Title:
Header Compression for BCMCS
Abstract:
This contribution proposes the adoption of an improved ROHC
unidirectional mode of operation for header compression in BCMCS.
Existing unidirectional mode of operation in ROHC does not work
efficiently when used over broadcast links with significant error rates and
scarce bandwidth. In our proposed improvement, static context
information will be sent in advance to the decompressor via BCMCS
information acquisition. The original ROHC compressor states and
decompressor states can still be used but with minor simplifications; less
information need to be sent over the unidirectional link to initialize or
periodically update the context, enabling shorter refreshing period,
which, in turn, leads to faster context damage recovery and shorter user
tune-in time.
Source:
Haipeng Jin and Jun Wang
QUALCOMM Incorporated
{haipengj, jwang}@qualcomm.com
Date:
September 15, 2003
Recommendation:
Review and adopt
Notice
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incorporate text or other copyrightable material contained in the contribution and any modifications
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any Organizational Partner’s standards publication even though it may include portions of the
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or in part such contributions or the resulting Organizational Partner’s standards publication.
QUALCOMM Incorporated is also willing to grant licenses under such contributor copyrights to third
parties on reasonable, non-discriminatory terms and conditions for purpose of practicing an
Organizational Partner’s standard which incorporates this contribution.
This document has been prepared by QUALCOMM Incorporated to assist the development of
specifications by 3GPP2. It is proposed to the Committee as a basis for discussion and is not to be
construed as a binding proposal on QUALCOMM Incorporated. QUALCOMM Incorporated specifically
reserves the right to amend or modify the material contained herein and nothing herein shall be
construed as conferring or offering licenses or rights with respect to any intellectual property of
QUALCOMM Incorporated other than provided in the copyright statement above.
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INTRODUCTION
This contribution proposes the use of an improved unidirectional (U) mode of ROHC for
header compression in BCMCS.
When used over error prone unidirectional links such as wireless broadcast links, the
ROHC U mode compression faces a very important tradeoff between efficiency and
reliability. When the periodic transition to initialization and refresh (IR) state in the
compressor is set to a long interval, fewer large IR packets are transmitted, leading to
higher bandwidth efficiency. However, since the wireless links have high error rate, it is
highly probable for the transmitted packets to be corrupted and cause repeated
decompression failures at the decompressor. Once it is forced back to no context (NC) state
by the failures, the decompressor will have to wait for a long time to receive the periodic IR
packets from the compressor in order to re-establish the context. All packets received
during this interval have to be discarded, causing disruption in services. In addition, the
long IR refresh interval will lead to long acquisition time when a MS initially tunes to or
switches back to the broadcast channel because the decompressor in the MS cannot be
updated as soon as possible. On the other hand, if the periodic transition to IR state in the
compressor is set to happen with a short interval, the decompressor will be able to recover
from context loss promptly, achieving higher reliability, and the tuning time for the MS will
also be short. However, the large number of IR packets sent will lead to much lower
efficiency. Therefore, there is tradeoff for sending frequent IR packet and bandwidth
efficiency.
In this contribution, we propose improvements to ROHC U mode in the initialization
procedure and the compression/decompression state machine to alleviate the dilemma in
setting the periodic update period and to make it more suitable for BCMCS. With our
proposed changes, less information need to be transmitted during periodic refresh while the
decompressor can still recover from context damage quickly and the MS can tune in faster.
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BACKGROUND ON ROHC
OA
OA
IR
OA
FO
SO
Timeout
Timeout/
Update
Timeout
/Update
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Figure 1 Compressor States and Logic (ROHC U Mode)
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In ROHC, when there is no return path available from the decompressor to the compressor,
U mode is used to perform header compression. The three compressor states, Initialization
and Refresh (IR) state, First Order (FO) state, and Second Order (SO) state, for ROHC
compression are shown in Figure 1. The compressor starts in IR state and transits
gradually to higher compression states. The purpose of the IR state is to initialize the
context at the decompressor or to refresh the context. In the IR state, the compressor sends
complete header information using IR packets. The purpose of the FO state is to efficiently
communicate irregularities in the packet stream. The compressor rarely sends information
about all dynamic fields, and the information sent is usually compressed. The compressor
enters SO state when the header to be compressed is completely predictable given the RTP
sequence number.
In U mode, the transition logic for compression states is based on three principles: the
optimistic approach, timeouts, and the need for update. Transition to a higher compression
state is carried out according to the optimistic approach principle. The compressor transits
to a higher state when it is fairly confident that the decompressor has received enough
information to correctly decompress packets according to the higher compression state.
