Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 Simulating Dynamic Bandwidth Allocation on Satellite

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)



Summary

• DVB-RCS overview

• Architecture of Tdma-dama– Extension to NS2– Demand Allocation Multiple Access– Rate Based allocation policy

• Validation & Measurements

• Conclusions



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



DVB-RCS network elements

TTRNCC

NCC

RNCC

Gateway

Forward Link

ReturnLink



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.)



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



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



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)



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



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



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.



Diagram of Tdma-dama

MAC buffer Requester

agent

TDMAfragmentation

and reassembly

Traffic terminal

satellite link

PHY Tx/Rx

Allocatoragent

Satellite network

Terrestrial network



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



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

)()()(



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)



Throughput Comparison (ON/OFF traffic)



Delay Comparison (ON/OFF traffic)



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



How to get the software?

The software is provided under a MIT free software license

http://votos.isti.cnr.it/simulator.htm

Documents

Istituto di Scienza e Tecnologie dellInformazione A Faedo Pisa –October 10th 2006Raffaello Secchi – WNS2 Simulating Dynamic Bandwidth Allocation on Satellite