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Networking with SONET/SDH
Debasish DattaDepartment of Electronics and Electrical
Communication EngineeringIndian Institute of Technology, Kharagpur
West Bengal 721302, India
E-Mail: ddatta@ece.iitkgp.ernet.in
Networking with SONET/SDHD. Datta, IIT Kharagpur 2
SONET/SDH Background
SONET: Synchronous optical network (ANSI)SDH: Synchronous Digital Hierarchy (ITU-T)
SONET and SDH are two sets of standard interfaces for synchronous optical network – designed to overcome the problems of Plesiochronous Digital Hierarchy (PDH)PDH at high bit ratesSONET and SDH interfaces define all layers from physical to higher layersBoth are synchronous networks and converge at 155MbpsDeployment in 1980sFiber-replacement for long-haul inter-city microwave links
Networking with SONET/SDHD. Datta, IIT Kharagpur 3
PDH Multiplexing Issues
Two bit streams perfectly synchronized with equal bit rateOutput has double bit rate w.r.t. input
Two bit streams with imperfect sync: Needs bit stuffingAt higher levels of multiplexing, complexity increases manifoldMux/Demux mountain – major hardware issue
Networking with SONET/SDHD. Datta, IIT Kharagpur 4
PDH Complexity
Multiplexing/Demultiplexing in PDH is cumbersomeLack of sync between incoming streams necessitates bit stuffing in multiplexed bit streamMakes it very difficult to take out a low bit-rate stream from a high-speed stream because of non-synchronized clocks (due to bit stuffing)This makes PDH MUX and DEMUX significantly complicatedEuropean: E1 (30chl, 2.048Mbps), E2 (120chl, 8.448Mbps), E3 (480chl, 34.368Mbps), E4 (1920chl, 139.264Mbps) E5 (7680chl, 565.148Mbps)American: T1 (24chl, 1.544Mbps), T2 (96chl, 6.312Mbps), T3 (672chl, 44.736Mbps), T4 (4032chl, 274.176Mbps)Bit rates do not scale up proportionately with number of channels due to stuffed bits – at high bit rates, needs different technique: SDH/SONET
Networking with SONET/SDHD. Datta, IIT Kharagpur 5
SONET/SDH FeaturesDesign and Management:
All clocks in network are synchronized with a simple control mechanism and rates are integral multiples of basic rates - Makes the design of SONET/SDH MUX and DEMUX simplerIncorporates byte interleaving and accommodate a given PDH frame as a whole in a bigger SDH/SONET frame without undoing PDH bit stuffing SONET and SDH standards incorporate extensive management info. for monitoring the performance, unlike PDH
Interoperability:It is very difficult to connect one vendor’s equipment to another’s because of nonstandard format on the transmission link of PDH –All vendors follow the same set of standards in SONET/SDH
Networking:SONET and SDH standards have evolved to incorporate specific network topologies (ring and linear, at times) and specific protection schemesServices restoration time is 50 ms for SONET/SDH and several seconds to minutes for PDH
Networking with SONET/SDHD. Datta, IIT Kharagpur 6
SONET/SDH: Basic STS Frame Structure
STS : Synchronous transport signalSOH : Section OverheadLOH : Line OverheadPOH : Path OverheadVT: Virtual Tributory
SOH+LOHPOH
3+1 86
90 columns
9 ro
ws
Payload
Frame Time = 125μs (Inter-sample period in 64kbps PCM signal)
OC-1: (90 × 9)/125 μs = 51.84Mbps (STS-1)
OC-n = OC-1 rate × nOC-3 => STS-3 => STM-1 at 155Mbps: SONET & SDH converge at 155Mbps
Big VT
Smaller VT
Small VT Small VT
Poin
ter
Poin
ter
Poin
ter
Networking with SONET/SDHD. Datta, IIT Kharagpur 7
OC-n Rates for SONET/SDH Networks
SONET Signal SDH Signal OC-n Bit rate (Mbps)
STS-1 - 51.84 (OC-1)
STS-3 STM-1 155.51 (OC-3)
STS-12 STM-4 622.08 (OC-2)
STS-24 - 1244.16 (OC-24)
STS-48 STM-16 2488.32 (OC-48)
STS-192 STM-64 9953.28 (OC-192)
STS-768 STM-256 39814.32(OC-768)
Networking with SONET/SDHD. Datta, IIT Kharagpur 8
Virtual Tributaries and VT Groups in OC1VT 1.5: 1.544Mbps, VT 2: 2.048Mbps, VT 3: 3.152Mbps, VT 6: 6.312MbpsHow a VT Group is formed:
A VT Group: 4 VT1.5, or 3 VT2, or 2 VT3 or 1 VT6 – All resulting a bit rate of 6.912MbpsOriginal PDH rates are modified to fit exactly integral multiples of PDH VTxframes into a SONET/SDH VT Group
7 such VT groups are plugged into a basic SONET frame: Takes 7 × 6.912 = 48.384 Mbps out of 51.84 Mbps
Implies 810 × (48.384/51.54) = 756 bytes for 7 VT Groups, i.e., 108 bytes for each VT Group810 – (90 × 4) Overhead = 774 bytes: 756 available for payload756 bytes allocated for VT GroupsOther allocations also possible – next slide
VT No. Original PDH Bit Rate (Mbps)
Modified SONET Bit Rate (Mbps)
Bit Rate of SONET VT Group(Mbps
VT 1.5 1.544 1.728 4 × 1.728 = 6.912
VT 2 2.048 2.304 3 × 2.304 = 6.912
VT 3 3.152 3.456 2 × 3.456= 6.912
VT 6 6.312 6.912 1 × 6.912= 6.912
Networking with SONET/SDHD. Datta, IIT Kharagpur 9
VT1.5
VT6
VT2
VT3
VT1.5 SPE
VT3 SPE
VT2 SPE
VT6 SPE
STS-1 SPE
VT group
STS-N
DS11.544 Mbps
DS26.312Mbps
DS1C3.152 Mbps
E12.048 Mbps
DS344.736 Mbps
ATM48.384 Mbps
E4139.264 Mbps
ATM149.76 Mbps
STS-3c SPE
STS-1
STS-3c
xN
xN/3
x7
x4
x1x2
x3
(SPE + path overhead)
Mapping of Low-speed PDH Streams into VTs For OCn
Networking with SONET/SDHD. Datta, IIT Kharagpur 10
Payload & Frequency Justification
POH
H1 H2 H3
H1 H2 H3
87 Columns
Wrapped up
H1, H2: Two 8-bit pointers to setup the starting point for a payload, just arrived
28 < 782 < 216
Starting point decided after the arrival without waiting for the beginning of next frame.