The compressor stays in the IR state until it assumes that the decompressor has received
the static context information. Similarly, the compressor transit from FO state to SO state
after it has confident that the decompressor has all parameters needed to decompress
according to a fixed pattern. The compressor normally obtains its confidence about
decompressor status by sending several packets with the same information according to the
lower compression state. While in SO state, the compressor must immediately transit back
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to the FO state when the header to be compressed does not conform to the established
pattern.
The decompressor states and logic are shown in Figure 2. The decompressor starts in the
lowest state, No Context (NC) state. Successful decompression will always move the
decompressor to the Full Context (FC) state. Repeated failed decompression will force the
decompressor to transit downwards to a lower state. In the NC state only IR packets, which
carry the static information fields, may be decompressed. All other packets received in the
NC state will be discarded.
To protect against decompressor failures, the compressor must periodically transit to lower
compression states. Particularly, periodic transition to the IR state is needed to refresh the
static context. Periodic transition to the IR state should be carried out less often than
periodic transition to the FO state.
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Success
No Static
Success
No Dynamic
NC
SC
K2 out of N2
failures
FC
K1 out of N1
failures
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Figure 2 Decompressor States and Logic (ROHC U Mode)
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PROPOSED ROHC U MODE IMPROVEMENTS FOR BCMCS
We propose the following improved ROHC U mode to be used for BCMCS to improve
efficiency and reduce tuning delay without reducing reliability.
Instead of using the IR packets sent during the IR state to establish and refresh the static
context, the BCMCS information acquisition procedure between the BCMCS controller and
the MS is used to convey the static information.
Typical static fields in RTP/UDP/IP protocol headers are listed as follows.
 IPv4 static fields
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
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
o Version
o Protocol
o Source Address
o Destination Address
UDP static fields
o Source Port
o Destination Port
RTP static fields
o SSRC (Synchronization Source)
Besides static header fields, the controller also needs to send some configuration
information to the MS via BCMCS information acquisition, for example, whether large CID
is being used, which profile is used, whether there is segmentation and what is the
expected maximum reconstruction unit.
Once the MS receives the header configuration information and static information, the MS
will keep it for the lifetime of the session. In the event of context damage, the decompressor
only needs to receive the dynamic information to re-establish the context. As a result, the
static context does not need to be sent periodically again. Both the compressor states
machine and decompressor states machine can be simplified as shown in Figure 3 and
Figure 4. In both figures, the states in ROHC U mode are still used with some minor
changes in the state transition logic. Dotted lines in Figure 3 and Figure 4 indicate the
state transitions that are no longer used; red lines indicate the state transitions that
remain to be used.
OA
OA
Initial Packet Received
IR
FO
SO
Timeout/
Update
Timeout
/Update
Timeout
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Figure 3 Improved Compressor States Machine
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Success
No Static
No
Dynamic
NC
Success
SC
static info
received
FC
K1 out of N1
failures
K2 out of N2
failures
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Figure 4 Improved Decompressor States Machine
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After initially sending the static context via signaling messages, the compressor enters the
FO state and sends IR-DYN packets which contain the dynamic part of the context and
other necessary information. Once it has confidence about the decompressor status by
following the optimistic approach principle, the compressor can transit to SO state and
perform optimal compression. To help the decompressor to quickly reach the FC state (with
small tuning or re-tuning time) and recover from context damage, the compressor must
periodically transit to FO state to refresh the dynamic context.
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The decompressor still starts from the IR state. Once it receives the static context through
the initial acquisition process, the decompressor enters the SC state. In the SC state, the
decompressor only attempts decompression on packets carrying seven or eight bits CRC as
specified by ROHC. Upon successful decompression, the decompressor transits to the FC
state. The decompressor performs decompression attempts until repeated failures force it
back to the SC state.
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SUMMARY
Our contribution achieves the following improvements when used for header compression
in BCMCS:
 The header compression configuration information and static context are
transmitted to the decompressor reliably during the BCMCS information acquisition
process. There is no more need to periodically refresh the static context using large
IR packets, thus leading to lower overhead.
 The compressor can periodically update the dynamic context with a higher
frequency. As a result, the decompressor can recover from context damage quickly
from the frequently transmitted dynamic context updates. High reliability is
achieved without severely sacrificing the efficiency. In addition, a new user joining
the broadcast service is also able to acquire the full context with a shorter delay.
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Note that the proposed improvements are not limited to RTP profile in ROHC U mode of
operation; they can be easily extended to other profiles as well as other header compression
algorithms.
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