Subject: guaranteed service vs CBR

From: [email-masked]
Date: Fri, 14 Jul 95 19:35 EDT

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.

---

From: Zheng Wang <[email-masked]>
Date: Mon, 17 Jul 95 11:59:56 +0100

>increase if sources are allowed to send in bursts.

You are absolutely right that guaranteed services (GS) and CBR
with edge buffer provide the same worst-case delay. Under
the worst case where all flows send the maximum they are allowed,
bursts will not enter the network so there is little difference.

However, when some flows send less than what they are allowed,
bursts will enter the network and use the idle bandwidth when
available.

In some sense, it is similar to work-conserving vs non-work-conserving.
Under the worst case, there is little difference since the
packets will depart at the same time. When there is spare
resources, the packets under work-conserving will depart
earlier than that under non-work-conserving.

Cheers
Zheng

Cheers
Zheng