Subject: Bandwith on Demand Subnetworks

From: Marie-Jose Montpetit <[email-masked]>
Date: Mon, 2 Aug 1999 14:41:44 -0700

This is a piece of text that Aaron Falk and I have put together for
inclusion, after discussion, to the pilc LINK draft to address Bandwidth on
Demand aspects.

Marie-Jose
_______________________________________________________________

"Bandwidth on Demand" Subnetworks
 
Wireless networks, including both satellite and terrestrial, may use
Bandwidth on Demand (BoD). Bandwidth on demand, which is implemented at the
link layer by Demand Assignment Multiple Access (DAMA) in TDMA systems, is
currently one of the proposed mechanism to efficiently share limited
spectrum resources amongst a large number of users.

The design parameters for BoD are similar to those in connection oriented
subnetworks however the implementation may be very different.  In BoD, the
user typically requests access to 
the shared channel for some duration, in the case of a rate-based mechanism,
or for a certain capacity (or volume) in the case of volume-based
mechanisms.

Hence, access may be allocated in terms of a period of time at a specific
rate (explicit request), a certain number of packets (volume request), or
until the user chooses to release the channel (explicit then implicit
request). Access may be coordinated through a central management entity
located at a network management center. It may also be shared amongst a
number of management entities, for example with a BoD processor on board the
satellite under the control of the network manager, or through using a
distributed algorithm amongst the users
using a round robin or hierarchical round robin distribution mechanism.

The resource shared may be a terrestrial wireless hop, a satellite uplink,
or an end-to-end satellite channel. Long delay BoD subnets pose problems
similar to the Connection Oriented
networks in terms of anticipating traffic arrivals. While connection
oriented subnets hold idle channels open expecting new data to arrive, BoD
subnets request channel access based on buffer occupancy (or expected buffer
occupancy) on the sending port. Poor performance will likely result if the
sender does not anticipate additional traffic arriving at that port during
the time it takes to grant a transmission request.

In effect, this is compounded by the segmentation of packets entering the
BoD subnet. A request may cover only one of the packet segments and the last
segments may experience a delay jitter of the full round trip to the BoD
processor before being sent (this if capacity is available).  It is 
recommended that the algorithm have the capability to extend a hold on the
channel for data
that has arrived after the original request was generated.  This may be done
by
piggybacking new requests on user data or by ensuring the signaling cycle to
be short enough to support high variability in the traffic statistics.

There are a wide variety of BoD protocols available and there has been
relatively little comprehensive research on the interactions  between the
BoD mechanisms and Internet protocol performance.

Issues that have to be addressed include:
- requesting methods to efficiently use the channel while allowing TCP to
attain full throughput especially in aggregated requesting schemes; this
addresses buffering and queue management in the requesting terminal and
should avoid snooping TCP but include some forms of statistical prediction
of traffic load
- effects of the signaling cycle on the slow start congestion avoidance
mechanisms of TCP: while the requests are processed the TCP will fill the
available capacity and move to congestion avoidance until more capacity is
made available this is a driver towards reducing BoD delay when available
- effect of multiple access on the performances: A tradeoff exists balancing
the time a user can be allowed to hold a channel to drain port buffers with
the additional imposed latency on other users who are forced to wait to get
access to the channel. 
- effects of BoD on long term correlation of IP aggregated traffic; recent
work has shown that BoD does not considerably reduce long range dependence
hence that buffering will need to be higher than predicted a impact
especially for onboard satellite switches

Common to all these issues is the desirable goal of designing mechanisms
that constrain
the BoD imposed latency and latency variation. This will be helpful in
preventing spurious timeouts from TCP and allow TCP connections to take full
advantage of the offered capacity.

References:

Celandroni N., Ferro E., Potorti F., and Maral G. Delay Analysis for
Inter-Lan 
Traffic Using Two Suitable TDMA Satellite Access Schemes. Int. Journal of
Satellite Communications, 15:141--153, 1997. 

Ors T., Sun Z., and Evans B.G. Analysis of a Mac Protocol To Guarantee Qos
for ATM over Satellite. Proceedings of ICC'98,  Atlanta, June 1998. 

Zeint T., Maral G., Brefort T., and Tondriaux M. Performance of
Combined/Fixed Reservation Assignment (CFRA) Scheme for Aggregate Traffic.
in COST  226 Integrated Space/Terrestrial Networks Final Symposium, Pages
183--199, Budapest, Hungary, 10-12 May 1995.

Hung, A, Montpetit, M.J. and Kesidis, G. "ATM via Satellite: Framework and
Implementation" Journal of Wireless Networks, 1998.

Qureshi, A. "The Effects of Bandwidth on Demand Protocols on Long-Range
Dependent Traffic Statistics." Workshop on Satellite Networks, ICC'99,
Vancouver, June 1999. 

__________________________________________
Marie-Jose Montpetit
Network Architecture
Teledesic LLC
1445 120th Ave NE Bellevue WA 98005
tel: +1-425-602-6594
fax: +1-425-602-6595
[email-masked]
www.teledesic.com