1. Cluster-based Multihop Networking with Controlled Quality of Service
March 2002
Cluster-based Multihop Networking with Controlled Quality of Service
Oliver Mülhens , Jörg Habetha, Wolfgang O. Budde
Philips Research, Aachen, Germany
Contact:

2. March 2002
Outline
The need for multi-cluster multi-hop communication in QoS-supporting BSSs (QBSSs)
Coverage area extension of a QBSS:  Forwarding mechanisms
Interconnection of QBSSs  Multi-hop ad hoc networking
Conclusions
Next steps

3. The need for multi-cluster multi-hop QoS-controlled WLANs
March 2002
The need for multi-cluster multi-hop QoS-controlled WLANs
Wireless QoS usually achieved through centrally-controlled clusters, e.g e QBSS
But QBSSs’ coverage area is limited to the radio range of the Hybrid Coordinator (HC)
Overall capacity for QoS-controlled traffic is given by the capacity of a single channel
Multi-cluster multi-hop communication is a necessity for QoS-enhanced wireless networks
Standardization needs to be started now!

4. Preface Multi-hop wireless networking… Our mission is to
March 2002
Preface
Multi-hop wireless networking…
is not just a particular improvement of a WLAN parameter or single implementation aspect
but adds a new dimension to next-gen WLANs
includes, but is not limited to, a/e/h
Our mission is to
Create awareness of the Multi-hop concept and its potential
Start a persistent discussion thread

5. Synopsis: Benefits of Multi-Cluster Multi-Hop QoS-controlled WLANs
March 2002
Synopsis: Benefits of Multi-Cluster Multi-Hop QoS-controlled WLANs
Coverage extension of QBSSs
Capacity increase due to smart clustering
E.g. different from single-channel IBSS
Wireless interconnection of QBSSs
More efficient use of spectrum in case of multi-hop connections (no collisions)
Can be built upon existing extensions
802.11e: QoS enhancements
802.11h: Frequency agility

6. Problem I: Limited coverage area of QBSSs
March 2002
Problem I: Limited coverage area of QBSSs
The higher the radio frequency, ...
the more line-of-sight the propagation,
the higher the free-space loss,
the higher the attenuation due to walls and other obstacles,
thus, the smaller the coverage area

7. Solution: Extension of Coverage Area by means of Forwarding
March 2002
Solution: Extension of Coverage Area by means of Forwarding
Coverage area of HC’s QoS management increased through forwarding of control information
City scenario: HC FN
QBSS coverage area increased through forwarding of user traffic
FN: Forwarding Node (FQSTA)

8. Problem II: Interconnection of QBSSs
March 2002
Problem II: Interconnection of QBSSs
As of today, QBSSs can only be interconnected through wired infrastructure
A-priori network planning required
Rather limited ad-hoc capabilities
No multi-hop mode defined for QBSSs

9. Solution: Multi-Channel Cluster Bridges
March 2002
Solution: Multi-Channel Cluster Bridges
A cluster bridge (CB) associates with overlapping QBSSs operating in different channels
Part-time presence in either of the two BSSs
Throughput optimization by predefined medium access opportunities
Requires some buffer space for transportation of packets from one BSS into the other

10. Properties of Clustered Wireless Networks
March 2002
Properties of Clustered Wireless Networks
The smaller the clusters, the more channels can be used in a given area (re-use)
Advantage: offered traffic per channel decreases
Option: spatial diversity improves re-use factor
The smaller the cluster, the higher modulation/coding can be used
Advantage: increased channel capacity
The smaller the cluster, the more hops needed
Dynamic clustering helps to maintain efficiency

11. Ex.: Capacity Increase over # Clusters
March 2002
Ex.: Capacity Increase over # Clusters
= Number of Clusters

12. Clustering in 802.11 Applicable diversity techniques
March 2002
Clustering in
Applicable diversity techniques
Frequency (channel)
Space
Code
Compliant with latest extensions
802.11h: frequency agility
802.11e: QoS enhancements (periodic beacons, reserved TXOPs)

13. March 2002
I. Coverage Extension
The forwarding QSTA (FQSTA) is a transparent representative of the HC
Selection and hand-off of FQSTA and HC.
QoS is guaranteed
Bandwidth-efficient
FQSTA HC

14. II. Wireless Interconnection of BSSs
March 2002
II. Wireless Interconnection of BSSs
Cluster 1
(frequency 1) HC CB HC
Cluster 2
(frequency 2)
CB: Cluster Bridge
HC: Hybrid Coordinator

15. III. Multi-Freq. Forwarding Example
March 2002
III. Multi-Freq. Forwarding Example
PCs/HCs operate on different frequencies
FQSTAs/CBs switch from one frequency to the other
FQSTAs/CBs switch during (E)DCF-phase
Switching Times TS and Waiting Times TW will occur
Freq 1
Freq 2
FQSTA/CB
Freq 1
Frq 2
Freq 1 T T T T S W S W

16. Work Items to be tackled
March 2002
Work Items to be tackled T4 T5
Dynamic clustering
Signalling procedures
Membership management
Interconnection of clusters
Forwarding procedure
Choice of forwarding stations
CB-installation and hand-off
Routing of packets
Type of algorithm
Address resolution T6
HC1 T3 T2 T1 T7 T8
HC2 T9
T11
T10
T12
HC3
T16
T13
T15
T14

17. March 2002
Conclusions
Multi-cluster multi-hop communication is indispensable for QoS-enhanced wireless networks
Centralized solutions based on PC/HC and Forwarding Stations / Cluster Bridges have been roughly sketched
Compliance with current features and extensions shown
Coverage extension and capacity increase for QoS-controlled networks is feasible

18. Next Steps... Discussion and standardization has to be started now
March 2002
Next Steps...
Discussion and standardization has to be started now
802.11a is in initial deployment
802.11e/h are settling these days
The road towards coverage extension and capacity increase must be presented in short term
Experience from simulation exists
Join forces!