Cellular Networks and Mobile Computing
COMS 6998-7, Spring 2014
Instructor: Li Erran Li
(lierranli@cs.columbia.edu)
http://www.cs.columbia.edu/~lierranli/coms69
98-7Spring2014/
3/10/2014:Future Directions of Cellular Networks
Outline
• Review of Previous Lecture
• Future Direction of Cellular Networks
– Introduction to SDN and NFV
– Software Defined Cellular Networks
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Cellular Networks and Mobile Computing (COMS
6998-7)
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Review of Previous Lecture
• What are the physical layer technologies in
LTE?
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LTE Physical Layer
• The key improvement in LTE radio is the use of OFDM
• Orthogonal Frequency Division Multiplexing
– 2D grid: frequency and time
– Narrowband channels: equal fading in a channel
• Allows simpler signal processing implementations
– Sub-carriers remain orthogonal under multipath
One resource block
propagation
One resource element
12 subcarriers during one slot
(180 kHz × 0.5 ms)
12 subcarriers
Time domain structure
Frame (10 ms)
One slot
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One OFDM
symbol
time
Cellular Networks and Mobile Computing
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Slot (0.5 ms)
Subframe (1 ms)
4
Review of Previous Lecture (Cont’d)
• What are the mobility protocols used in
cellular networks?
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Mobility Protocol: GTP
SGi
HSS
PDN GW
GTP
S5
Gn
GTP
SGW
S11
MME
SGSN
MSC
IuCS
IuPS
S1-U
S1-CP
RNC
GTP
Iub
eNodeB
NodeB
UE
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Courtesy: Zoltán Turányi
6
Mobility Protocol: Proxy Mobile
IP (PMIP)
SGi
HSS
PDN GW
S5
S2
PMIP
PMIP
SGW
S11
S1-U
Non-3GPP Access
(cdma2000, WiMax, WiFi)
MME
S1-CP
GTP
eNodeB
UE
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Cellular Networks and Mobile Computing
(COMS 6998-7)
EPC – Evolved Packet Core
Courtesy: Zoltán Turányi
7
Review of Previous Lecture (Cont’d)
• Is carrier sensing multiple access (CSMA) used
in cellular networks?
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Random Access
Why not carrier sensing like
WiFi?
• Base station coverage is much
larger than WiFi AP
Base station
– UEs most likely cannot hear
each other
• How come base station can
hear UEs’ transmissions?
UE 2
UE 1
– Base station receivers are much
more sensitive and expensive
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Review of Previous Lecture (Cont’d)
• What is the current LTE network architecture
and its problems?
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Current LTE Architecture
• No clear separation of control plane and data plane
• Hardware centric
Control Plane
Data Plane
Mobility
Management
Entity (MME)
User
Equipment
(UE)
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Home
Subscriber
Server
(HSS)
Policy Control and
Charging Rules
Function (PCRF)
Base
Serving
Station
(eNodeB)
Gateway
(S-GW)
• Problem with Intertechnology (e.g. 3G
to LTE) handoff
• Problem of inefficient
radio resource
allocation
Packet Data
Network
Gateway
(P-GW)
11
Outline
• Review of Previous Lecture
• Future Direction of Cellular Networks
– Introduction to SDN and NFV
– Software Defined Cellular Networks
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Source: Nick Mckeown, Stanford
Routing, management, mobility management,
access control, VPNs, …
Feature
Feature
Million of lines
of source code
6,000 RFCs
Billions of gates
Bloated
OS
Custom Hardware
Power Hungry
• Vertically integrated, complex, closed,
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proprietary
• Networking industry with “mainframe” mind-set
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Source: Nick Mckeown, Stanford
The network Should Change to
Feature
Feature
Network OS
Feature
Feature
OS
Feature
Feature
Custom Hardware
OS
Feature
Feature
Custom Hardware
OS
Feature
Custom Hardware
Feature
OS
Feature
Feature
Custom Hardware
OS
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Custom Hardware
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Source: Nick Mckeown, Stanford
Software Defined Network (SDN)
3. Consistent, up-to-date global network view
Feature
Feature
2. At least one Network OS
probably many.
