Sonet

SONET/SDH
Yaakov (J) Stein
Chief Scientist
RAD Data Communications

Y(J)S SONET Slide 2
Course Outline
Background (analog telephony, TDM, PDH)
SONET/SDH history and motivation
Architecture (path, line, section)
Rates and frame structure
Payloads and mappings
Protection and rings
VCAT and LCAS
Handling packet data

Y(J)S SONET Slide 3
Background

Y(J)S SONET Slide 4
The PSTN circa 1900
pair of copper wires
“local loop”
manual routing at local exchange office (CO)
• Analog voltage travels over copper wire end-to-end
• Voice signal arrives at destination severely attenuated and distorted
• Routing performed manually at exchanges office(s)
• Routing is expensive and lengthy operation
• Route is maintained for duration of call

Y(J)S SONET Slide 5
Telephony Multiplexing
1900: 25% of telephony revenues went to copper mines
 standard was 18 gauge, long distance even heavier
 two wires per loop to combat cross-talk
 needed method to place multiple conversations on a single trunk
1918: “Carrier system” (FDM)
 5 conversations on single trunk
 later extended to 12 (group)
 still later supergroups (60), master groups (60)), …
f
channels
8 kHz
12 kHz
4 kHz
16 kHz
20 kHz

Y(J)S SONET Slide 6
The Digitalization of the PSTN
Shannon (Bell Labs) proved that
Digital communications
is always better than
Analog communications
and the PSTN became digital
Better means
 More efficient use of resources (e.g. more channels on trunks)
 Higher voice quality (less noise, less distortion)
 Added features
After the invention of the transistor, in 1963 T-carrier system (TDM)
 1 byte per sample – 8000 samples per second
 T1 = 24 conversations per trunk
 2 groups per cable!
t
timeslots

Y(J)S SONET Slide 7
and switching became easier too
Complexity increases rapidly with size
1 2 4 5 6 7 83
1
2
3
4
5
6
7
Analog Crossbar switch Digital Cross-connect (DXC)
processor
t
1 2 3 4 5
t
2 1 5 4 3

Y(J)S SONET Slide 8
Optimized Telephony Routing
Circuit switching (route is maintained for duration of call)
Route “set-up” is an expensive operation, just as it was for manual switching
Today, complex least cost routing algorithms are used
Call duration consists of set-up, voice and tear-down phases

Y(J)S SONET Slide 9
The PSTN circa 1960
local loop
subscriber line
automatic routing through universal telephone network
• Analog voltages used throughout, but extensive Frequency Division Multiplexing
• Voice signal arrives at destination after amplification and filtering to 4 KHz
• Automatic routing
• Universal dial-tone
• Voltage and tone signaling
• Circuit switching (route is maintained for duration of call)
trunks
circuits

Y(J)S SONET Slide 10
The Present PSTN
subscriber line
• Analog voltages and copper wire used only in “last mile”,
but core designed to mimic original situation
• Voice signal filtered to 4 KHz at input to digital network
• Time Division Multiplexing of digital signals in the network
• Extensive use of fiber optic and wireless physical links
• T1/E1, PDH and SONET/SDH “synchronous” protocols
• Signaling can be channel/trunk associated or via separate network (SS7)
• Automatic routing
• Circuit switching (route is maintained for duration of call)
• Complex routing optimization algorithms (LP, Karmarkar, etc)
PSTN Network
class 5 switchclass 5 switch
tandem switch
last mile

TDM timing
Time Domain Multiplexing relies on all channels (timeslots)
having precisely the same timing (frequency and phase)
In order to enforce this
the TDM device itself frequently performs the digitization
analog
signals
digital
signals

if the inputs are already digital
If the TDM switch does not digitize the analog signals
then there can be a problem
the clocks used to digitize do not have identical frequencies
we get byte slips! (well, actually, we can get bit slips first …)
exaggerated pictorial example
Numerical example:
clock derived from 8000 Hz. quartz crystal
typical crystal accuracy =  50 ppm
So 2 crystals can differ by 100 ppm
i.e. 0.8 samples / second
So difference is 1 sample after 1 ¼ seconds
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
3
4
4
4
4
4
4
5
5
5
6
6
6
5
5
5
7
7
7
6
6
6
8
8
8
9
9
9
7
7
7
9
8
8
component
signals
TDM

