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Roy YatesECE/WINLAB, Rutgers
NSF Workshop on Ultra Low Latency Wireless Networks March 26, 2015
Age of Information Status Updating Systems and Networks
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Motivation
• 50 years of rate maximization– at the expense of delay
• long (coded) packets on wireless channels,• ARQ • video streaming with large delays to absorb
packet jitter• Caching to compensate for network latency
• high throughput “best-effort” networks
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Applications(The Samsung 5G List)
• V2X – timely delivery of critical messages for traffic safety
• Mission-Critical IoT (M2M)– mission-critical systems – process monitoring/detection and
disaster response
• Virtual/Augmented Reality– seamless virtual/real-world interaction
• Real-time remote access (tactile feedback)– long range, real-time control for remote surgery, driving,
etc.
• Everything-on-Cloud– instantaneous cloud-based services/multimedia content
~1 ms
1-10 ms
10-100ms
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Applications(The Samsung 5G List)
• V2X – timely delivery of critical messages for traffic safety
• Mission-Critical IoT (M2M)– mission-critical systems – process monitoring/detection and
disaster response
• Virtual/Augmented Reality– seamless virtual/real-world interaction
• Real-time remote access (tactile feedback)– long range, real-time control for remote surgery, driving,
etc.
• Everything-on-Cloud– instantaneous cloud-based services/multimedia content
~1 ms
1-10 ms
10-100ms
57 mph=
1 inch/ms
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Applications(The Samsung 5G List)
• V2X – timely delivery of critical messages for traffic safety
• Mission-Critical IoT (M2M)– mission-critical systems – process monitoring/detection and
disaster response
• Virtual/Augmented Reality– seamless virtual/real-world interaction
• Real-time remote access (tactile feedback)– long range, real-time control for remote surgery, driving,
etc.
• Everything-on-Cloud– instantaneous cloud-based services/multimedia content
• Wireless Network on Chip – Cloud on Chip?
~1 ms
1-10 ms
10-100ms
?? ms
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Network Delay(H. Viswanathan, Bell Labs)
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Remote Surgery
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Remote Surgery
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PHY
• Wider Channels (but not UWB )– 30+ GHz mmWave
• M2M: reliability is essential
• Practice: Emerging low latency 5G– Channel Estimation, Modulation, Coding,
Framing • (check out Fettweis CTW 2013 talk)
• Theory:– Delay-Limited Capacity, Short blocklength
source/channel coding
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Latency-Sensitive MAC
• Practice:– 2G/3G packet voice MAC– LTE Scheduling– Sleep protocols!
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Latency-Sensitive MAC
• Practice:– 2G/3G packet voice MAC– LTE Scheduling– Sleep protocols!
• What Randy said: “CSMA style random access seems ill-matched to low latency unless the network is very underutilized.”
• Theory: Are rate/delay tradeoffs fundamental?
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Large Networks (Hundreds of cars)
Frequent Updates
Reliability and Timeliness are required
V2V Safety Messaging
Source
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• DSRC standard MAC protocol• Message Scheduling, Forwarding/Piggybacking• Power/rate adaptation, coverage …
• Performance Metrics?
V2V Safety Messaging
Source
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• Car u sends updates to car v• Updates pass through network/service
system• Car v wants latest state information.
• Metric: Age of the latest status update
Network
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Update Age
D(t) UpdateSent
Received
tt1 t2t1’ t2'
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Update Age
D(t)
• Low Update Rate Age gets large between
updates
UpdateArrival
Departure
tt1 t2t1’ t2'
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Update Age
D(t)
• High Update Rate Queueing Delay
t1 t2 t1’ t2't3 t3'
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Average Update Age
D(t)
• Update Rate:• High Queueing delays• Low Infrequent
updates
High Average Age
Average Age
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FCFS Average Update Age
D(t)
𝑋 𝑇
• X= Interarrival Time
• T= System Time
• Weak ergodicity requirements• E[XT ] is tricky, negative correlation
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Average Age
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Average Age
Nothroughput/
delaytradeoff
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Other Age Metrics
Average Peak AgeD1
D(t)
D(t)
D2
D3
D* P[ (D t)>D* ]
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Competing Updates
• How often is too often?
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Multiple Sources
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Multiple Sources
Models for Source 2:• Competing status updater• Other traffic
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Multiple SourcesFCFS Status Age Region
OptimalSharing
Nash Equilibriu
m
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𝜇Source
1
Source2
Monitor
• Queueing delays increases status age
• Reduce/Eliminate the queues?• “Packet Management”
[Costa, Codreanu, Ephremides ISIT’14]
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LCFSPre-emption & Discarding
(No Queueing)
𝜇Source
1
Source2
Monitor
l1
l2
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V2V Safety Messaging
Network
• Multiple Sources• Fast local server interface • Slow Server
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Multiple SourcesFCFS/LCFS Age
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Timely CompressionA Status Updating Problem
• Encoder input symbol = status update • Age = Decoder symbol lag• Block coding Bursty bit arrivals at FIFO buffer
Bit pipe queueing delay Decoding delay
• [Sahai&Cheng ISIT’07]
a1a2a3…
Encoder FIFObuffer
Rate Rbit pipe Decoder
a1a2a3…01 110 11…
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Timely CompressionA Status Updating Problem
Encoder
01 110 11…
FIFObuffer
Rate Rbit pipe Decoder
a1 a2 a3 a4 a5 a6 a7 …
ta1 a2 a3a4 …
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Timely CompressionHuffman Block Coding Example
Channel Rate R
Sta
tus
Ag
e
High rate pipe:
use small blocks
Low rate pipe:
Use large blocks
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Summary
• Information Age Minimization– Match the load to the network/system
• “Rate” is an input for controlling delay – Redesign the system
• Give priority to timely updates• Packet Management
• Ultra Low Latency Networks– Sub 1ms latency applications?
• Better Theory for Network Latency
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