H3 takes care of slower or faster incoming payload data stream by putting an additional byte in the beginning, or by removing one byte at the beginning of the payload
This additional action of H3 is carried out after every 4th frame to offer a 320 ppm clock jitter tolerance – much higher than permitted clocks in SONET networks (more on next slide)
SOH, POH PAYLOAD
Networking with SONET/SDHD. Datta, IIT Kharagpur 11
Jitter Tolerance with H3 Byte
How SONET can ensure 320ppm jitter tolerance over 4 consecutive basic frames:
Four SPE frames nominally contain: 4 × 9 × 87 = 3132 bytes of dataOffers a bit rate = (3132 × 8) / (4 × 125 × 10 -6) = 50.112 Mbps
Positive byte stuffing by H3 for slower incoming data:Offers a bit rate = (3131 × 8) / (4 × 125 × 10-6) = 50.096 Mbps
Negative byte stuffing by H3 for faster incoming data:Offers a bit rate = (3133 × 8) / (4 × 125 × 10-6) = 50.128 Mbps
This arrangement offers a range of clock rates of 50.112 ± 0.016 MbpsOffers 2 × 0.016 = 0.320 Mbps → 320 ppm jitter tolerance, while SONET/SDH clocks are designed to have a spec of 20 ppm with a normal distribution; so even 4σ (=80ppm) remains far below 320 ppm giving an excellent control on clock jitter
Networking with SONET/SDHD. Datta, IIT Kharagpur 12
SONET/SDH Network Topology Examples
Ring,OC-3/12
ADM
ADM
ADM Ring
BLSRRing
OC-12/48 ADMADM
ADM
ADMADM DCS
LTEADM
ADM
ADM
DCS
ADM
LTE
ADMADM
ADM
LTE
ADM
ADM Ring, OC-12/48
UPSR
Linear ADM
Ring: Failure survivability
Networking with SONET/SDHD. Datta, IIT Kharagpur 13
SDH/SONET Connectivity Example
T1/T3
OC-3
T1/T3
ATM (OC3)
DS0
ISDN
CO
SONET/SDHBackbone
(TDM-based)
SONET/SDHBackbone
(TDM-based)
EOS
Voice (circuit-switched) as well as data (packet-switched) services through TDM connectivityIP-over-ATM-over-SDH/SONETEthernet-over-SONETISDN, ADSL, voice through SDH/SONET
ADSL
Ethernet
TAComputer
EthernetRouter
Networking with SONET/SDHD. Datta, IIT Kharagpur 14
SONET/SDH: Different Layers
Physical layer Physical layer
Path layer
Physical layer
Line layer
Section layerSection layer
Physical layer
Line layer
Section layer
Path layer
Line layer
Section layer
SONETterminal
SONETADM
SONETterminal
Regenerator
Connection
* Thick line indicates a SONET connection
Path
Line(between ADMs)
(between terminals)
Section
(between regens.)
Networking with SONET/SDHD. Datta, IIT Kharagpur 15
SONETnetwork
ATMswitch ATM
switch
Data linkNetwork
Physical
NetworkData link
ATMSO
NET
Layered view
Path
Line section
ATMnetwork
IP routerIP router
Data linkNetwork
Physical
NetworkData link
IP
ATM
Layered view
ATM-over-SONET Network
IP-over-ATM Network
Heterogeneous Networks: ATM-SONET, IP-ATM
Networking with SONET/SDHD. Datta, IIT Kharagpur 16
Data linkNetwork
Physical
Network
Data link
Network
SON
ET
ATM
IP
Section
Path
Line
Physical
Amplifier section
Multiplex section
Channel
SON
ET layer
Optical layer
SONET with an underlying optical layer (WDM)
Data link
IP-over-ATM-over-SONET
Heterogeneous Networks: IP-ATM-SONET, SONET-WDM
Networking with SONET/SDHD. Datta, IIT Kharagpur 17
Second-generation Optical Network Layers
Multiplex section
Amplifier section
Channel(wavelength)
Multiplex section
Channel
Amplifier section
Amplifier section
Amplifier section
Multiplex section
WDMnode
WDMnode
WDMnode
Amplifier
* Thick line indicates optical layer connection
Connection
Networking with SONET/SDHD. Datta, IIT Kharagpur 18
IP-over-WDM: Evolving Scenario
IP
ATM
SONET/SDH
WDM
IP/MPLS
SONET/SDH
WDM
IP/GMPLS
Thin SONET/SDH
WDM+OXC
IP/GMPLS
WDM+OXC
GMPLS (Generalized multiprotocol label switching): an extension of MPLS concept with switching capabilities at multiple levels:
Packet switching capability, Time slot switching capability (TDM), Wavelength switching capability, Fiber switching capability
Also: EOS-over-WDM, POS-over-WDM?
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