Protection concerns using
Switched Ethernet as internal bus
for a Telecommunication Network
Element
Jukka Lehtniemi
25 March, 2008
Supervisor: Professor Jörg Ott
Instructors: LicPhil Stefan Wiklund
MSc Juha Eloranta
Outline
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Introduction
Telecommunication Computer and AdvancedTCA
Evolution of Ethernet Technology
Spanning Tree Protocol
Other Link Protection Technologies
Network Element Topology Considerations
Link Protection Solutions for Network Element
Conclusions
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Introduction
 Is a switched Ethernet based internal bus good enough
solution in terms of link protection in a
telecommunication network element?
 Different link protection mechanisms available and the
suitability for use as a telecommunication element
internal bus
 The network node architecture is assumed according to
the AdvancedTCA specification
 Literature study & comparison
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Telecommunication Computer
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Boards
Multiprocessor computing
cluster
Variety of different
processors: general purpose
processors, network
processors, DSPs etc.
Variety of different network
interfaces: TDM, ATM,
Ethernet
Inter
Redundancy
Subrack
Modular board & subrack Link
architecture in a server
cabinet
Resources connected
together by internal
(switched) bus
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Backplane
Subrack
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Advanced Telecommunication
Computing Architecture
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Open hardware framework
Specified by PCI Industrial Computer Manufacturers Group - a
consortium of over 450 hardware manufacturers
Designed for reliability concerned applications
Mechanical design, platform management, data transport etc.
Flexible and loose specifications: profiled further by SCOPE
Alliance
AdvancedTCA specifies two subrack level interconnection busses:
base interface and fabric interface.
Ethernet is the selected technology for the base interface. It is
generally also seen as the most promising candidate for the fabric
interface
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Ethernet Evolution: Shared Bus
Collision Domain
Base-2
Base-T
Repeater 1
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Ethernet Evolution: Switched Bus
Collision Domains
Base-T
Switch 1
Switch 2
Full Duplex – CSMA/CD disabled
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Ethernet Evolution
 The Ethernet technology has evolved from a simple
media sharing LAN solution to a versatile and diverse
family of frame based computer networking
technologies.
 From the user protocol perspective it still provides a
transparent and compatible frame transmission service
 Key enhancements
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Switching
Microsegmentation
Full Duplex
Flow Control
Virtual LANs
Connectivity Fault Management
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Spanning Tree Protocol
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Topology loops in a switched
Ethernet LAN will cause frame
multiplication and thus they are
unacceptable
The Spanning Tree Protocol (STP)
operated by the switches will
eliminate such loops by blocking
redundant links in the physical
topology and restricting the active
topology to a simply and fully
connected tree
 STP provides topology
recovery in the case of link
failure. This can be used as
a link protection solution
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DP
DP
Root
B1
Bridge
RP
RP
BP
DP
DP
DP
B2
DP
RP
BP
RP
BP
DP
B5
DP
DP
BP
RP
BP
B4
Jukka Lehtniemi
DP
B3
B6
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Spanning Tree Protocol
 STP propagates the topology information by
continuous transmission of distance vectors
 New protocol version called Rapid STP (RSTP) has
been standardized
 Proposal-agreement based communication of RSTP
responds faster to topology changes
 Multiple STP (MSTP) protocol defines per VLAN
spanning trees
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Link Aggregation
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Link Aggregation is defined by IEEE 802.3
Allows bundling multiple physical links to a single logical link
Bandwidth extension
Redundancy
Link Aggregate
Switch 1
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Switch 2
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Resilient Packet Ring
Wrapping
Steering
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RPR is specified by IEEE
802.17
Dual Ring topology
Service priorities and fairness
Topology discovery and
continuity check
Healing strategies: Steering
and Wrapping
50 ms protection time against
link failure
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Update!
Span
Failure
Update!
Update!
Update!
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Ethernet Protection Switching
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ITU-T Rec. G.8031 / Y.1342
Linear Protection Switching
VLAN based
Point-to-Point connections
Failure detection by Ethernet CFM
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Ethernet Automatic Protection
Switching
 By Extreame Networks
 Switched Ethernet, Ring topology
 One way transmission: primary port transmitting and
secondary port blocking
 Continuous health check polling by Master Node
 Recovery based on filtering database flush ordered by
Master Node and standard Ethernet address learning
mechanisms
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AdvancedTCA Subrack
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Two switch boards (per interface)
Dual Star topology
BASE-T specified, BASE-KX or BASE-KR likely in future
8 uplink/interconnect ports per switch board defined by SCOPE
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5 Subrack Topology: Bus & Ladder
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5 Subrack Topology: Star
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5 Subrack Topology: Full Mesh
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5 Subrack Topology: Ring
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Link Protection Solutions for
Multisubrack AdvancedTCA
Network Element: Requirements
 Primary requirement: < 50 ms protection switching time
 Manageable complexity
 Reasonable design, manufacturing and maintainance
costs
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Solutions
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•Point-to-point: Too fine
grained (complex) total
solution
Link Aggregation
Ethernet Protection Switching
Ethernet Automatic Protection Switching
Redundant Packet Ring
Physically Distinct Switching Planes
(Rapid) Spanning Tree
•Master Node is
a single point of failure
•No guaranteed switch
over time < 50 ms
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Recommended solutions: RPR
 Guaranteed switch over time
 No need for protection switching
supervision clients on the boards: A
board may transmit using either of the
subrack bridges at any time
 Extendibility: no practical restrictions
on the amount of subracks to be
connected to the ring interconnect
 Equipment prices are presumable
higher
 Additional LAN technology layer and
tunnelling of Ethernet frames on the
ring
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Recommended solutions:
Physically Distinct Switch Planes
 Pure Ethernet solution
 Prices of the equipment probably
lower
 Topology extension by number of
subracks in case of the redundant
star topology may require additional
interface boards in central subrack
 Supervision and switch-over
mechanism needs to be in place on
each board
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Conclusions
 Variety of link protection solutions for Ethernet exists
 Most of them do not meet the requirements for a
telecommunication network element
 Recommended solutions identified by the study:
– RPR interconnect
– Construct of redundant, distinct, loopfree switching planes
 Further analyzing of these alternatives with proper
simulations or real hardware testing is suggested
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