Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Simulating Dynamic Bandwidth Allocation on Satellite Links
Alberto Gotta, Francesco Potortì, Raffaello Secchi
Italian National Research Council (CNR)Istituto di Scienza e Tecnologie dell’Informazione “A. Faedo” (ISTI)
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Summary
• DVB-RCS overview
• Architecture of Tdma-dama– Extension to NS2– Demand Allocation Multiple Access– Rate Based allocation policy
• Validation & Measurements
• Conclusions
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
DVB-RCS system
• DVB-RCS (ETSI EN 301 790)
– Bidirectional channels over satellite links (Meshed Networks)
– It defines an MF-TDMA medium access control
– It guarantees IP/DVB connectivity via satellite links
RETURN LINK
DVB-RCS
FORWARD LINK
DVB-S
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
DVB-RCS network elements
TTRNCC
NCC
RNCC
Gateway
Forward Link
ReturnLink
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Tdma-dama objectives
• Deployment of DVB-RCS features into ns-2– Integration of TDMA access scheme into the ns-2
satellite environment– Development of bandwidth allocation methods (CRA,
RBDC, VBDC, FCA)
• Study of the interaction between transport and satellite MAC layer– Congestion control (TCP, TCP friendly) over satellite
links with dynamic bandwidth allocation– Impact of MAC delay over delay-critical applications
(e.g VoIP, video-conferencing, etc.)
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Tdma-dama in ns-2
• The current satellite module of ns-2 supports both geostationary satellite and polar orbiting LEO constellations with multiple users per downlink/uplink and asymmetric links
• Tdma-dama is a module in the MAC layer block which supports dynamic bandwidth allocation mechanisms – IP packets fragmentation and encapsulation into a
TDMA frame– Fully customizable from Otcl Interface
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
DAMA allocation principle
• Each traffic terminal periodically submits to a master terminal an allocation request based on the incoming traffic rate or traffic backlog at the terminal
Traffic Terminals
(Requesters) Master(Allocator)request
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
DAMA allocation principle
allocation
• The master station collects the request of all the active traffic terminals, computes a burst time plan and broadcasts it back to the terminals
Traffic Terminals
(Requesters) Master(Allocator)
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Drawbacks of Dynamic Allocation
• The time interval between the TTs requesting a given bandwidth and the system assigning it is called allocation delay
• Due to the large allocation delay on the satellite link, we can observe that the transmission delay dramatically increases when the rate of traffic incoming to a TT varies rapidly
– if the input rate increases in a time shorter than the allocation delay, the excess traffic is buffered at the traffic terminal, thus increasing the queuing delay
• Our simulative analysis confirms experimental findings
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Bandwidth Allocation Methods
The following methods can be combined in many ways:• Constant Rate Assignment (CRA)
– Bandwidth is negotiated between the traffic terminal and the allocator at the beginning of each connection
• Rate Based Dynamic Capacity (RBDC)– Each Traffic terminals submits to the allocator a bandwidth
request message based on the rate of local incoming traffic
• Volume Based Dynamic Capacity (VBDC)– Each terminal dynamically signals the data volume needed to
empty its buffer
• Free Capacity Assignment (FCA)– No explicit request comes from the terminals. Unused bandwidth
is assigned automatically by the allocator to the traffic terminals according to some fairness criteria
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Tdma-dama Design
• Tdma-dama consists of two customizable C++ classes of agents– The Requester agent, one for each satellite
terminal, is responsible for calculating the bandwidth needed by the terminal and for submitting a request to the allocator.
– The Allocator agent computes the burst time plan, that is, the TDMA frame allocation for all stations.
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Diagram of Tdma-dama
MAC buffer Requester
agent
TDMAfragmentation
and reassembly
Traffic terminal
satellite link
PHY Tx/Rx
Allocatoragent
Satellite network
Terrestrial network
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Otcl Interface
• A single system-wide Allocator is installed in a terminal:set alc [$ter install-allocator Allocator/Proportional]
• One Requester is installed into each terminal: set req [$ter install-requester Requester/RDBC]
• The definition of Requester and Allocator must specify the subclass of the objects. The object name can be used at run time to dynamically change the properties of Allocator and Requester
Variable name Meaning
max_slot_num_ slots per frame
slot_packet_len_ size of a slot
num_frame_ allocation period
bandwidth_ satellite link capacity
main parameters
• The MAC/TDMA block is created through the procedure node-config consistently with the standard ns-2 procedure and configured through a set of class parameters
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Example: RBDC Requester
• A generic rate- and volume-based request:
where R(t) is the mean incoming traffic rate and q(t) is the incoming queue length at the requesting terminal
• This expression can be interpreted as a proportional-integral controller of output rate (q(t) being the integral part), which improves the stability of the queue
• Stability can be further improved by providing a minimum share to each station in addition to the requested bandwidth
sreq T
tqtRtR
)()()(
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Test-bed platform for validation
DVB-RCS DVB-S
channel capacity 2.1 Mb/s
active stations 18
available bandwidth ~1.2 Mb/s
slots per frame 48
frame duration 273 ms
bandwidth granularity 44 kb/s
guaranteed bandwidth 44 kb/s
mean RTT 840 ms
testbed specifications
LANEthernet
LANEthernet
• Medium Access Control TDMA/TDM
• The bandwidth allocation uses a DAMA scheme
• It implements a variant of Rate Based Dynamic Capacity (RBDC)
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Throughput Comparison (ON/OFF traffic)
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Delay Comparison (ON/OFF traffic)
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
Conclusions
• We described Tdma-dama, a simulator of TDMA satellite MAC based on ns-2 that allows investigation of a large class of DAMA schemes, including those outlined in the DVB-RCS standard
• We validated the simulator through measurements performed on the Skyplex Data network, which implements a variant of the RBDC allocation mechanism– The statistics considered were throughput and one-way delay – The simulator was able to accurately reproduce the dynamics of the
real system
• The simulations highlight that DAMA algorithms are responsible for the large delay (several times the propagation delay) that may occur in response to abrupt variations of input rate
Raffaello Secchi – WNS2 Istituto di Scienza e Tecnologie
dell’Informazione “A Faedo”Pisa –October 10th 2006
How to get the software?
The software is provided under a MIT free software license
http://votos.isti.cnr.it/simulator.htm