Open- and closed-source
Network OS
1. Open interface to packet forwarding
Packet
Forwarding
Packet
Forwarding
Packet
Forwarding
Packet
Forwarding
Packet
Forwarding
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Network OS
Source: Nick Mckeown, Stanford
Network OS: distributed system that creates a
consistent, up-to-date network view
– Runs on servers (controllers) in the network
– Floodlight, POX, Pyretic, Nettle ONIX, Beacon, … +
more
Uses forwarding abstraction to:
– Get state information from forwarding elements
– Give control directives to forwarding elements
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Source: Nick Mckeown, Stanford
Software Defined Network (SDN)
Control Program A
Control Program B
Network OS
Packet
Forwarding
Packet
Forwarding
Packet
Forwarding
Packet
Forwarding
Packet
Forwarding
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Source: Nick Mckeown, Stanford
Control Program
Control program operates on view of network
– Input: global network view (graph/database)
– Output: configuration of each network device
Control program is not a distributed system
– Abstraction hides details of distributed state
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Source: Nick Mckeown, Stanford
Forwarding Abstraction
Purpose: Abstract away forwarding hardware
Flexible
– Behavior specified by control plane
– Built from basic set of forwarding primitives
Minimal
– Streamlined for speed and low-power
– Control program not vendor-specific
OpenFlow is an example of such an abstraction
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Network Functions
Virtualisation Approach
Session Border
Controller
WAN
Acceleration
Message
Router
Independent
Software Vendors
CDN
Carrier
Grade NAT
DPI
Tester/QoE
monitor
Firewall
SGSN/GGSN
PE Router
BRAS
Radio Network
Controller
Orchestrated,
automatic
remote install
hypervisors
Generic High Volume Servers
Generic High Volume Storage
Classical Network Appliance
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Approach
(COMS 6998-7)
Generic High Volume 20
Ethernet Switches
Outline
• Review of Previous Lecture
• Future Direction of Cellular Networks
– Introduction to SDN and NFV
– Software Defined Cellular Networks
• Radio Access Networks
• Cellular Core Networks
• Cellular Wide Area Networks
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A Clean-Slate Design:
Software-Defined Radio Access
Networks
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Carrier’s Dilemma
Exponential Traffic Growth
8
Exabyte
11.2
12
Annual Growth 83%
10
Shannon
6
Shannon (3dB)
4
6
4.7
2.8
0.5 0.9
0.0 0.0 0.1 0.2
2
1.6
1
2017
2016
2015
2014
2013
2012
2011
2010
2009
2008
0
2007
4G
3
0
-15
-12.5
-10
-5
-2.5
0
2.5
5
7.5
10
12.5
15
17.5
20
4
•
7
5
7.4
8
2
Limited Capacity Gain
Poor wireless connectivity if left unaddressed
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LTE Radio Access Networks
• Goal: high capacity wide-area wireless network
– Dense deployment of small cells
Base Station (BS)
Serving Gateway
Packet Data
Network Gateway
User Equipment (UE)
Serving Gateway
access
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Internet
core
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Dense and Chaotic Deployments
• Dense: high SNR per user leads to higher
capacity
o
Small cells, femto cells, repeaters, etc
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Problems
•
Current LTE distributed control plane is ill-suited
o Hard to manage inter-cell interference
•
o Hard to optimize for variable load of cells
Dense deployment is costly
o Need to share cost among operators
o Maintain direct control of radio resources
o Lacking in current 3gpp RAN sharing standards
26
SoftRAN: Big Base Station Abstraction
Big Base Station
Radio Element 1
time
controller
frequency
Radio Element 2
time
time
frequency
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Radio Element 3
time
frequency
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Radio Resource Allocation
3D Resource Grid
time
Flows
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SoftRAN: SDN Approach to RAN
Coordination :
X2 Interface
Control Algo
Control Algo
PHY & MAC
PHY & MAC
Control Algo
PHY & MAC
BS1
BS3
Control Algo
Control Algo
PHY & MAC
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BS2
BS5
PHY & MAC
BS4
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SoftRAN: SDN Approach to RAN
Control Algo
Operator Inputs
Network OS
RadioVisor
PHY & MAC
PHY & MAC
PHY & MAC
RE3
RE1
RE5
PHY & MAC
Radio Element
(RE) 3/10/14
RE2
PHY & MAC
RE4
30
SoftRAN Architecture Summary
CONTROLLER
RAN Information Base
Periodic Updates
Controller
API
•
•
•
RADIO ELEMENTS
Interference
Map
Bytes
Rate
Queue
Size
Flow
Records
Network
Operator
Inputs
QoS
Constraints
Radio
Element
API
3/10/14
Radio Element
3D Resource Grid
POWER
FLOW
Radio Resource
Management
Algorithm
Frequency
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31
SoftRAN Architecture: Updates
• Radio element -> controller (updates)
– Flow information (downlink and uplink)
– Channel states (observed by clients)
• Network operator -> controller (inputs)
