TOWARD A COMPLETE CONTROL
FRAMEWORK FOR ADAPTIVE CAPACITY
ALLOCATION IN MOBILE DVB-RCS
Nedo Celandroni*, Franco Davoli°, Erina Ferro*,
Alberto Gotta*, Raffaello Secchi*
*ISTI-CNR, Area della Ricerca del C.N.R., Via Moruzzi 1,
I-56124 Pisa, Italy
°DIST-University of Genoa and CNIT (Italian National
Consortium for Telecommunications), Italy
September 26th 2007
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Objectives
• We address the general problem of controlling and optimizing
access and bandwidth sharing among different applications in
mobile DVB-RCS
• This work deals explicitly with the dynamic assignment of
bandwidth to elastic (TCP) traffic on the uplink. Different access
techniques are compared:
– Random Access: Diversity Slotted Aloha (DSA)
– Deterministic Access: Demand Assignment Multiple Access (DAMA)
• The convenience of one technique with respect to the other as
concerns TCP performance is determined by:
– Bandwidth Cost per connection
• Total number of slots allocated in the case of DAMA
• Data successfully received in DSA case divided by DSA throughput
– Average Completion Time
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MUltimedia MObile Bandwidth
ALlocation (MUMOBAL) architecture
•
Class-based Dynamic Bandwidth Allocation
–
•
One portion of bandwidth for each typology of traffic and moving boundaries between
bandwidth portions
In each portion, the Network Control Centre (NCC) assigns bandwidth to the relevant
traffic classes, by taking into account their QoS requirements and traffic characteristics
•Collision free Optimal controller:
• Optimal DBA Control Algorithm
for Service Level Agreement
•Collision free DAMA:
• Rate Based Dynamic Capacity
• Volume Based Dynamic Capacity
• FCA option
•Contention access DSA:
• Contention-based MAC
Collision free
Optimal Controller
Collision free
Optimal Controller
September 26th 2007
•EF traffic:
• Voice, video & Real Time
applications with QoS requirements
•Large volume AF & BE traffic:
• web applications, video streaming,
interactive and peer-to-peer, etc...
•Small volume BE traffic:
• web-browsing, e-mail, signaling, etc…
Collision free
DAMA
Collision free
DAMA
Contention access
DSA
Contention access
DSA
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DAMA scheme
•
DAMA scheme adopted resembles the FIFO Ordered Demand Assignment
(FODA)
– N. Celandroni, E. Ferro, “The FODA-TDMA satellite access scheme:
presentation, study of the system and results”, IEEE Transactions on
Communications, December 1991
•
•
The bandwidth requests, contained in each packet header, is done by each
station taking into account the instantaneous queue size and a term
proportional to the incoming traffic
The NCC performs the bandwidth assignment by cyclically scanning all
requests and giving each station an amount of bandwidth proportional to the
request
– The assigned amount of bandwidth is subtracted from the request for the next
assignment cycle; successive requests coming from the stations override the
current residual values of the previous ones
– The length of the cycle is dynamically varied according to the overall system load
– A lower bound on the assignment is set as a minimum guaranteed bandwidth,
whereas an upper bound is given by the capacity of a carrier
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Diversity Slotted Aloha (1/2)
i-th frame
time
time slot
DSA for k=2
A packet is sent twice
station #1
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station #2
station #3
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Diversity Slotted Aloha (2/2)
•
The NCC maintains a target
throughput by allocating the
number of DSA slots per frame that
are necessary to achieve it; it
corresponds to the average
number of successful slots in a
frame divided by the target
throughput itself
•
For design purposes, we can
consider the asymptotic values of
DSA collision probability and
throughput as functions of G, the
number of users per slot, and k, the
number of copies delivered
September 26th 2007

pc(k )  1 ekG
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
k
 (k )  G  1  pc(k ) 
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Case study: mobile user acts as an FTP
client
•
Assumptions
– Users are synchronized with the system and receive the burst time plan, sent
at each frame time by the NCC in TDM mode (DVB-S2)
– Any user is allowed to transmit in a frame only upon receiving a valid burst
time plan for that frame
•
DAMA first access & connection termination
– A user issues the initial request for bandwidth on reception of a SYN
message from the TCP server by sending a request packet in the DSA area
– Successive requests for bandwidth are piggy-backed with data
– A user releases the bandwidth when the data transfer is completed
•
System parameters
– DAMA: the minimum amount of bandwidth equal to one slot every m frames
allocated by the NCC upon the reception of the access request
– DSA: the target throughput and, consequently, the collision probability that
derives by the best choice of the k value
• Target throughput 10%  k=4, collision probability = 0.012
• Target throughput 20%  k=3, collision probability = 0.121
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Simulative Scenario (ns-2)
LMS channel parameters
TDMA/DSA
DOWNLINK: TDM
UPLINK:
ON/OFF
mobile
channel
TNB
TBL
Blocking
prob.