The fix
We must ensure that all the clocks have the same frequency
Every telephony network has an accurate clock called
a “stratum 1” or “Primary Reference Clock”
All other clocks are directly or indirectly locked to it (master – slave)
A TDM receiving device can lock onto the source clock
based on the incoming data (FLL, PLL)
For this to work, we must ensure that the data has enough transitions
(special line coding, scrambling bits, etc.)
1
0
transitions no transitions

Comparing clocks
A clock is said to be isochronous (isos=equal, chronos=time)
if its ticks are equally spaced in time
2 clocks are said to be synchronous (syn=same chronos=time)
if they tick in time, i.e. have precisely the same frequency
2 clocks are said to be plesiochronous (plesio=near chronos=time)
if they are nominally if the same frequency
but are not locked

PDH principle
If we want yet higher rates, we can mux together TDM signals (tributaries)
We could demux the TDM timeslots and directly remux them
– but that is too complex
The TDM inputs are already digital, so we must
– insist that the mux provide clock to all tributaries
(not always possible, may already be locked to a network)
OR
– somehow transport tributary with its own clock
across a higher speed network with a different clock
(without spoiling remote clock recovery)

PDH hierarchies
64 kbps
2.048 Mbps 1.544 Mbps 1.544 Mbps
6.312 Mbps 6.312 Mbps8.448 Mbps
34.368 Mbps
139.264 Mbps
44.736 Mbps 32.064 Mbps
97.728 Mbps274.176 Mbps
CEPT N.A. Japan
4
3
2
1
0
level
* 30
* 24
* 24
* 4
* 4
* 4
* 4
* 7
* 6
* 4
* 5
* 3
E1
E2
E3
E4
T1
T2
T3
T4
J1
J2
J3
J4

Framing and overhead
In addition to locking on to bit-rate
we need to recognize the frame structure
We identify frames by adding Frame Alignment Signal
The FAS is part of the frame overhead (which also includes "C-bits", OAM, etc.)
Each layer in PDH hierarchy adds its own overhead
For example
 E1 – 2 overhead bytes per 32 bytes – overhead 6.25 %
 E2 – 4 E1s = 8.192 Mbps out of 8.448Mbps
so there is an additional 0.256 Mbps = 3 %
altogether 4*30*64 kbps = 7.680 Mbps out of 8.448 Mbps
or 9.09% overhead
What happens next ?

PDH overhead
Overhead always increases with data rate !
digital
signal
data rate
(Mbps)
voice
channels
overhead
percentage
T1 1.544 24 0.52 %
T2 6.312 96 2.66 %
T3 44.736 672 3.86 %
T4 274.176 4032 5.88 %
E1 2.048 30 6.25 %
E2 8.448 120 9.09 %
E3 34.368 480 10.61 %
E4 139.264 1920 11.76 %

OAM
analog channels and 64 kbps digital channels
do not have mechanisms to check signal validity and quality
thus
 major faults could go undetected for long periods of time
 hard to characterize and localize faults when reported
 minor defects might be unnoticed indefinitely
Solution is to add mechanisms based on overhead
as PDH networks evolved, more and more overhead was dedicated to
Operations, Administration and Maintenance (OAM) functions
including:
 monitoring for valid signal
 defect reporting
 alarm indication/inhibition (AIS)

PDH Justification
In addition to FAS, PDH overhead includes
justification control (C-bits) and justification opportunity “stuffing” (R-bits)
Assume the tributary bitrate is B  T
Positive justification
payload is expected at highest bitrate B+T
if the tributary rate is actually at the maximum bitrate
then all payload and R bits are filled
if the tributary rate is lower than the maximum
then sometimes there are not enough incoming bits
so the R-bits are not filled and C-bits indicate this
Negative justification
payload is expected at lowest bitrate B-T
if the tributary rate is actually the minimum bitrate
then payload space suffices
if the tributary rate is higher than the minimum
then sometimes there are not enough positions to accommodate
so R-bits in the overhead are used and the C-bits indicate this
Positive/Negative justification
payload is expected at nominal bitrate B
positive or negative justification is applied as required

