[From the Kitchen]
Currently the T1E1.413 is working overtime on the Issues of ATM over ADSL.
There was to be a meeting in San Jose hosed by Amati, but Due to the heavy travel schedule of many of participants , the decision was made to have a conference call on January 14th in place of the in-person 2-day meeting.
Key Items for Issue 2 T1.413:
Rate adaptation
-at startup
-dynamic (on-the-fly)
ATM transport via ADSL
Reduced overhead
Interoperability
-issue 1 to issue 2
-start-up capability/version negoitiation
-multivendor interworking
POTS Splitter, specification of interface to external LPF
Network bit timing to customer equipment
Self crosstalk performance requirements
Transmit PSD mask
Higher speeds
ATU-R data registers
The next meeting is FEB 3-7 in Austin, Tx.
HOSTED BY: NetSpeed, Inc.
MEETING PLACE: DoubleTree Hotel
6505 Interstate Highway 35 North
Austin, TX 78752
Phone: 512-454-3737
Fax: 512-454-6915 ( put Name/Guest )
CONTACT: Vernon Tice - NetSpeed
Phone: 512-257-4779
Fax: 512-249-8506
Email: vtice@netspd.com
A sneak peek at some of the issues leading up to the conference call:
JW@KSC
From Friday Jan 10th 1997
Effects of ADSL Dynamic Rate Adaption on ATM Traffic Classes
Alan J Weissberger, Dev Bhattacharya (NEC-Electronics),
John Cioffi (Amati Communications)
References: T1E1.4/96-252, 303, 304, -----374, ADSLF/96-90
Abstract
We analyze the effects of rate adaption on the 5 types of ATM traffic
classes (see 96-251R1) and conclude that, with some constraints on ATM performance, it's possible to support most ATM applications under dynamic rate adaption.
Discussion
1. If ADSL only supports non-real time applications (e.g. UBR, ABR or non rt VBR) then rate adaption should not pose a significant problem. For UBR and ABR traffic, there may be data cells lost during
the RA process and Transport layer (e.g. TCP) retranmission will be
required.
For ABR, there may be lost RM cells and the RM cell round trip time may lengthen if the ADSL channel rate is adapted downward. Therefore, convergence to the correct ABR source transmission rate may take
longer than would otherwise be the case. In certain instances, the Minimum Cell Rate might be zero and the Peak Cell Rate (PCR) = Initial or Actual Cell Rate, might not be able to be sustained if the ADSL channel rate is adapted downward.
For non rt VBR, we suggest the bandwidth management entities (CPM and CBM) add up the PCRs on all outstanding PVCs and SVCs and NOT permit additional SVCs to be created if the sum of all the PCRs exceeds a predefined threshold, which is less than the available channel bandwidth. This builds in an additional tolerance for downward
RA without breaking the PCR guarantee in the ATM traffic contract for
established virtual circuits. Additionally, traffic shaping by the source transmitter is recommended to reduce the PCR and maximum burst size for each non rt VBR virtual circuit.
For UBR, ABR, and non rt VBR traffic, it might be desireable to suspend cell specific TC to T1.413 generic TC transmission during the dynamic rate adaption process. A signal from the T1.413 PMD indicating dynamic RA in progress would be used to freeze TC transmission, which would be resumed when the signal is negated. This would eliminate unnecessary cell loss and the resultant TCP retransmissions, resulting in higher "goodput." It would also ease the networks requirement to do Early Packet Discard in the DSLAM or ATM edge switch.
2. We suggest that the ADSL network operator build a safety factor into the provisoned permissible channel rates to be agreed upon at start-up. This will enable small decreases in noise margin to be tolerated while maintaining a 10-7 BER. This will facilitate support of CBR and rt VBR virtual circuits (SVC/PVCs), which may not be capable of rate adaption. In other words, loop attenuation/ margin, worst case cross talk and external noise should be considered before permitting an overly optimistic start-up rate (in each direction of transmission).
3. CBR and rt VBR traffic requiring guaranteed bandwidth should be assigned to the Fast path channel which should NOT be greatly effected by the aggregate ADSL rate change. Any rate adaption on this channel should be kept small and data in the Fast path buffer should not be
dropped. Examples of CBR/ rt VBR applications for the Fast Path include: video conferencing, real time medical imaging, compressed digital voice/audio, real time broadcast video.
Note that while the H.320 video/audio codec can accept rates in n x 64Kbps increments, it can do so only on call establishment and NOT during the digital information transfer phase of a call.
4. MPEG2 CBR or rt VBR over AAL 5 may be able to accept a downward rate adaption of about 2 Mbps e.g. from 6 Mbps to 4 Mbps. Any lower rate would probably necessitate clearing (SVC) or resetting (PVC) of the virtual circuit. For these applications, the Interleaved path might be desireable (in the downstream direction) to provide
better immunity against impulse noise. We recommend the Interleaved path take the majority of the ADSL aggregate rate change, while preserving the Fast path channel rate as noted in 3. above. |
