Subject: Re: packet loss probability

From: [email-masked]
Date: Thu, 18 Nov 1999 09:51:48 +0530

[giacomon]

> Hi,
>
> Assume packet size = p bits. (note that p is only useful payload)
> Assume bit error rate = b (after taking into account the advantage gained by
> FEC, etc.)
>
> PER = 1 - (1 - b)^^p (^^ Indicates exponential)(superscript:  )
> For small b, this comes to approximately bp.
>

I agree with you,
but then the question is:
"what is the typical value of b after taking
the advantage of FEC into account"?

[asaha] By my experience, 10^^-5 is a reasonable figure. Depends a lot on
the satellite link budget and type of the terminal. For a mobile terminal
10^^-5 is probably pretty good, whereas for a fixed terminal with
a directional antenna it can be two orders of magnitude better (10^^-7 ??).

As Lloyd Wood and others have pointed out, this figure gives the raw packet
error rate. The impact on the transport protocol (TCP, what have you) is
different. For example, if your TCP MTU is say 576 octets, and the size of
the packets at layer 2 over the radio link are 100 octets we could compute:
(a) approx 40% of the TCP packets would be less than 100 octets. These map
to acks, HTTP get messages, cache validation queries, etc. These would show
the b*p packet error rate.
(b) The rest of the TCP packets, corresponding to HTTP data, FTP data, etc.
would be the size of the MTU. These would be broken up into an average of,
say, 5.5 packets over the radio link. These would show a packet error rate
of 5.5*b*p.

You can check out the data dumps available at the lbl.gov site to get a good
idea of the TCP packet size distributions.

So overall you are getting 0.4*b*p + 0.6*5.5*b*p = 3.7*b*p

Again, it might be the case that the ack path has different characteristics
than the main transmission path. This will also have effects on the perceived
packet loss rate at the transport layer level.

Regards,
Abheek Saha

---

From: Jamshid Mahdavi <[email-masked]>
Date: 19 Nov 1999 10:02:36 -0800

Just a small nit:

[email-masked] writes:

...
> the b*p packet error rate.

The times when I have looked, I haven't seen an HTTP get message which
is smaller than 100 bytes.  I doubt if this is even possible.  Off the
top of my head, typical sizes are 200-500 bytes.

--Jamshid