– QoS requirements
– Flow preferences
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SoftRAN Architecture: Controller Design
• RAN information base (RIB)
– Update and maintain global network view
• Interference map
• Flow records
• Radio resource management
– Given global network view: maximize global utility
– Determine RRM at each radio element
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SoftRAN Architecture: Radio Element API
• Controller -> radio element
– Handovers to be performed
– RF configuration per resource block
• Power allocation and flow allocation
– Relevant information about neighboring radio
elements
• Transmit Power being used
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34
Refactoring Control Plane
• Controller responsibilities:
- Decisions influencing global network state
• Load balancing
• Interference management
• Radio element responsibilities:
- Decisions based on frequently varying local
network state
• Flow allocation based on channel states
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SoftRAN Advantages
• Logically centralized control plane:
– Global view on interference and load
• Easier coordination of radio resource management
• Efficient use of wireless resources
– Plug-and-play control algorithms
• Simplified network management
– Smoother handovers
• Better user-experience
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SoftRAN: Evolving the RAN
• Switching off radio elements based on load
– Energy savings
• Dynamically splitting the network into Big-BSs
– Handover radio elements between Big-BSs
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Implementation: Modifications
• SoftRAN is incrementally deployable with
current infrastructure
– No modification needed on client-side
– API definitions at base station
• Femto API : Standardized interface between scheduler
and L1 (http://www.smallcellforum.org/resourcestechnical-papers)
• Minimal modifications to FemtoAPI required
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RadioVisor Design
•
Slice manager
o
Traffic to
Slice
Mapping
3D Resource
Grid
Allocation &
Isolation
RadioVisor
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•
Slice
Manager
•
Slice configuration, creation,
modification, deletion and multi-slice
operations
Traffic to slice mapping at RadioVisor
and radio elements
3D resource grid allocation and
isolation
o
Considers traffic demand, interference
graph and policy
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Slice Manager
•
•
•
Slice definition
o Predicates on operator, device, subscriber, app
attributes
o
A slice can be all M2M traffic of operator 1
Slice configuration at data plane and control plane
o PHY and scheduler: narrow band PHY for M2M slice
o Interference management algorithm
Slice algebra to support flexible slice operations
o Slice merge, split, (un)nest, duplicate
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•
•
Slices present resource
demands every time window
Max min fair allocation
Example
o Red slice entitles 2/3 and
demands 2/3 RE1 only
o Blue slice entitles 1/3 and
demand 1/3 RE2 and 1 RE3
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Interference
Edge
Radio
Element 1
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Radio
Radio
Element 2 Element 3
Frequency
•
Resource Grid Allocation and
Isolation
41
Conclusion
•
•
•
Dense deployment calls for central control of radio
resources
Deployment costs motivate RAN Sharing
We present the design of RadioVisor
o Enables direct control of per slice radio resources
o Configures per slice PHY and MAC, and
interference management algorithm
o Supports flexible slice definitions and operations
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A Clean-Slate Design:
Software-Defined Cellular Core
Networks
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Cellular Core Network Architecture
Base Station (BS)
Serving Gateway
Packet Data
Network Gateway
User Equipment (UE)
Serving Gateway
access
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Internet
core
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SoftCell Overview
Simple hardware
+ SoftCell software
Controller
Interne
t
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SoftCell Design Goal
Fine-grained service policy for diverse app needs
»
»
Video transcoder, content filtering, firewall
M2M services: fleet tracking, low latency medical
device updates
with diverse needs!
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Computing (COMS 6998-7)
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Characteristics of Cellular Core
Networks
1. “North south” traffic pattern
2. Asymmetric edge
3. Traffic initiated from low-bandwidth access
edge
Gateway Edge
Internet
~1 million Users
~10 million flows
~400 Gbps – 2 Tbps
Access
Edge
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~1K Users
~10K flows
Cellular Networks and Mobile
~1 – 10 Gbps
Computing (COMS 6998-7)
47
Challenge: Scalability
Packet classification: decide which service policy
to be applied to a flow
» How to classify millions of flows per second?