SONET/SDH
motivation and history

First step
With the disvestiture of the US Bell system a new need arose
MCI and NYNEX couldn’t directly interconnect optical trunks
Interexchange Carrier Compatibility Forum requested T1 to solve problem
Needed multivendor/ multioperator fiber-optic communications standard
Three main tasks:
 Optical interfaces (wavelengths, power levels, etc)
proposal submitted to T1X1 (Aug 1984)
T1.106 standard on single mode optical interfaces (1988)
 Operations (OAM) system
proposal submitted to T1M1
T1.119 standard
 Rates, formats, definition of network elements
Bellcore (Yau-Chau Ching and Rodney Boehm) proposal (Feb 1985)
proposed to T1X1
term SONET was coined
T1.105 standard (1988)

PDH limitations
Rate limitations
 Copper interfaces defined
 Need to mux/demux hierarchy of levels (hard to pull out a single timeslot)
 Overhead percentage increases with rate
At least three different systems (Europe, NA, Japan)
– E 2.048, 8.448, 34.348, 139.264
– T 1.544, 3.152, 6.312, 44.736, 91.053, 274.176
– J 1.544, 3.152, 6.312, 32.064, 97.728, 397.2
So a completely new mechanism was needed

Idea behind SONET
Synchronous Optical NETwork
 Designed for optical transport (high bitrate)
 Direct mapping of lower levels into higher ones
 Carry all PDH types in one universal hierarchy
– ITU version = Synchronous Digital Hierarchy
– different terminology but interoperable
 Overhead doesn’t increase with rate
 OAM designed-in from beginning

Standardization !
The original Bellcore proposal:
 hierarchy of signals, all multiple of basic rate (50.688)
 basic rate about 50 Mbps to carry DS3 payload
 bit-oriented mux
 mechanisms to carry DS1, DS2, DS3
Many other proposals were merged into 1987 draft document (rate 49.920)
In summer of 1986 CCITT express interest in cooperation
 needed a rate of about 150 Mbps to carry E4
 wanted byte oriented mux
Initial compromise attempt
 byte mux
 US wanted 13 rows * 180 columns
 CEPT wanted 9 rows * 270 columns
Compromise!
 US would use basic rate of 51.84 Mbps, 9 rows * 90 columns
 CEPT would use three times that rate - 155.52 Mbps, 9 rows * 270 columns

SONET/SDH
architecture

Layers
SONET was designed with definite layering concepts
Physical layer – optical fiber (linear or ring)
– when exceed fiber reach – regenerators
– regenerators are not mere amplifiers,
– regenerators use their own overhead
– fiber between regenerators called section (regenerator section)
Line layer – link between SONET muxes (Add/Drop Multiplexers)
– input and output at this level are Virtual Tributaries (VCs)
– actually 2 layers
 lower order VC (for low bitrate payloads)
 higher order VC (for high bitrate payloads)
Path layer – end-to-end path of client data (tributaries)
– client data (payload) may be
 PDH
 ATM
 packet data

SONET architecture
SONET (SDH) has at 3 layers:
 path – end-to-end data connection, muxes tributary signals path section
– there are STS paths + Virtual Tributary (VT) paths
 line – protected multiplexed SONET payload multiplex section
 section – physical link between adjacent elements regenerator section
Each layer has its own overhead to support needed functionality
SDH terminology
Path
Termination
Path
Termination
Line
Termination
Line
Termination
Section
Termination
path
line line line
ADM ADMregenerator
section section sectionsection

STS, OC, etc.
A SONET signal is called a Synchronous Transport Signal
The basic STS is STS-1, all others are multiples of it - STS-N
The (optical) physical layer signal corresponding to an STS-N is an OC-N
SONET Optical rate
STS-1 OC-1 51.84M
STS-3 OC-3 155.52M
STS-12 OC-12 622.080M
STS-48 OC-48 2488.32M
STS-192 OC-192 9953.28M
* 3
* 4
* 4
* 4

rates
and
frame structure

SONET / SDH frames
Synchronous Transfer Signals are bit-signals (OC are optical)
Like all TDM signals, there are framing bits at the beginning of the frame
However, it is convenient to draw SONET/SDH signals as rectangles
framing

