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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: [email protected]

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


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


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


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


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


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)


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


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


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


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


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


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


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.)


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


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


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


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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Documents

vt group

section layer

line layer

bit rate

bit rate

bit stuffing

layered view

vt groups