Traffic steering: generate switch rules to implement
policy paths, e.g. traversing a sequence of
middleboxes
» How to implement million of paths?
• Limited switch flow tables: ~1K – 4K TCAM, ~16K – 64K
L2/Ethernet
Network
dynamics: setup policy paths for new
3/10/14
users and new flow?
48
SoftCell: Design-in-the-Large
Controller
1. Scalable system design
»
»
Classifying flows at access
edge
Offloading controller tasks
to switch local agent
2. Intelligent algorithms
»
»
LA
LA
Gateway Edge
LA
Enforcing policy
LA
consistency under mobility
Multi-dimension
Access
Edge
aggregation to reduce
~1K Users
~10K flows
switch rule entries
~1 million
Users
~10 million
flows
~up to 2 Tbps
~1 – 10 Gbps
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Multi-Dimensional Aggregation
Use multi-dimensional tags rather than flat tags
Policy Tag
BS ID
User ID
Aggregate
Aggregate
Aggregate
flows that
flows going
flows going
share a
to the same to the same
common
Users.
(group of)
policy (even
base
across Users
stations
Exploit
andlocality
BSs)in network topology and traffic pattern
Selectively match on one or multiple dimensions
» Supported by the multiple tables in today’s switch chipset
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Conclusion and Future Work
• SoftCell uses commodity switches and middelboxes to build
flexible and cost-effective cellular core networks
• SoftCell cleanly separates fine-grained service policies from
traffic management policies
• SoftCell achieves scalability with
Data Plane
Control Plane
Asymmetric Edge Design
Multi-dimensional Aggregation
Hierarchical Controller Design
• Deploy SoftCell in real test bed
• Exploit multi-stage tables in modern switches
3/10/14 –
Reduce m×n rules to m+n rules
51
A Clean-Slate Design:
Software-Defined WAN
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Current Mobile WANs
• Organized into rigid and very large regions
• Minimal interactions among regions
• Centralized policy enforcement at PGWs
Two Regions
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Mobile WANs Problems
• Suboptimal routing in large carriers
– Lack of sufficiently close PGW is a major cause of
path inflation
• Lack of support for seamless inter-region
mobility
– Users crossing regions experience service
interruption
• Scalability and reliability
– The sheer amount of traffic and centralized policy
enforcement
• Ill-suited to adapt to the rise of new
applications
– E.g., machine-to-machine
– All users’ outgoing traffic traverses a PGW to the
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SoftMoW Motivation
Question: How to make the packet core scalable, simple,
and flexible for tens of thousands of base stations and
millions of mobile users?
• Mobile networks should have fully connected core
topology, small logical regions, and more egress points
• Operators should leverage SDN to manage the whole
network with a logically-centralized controller:
– Directs traffic through efficient network paths that
might cross region boundaries
– Handles high amount of intra-region signaling load
from mobile users
– Supports seamless inter-region mobility and
optimizes its performance
– Performs network-wide application-based such as
region optimization
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SoftMoW Solution
• Hierarchically builds up a network-wide control plane
– Lies in the family of recursive SDN designs (e.g. XBAR,
ONS’13)
• In each level, abstracts both control and data planes
and exposes a set of “dynamically-defined” logical
components to the control plane of the level above.