SONET STS-1 frame
Each STS-1 frame is 90 columns * 9 rows = 810 bytes
There are 8000 STS-1 frames per second
so each byte represents 64 kbps (each column is 576 kbps)
Thus the basic STS-1 rate is 51.840 Mbps
90 columns
9rows
framing

SDH STM-1 frame
Synchronous Transport Modules are the bit-signals for SDH
Each STM-1 frame is 270 columns * 9 rows = 2430 bytes
There are 8000 STM-1 frames per second
Thus the basic STM-1 rate is 155.520 Mbps
3 times the STS-1 rate!
270 columns
9rows
…

SONET/SDH rates
STS-N has 90N columns STM-M corresponds to STS-N with N = 3M
SDH rates increase by factors of 4 each time
STS/STM signals can carry PDH tributaries, for example:
 STS-1 can carry 1 T3 or 28 T1s or 1 E3 or 21 E1s
 STM-1 can carry 3 E3s or 63 E1s or 3 T3s or 84 T1s
SONET SDH columns rate
STS-1 90 51.84M
STS-3 STM-1 270 155.52M
STS-12 STM-4 1080 622.080M
STS-48 STM-16 4320 2488.32M
STS-192 STM-64 17280 9953.28M

SONET/SDH tributaries
E3 and T3 are carried as Higher Order Paths (HOPs)
E1 and T1 are carried as Lower Order Paths (LOPs)
(the numbers are for direct mapping)
SONET SDH T1 T3 E1 E3 E4
STS-1 28 1 21 1
STS-3 STM-1 84 3 63 3 1
STS-12 STM-4 336 12 252 12 4
STS-48 STM-16 1344 48 1008 48 16
STS-192 STM-64 5376 192 4032 192 64

Synchronous Payload Envelope
STS-1 frame structure9rows
Transport
Overhead
TOH
6rows3rows
Section overhead is 3 rows * 3 columns = 9 bytes = 576 kbps
framing, performance monitoring, management
Line overhead is 6 rows * 3 columns = 18 bytes = 1152 kbps
protection switching, line maintenance, mux/concat, SPE pointer
SPE is 9 rows * 87 columns = 783 bytes = 50.112 Mbps
Similarly, STM-1 has 9 (different) columns of section+line overhead !
90 columns
9rows

STM-1 frame structure
Section
Overhead
SOH
STM-1 has 9 (different) columns of transport overhead !
RS overhead is 3 rows * 9 columns
Pointer overhead is 1 row * 9 columns
MS overhead is 5 rows * 9 columns
SPE is 9 rows * 261 columns
…
270 columns
RSOH
MSOH

Even higher rates
3 STS-1s can form an STS-3
4 STM-1s (STS-3s) can form an STM-4 (STS-12)
4 STM-4s (STS-12s) can form an STM-16 (STS-48)
etc. for STM-N (STS-3N)
The procedure is byte-interleaving
9 rows
9*N
columns
270*N columns

Byte-interleaving
. . .

Scrambling
SONET/SDH receivers recover clock based on incoming signal
Insufficient number of 0-1 transitions causes degradation of clock performance
In order to guarantee sufficient transitions, SONET/SDH employ a scrambler
 All data except first row of section overhead is scrambled
 Scrambler is 7 bit self-synchronizing X7 + X6 + 1
 Scrambler is initialized with ones
A short scrambler is sufficient for voice data
but NOT for data which may contain long stretches of zeros
When sending data an additional payload scrambler is used
 modern standards use 43 bit X43 + 1
 run continuously on ATM payload bytes (suspended for 5 bytes of cell tax)
 run continuously on HDLC payloads
Z-43
Xn Yn = Xn + Yn-43

STS-1 Overhead
The STS-1 overhead consists of
 3 rows of section overhead
– frame sync (A1, A2)
– section trace (J0)
– error control (B1)
– section orderwire (E1)
– Embedded Operations Channel (Di)
 6 rows of line overhead
– pointer and pointer action (Hi)
– error control (B2)
– Automatic Protection Switching signaling (Ki)
– Data Channel (Di)
– Synchronization Status Message (S1)
– Far End Block Error (M0)
– line orderwire (E2)
A1 A2 J0
B1 E1 F1
D1 D2 D3
H1 H2 H3
B2 K1 K2
D4 D5 D6
D7 D8 D9
D10 D11 D12
S1 M0 E2
section
overhead
line
overhead

