
T323 Modify complete timer Time the network waits after sending a MODIFY message
during call mode changes, until receiving a MODIFY
COMPLETE or MODIFY REJECT message before
10 seconds

2. Timer Name Description Value
initiating call clearing procedures.
T3101 Immediate assignment
timer
Time the network waits after sending the IMMEDIATE
ASSIGNMENT or IMMEDIATE ASSIGNMENT
EXTENDED message until the main signalling link is
established before releasing the newly allocated
channels.
1 second
T3103 Handover timer
Time the network waits after transmitting a HANDOVER
COMMAND message until receiving HANDOVER
COMPLETE or HANDOVER FAILURE or the MS re-
establishes the call before the old channels are released.
If the timer expires and the network has not received a
correctly decoded L2 (format A or B) or TCH frame, then
the newly allocated channels are released.
2 seconds
T3105 Physical information
repetition timer
Time the network waits after sending the
PHYSICAL INFORMATION message until receiving a
correctly decoded L2 (format A or B) or TCH frame. This
occur before re-transmitting the
PHYSICAL INFORMATION message or releasing the
newly allocated channels.
50 ms
T3107 Channel assignment
timer
Time the network waits after transmitting an
ASSIGNMENT COMMAND message until receiving
the ASSESSMENT FAILURE message or the MS re-
establishes the call before releasing the old and the new
channels.
3 seconds
T3109 Signaling
disconnection timer
Time the network waits after sending the CHANNEL
RELEASE message before disconnecting the signalling
link.
5 seconds
T3111 Channel deactivation
after disconnection timer
Time the network waits after disconnecting the signalling
link before deactivating the channel.
500 ms
T3113 Paging timer
Time the network waits after transmitting the PAGING
REQUEST message until receiving the
PAGING RESPONSE message. This occurs before re-
transmitting the PAGING REQUEST (if the maximum
number of re-transmissions have not been exceeded).
5 seconds
T3212Locationupdate timer The location update timer is set to zero,
periodic location update by the MS are disabled. If the MS
camps to the BCH and decodes a new MCC or MNC from
the one it last camped on, it should perform
zero = infinite time

3. Timer Name Description Value
a location update.
T3250 TMSI reallocation
timer
Time the network waits after sending the TMSI
REALLOCATION COMMAND until receiving TMSI
REALLOCATION COMPLETE. This occurs before
aborting theprocedure and releasing the Radio Resource
connection.
5 seconds
T3260 Authentication
response timer
Time the network waits after an AUTHENTICATION
REQUEST until receiving AUTHENTICATION
RESPONSE. This occurs before aborting
the procedure and releasing the Radio Resource
connection.
5 seconds
Timer T200 - FACCH and SDCCH supervision timer
There are two timers called T200s and T200f associated
with sdcch and facch respectively for the supervision of messages which transfer from
BTS to mobile.
Range for both of this timer are from 220 to 1260 with step of 20 and Unit is 20ms.
Overview:-
This timer is used on different control channel types and determines the waiting time
for a layer 2 frame acknowledgement.
During any dedicated connection the BTS and the MS exchange specific LAPDm
signalling messages in the so-called in 'acknowledged mode'. This mode is started with
an SABM and normally ends with the LAPDm DISCONNECT.All messages within this
'acknowledged mode' are checked by
the LAPDm flow control mechanisms based on sequence numbers that are assigned to
each transmitted LAPD message. Only for those messages that are transmitted in
acknowledged mode the timer T200 is used as waiting time for the layer2
acknowledgement to a transmitted message.

4. MS also uses the layer 2 acknowledgement mechanisms based on T200 in the same
way as the BTS. In the MS, however, the T200 timer values are not variable but
hardcoded.
Functional description:-
When the BTS transmits a DL layer-2 frame on the air (e.g. an I-frame with an
embedded layer 3 message) the BTS starts the timer T200 and waits for the
acknowledgement frame from the MS. The acknowledgement frame can be another I-
frame sent by the MS in the UL (if the MS has to transmit a layer-3 DTAP message
anyway) as well as RECEIVE READY (RR) if there is nothing else to be transmitted on
layer 3. If the acknowledgement from the MS is not received within T200 the
transmission of the layer 2 frame is repeated and T200 restarted. The total number of
repetitions is restricted to N200, a fixed value for the BTS side specified by GSM which
depends on the following control channel types:
N200(SDCCH)=23,
N200(SACCH)=5,
N200(FACCH/FR)=34,
N200(FACCH/HR)=29
If T200 expires after the last layer 2 frame repetition the BTS sends an ERROR
INDICATION with cause 'T200 expired N200+1 times'
(followed by a RELEASE INDICATION) message to the BSC, which releases the
associated resources.
Read more about other GSM timers:-
GSM timers
Optimized value and uses:-
Based on project trial optimized value for both timers are 800ms.
You can change value of this timer to improve SDCCH drop, TCH
assignment, HOSR, TCH Drop.
#T200 is a timer of acknowledgement trasmission from BTS to MS through LAPD. When BTS transmits a