– Virtual Base stations (VBS), Gigantic Switches (GS),
and Virtual Middleboxes (VMB)
Union of
Coverage
Latency
Matrix
Sum of
capacities
VBS
GS
VMB
Core Net
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Radio Net
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Policy
56
SoftMoW Solution
• New Dynamic Feature: In each level, the
control logic can modify its logical
components for optimization purposes
– E.g., merge/spilt and move operations
GSW2
GSW1
VBS1
GSW1
VBS1
VBS2
GSW1
GSW3
Merge/Split
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GSW2
VBS2
VBS3
GSW2
Move and Split
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VBS3
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First Level-SoftMoW Architecture
• Replace inflexible and expensive hardware devices (i.e.,
PGW, SGW) with SDN switches
• Perform distributed policy enforcement using middle-box
instances
• Partition the network into independent and dynamic logical
regions
• A child controller manages the data plane of each regions
Events GS Rules &
Actions
Bootstrapping
phase:
based on location
and processing
capabilities of child
controllers
Agent A
Child A
NIB
E2
E3
Boundary
M
1
Region A
E1
M
M
3/10/14
Local
Apps
M
2
4
BS1
M
3
Region B
I1
7
9
5
BS2
M
BS3
6
BS4
E4
M
M
8
M
10
BS5
M
BS6
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Second Level-SoftMoW Architecture
• A parent runs a global link discovery protocol
– Inter-region links are not detected by BDDP and LLDP
• A parent participates in the inter-domain routing protocol
• A parent builds virtual middlebox chains and egresspoint policies, and dictates to GSs
Events GS Rules &
Actions
Agent A
I-Mobility
Manager
Local
Apps
Middlebox Egress
Optimizer Selection
Child A
NIB
E2
E3
Boundary
M 1
Region A
E1
M 3
M 2
M
4
BS13/10/14
6
Region B
I1
7
E4
M
8
M
GS
Protocol
E1
M
9
5
BS2
BS3
BS4
M
10
BS5
BS6
E2
E3
E4
-----
M M
M M
M
BGP
sessions
Parent
NIB
M
Region
Optimizer
2M M
Internal
VBS1
GSA
Border
VBS
1
I1
GSB
2M M
Border Internal
VBS
2 VBS2
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Hierarchical Traffic Engineering
• A parent pushes a global label into each traffic group
• Child controllers perform label swapping
o Ingress point: pop the global label and push some local labels for
intra-region paths
Events GS Rules &
o Egress point: pop the local labels and push
back
the global label
Actions
Push W
I-Mobility
Manager
Middlebox
Optimizer
Agent A
Egress
Selection
Region
Optimizer
Parent
NIB
GS
Protocol
E1
E2
E3
2M M
Latency
(P1,E2)=300
Latency
(P1,E4)=100
GSA
Internal
VBS1
Push W
3/10/14
Border
VBS
1
I1
E2
GSB
E3
Boundary
M 1
Region A
E1
E4
M M
M M
Child A
NIB
Pop W2
Push W
-----
Web
Voice
BGP
sessions
Local
Apps
Pop W1
M 2
Region B
I1
M 3
M
6
7
E4
M
8
M
2M M
GS
Rules
Border Internal
VBS
VBS2
2
M
4
BS1
Pop W
Push W1
M
5
BS2
Pop W
9
BS3
BS4
M
M
10
BS5
BS6
Push W2
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Time-of-day Handover Optimization
Q: How can an operator reduce inter-region
handovers in peak
E
A
M
GSA
M M
VBS1
VBS1
VBS2
Border
VBS2
Abstraction
update
coordination
Child A
Child B
E2
Parent
E3
E4
E3
Boundary
M
M 1
Region A
E1
E2
Internal
VBS2
VBS2
Handover
graph
E1
3M M
GSB
Border
VBS
1
Min Cut
300 Border 1000 Border 2000 Internal
Internal
E4
M 2M
E3
E2
1
hours?
GS
M 2
Region B
I1
M 3
6
7
M
8
M
M M
M M
2M M
GSA
I1
GSB
2M M
GS Rule:
Move Border VBS1
M
4
M
BS2
BS1
Internal
VBS1
Border
Border Internal
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VBS1
VBS2 VBS
2
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New
Border
9
5
BS3
Old
Border
BS4
M
M
10
BS5
BS6
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Conclusion
SoftMoW:
• Brings both simplicity and scalability to the
control plane of very large cellular networks
– decouples control and data planes at multiple levels
( focused only on two levels here)
• Makes the deployment and design of networkwide applications feasible
– E.g., seamless inter-region mobility, time-of-day
handover optimization, region optimization, and
traffic engineering
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Summary
• Mobile computing depends on cellular
networks
• Cellular network performance still far from
meeting demands of mobile computing
• Cellular network architecture is evolving to
meet demands of mobile computing
– SDN and NFV
• AT&T’s domain 2.0
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Questions?
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Home
Subscriber
Server
(HSS)
Mobility
Management
Entity (MME)
User
Equipment
(UE)
Base
Station
(eNodeB)
Policy Control and
Charging Rules
Function (PCRF)
Serving
Gateway
(S-GW)
Packet Data
Network
Gateway
(P-GW)