STM-1 Overhead
A1 A1 A1 A2 A2 A2 J0 res res
B1 m m E1 m F1 res res
D1 m m D2 m D3
B2 B2 B2 K1 K2
D4 D5 D6
D7 D8 D9
D10 D11 D12
S1 M1 E2
RSOH
MSOH
SOH
m
– media
dependent
(defined for
SONET radio)
res
– reserved for
national use
AU pointers

A1, A2, J0 (section overhead)
A1, A2 - framing bytes
 A1 = 11110110
 A2 = 00101000
SONET/SDH framing always uses equal numbers of A1 and A2 bytes
J0 - regenerator section trace (in early SONET - a counter called C1)
enables receiver to be sure that the section connection is still OK
enables identifying individual STS/STMs after muxing
J0 goes through a 16 byte sequence
MSBs are J0 framing (1000…00)
Cs are CRC-7 of previous frame
S are 15 7-bit characters
section access point identifier
SSSSSSS0
SSSSSSS0
C7C6C5C4C3C2C11
…

B1, E1, F1, D1-3 (section overhead)
B1 – Byte Interleaved Parity-8 byte
even parity of bits of bytes of previous frame after scrambling
only 1 BIT-8 for multiplexed STS/STM
E1 – section orderwire
64 kbps voice link for technicians
from regenerator to regenerator
F1 – 64 kbps link for user purposes
D1 + D2 + D3 – 192 kbps messaging channel
used by section termination as Embedded Operations Channel (SONET)
or Data Communications Channel (SDH)

Pointers (line overhead)
In SONET, pointers are considered part of line overhead
For STS-1, H1+H2 is the pointer, H3 is the pointer action
H1+H2 indicates the offset (in bytes) from H3 to the SPE
(i.e. if 0 then J1 POH byte is immediately after H3 in the row)
4 MSBs are New Data Flag, 10 LSBs are actual offset value (0 – 782)
When offset=522 the STS-1 SPE is in a single STS-1 frame
In all other cases the SPE straddles two frames
When offset is a multiple of 87, the SPE is rectangular
To compensate for clock differences
we have pointer justification
When negative justification
H3 carries the extra data
When positive justification
byte after H3 is stuffing byte

SONET Justification
If tributary rate is above nominal, negative justification is needed
When less than 8 more bits than expected in buffer
 NDF is 0110
 offset unchanged
When 8 extra bits accumulate
 NDF is set to 1001
 extra byte placed into H3
 offset is decremented by 1 (byte)
If tributary rate is below nominal, positive justification is needed
When less than 8 fewer than expected bits in buffer
 NDF is 0110
 offset unchanged
When 8 missing bits
 NDF is set to 1001
 byte after H3 is stuffing
 offset is incremented by 1 (byte)
H1 H2 extra …
H1 H2 H3 stuff …

B2, K1, K2, D4-D12 (line overhead)
B2 – BIP-8 of line overhead + previous envelope (w/o scrambling)
N B2s for muxed STM-N
K1 and K2 are used for Automatic Protection Switching (see later)
D4 – D12 are a 576 Kbps Data Communications Channel
between multiplexers
usually manufacturer specific OAM functions

S1, M0, E2 (line overhead)
S1 – Synchronization Status Message
indicates stratum level (unknown, stratum 1, …, do not use)
M0 – Far End Block Error
indicates number of BIP violations detected
E2 – line orderwire
64 kbps voice link for technicians
from line mux to line mux

Payloads
and
Mappings

STS-1 HOP SPE structure
We saw that the pointer the line overhead points to the STS path overhead POH
(after re-arranging) POH is one column of 9 rows (9 bytes = 576 kbps)

STS-1 HOP
1 column of SPE is POH
2 more (“fixed stuffing”) columns are reserved
We are left with
84 columns = 756 bytes = 48.384 Mbps for payload
This is enough for a E3 (34.368M) or a T3 (44.736M)
1 875930

