Subject: guaranteed service vs CBR

From: "Roch Guerin ((914) 784-7038)" <[email-masked]>
Date: Fri, 14 Jul 95 19:54:02 EDT

Reference:  Your note of Fri, 14 Jul 95 19:35 EDT

Correct for the very limited set of service disciplines your assumptions
allow, but incorrect for general policies.  Btw, you need at least
D_i > b_i/r_i and even that may not suffice if you take packetization
into account (error terms).  In any case, this is quite restrictive.
Roch

*************** Referenced Note ***************

From: tse at research.att.com
Date: Fri, 14 Jul 95 19:35 EDT
To: [email-masked]
Subject: guaranteed service vs CBR

Hopefully this will clarify some of the current discussions
comparing guaranteed service with CBR having buffers at network edge.

Assumptions:

1) Each user has leaky bucket parameters (b_i,r_i), and a hard
network delay requirement D_i.

2) Each network element provides a share of bandwidth
r_i to user i (i.e. user i is entitled to this much whenever he
wants it.)

3) We want to guarantee zero loss (due to violation of delay
bound) to each user, even in the worst-case.

Then the maximum number of users that can be admitted into such a
network is the same as the number that can be admitted into a
CBR network with the same link bandwidth, no network buffering,
and buffering of size b_i for each user at the edge.

This result follows from the facts that 1) each link has to have
bandwidth at least the sum of the r_i's for all the users sharing that
link; 2) the worst case network delay experienced by user i is b_i/r_i
(Parekh and Gallager). But this is also the worst case delay if
user i is instead given a CBR bandwidth r_i throughout its route and a
network edge buffer of size b_i. Thus, a CBR network with the same
link bandwidths can provide the same worst-case delay, and hence
can do just as well. Thus, the utilization of the network does NOT
increase if sources are allowed to send in bursts.

The reason that there is no gain in performance is because
we have a hard delay requirement and zero loss requirement,
and hence only interested in the worst-case delay, which occurs
when the sources conspire to burst. If we relax either of these
requirements, sharing buffers will help.