Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 Simulating Dynamic Bandwidth Allocation on Satellite Links Alberto Gotta, Francesco Potortì, Raffaello Secchi Italian National Research Council (CNR) Istituto di Scienza e Tecnologie dellInformazione A. Faedo (ISTI)

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 Summary DVB-RCS overview Architecture of Tdma-dama –Extension to NS2 –Demand Allocation Multiple Access –Rate Based allocation policy Validation & Measurements Conclusions

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 DVB-RCS system DVB-RCS (ETSI EN ) –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

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 DVB-RCS network elements TT RNCC NCC RNCC Gateway Forward Link Return Link

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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.)

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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)

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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.

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 Diagram of Tdma-dama MAC buffer Requester agent TDMA fragmentation and reassembly Traffic terminal satellite link PHY Tx/Rx Allocator agent Satellite network Terrestrial network

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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 nameMeaning 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

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 Test-bed platform for validation DVB-RCS DVB-S channel capacity2.1 Mb/s active stations18 available bandwidth~1.2 Mb/s slots per frame48 frame duration273 ms bandwidth granularity44 kb/s guaranteed bandwidth44 kb/s mean RTT840 ms testbed specifications LAN Ethernet LAN Ethernet Medium Access Control TDMA/TDM The bandwidth allocation uses a DAMA scheme It implements a variant of Rate Based Dynamic Capacity (RBDC)

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 Throughput Comparison (ON/OFF traffic)

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 Delay Comparison (ON/OFF traffic)

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 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

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 How to get the software? The software is provided under a MIT free software license