5. acknowledgement message to MS, T200 times is started in LAPD. If this acknowledgement message is not received
by MS until T200 timer is expired then the transmission will be repeated (retransmission).The maximum
retransmission is N200+1 times. T200 will be restarted in each retransmission. So, N200 is the maximum number of
retransmission of LAPD (Layer-2). If until N200+1 times of retransmission the acknwledgement message, MS still
doesn’t receive this message, then the connection will be dropped.
This problem is may be caused by hardware problem, bad condition of Radio, or imbalance link (UL and DL).
#L2 - Link Release Timer T200 (ms)
Parameter group
ISDN Signaling
Access level
Field
Possible values
Integer multiples of 5, in the range 700 through 2000
Defaults
1000
Explanation
Specifies the value of the T200 timer, which acts as a retry time when attempting to
establish the L2 data link. On starting to establish the link, the timer is started. The
timer is stopped when the switch responds. On expiry, another L2 link establish
message is sent to the switch and the timer is restarted. This is done for a particular
number of retries until it is assumed that the switch is not ready to activate L2.
#T200 is a timer of acknowledgment transmission from BTS to MS through
LAPD. When BTS transmits a acknowledgment message to MS, T200 times is
started in LAPD. If this acknowledgment message is not received by MS until
T200 timer is expired then the transmission will be repeated
(retransmission).The maximum retransmission is N200+1 times. T200 will be
restarted in each retransmission.

6. N200 is the maximum number of retransmission of LAPD (Layer-2). If until
N200+1 times of retransmission the acknowledgment message, MS still doesn’t
receive this message, then the connection will be dropped.
#
Makarand
1st Query
T200 is a basic timer in the LAPDm & LAPD protocol (default value 240ms). T200 is the time between
retransmissions, T200 is started for every transmission of a frame, and if the timer expires the frame is
retransmitted. If the error persists after N200 retransmissions a link failure is detected and the data link is
re-established. If the contact between the BSC and the BTS (or BTS & the MS)is broken the protocol will
detect a link failure in approximately one second when using default values, calculated as (N200+1)*T200
+ transmission and processing time.
2nd Query
It is just the other way round.Data link failure is the cause of HO failure
3rd Query
As HO process is break before make,so the data link in case of HO is from MS to target BTS.
After the HO command ,if a data link failure is observed,HO faiure happens,& then either return to old
channel or call drop happens.
Br
Bijoy
Makarand - 3rd September 2010 (01:17 GMT)
Hi bijoy,
exactly when t200 starts?
Is ho failure is one of the cause for data link failure?
In ho failure which data link gets broken ,i think it must be at source side , then how it gets restored as its
nt necessary that call may drop after data link failure.

7. Bijoy - 29th July 2010 (09:12 GMT)
Hi
TTALONG in the BSC controls the value of the time-alignment timer.Time Alignment is a mechanism
used between the BTSs and the transcoders to minimize the time speech and data frames are stored in
the BTS before being sent out on the radio interface.The time-alignment timer TTA is located in the BTS
and it starts when the BTS sends time-alignment change information to the transcoder and it stops after
receiving a response from the transcoder.
TTALONG will be used for TGs where SIGDEL is set to LONG.SIGDEL is set to LONG for Satellite A-
bis.There are Ericsson specific parameters.
#What is Optimum Value of T200?
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we like to know the optimum value for T200 on LAPDm. All vendors have
different default values (Siemens 145 ms, Nokia 220 ms, Satellite Abis 400
ms etc.) for this timer - but which value is the best to reduce SDCCH drops
and to keep the retransmissions at an acceptable level ?
Example: SDCCH/8
During a 51er multiframe the SDCCH/8 occupies four consecutive TDMA
frames (four bursts are sent). Than the MS / BTS has to wait for the
next 51er multiframe (i.e. 235 ms) before the next Layer 2 frame could be
sent. 145 ms / 220 ms are shorter than the 51er multiframe (235 ms) so in
case of an missingacknowledgement this is always a T200 expiry. The SDCCH
drop will occur if T200 expired N200+1 times. If the T200 is increased (for
example to 500 ms) we have two 51er multiframe to get
the acknowledgement and the SDCCH drops are reduced.
The Qos Stats show a clear & strong corelation between the KPI and the parameter
T200. That's amazing !

