Subject: [AVT] RTP packet format for events in RFC2833

From: "Wong, David" <[email-masked]>
Date: Tue, 3 Sep 2002 18:59:09 -0400

I'm confused on how to update values in the RTP header (timestamp) and event
block (duration) when transmitting an event.
 
Let's take the example in section 3.8 of RFC 2833 (or the draft).
 
I. Consider the digit event "9" at time 0 that lasts 200 ms.
 
I have two interpretations for how the RTP header timestamp and the event
duration should be updated. Which one is correct (assuming an interpacket
delay of 50 ms, or 400 timestamp units)?
 
1. The RTP header timestamp is updated by 400 timestamp units per packet,
and the duration is the same for each packet during the event (400 timestamp
units).
 
RTP seq#        RTP timestamp        duration
---------------        ---------------------        ------------
      0                        0                     400
      1                        400                 400
      2                        800                 400
      3                        1200               400
 
2. The RTP header timestamp is the same for all of the packets during the
event (0) and the duration increments by 400 timestamp units per packet.
 
RTP seq#        RTP timestamp        duration
---------------        ---------------------        ------------
      0                        0                     400
      1                        0                     800
      2                        0                     1200
      3                        0                     1600

II. For the last packet for the event, seq # 3 in either example, how is
this packet retransmitted? Are the recommended three retransmissions
transmitted at 50 ms apart or are they retransmitted immediately? Are the
RTP sequence numbers updated for each retransmission?
 
III. What should be transmitted between the end of digit "9" and the
beginning of digit "1", which happens at time 800 ms? Can nothing be
transmitted or should silence be transmitted? If silence is transmitted
between events, then shouldn't silence be transmitted even when the digit
"9" is being transmitted (from the viewpoint that this may be a VOIP phone
that isn't transmitting any audio, I would think that silence would always
be transmitted that a person is not speaking).

David Wong
Software Engineer
Communications Infrastructure Test Group

p| 978.661.1241 f| 978.988.0148
email | [email-masked]
205 Lowell Street | Wilmington, MA | 01887

E M P I R I X
http://www.empirix.com <http://www.empirix.com/>

---

From: Bino George <[email-masked]>
Date: Thu, 05 Sep 2002 10:27:18 +0530

Hello :
        Obviously the second interpretation is correct. Logically an
event is generated at a specific RTP time stamp and  after that only RTP
sequence number and the event duration is updated.
     From RFC 2833 :
           If the event continues more than one period, the source
generating the events should send a new event packet with the RTP
timestamp value corresponding to the beginning of the event and the
duration of the event increased correspondingly.

     I think retransmission of the last packet of the event is a
nuisance{ for  transmitter logic } , the receiver can take care this
issue.

bino

I'm confused on how to update values in the RTP header (timestamp) and
event block (duration) when transmitting an event.

I have two interpretations for how the RTP header timestamp and the
event duration should be updated. Which one is correct (assuming an
interpacket delay of 50 ms, or 400 timestamp units)?

1. The RTP header timestamp is updated by 400 timestamp units per
packet, and the duration is the same for each packet during the event
(400 timestamp units).

2. The RTP header timestamp is the same for all of the packets during
the event (0) and the duration increments by 400 timestamp units per
packet.

II. For the last packet for the event, seq # 3 in either example, how is
this packet retransmitted? Are the recommended three retransmissions
transmitted at 50 ms apart or are they retransmitted immediately? Are
the RTP sequence numbers updated for each retransmission?

III. What should be transmitted between the end of digit "9" and the
beginning of digit "1", which happens at time 800 ms? Can nothing be
transmitted or should silence be transmitted? If silence is transmitted
between events, then shouldn't silence be transmitted even when the
digit "9" is being transmitted (from the viewpoint that this may be a
VOIP phone that isn't transmitting any audio, I would think that silence
would always be transmitted that a person is not speaking).

David Wong
Software Engineer

E M P I R I X
http://www.empirix.com

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