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Patch Panels in the Sky:
A Case for Free-Space Optics
in Data Centers
Navid Hamed Azimi, Himanshu Gupta
Vyas Sekar, Samir Das
Datacenter network design is hard!
Performance
Cost
Energy
Cabling
Expandability
Cooling
Adaptability
2
Existing Choices
Cost
Core
Top of Rack (ToR)
…
Over subscribed
(tree, leaf spine,…)
Performance
3
Existing Choices
Cost
Core
Top of Rack (ToR)
…
Over provisioned
(Fattree, Jellyfish,…)
Over subscribed
(tree, leaf spine,…)
Performance
4
Existing Choices
Cost
Over provisioned
(Fattree, Jellyfish,…)
Core
Augmented
(c-Through , 3D Beam)
Top of Rack (ToR)
…
Over subscribed
(tree, leaf spine,…)
Performance
5
Existing Choices
Cost
Over provisioned
(Fattree, Jellyfish,…)
Augmented
(c-Through , 3D Beam)
Over subscribed
(tree, leaf spine,…)
Can we
get here?
Performance
6
Our Vision
• Core  Coreless
• Wired  Wireless
• Static  Flexible
…
7
Potential Benefits of This Vision
Coreless
Wireless
Flexible
Performance
Cost
Cabling
Cooling
Energy
Expandability
Adaptability
8
Challenges in Realizing the Vision
• Flexible wireless links
• Network management
9
Outline
• Flexible wireless links
• Network management
• Feasibility & cost
10
Traditional RF is not Sufficient
…
• Wide beam
High interference
11
Traditional RF is not Sufficient
…
• Wide beam
• Difficult to narrow
High interference
12
Solution: Free Space Optical Links (FSOs)
…
• Easy to narrow beam
• No interference
• High throughput
over long range
13
Concerns with FSOs
•
•
•
•
Cost : $30K
Size : 3 ft³
Power: 60w
Flexibility
14
Why Size, Cost, Power Can be Reduced?
• Traditional use : outdoor, long haul
‒ High power
‒ Weatherproof
• Data centers: indoor, short haul
‒ Low power
• Feasible roadmap via commodity fiber optics
‒ E.g. Small form transceivers (Optical SFP)
15
Proof of Concept (1 Gbps)
Fiber connected
to SFP (TX)
Lens
Mirror
SFP (RX)
Lens
Mirror
SFP : 2x$40
Lens : 2x$30
16
FSO Link Throughput
PDF
1
0.8
0.6
Optical fiber
FSO
0.4
0.2
0
<924
924- 928
928- 932 >932 Mbps
Throughput
FSO link is the same as optical cable
17
FSO Link Misalignment Tolerance
• Effect of vibrations, etc.
Movement tolerance
– 0.7mm at destination
– 3mm at source
3 mm
TX
0.7 mm
RX
FSO link is stable
18
FSOs (1Gbps)
•
•
•
•
Cost : $30K
Size : 3 ft³
Power: 60w
Flexibility
•
•
•
•
Cost : $140
Size : 1x4 in
Power: 0.3w
Flexibility
19
Flexibility via Switchable Mirror
• Switchable Mirror: glass
• Electronic control, low latency
mirror
Ceiling Mirror
SM in “mirror”
mode
Receiver 1
Reciever2
Sender
20
Flexibility via Switchable Mirror
• Switchable Mirror: glass
• Electronic control, low latency
mirror
Ceiling Mirror
SM in “mirror”
mode
Receiver 1
Sender
Receiver 1
Reciever2
Reciever2
21
Outline
• Flexible wireless links
• Network management
• Feasibility & cost
22
Management: Pre-Configuration
Input: FSO#, SM#,…
Output: Candidate links (Alignment of each SM)
Receiver 1
Reciever2
Sender
23
Management: Reconfiguration
Input: Traffic
Output: Run-time topology
Receiver 1
Reciever2
Sender
24
Current Solutions: Simple Heuristics
• Pre-Configuration : Regular Random Graph
• Reconfiguration
Only reconfigure for large flows
Only delete a link if network remains connected
25
Outline
• Flexible laser link
• Management
• Feasibility & cost
26
Throughput
Average Throughput Per
Server (Gbps)
• 512 racks, 48 server/rack, hotspots traffic, flow level simulations
2
Fattree 2G
3D-Beamforming
1.5
c-through
1
FSO
0.5
0
(4,5)
(4,10)
(8,5)
(12,5) (8,10) (16,5) (12,10) (16,10)
(Hot Spots %, Average Load per Hot spot (Gbps))
FSO based 2-4x improvement
27
Cost Comparison
Architecture
Cost
Effective Bisection
Bandwidth
Fattree 1 Gbps
$ 13 M
1
FSO based
$ 38 M
8.5 Gbps
Fattree 10 Gbps
$ 57 M
10 Gbps
Gbps
FSO based: Near 10G performance with 65% of cost
28
Conclusions
• Data center network is challenging
• Our vision: Fully flexible, coreless, wireless network
• Key enablers : FSO + switchable mirrors
• Early promise: ~ 10Gbps performance @ 65% cost
• Future work: Improve FSO, better management
29