If an MS wants to open a new UL TBF, it can either use (P)RACH or
request one in a Ack/Nack message for a DL TBF (PACCH). When a TBF
becomes idle (LCC queue is empty but the TBF is kept open), there
aren't any Ack/Nack requests that can be used by the MS to ask for an
UL TBF, therefore it has to use the RACH. This leads to many RACH
requests even for a single HTTP transaction, so it takes some time to
retrieve even a simple web page.
This commit modifies the scheduler to regularly send Ack/Nack
requests on idle DL TBFs. It does so by extending the priority based
scheduling algorithm to have 5 priority levels (highest priority
first):
- Control block is pending
- High age (100%) threshold reached (-> request Ack/Nack)
- Data is waiting or there are pending Nacks
- Low age (200ms) threshold reached (-> request Ack/Nack)
- Pending Nacks that have been resent already
- None of the above (-> send DL dummy control block)
The 'age' refers to the time since since the last control block has
been sent on the TBF. This high age threshold is set to
dl-tbf-idle-time or to 50% of T3190 (whichever is smaller), aiming
for at least a poll (and TBF shutdown) after the TBF has expired and
to safely prevent expiry of T3190. So if dl-tbf-idle-time > 200ms,
there will be a poll every 200ms and a final poll after
dl-tbf-idle-time. On high load, the interval between polls can get
higher, but the 'high age' poll should be in place.
This commit implements the scheduling with respect to GSM 44.060,
9.3.1a ("Delayed release of downlink TBF").
Ticket: #556
Sponsored-by: On-Waves ehf
Currently a lot of LLC dummy commands will be generated while waiting
for an ACK for the DL TBF, even if there are blocks that could be
resent instead.
This patch modifies create_dl_acked_block to only call
create_new_bsn() if there is unsent LLC data or m_window is empty.
If the TBF is in state FLOW, no unsent LLC data is left, but there
are blocks left in m_window, those are resent instead.
Sponsored-by: On-Waves ehf
Currently a DL TBF is immediately closed, when the LLC queue is
drained. This will lead to a new DL assignment if data is received
afterwards. In addition, it is not possible to keep the PACCH open
to poll the MS for UL establishment requests there.
GSM 44.060, 9.3.1a suggests to delay the release of an inactive TBF
for some time (max 5s).
This commit mainly changes create_new_bsn() to send LLC dummy
commands as filler if no LLC data is available until keep_open()
returns false. The keep_open() functions returns true unless a
configurable time has passed after the LLC data store drained. By
default, that time is not set which causes keep_open() to always
return false, so that delayed release is effectively disabled.
The following VTY commands are added:
- dl-tbf-idle-time <1-5000> to set the delay in ms
- no dl-tbf-idle-time to disable delayed release
Ticket: #556
Sponsored-by: On-Waves ehf
If a BSN is going to be created but there is no frame stored in
m_llc, an empty LLC message would be created. This shouldn't happen
currently, but this will be a common case, when delayed TBF release
is implemented.
This commit changes create_new_bsn() to create an LLC dummy
command in that case and to put it into the frame buffer.
Sponsored-by: On-Waves ehf
Currently if append_data() is used when there is no LLC data in the DL TBF,
it will either call reuse_tbf() which in turn will call put_frame(),
or it will append the LLC message to the queue, even if the queue and
the frame buffer are empty. This only happens with the test case so
far, but this would change when idle DL TBFs are kept open for some
time. It results in empty LLC message being sent to the MS (see log
below).
This commit changes append_data to check for this case and to
eventually use put_frame() instead of appending the LLC data to the
queue.
Addresses:
TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW) downlink (V(A)==0 ..
V(S)==0)
- Sending new block at BSN 0
-- Chunk with length 0 is less than remaining space (20): add length
header to to delimit LLC frame
Complete DL frame for TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW)len=0
- Dequeue next LLC for TBF(TFI=0 TLLI=0x00000000 DIR=DL STATE=FLOW)
(len=200)
Sponsored-by: On-Waves ehf
This functions calculates the number of frames that have passed since
the last DL poll (RRBP flag set) has been sent. It returns a value
less than zero (fn_now - fn_sched) if the block has been scheduled but
not yet sent.
