The L*IP Access System

Transcription

1 *IP Satellite System The *IP Access System Prototype built for ESA, ARTES-5 contract Meshed MF-TDMA, over GEO Optimized for IP QoS DAMA MF-TDMA modem supports up to 4 Msymb/s QPSK, Turbo codec Fade mitigation techniques A ar Satellite etwork, e.g. DVB/RCS A Satellite etwork with Meshed Topology, like *IP Comparison ar versus Meshed Topology in Satellite etworks Problems of star topology with DVB as forward channel: fixed allocation of forward / return bandwidths double hop for communication between 2 user terminals twice the delay and twice the bandwidth considerable large minimum bandwidth for DVB expensive hub, even for small networks but: receiver in terminals are cheap (mass market) Reference Scenario of the *IP Meshed Satellite etwork Meshed: flexible bandwidth allocation in forward and return single hop between slaves but: slaves are more complex

2 Subdivision of the Satellite Resource in Time and Frequency Subdivision of the Satellite Resource in Time and Frequency (2) Carriers can have different size: 1, 2 or 4 Msymb/s Support of stations with different equipment (antenna, amplifier) For example: a low cost station can only transmit on a 1 Msymb/s carrier frame slot time Subdivision of the Satellite Resource in Time and Frequency (3) TDMA frame length: 20ms Time slot length 125us 160 slots per frame s length in integer multiples of the slot length Bursts can be 1 to 159 slots long Efficiency increases with the size of the! The Allocation Plan R reference AP transmitting the allocation plan BW requests from the slaves S1 S2 data from station 1 to station 2 Multiplexing on the Satellite (or: How can we increase the size?) Connection-Oriented Flow Identification Virtual Connections (VC) = IP Packet Flows defined unidirectional; specified by TX+RX station and QoS parameters S2 S3 Pipes = Burst Flows multiplexes of VCs S4 specified by TX station + RX stations Multiplexing!

3 Connection-Oriented Flow Identification (2) Multiplexing of different kind of traffic to different station into a single Demand Assignment Multiple Access (DAMA) Principle Architecture of a Terminal/ation Traffic Handling in a Terminal Packet Switching Traffic Queuing Packets wait in queues until BW is available (DAMA!) IP packets are switched to associated VCs, based on QoS class Routing (receiver station) Different queues for different QoS classes, High priority traffic may overtake Packets are dropped, if incoming traffic rate is higher than available BW

4 Traffic Policing with the Token Bucket Algorithm Traffic Scheduling / Multiplexing Execution of QoS parameters TMR: Traffic mean rate MBS: Maximum size Incoming packets may pass the server, if enough tokens are available Question: Which queue should be served next? Decision based on QoS parameters of the VC Traffic s up to MBS are accepted On the average, traffic rate is limited to TMR Fragmentation & Reassembly (Encapsulation) TDMA Burst Format Burst length is in most cases different from IP packet length IP packets have to be fragmented at the transmitter and reassembled at the receiver Encapsulation introduces additional overhead Preamble Burst Payload TDMA Burst Guard Time ATM/AA5 Encapsulation ATM Adaptation ayer 5 (AA 5) Payload (IP packet) PA SAR D Tr. 48 Bytes 48 Bytes 48 Bytes 48 Bytes UU CPI ength CRC UU User to user indication 8 bit CPI Common part indicator 8 bit ength ength of payload 16 bit CRC Cyclic Redundancy Check 32 bit

5 Format of the ATM Cell Unidirectional ightweight Encapsulation (UE) 5 byte header 48 byte payload VPI VCI PT C HEC data P 1 8 bit VPI Virtual Path Identifier 12 bit VCI Virtual Circuit Identifier 16 bit PT Payload Type 3 bit CP Cell oss Priority 1 bit HEC Head Error Control 8 bit SDU Format of UE ow Overhead Encapsulation (OE) D length type CRC-32 Destination address present flag (1 bit) length of the (15 bit) type of the (16 bit IAA EtherType) Protocol Data Unit cyclic redundancy checksum (32 bit) OE - SDU Format OE - Fragment Format Fragment Header (4 bytes) Payload ( bytes) type 2 byte CRC byte VCD B o S E o S E 11 HEC 8 data type CRC-32 Protocol Type (optional) Cyclic redundancy checksum (32 bit) VCD BoS EoS E HEC Virtual Connection Descriptor Begin of SDU End of SDU ength of payload in Bytes Header Error Control

6 and encaps. Overhead 25% 20% 15% 10% 5% 0% Comparison of ATM, UE and OE average payload size [bytes] ATM UE OE Comparison of ATM, UE and OE (2) ATM is due to its large header compared to its small body inefficient UE is nearly as efficient as OE, but can only support sizes that are multiples of the MPEG-2 cell size results in efficiency problems with VoIP traffic OE is most efficient and supports any size fits into the allocation plan grid with fixed slot sizes Calculation of the Total Bandwidth Calculation of the signaling overhead (ref, allocation plan, BW requests) Calculation of the overhead ( unique word, guard time) Calculation of the encapsulation overhead (fragment header, SDU overhead) Assumption: Each active pipe is assigned one per frame How many pipes do we need? umber of stations RCVp Average number of receivers per station Pp Average number of pipes per station B tot Total bandwidth in the satellite network Chn umber of carriers k Rate of over-admission B pipe RCVp k B = B pipe tot Pp B tot chn Pp k chn RCVp How many pipes do we need? (2) =100 RCVp = 3 Chn =10 k chn RCVp Pp 1. 5 k = 5 Signaling Overhead ref ength of reference AP ength of AP BWreq ength of BW request ote that this is a lower bound on the number of pipes In practice, 2-3 pipes per station are realistic ref = const AP ~ BWreq ~ Pp Pp O sig f = (,, ) ref B AP tot BWreq

7 signaling overhead Signaling Overhead 12% 10% 8% 6% 4% 2% 0% available bandwidth per station [kbit/s] Burst Overhead Preamble Burst Payload TDMA Burst Average length of Bursts b Equivalent overhead bits b O = Guard Time Burst Overhead (2) η umber of s per second Fr umber of frames per second sig = 1 O sig Signaling efficiency = Pp sig B = η tot Fr overhead 12% 10% Burst Overhead (3) 8% 6% 4% 2% 0% payload [bytes] OE - Overhead Calculation Average length of b SDU umber of SDU overhead bits n frg Average number of fragments per b frg Overhead bits of fragment header Average length of b Burst overhead bits Total Overhead Calculation ηsig = 1 O sig Signaling efficiency η =1 O Burst efficiency ηoe =1 O OE OE efficiency n frg + b b SDU +1 O OE =1 + b SDU + b frg n frg Otot =1 η η sig η OE

8 total MAC overhead Total Overhead 15% 10% 5% 0% average available bandw idth per station [kbit/s]