Subject: Re: [Diffserv] Comments on VW EF draf

From: "James M. Polk" <[email-masked]>
Date: Sun, 26 Mar 2000 22:57:33 -0600

Shahram 

In IP, using a G.711 codec with Zero silence suppression-- with the IP/UDP/RTP
headers (40 byte header to go along with the 160 byte payload) -- it does
require 80kbps with a 20ms clock (the math proves it). TDM doesn't require any
headers, just sample rate, so it is just payload. Why IP Header compression
isn't included for comparison is likely because (#1) that isn't  Routable, (#3)
RFC2508 is for Point to Point links -- not Switched or multipath, (#3) Incurs a
Latency hit in the set up of the header information at both ends of a Point to
Point Link, (#4) is (I believe) only used between Layer 3 devices because
that's what you're compressing because of the requirement for L3 awareness.

Keep in mind that in the 80kbps calculation -- NO Layer 2 headers are accounted
for (something I've never agreed with when people use this throughput value).
You will always require a L2 Header -- although it might change with the type
of media you use on a per link basis. Ethernet is the worst with this -- but
basically the fastest transport (so it doesn't matter as much) at (I believe)
approximately 126kbps throughput for a G.711 codec.

All that said -- I'm merely arguing the facts of IP Codec theory (and
application) and have no other biases towards advocating or rebuking what is
written in this draft.

At 02:20 PM 3/26/2000 -0800, Shahram Davari wrote:
>Hi,
>
>I have a few questions/comments on the VW-BA draft:
>
>1) In section 2.6 the voice capacity of IP network is compared to that
>of a TDM Telco trunk, and it was concluded that there is a 25% BW
>penalty for IP. However, it is not clear why for IP a 200 byte per 20 msec
>(equivalent to 80 kbit/sec rate) is assumed, while for the TDM a 64Kbit/sec
>rate. Is this overhead due to RTP/UDP/IP header? if so why header
>compression is not considered for the comparison?
>
>2) Section 6.1.1.1 says that:
>
>"in order to get the worst jitter a packet from each ingress must arrive at
>each of the M border routers at the same time and must be transmitted to the
>interior router queue for the bottleneck link B at the same time"
>
>Since this section is about dumpbell bottle neck, why is it necessary for
>the packets to arrive at the ingress routers at the same time? to achieve
>the worst jitter case we only need the packets to arrive at the 
>bottleneck at the same time, don't we?
>
>3) Section 6.1.1.1 says "bottleneck B" while the figure shows the
>bottleneck link as "L". Which one is correct?
>
>4) Section 6.1.1.1 says: "The links between the border routers and the
>bottleneck router must be enough larger than B that the packets are
>still sitting in B's queue when our (A,Z) packet arrives at the end of
>a burst of N, that is L>NxB"
>
>- What does B's queue mean does it mean the queue for the bottleneck L?
>
>- instead of the sentence "must be enough larger" isn't it better
> to say "must be just large enough".
>
>- How is the L>NB equation derived? It doesn't seem correct to me because,
>if L is infinitely large, then any packet arriving at bottleneck won't see
>any other packets in front of it.
>
>According to my calculations if you have M links arriving at the
>bottleneck point and each carrying a burst of N packets, then in order
>for the last packet in a burst to experience jitter due to the other
>packets in front of it, the time to transmit one packet over bottleneck
>link "L" must not exceed the time that it takes to receive a burst of
>N packets. That is:
>
>S/L < NS/B  =>  L<B/N
>
>4) Last paragraph of section 6.1.1 says " the amount of time to send an
>EF packet on each link can be written as fT/(total no. of EF-marked flow
>crossing the link). Is this always true. Don't you think that this holds
>only when the full f% capacity of the link is used by EF.
>
>5) Section 6.1.1.2 finds the worst case jitter as fT. But the implicit
>assumption is that an EF flow won't encounter another EF flow more than
>once. However, in multipath routing an EF-flow may encounter another
>EF flow many times. For example consider the following scenario:
>
>          X                    X
>     /--------->\          /-------->\
>    /            \        /           \
>-->A              B----> C             D------>
>    \            /        \           /
>     \--------->/          \-------->/
>         Y                      Y
>
>Flow X and Y encounter each other more than once.
>
>
>I appreciate the authors reply.
>
>Regards,
>
>-Shahram
>
>_______________________________________________
>diffserv mailing list
>[email-masked]
>http://www.ietf.org/mailman/listinfo/diffserv
>Archive: http://www-nrg.ee.lbl.gov/diff-serv-arch/
>
>

*************************************
"At the end of the day... the most committed win!"

James M. Polk
Sr. Product Manager, Multiservice Architecture and Standards
Enterprise Voice Business Unit
Cisco Systems
Dallas, Texas
w) 972.813.5208
f)  972.813.5280
www.cisco.com

---

From: Shahram Davari <[email-masked]>
Date: Mon, 27 Mar 2000 14:09:05 -0800

Hi James,
 
Thanks for your reply. Don't you think these assumptions for G.711 coding
should be incorporated in the draft?
 
Regards,
-Shahram

-----Original Message-----
From: James M. Polk [mailto:[email-masked]]
Sent: Sunday, March 26, 2000 11:58 PM
To: Shahram Davari; [email-masked]
Subject: Re: [Diffserv] Comments on VW EF draf

Shahram 

In IP, using a G.711 codec with Zero silence suppression-- with the
IP/UDP/RTP headers (40 byte header to go along with the 160 byte payload) --
it does require 80kbps with a 20ms clock (the math proves it). TDM doesn't
require any headers, just sample rate, so it is just payload. Why IP Header
compression isn't included for comparison is likely because (#1) that isn't
Routable, (#3) RFC2508 is for Point to Point links -- not Switched or
multipath, (#3) Incurs a Latency hit in the set up of the header information
at both ends of a Point to Point Link, (#4) is (I believe) only used between
Layer 3 devices because that's what you're compressing because of the
requirement for L3 awareness.

Keep in mind that in the 80kbps calculation -- NO Layer 2 headers are
accounted for (something I've never agreed with when people use this
throughput value). You will always require a L2 Header -- although it might
change with the type of media you use on a per link basis. Ethernet is the
worst with this -- but basically the fastest transport (so it doesn't matter
as much) at (I believe) approximately 126kbps throughput for a G.711 codec.

>EF flow many times. For example consider the following scenario:
<http://www.ietf.org/mailman/listinfo/diffserv> 
>Archive: http://www-nrg.ee.lbl.gov/diff-serv-arch/
<http://www-nrg.ee.lbl.gov/diff-serv-arch/> 

James M. Polk
Sr. Product Manager, Multiservice Architecture and Standards
Enterprise Voice Business Unit
Cisco Systems
Dallas, Texas
w) 972.813.5208
f)  972.813.5280
www.cisco.com <http://www.cisco.com/>