Warm Re-Start
`
`Tl El.4/97-J 61 Rl
`
`T1El.4/97-161Rl
`
`STANDARD PROJECT:
`
`Asymmetric Digital Subscriber Line (ADSL)
`
`TITLE:
`
`Warm Re-Start for ADSL
`
`SOURCE:
`
`Deutsche Telekom AG
`
`Dr. Werner Henkel
`Technologiezentrum
`P.O. Box 10 00 03
`D-64276 Darmstadt
`Germany
`
`E-mail: henkelw@tzd.telekom.de
`Phone: +49 6151 83 5006
`Fax:
`+49 6151 83 3035
`
`Amati Communications Corporation
`
`PeterS. Chow
`2043 Samaritan Drive
`San Jose, CA 95124
`
`E-mail: peter@amati.com
`Phone: ( 408) 879-2043
`Fax:
`(408) 879-2906
`
`DATE:
`
`September 22-26, 1997
`
`DISTRIBUTION TO:
`
`T1El.4
`
`ABSTRACT
`
`For wide acceptance of ADSL, the modem startup time is an important parameter. ANSI Tl.413
`issue I standard has an initialization procedure that will take approximately 7 seconds, which can
`be significantly reduced in a warm re-start situation. During a warm re-start, the modem will
`attempt to use previously calculated values of the equalizer coefficients, bit and power
`allocations, and ~ther system parameters. This contribution outlines the reasons for a warm re(cid:173)
`start procedure, suggests locations within initialization that can be shortened considerably in
`principle, and recommends to at least have this issue placed on the living list of the issue 3
`standard, if it cannot be included in the issue 2 standard.
`
`NOTICE
`
`This contribution has been prepared to assist the Standards Committee Tl - Telecommunications. This
`document is offered to the committee as a basis for discussion and is not a binding proposal. The
`requirements are subject to change in form and numerical value after further study. The rights to add to,
`amend, or withdraw the statements contained herein are reserved.
`
`

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`• • •• ...-' -. VJ J\.J
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`1. Background
`
`The current T 1.413 issue 1 ADSL standard requires an initialization procedure that will have a
`minimum duration of approximately 28,000 DMT symbols (some with cyclic prefix and so,me
`without), which is equivalent to about 7 seconds. In the worst case, a standard compliant ADSL
`modem may take up to approximately 10 seconds for initialization. Compared to traditional
`voice band modem connect time, this is quite reasonable; however, it may not be acceptable in
`the cases of a sync loss or unrecoverable micro-interruptions. While these unrecoverable error
`events should not hap,.pen very often, it would be of great benefit to the operators deploying
`ADSL modems, if a warm re-start procedure is available that requires significantly shorter
`amount of training time. In fact, certain operators, such as DTAG, may not be able to deploy any
`ADSL modem without this feature.
`
`2. Potential Savings During Initialization Using Warm Re-Start
`
`Most of the initialization time is consumed by the transceiver training and the channel analysis
`phases; e.g., the various Reverb phases (512 to 4096 DMT symbols) and the Medley phase
`( 163 84 DMT frames). The ultimate goal of these phases of initialization is to come up with the
`correct echo canceller settings, equalizer coefficients, and the bit and gain allocations based on
`the measured SNRs. However, in principle, these training stages can be shortened significantly if
`we make the assumption that the channel has not changed significantly and that the error event is
`caused by temporary anomalies in the time domain. While we still need to acquire sample and
`frame synchronization, the time required for echo canceller training, time domain equalizer
`training, SNR calculations, and the bit and gain allocations can be much shorter than a full
`modem initialization, if we use the previously saved settings, either as a starting point for various
`adaptive training algorithms or as the actual settings. If the warm re-start procedure fails, then a
`fallback mode should be implemented, leading to the standard initialization procedure, where all
`the coefficients will be determined from scratch.
`
`Another area of the current initialization process that can be enhanced is before the bit allocation
`has been calculated, information transfer is carried out at a rate of one bit per DMT symbol. This
`is quite inefficient, and a worthwhile alternative during warm re-start would be to allow for a
`worst case bit allocation, based on the stored SNR table, which should still be significantly more
`efficient than transferring 1 bit per DMT symbol.
`
`3. Recommendation
`
`Given the tight schedule, we understand that a full warm re-start specification may not be
`possible for inclusion in the issue 2 standard. However, if there is enough support, we would like
`to propose inclusion of warm re-start as an informative annex (with text to be provided in the
`next meeting). In the very least, we strongly recommend that warm re-start be placed on the
`living list for the issue 3 T 1.413 ADSL standard.
`
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