Annex 6 to AVC-743R

ATM performance assumptions

Source	:	Q.2/15

Title	:	Liaison on ATM Performance Assumptions

Purpose	:	Response to a previous Liaison and request for comments

Contact	:	Sakae OKUBO
			Rapporteur for Q.2/15
			Graphics Communication Laboratories
			6F ANNEX TOSHIN BLDG, 4-36-19 Yoyogi,
			Shibuya-ku, Tokyo  151 Japan
				Phone	:  +81 3 5351 0181
				Fax	:  +81 3 5351 0185
				e-mail	:  okubo@gctech.co.jp

We highly appreciate your informative input regarding our questions on our ATM Performance Assumptions which were reflected in AVC-635 (July 5, 1995 - DRAFT V1.0).  Based on your input, we plan to modify our ATM performance assumptions as follows:

1)	CDV values will be expressed as a function of the cell transfer time (T).  We will also note in the new version of AVC-635 that (a) the shown values are applicable for high bitrate links (e.g., 155 Mb/s OC-3) and not for lower speed links for which the CDV value can be much higher than the assumed numbers, and (b) these CDV assumptions are for CBR traffic only.

2)	Worst, average, and best case CDV assumptions will be modified to [400-1000T], [200-400T], and [150-200T], respectively.

3)	Our worst case BER assumption will be modified to 10-7 as you recommended.

4)	Our worst case delay assumption will be modified to 10 ms, and the best case assumption to 1 ms.  (Both numbers exclude propagation delays)

5)	Recommendation G.826 will be referenced for the definition of SES.

In response to your questions, we have the following clarifications:

1)	Currently, we are in the process of defining a FEC frame for our broad band audiovisual terminals (H.32X).  This FEC frame will occupy the payload of several (around five) ATM cells.  It is not clear, however, if the selected FEC frame format will align itself with an (integer) multiple of ATM cells or not.  Once we make our final decision of the FEC frame format, we would be more than happy to provide you with our preferred values for the N and M parameters of the SECB definition.

2)	Normally, the audiovisual terminals that we are concerned with interwork among themselves over N-ISDN and/or B-ISDN networks.  However these terminals have to also interwork  with ordinary telephones connected to the analogue network.  This is the main reason for our question about end-to-end delays.  Our concern is that if the delay is on the order of 10 ms then echo cancellers would be required when communicating with ordinary telephones.

3)	Time stamps with absolute values (i.e., not differential values as in SRTS) will be included in our payload.  Although the particular implementation of the timing recovery method used by the decoder is not standardized, a typical implementation would use the time stamps values in conjunction with a low-pass filter and a PLL to derive the timing.  The value of the CDV influences the buffer size needed to buffer the incoming payload (which include the time stamps), and the length of the LPF used to smooth the variation between the time stamp values and the locally derived timing.  Based on this typical implementation, our objective is to minimize the buffer size and the length of the LPF (in terms of the number of taps in the time-domain).  Therefore, our desire is to have the CDV values as small as possible.

4)	Based on our current understanding of our ATM performance assumptions, and with the help of your input, we have made the following conclusions:

	a)	The recovery of lost cells will not be necessary in our broadband audiovisual terminals (H.32Y and H.32X).

	b)	Protection against error events will be needed.  This is the reason behind our attempt in defining a FEC mechanism for H.32X terminals.  For H.32Y terminals, it is expected that the FEC capability included in Recommendation H.261 will be adequate.

	c)	The assumed end-to-end delays do not represent any problem for our typical audiovisual services.

Finally, we would appreciate your help in clarifying the following issues:

1)	Our previous assumption for the average BER was 10-7.  After your input, this value became the worst case scenario.  What would you recommend for an average value of the BER parameter?  Also, what would be your advice regarding a best case BER assumption?

2)	Similar question (although not as critical), regarding the average (or typical) end-to-end delay.

3)	Do you have any advice regarding CDV values for the lower bitrate links (e.g., DS-1 and DS-3 rates)?

4) 	You mentioned the low and high-frequency CDV types in your liaison.  Is it possible to control one type of CDV at the expense of the other type?

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