At ECOC 2019, Jörg-Peter Elbers talked strategies and technologies for ensuring the low levels of latency needed in tomorrow's mobile and data center interconnect networks.

1. Low latency for DCI and mobile applications
Jörg-Peter Elbers
ECOC 2019
Why and how?

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Is low latency just another …

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Mobile network infrastructure Real-time (mobile) applications
High-frequency trading (HFT) High-performance computing
A few examples
Low latency – Where do we need it?
“A one-millisecond
advantage in trading
applications can be worth
USD 100 million a year.”

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Low latency in DCI applications

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Applications require a maximum latency and controlled latency variation
Low latency is more than just fast transmission …
Framing
FEC
Link control
Frame forwarding
Switching
Address lookup
Packet forwarding
Routing
TCP windowing
Flow control
Packet retransmission
Application
Presentation
Session
Less latency
More latency
1
2
3
4
5
6
7 Speed-up in high-frequency trading
broker's
contribution
exchange
response time
round trip time order processing
time
2007
2010500ms
450ms
850ms
100ms
10ms 30ms
60ms
250µs

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Latency close to fiber
propagation delay
(5µs/km)
Straight line
connection
Minimization of
equipment delay
1300km link for HFT, approx. 13ms round-trip time
… but transmission line can be critical
Chicago
Futures market
New York
NYSE, NASDAQ

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10G transparent regenerator-based design performs best
Directions to beat the fiber delay: µwave, free-space optics, low-latency fibers
Low-latency design considerations
[B. Teipen et al., ECOC’12]
X
p
dr
X
p
dr
X
p
dr
X
p
dr
X
p
dr
X
p
dr
10G transparent 0.70µs
X
p
dr
X
p
dr
100G OTN + SD-FEC 15.24µs
100G OTN + SD-FEC * 11.44µs (* reduced number of iterations)
FBGFBG FBGFBGFBGFBGFBGFBGFBG
X
p
dr
X
p
dr
10G OTN 3.09µs
10G OTN + GFEC 9.39µs
Tab. 2: Latencies for system building blocks
System Component Latency
Simple transparent 10G Regen 10 ns
Optical Mux/Demux 20 ns
Raman Amplification 30 ns
DCM (grating) 50 ns
10 Gb/s OTN line card 2.0 μs
100 Gb/s OTN line card w/ equalization ~ 9 μs
10 Gb/s 5.6dB NECG G.709 FEC 6.3 μs
100 Gb/s 11.1dB NECG SD-FEC ~5.6 μs
Recipe for low latency:
Limit excess fiber
and electronic processing
10 span system, 20dB loss per span

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Rule of thumb: a few µs per network element
Ethernet aggregator latencies – examples
[iCirrus paper Fronthaul Evolution: From CPRI to Ethernet“, OFT 2015]

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Low latency in mobile applications

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~10ms round-trip time~1ms round-trip time ~5ms round-trip time
Very low latency Low latencyUltra low latency
There will not be a one-size-fits-all configuration
Latency determines location of RAN functions
RU: radio unit
DU: distributed unit
CU: central unit
MEC: multi-access edge computing
UPF: user plane function
RU: radio unit
DU: distributed unit
CU: central unit
MEC: multi-access edge computing
UPF: user plane function
RU: radio unit
DU: distributed unit
CU: central unit
MEC: multi-access edge computing
UPF: user plane function
~1000
sites
~100
sites
~10
sites
~1000
sites
~100
sites
~10
sites
~1000
sites
~100
sites
~10
sites
Source: NGMN Overview on 5G RAN Functional Decomposition

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Ethernet is simplest; TDM-PON, FlexE and OTN add additional protocol layers
5G transport layer stack options
Optical underlay
Backhaul Fronthaul
F1
Fx
eCPRI
CPRI
IEEE1914.3
NGS1
Timingandsync Ethernet
802.1 CM
Grey optics WDM
Fiber
TDM-PON
FlexE2.1 or OTN
OAMandserviceassurance

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With eCPRI, 5G goes Ethernet
eCPRI leverages Eth transport & OAM and offers ~10x reduction in bandwidth

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Maximum latency on the data path plus accurate timing delivery
eCPRI latency and timing requirements
CoS Traffic
Max. one-
way frame
delay
Use case
Max. one-
way frame
loss ratio
High25
User plane
(fast)
25µs
Ultra-low latency
applications
10-7
High100 100µs
Full LTE or NR
performance
High200 200µs
Installations with
long fiber links
High500 500µs
Large latency
installations
Medium
User plane
(slow),
C&M plane
(fast)
1ms All 10-7
Low C&M plane 100ms All 10-6
Category
Maximum time error |TE| at UNI
Maximum
time
alignment
error TAE
between
antenna
ports
T-TSC in radio equipment
T-TSC in
transport
network
T-TSC with
|TEmax|=70n
s
(Class B)
T-TSC with
|TEmax|=15n
s
A+ (relative) n/a n/a 20ns 65ns
A (relative) n/a 60ns 70ns 130ns
B (relative) 100ns 190ns 200ns 260ns
C (absolute) 1100ns 3µs

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Limiting latency variation in x-haul network
Low-latency timing-accurate mobile x-haul based on SDN-enabled 100G Ethernet aggregator
RoE
BH
TrafficAnalyzer
10G
100G100G
10G
1GIEEE 1588v2 PTP
(grand master)
IEEE 1588v2 PTP
slave (probe)
10G10G
10G10G
10G 10G
BBU
GNSS
RRH
PTP
10G 10G
Central Office Remote Node
…
RoE
RoE
NETCONF/YANG
Controller
100G100G
FUSION
IP Core
MAC 100G PHY
MAC
10G
PHY
MAC
10G BH
…
x6…8
10G
PHY
100G MAC100G PHY
FUSION
IP Core
MAC
MAC
100G Aggregator Node (time sensitive)
1G PTP
100GMAC 100G PHY
10G PHY MAC10G FH
10G FH
…
x4
1G PHY
10G PHY
…
10G
10G
…
Backhaul
Service Configuration
and Monitoring
…
…
…
PTP
TrafficEmulator
…
Traffic
Generator
100G Transport Node (time sensitive)
…
…
SM
GST 100GbE ingress
treated as GST
FH bounded delay aggregation
Dagg = Store-fw MTU@10G +
serve all other streams
(F  1) MTU@100Gbps +
transmission of packet
MTU@100Gbps 100G
1G
Paper Tu3B.3
Fronthaul traffic
(1522 Byte MTU):
• <3.1µs agg+deagg latency
(1µs from MAC/PHY)
• <0.6µs transit node latency
• 5µs per fiber-km
PTP traffic:
• <±75ns time error
(w/o additional means)
Best
demo
award

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Low latency (mobile) application
Video surveillance demo at EUCNC 2019
Virtual machine
Video
management
system
Compute node
AMEN
(access metro edge node)
Virtual machine
Video analytics
Compute node
MCEN
(metro core edge node)
Core networkRemote client
Fixed camera
Metro network
controlled by NFV network service
Demonstrated here separately with partially emulated data
plane (ROADMs, transceivers, …)
PTZ camera
High bandwidth video and low latency control traffic

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Summary

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Conclusions
Required latency and its value depends on the application
The network is only one contributor
Lower latency is often about simplification
Trade-off between dedicated solutions and economies of scale
More network layers may help short-term but can block future evolution
Latency becomes additional network dimensioning parameter
If you cannot measure it, you cannot monetize it

18. Thank you
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jelbers@advaoptical.com