Latency Scenarios of Bridged Networks

Transkript

1 Latency Scenarios of Bridged Networks Christian Boiger Real Time Communication Symposium January 2012 Munich, Germany D E G G E N D O R F U N I V E R S I T Y O F A P P L I E D S C I E N C E S

2 AVB Gen1 Mechanisms AVB Gen1 supports 2 traffic classes (Class A and Class B) Class A stream traffic has the highest priority (above Class B and strict priority traffic) Max 75% of the bandwidth can be allocated for AVB streams The packets (Class A) are transmitted with a rate of 8000 packets/s (i.e. one packet every 125µs) The packets are shaped by the Credit Based Shaper (CBS) in order to prevent bursts (and allow non AVB stream traffic) 2

3 AVB Gen1 Class A Credit Based Shaper (CBS) ingress: credit sendslope idleslope egress: 2/0 1/0 0/0 2/1 1/1 0/1 3

4 AVB Gen1 Class A Credit Based Shaper (CBS) ingress: credit idleslope sendslope egress: 2/0 1/0 interfering non AVB stream frame 0/0 2/1 1/1 0/1 4

5 AVB Gen1 Latency 12/1 11/1 10/1 9/1 8/1 7/1 6/1 5/1 4/1 3/1 2/1 1/1 0/ /0 11/0 10/0 9/0 8/0 7/0 6/0 5/0 4/0 3/0 2/0 1/0 0/ /1 11/1 10/1 9/1 8/1 7/1 6/1 5/1 4/1 3/1 2/1 1/1 0/1 12/2 11/2 10/2 9/2 8/2 7/2 6/2 5/2 4/2 3/2 2/2 1/2 0/2 12/3 11/ /0 11/0 10/0 9/0 8/0 7/0 6/0 5/0 4/0 3/0 2/0 1/0 interfering non AVB stream frame 0/0 12/1 11/1 10/1 9/1 8/1 7/1 6/1 5/1 4/1 3/1 2/1 1/1 0/1 12/2 11/2 10/2 9/2 8/2 7/2 6/2 5/2 Equation: Max Latency(Talker) = tdevice + t(maxframesize+preamble+sfd+ipg) + (t(allstreams)-t(maxframe+preamble+sfd+ipg))* trate/abandwidth + t(maxframe+preamble+sfd) Worst case talker latency: Fast Ethernet: µs Gigabit Ethernet: µs 5

6 AVB Gen1 Network Latency Class A latency goal: 2ms / 7 hop T Talker L Listener T B B B B B B L B Bridge The general assumption is that the average bridge latency equals the worst case talker latency But many crossing streams in big networks can increase the latency beyond 2ms / 7 hop The CBS guarantees a deterministic latency Jitter can be in the same order than the latency Class B supports 50ms / 7 hops 6

7 Possible Latency Improvements 7

8 AVB Gen2 Proposals Pre-Emption 8

9 credit H o c h s c h u l e D e g g e n d o r f H o c h s c h u l e f ü r a n g e w a n d t e W i s s e n s c h a f t e n AVB Gen2 Proposals Credit Based Shaper with Pre- Emption /0 11/0 10/0 9/0 8/0 7/0 6/0 5/0 4/0 3/0 2/0 1/0 non AVB stream frame 0/0 12/1 11/1 10/1 9/ /0 11/0 10/0 9/0 8/0 7/0 6/0 5/0 4/0 3/0 2/0 1/0 #1 0/0 12/1 11/1 10/1 9/1 #2 8/1 7/1 6/1 5/1 4/1 #3 3/1 2/1 1/1 0/1 12/2 #4 11/2 non AVB stream frame 9

10 AVB Gen2 Proposals Credit Based Shaper with Pre- Emption Worst case talker latency (Class A): Fast Ethernet: µs (AVB Gen 1: µs) Gigabit Ethernet: µs (AVB Gen 1: µs) Less problems in bigger networks, especially Fast Ethernet networks But still not really Ultra-Low Latency (UL) 10

11 AVB Gen2 Proposals Time Aware Shaper Ingress: non Egress: non Ingress: non Egress: non t0 tmaxframe t0 11

12 AVB Gen2 Proposals Time Aware Shaper Ingress: non 125 µs µs non Egress without pre-emption UL cla Egress with pre-emption non UL class frame (framelet) non (framelet) UL cla t0 tminframelet t0 t0 tpre-emption t0: is ready to be transmitted and UL class queue is de-blocked t0-tminframelet: Egress port is blocked for non UL class t0-tpre-emption: Pre-emption mechanism is activated 12

13 AVB Gen2 Proposals Time Aware Shaper Resulting latency: Latency = tdevice + tulclassframe + tjitter tdevice tdevice = device specific latency (e.g. MAC, switching delay) tulclassframe = time to transmit the tjitter= time to compensate the time synchronization error Lowest latency for store-and-forward Lowest delivery variation Fast Ethernet latency: 19.72µs/hop Gigabit Ethernet latency: 2.38µs/hop 13

14 Even Smaller Latencies with Cut-thourgh Cut-through doesn t improve the worst case latency in normal networks But cut-through improves the latency if the port is idle when the frame is ready to be transmitted Time Aware Shaping guarantees that the port is idle Even lower latencies with cut-through in combination with Time Aware Shaping Fast Ethernet: 10.24µs Gigabit Ethernet: 1.536µs 14

15 A Latency Option for Every Application Fast Ethernet per hop latencies for a 150 byte frame: Credit Based Shaper AVB Gen1: 250,28µs Credit Based Shaper w/pre-emption: µs Time Aware Shaper in the end-stations but only pre-emption in the bridges: 30.56µs ++ Time Aware Shaper: 19.72µs (jitter=1µs) Time Aware Shaper w/cut-through: 10.24µs tdevice = 5.12 µs; min framelet = 128 byte; cutthrough time = 5.12µs; cable delay not included 15

16 A Latency Option for Every Application Gigabit Ethernet per hop latencies for a 150 byte frame: Credit Based Shaper AVB Gen1: µs Credit Based Shaper w/pre-emption: µs Time Aware Shaper in the end-stations but only pre-emption in the bridges : 3.57µs ++ Time Aware Shaper: 2.38µs (very low jitter) Time Aware Shaper w/cut-through: 1.536µs tdevice = µs; min framelet = 128 byte; cut-through time = 0.512µs; cable delay not included 16

17 Thank You 17