If the function is called before the first data block has been sent
it will return -1.
If the function is called before the first DL poll is sent, it
returns the number of frames since the first data block has been
sent.
Sponsored-by: On-Waves ehf
Some properties of a DL TBF are explicitly calculated within modules
using DL TBFs.
This commit introduces the methods need_control_ts(), have_data(),
is_control_ts() to hide internals of the DL TBF implementation.
Sponsored-by: On-Waves ehf
Currently the DL Ack/Nack is not requested, if the last chunk of the
final LLC frame in the TBF fits exactly into the remaining space of
the RLC block. It is also not requested, if the RRBP flag cannot be
set due to scheduling issues (single block allocation, polling
already pending for this TBF, ...). So up to POLL_ACK_AFTER_FRAMES
(20) RLC data blocks will have to be resent, before requesting the
Ack/Nack will be retried.
This commit removes the first_fin_ack parameter of
create_dl_acked_block entirely and adds a request_dl_ack() method
that should be called, when the TBF's state changes to FINISHED. This
request will be reset, when the block has been scheduled
successfully. Since the first_fin_ack hasn't been set if chunk ==
space, calling request_dl_ack() on both places in create_new_bsn()
when the state is changed to FINISHED will also fix the first issue
described above.
Note that DL scheduling might not be fair concerning access to
first_ts when multiple TS are used for a TBF. In theory, a TBF might
never get access to first_ts in the worst case.
Sponsored-by: On-Waves ehf
Currently two DL TBF objects with the same TLLI exist after reuse_tbf
which make the result of tbf_by_tlli undefined. This leads to several
DL TBFs belonging to the same TLLI.
This commit extends tbf_by_tlli to check the m_tlli_valid flag for
DL, too. That flag is set to 0 in reuse_tbf to mark the 'old' DL TBF
as invalid after its LLC data has been copied to the new one.
Sponsored-by: On-Waves ehf
If the PCU didn't receive the downlink ack/nack when it was polled there
is no reason to assume it will arrive at a later time. In that case
N3105 is already increased, but the next block sent should have a valid
RRBP set as well to poll again.
This can happen if either the BSN with the valid RRBP gets lost on the
downlink or the DL ACK/NACK gets lost on the uplink path.
Ticket: SYS#382
Sponsored-by: On-Waves ehf
There are a couple of possibilities where one TBF is used to assign a
new one:
1. Assign a DL TBF from a UL TBF
2. Assign a UL TBF from a DL TBF
3. Assign a DL TBF from a DL TBF which is in wait-release state (T3193 is
running)
In these cases the assignment is sent on the existing TBF and triggers
the assignement of the new TBF (with different TFI/direction).
The current code detects these situations by looking at dl/ul_ass_state
and then chosing the TBF with the opposite direction (DL/UL) that has
the same TLLI. This does not work in the case 3 above where a new DL TBF
is triggered for a DL TBF. The current code reuses the old TBF (and
TFI), but this violates the spec.
This patch introduces a m_new_tbf member which is set to the new TBF to
be assigned. When receiving a control ack the code looks up the
n_new_tbf member of the tbf that requested the control ack and completes
the ul/dl assignment. If the old TBF was in the wait release state
(T3193 is running) it is released.
From 3GPP TS 04.60 9.3.2.6:
"""
If the network has received the PACKET DOWNLINK ACK/NACK message with
the Final Ack Indicator bit set to '1' and has new data to transmit for
the mobile station, the network may establish a new downlink TBF for the
mobile station by sending the PACKET DOWNLINK ASSIGNMENT or PACKET
TIMESLOT RECONFIGURE message with the Control Ack bit set to '1' on
PACCH. In case the network establishes a new downlink TBF for the mobile
station, the network shall stop timer T3193.
"""
reuse_tbf() is modified to allocate a new TBF with a new TFI and trigger
a dl assignment for that TBF on the old TBF. All pending data is moved
to the new TBF.
Ticket: SYS#382
Sponsored-by: On-Waves ehf
Jacob Erlbeck
Daniel Willmann