8. Regarding the Ack from the BTS, I'm not sure (and I'm tired to look in the
3GPP specs :) ). Take the subchannel "0" from the SDCCH ts.
in DL : the BTS sends SDCCH/0 on burst 0, 1, 2, 3 and the SACCH/0 on burst
32, 33, 34, 35
in UL : th MS sends SDCCH/0 on burst 15, 16, 17, 18
and the SACCH/0 on 47, 48, 49, 50
And I ***believe*** that the Lapdm acknowledgments can be sent on either
the SACCH or the SDCCH, since both of them are sent over the same LapDm
link. I'm not sure at all about this though, but it sounds logical.
The value of the SDCCH Drop due to Radio failures (in ALU) is usually around
1% in a fairly good network. The SDCCH Drop due to Radio Failures is a
counter that encompasses both the Radio Link Timeout and the
"T200*N200+1 times" failures.
I am not able to test your changes because I am not working on a live
network (i am a gsm trainer, living in a world of theory...)
T200 = 220ms for sDCCH SAPI0
= 450ms for SDCCH SAPI3
= 900ms for SACCH associated to SDCCH
Looking at those timers, I fear that the LapDm link for SDCCH is different
than the LapDm link for the SACCH of the SDCCH... I'll let you look it up
though :)
#T200 SDCCH_SAPI0, T200 SACCH SDCCH, T200 SDCCH_SAPI3, and N200 of
SDCCH
After the SDCCH is seized, the transmit end must set the timer T200 when it
sends a signaling message. When the timer expires, the transmit end regards that
the receive end does not receive the message, and then the transmit end

9. retransmits the message. The number of retransmissions depends on N200. If
T200 x N200 is set to a small value, the timer expires before the transmit end
receives an acknowledgement from the receive end. In this case, a forced call
drop occurs. Therefore, the call drops on SDCCHs increase.
#
It can also be related to Lapd but that's not the same Timer. Lapd T200 is lapd N200 are different from
T200 and N200.
DangerousMinds - 27th June 2008 (10:57 GMT)
Thanks again. Someone was telling me this is related to the LapD Signalling and T200 expiry means
failure on the LapD interface. is this true?
Michele - 26th June 2008 (12:45 GMT)
T200 is an air interface timer regulating the communication between MS and BTS. It defines the rate of
repetition of a message (on BTS side) on the air if it is not acknowledged by MS.
If the message was repeated N200 times and no answer was received by MS, an error indication with
cause T200 expired is provided to BSC.
T200=ny1 * N200 where ny1 is configurable on BTS database
DangerousMinds - 25th June 2008 (09:24 GMT)
Hi, Can anyone please explain what does N200 timeout mean? And what should be done to rectify this
problem? Also, is this related in any way to the T200 timer expiry? Thanks!
#Dears,
Am doing a DT analysis and i found that for some reason i keep having N200 timeout .
as all know the N200 is a layer 2 timer and if define the number of retransmission in LAPD frams .
LAPD is between the BTS and BSC , so my question what could cause the timer to finish.

10. I think , it could be no stability in transmission (E1) , what you think ??
please advice
:confused:
RF engineer
2010-02-22, 06:34 PM
Dears,
Am doing a DT analysis and i found that for some reason i keep having N200 timeout .
as all know the N200 is a layer 2 timer and if define the number of retransmission in LAPD frams .
LAPD is between the BTS and BSC , so my question what could cause the timer to finish.
I think , it could be no stability in transmission (E1) , what you think ??
please advice
:confused:
N200 or T200 ?
BR
RF
EngSoft
2010-02-22, 10:25 PM
N200 . T200 is other thing
RF engineer
2010-02-23, 12:08 AM
N200 . T200 is other thing
Hi
No, both of them related to each other but i never faced such a problem for N200 and face T200 expired
may be other colleague may be helping
BR
RF
hassen
2010-02-23, 04:44 AM
Hi
the mobile station stops the transmitting of measurement reports
the BTS never get an acknowledge on a Layer 2 message after the time T200
LAPD N200 is the maximum number of retransmissions
br hassen
tukangoptim
2010-02-23, 05:17 AM
example time out when channel ass cmd during HO.
mostly due to radio quality. or if TRanscodre timer also expired abis subslot perhaps problem.
[Only registered and activated users can see links]
wael
2011-01-14, 08:30 PM
Usually this timer N200 come with T200 during establish the SABAM message which is in Layer 2 during the
assaying the channels in the call setup(SDCCH) or during the handover(FACH/FullTCH, FACH/half TCH ) , in
the TEMs will see the N200 time out come with handover failure and with data link failure and like this case

11. can be come due to :
· Link transmission
· TEMS ( there is problem in the TEMS)
· hardware issue in the Maine processer in the BTS itself causing delay .
· Bad RxQual UL
kadry
2011-01-15, 10:01 PM
N200:means the number of resend SDCCH to moble station
The value specifies the maximum number of retransmitting a frame
Timer T200 has expired N200 +1 times (MS does not respond any more).
check if there is:
bad coverage
high interference
hardware faulty
thanks
#