Highway
3.02 s
0.36 s
0.107
Rural
2.03 s
0.55 s
0.215
Environment
HUB station
SERVER
Terrestrial
Segment (50
ms)
CLIENT
Internet
Gateway
RTT=600 ms, Segment size=1416B (equivalent to 8
DVB packets), Frame size=20 ms, TCP/Sack
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Short-lived connections (highway)
Average Uplink Bandwidth Cost
700
DSA 10%
DSA 20%
TDMA 1f/s
TDMA 2f/s
TDMA 4f/s
TDMA 8f/s
500
400
12
Completion Time [s]
DVB packets/connection
600
Average Completion Time
14
300
200
10
8
DSA 10%
DSA 20%
TDMA 1f/s
TDMA 2f/s
TDMA 4f/s
TDMA 8f/s
6
4
100
2
0
0
20
40
60
80
0
Connection Length [kB]
September 26th 2007
20
40
60
80
Connection Length [kB]
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Short-lived connections (rural)
Average Uplink Bandwidth Cost
700
DSA 10%
DSA 20%
TDMA 1f/s
TDMA 2f/s
TDMA 4f/s
TDMA 8f/s
500
400
22
Completion Time [s]
DVB packets/connection
600
Average Completion Time
300
200
17
DSA 10%
DSA 20%
TDMA 1f/s
TDMA 2f/s
TDMA 4f/s
TDMA 8f/s
12
7
100
2
0
0
20
40
60
80
0
40
80
60
Connection Length [kB]
Connection Length [kB]
September 26th 2007
20
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Long-lived connections (highway)
45000
40000
35000
30000
25000
20000
15000
10000
5000
0
1600
1400
DSA
TDMA
DSA
1200
TDMA
1000
800
600
400
200
0
10% 20%
•
Completion Time
duration [s]
DVB packets/connection
Bandwidth Cost
1 f/s 2 f/s 4 f/s 8 f/s
10% 20%
1 f/s 2 f/s 4 f/s 8 f/s
DSA may not be suitable for long-lived connections in highway
–
DSA 10% is outperformed by DAMA 1f/s in both duration and cost, while
DSA 20% has a duration very close to DAMA 4f/s, but the latter exhibits
a significantly lower cost
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Long-lived connections (rural)
Completion Time
90000
80000
3000
70000
60000
50000
40000
DSA
TDMA
30000
20000
TDMA
2000
1500
1000
500
10000
0
0
10% 20%
•
DSA
2500
duration [s]
DVB packets/connection
Bandwidth Cost
1 f/s 2 f/s 4 f/s 8 f/s
10% 20%
1 f/s 2 f/s 4 f/s 8 f/s
In a rural environment
–
DSA 10% and DAMA 2f/s exhibit very similar performance in both
duration and cost, while DSA 20% has a duration similar to DAMA 8f/s,
but the latter exhibits a significantly lower cost
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Summary of simulations results
1. The smaller the size of TCP messages, the higher the gain of DSA with
respect to DAMA in bandwidth expenditure
2. Since DSA cost depends on data successfully delivered only, it is less
sensible to changes in the channel blocking probability
3. DSA is convenient for very small transfers in terms of completion time in
both environments as well
–
the connection duration in DAMA 1f/s is comparable with DSA 10% in that
very low losses of ACKs due to collision cause negligible performance
degradations to TCP
4. Limited to our investigations, DSA is not convenient for long-lived
connections in both environments
5. The choice of system parameters is a matter of more complex
investigations (e.g. the mobile acts as server). Nevertheless, once the
parameters have been selected, cost diagrams will be used to evaluate
convenience thresholds. For instance
–
In case DAMA 4f/s and DSA10% are selected as the most convenient
options, the threshold file size is about 17 kB and 65 kB for highway and rural
environments, respectively
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Conclusions & Future works
•
We analyzed the behaviour of two different access methods, based on DSA
and DAMA respectively, in providing the basis for a FTP
– The two methods have been compared in terms of bandwidth cost and
connection completion time under the case of users acting as clients
– Simulation results have shown an advantage of DSA over DAMA up to certain
convenience thresholds in the file size as functions of DSA throughput and
DAMA m parameter
– The non compliance with the convenience thresholds would imply an increased
cost for users and, likely, higher revenue for the service provider
– DSA does not result convenient for long-lived connections in both environments
•
This study will be completed with the analysis of the case in which the
mobile user acts as a server
– Server and client cases are necessary to operate the choice of the target
throughput in DSA and the m factor in DAMA
– For the server case, in order to obviate to the collisions’ effect in DSA, a packet
level FEC might be necessary
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