STS-1 Path overhead
1 column of overhead for path (576 Kbps)
POH is responsible for
– path type identification
– path performance monitoring
– status (including of mapped payloads)
– virtual concatenation
– path protection
– trace
J1
B3
C2
G1
F2
H4
F3
K3
N1
POH

J1, B3, C2 (path overhead)
J1 – path trace
enables receiver to be sure
that the path connection is still OK
B3 – BIP-8 even bit parity of bytes
(without scrambling)
of previous payload
C2 – path signal label
identifies the payload type
(examples in table)
C2
(hex)
Payload type
00 unequipped
01 nonspecific
02 LOP (TUG)
04 E3/T3
12 E4
13 ATM
16 PoS – RFC 1662
18 LAPS X.85
1A 10G Ethernet
1B GFP
CF PoS - RFC1619

G1, F2, H4, F3, K3, N1 (path overhead)
G1 – path status
conveys status and performance back to originator
4 MSBs are path FEBE, 1 bit RDI, 3 unused
F2 and F3 – user specific communications
H4 – used for LOP multiframe sync and VCAT (see later)
K3 (4 MSBs) – path APS
N1 – Tandem Connection Monitoring
Messaging channel for tandem connections

LOP
To carry lower rate payloads, divide the 84 available columns
into 7 * 12 interleaved columns, i.e. 7 Virtual Tributary (VT) Groups
VT group is 12 columns of 9 rows, i.e. 108 bytes or 6.912 Mbps
VT group is composed of VT(s)
 there are different types of VT in order to carry different types of payload
 all VTs in VT group must be of the same type (no mixing)
 but different VT groups in same SPE can have different VT types
A VT can have 3, 4, 6 or 12 columns
1 875930 1 2 3 4 5 6 7
7 VTGs

SONET/SDH : VT/VC types
VT/STS VC column
rate
payload
VT 1.5 VC-11 3 1.728 DS1 (1.544)
VT 2 VC-12 4 2.304 E1 (2.048)
VT 3 6 3.456 DS1C (3.152)
VT 6 VC-2 12 6.912 DS2 (6.312)
STS-1 VC-3 48.384 E3 (34.368)
STS-1 VC-3 48.384 DS3 (44.736)
STS-3c VC-4 149.760 E4 (139.264)
LOP
HOP
standard PDH rates map efficiently into SONET/SDH !
4 per group
3 per group
2 per group
1 per group

Y(J)S APS Slide 57
LO Path overhead
LOP OH is responsible for timing, PM, REI, …
LO Path APS signaling is 4 MSBs of byte K4
V5
J2
N2
K4
V1 pointer
V2 pointer
V3 pointer
V4 pointer
VC11 – 25B
VC12 – 34B
125 msec
500 msec
H4=XXXXXX00
H4=XXXXXX01
H4=XXXXXX10
H4=XXXXXX11
VC11 – 27B
VC12 – 36B

Payload capacity
VT1.5/VC-11 has 3 columns = 27 bytes = 1.728 Mbps
but 2 bytes are used for overhead (V1/V2/V3/V4 and V5/J2/N2/K4)
so actually only 25 bytes = 1.6 Mbps are available
Similarly
VT2/VC-12 has 4 columns = 36 bytes = 2.304 Mbps
but 2 bytes are used for overhead
So actually only 34 bytes = 2.176 Mbps are available

LOP overhead
V5 consists of
 BIP (2b)
 REI (1b)
 RFI (1b)
 Signal label (3b) (uneq, async, bit-sync, byte-sync, test, AIS)
 RDI (1b)
J2 is path trace
N2 is the network operator byte
– may be used for LOP tandem connection monitoring (LO-TCM)
K4 is for LO VCAT and LO APS

SDH Containers
Tributary payloads are not placed directly into SDH
Payloads are placed (adapted) into containers
The containers are made into virtual containers (by adding POH)
Next, the pointer is used – the pointer + VC is a TU or AU
Tributary Unit adapts a lower order VC to high order VC
Administrative Unit adapts higher order VC to SDH
TUs and AUs are grouped together until they are big enough
We finally get an Administrative Unit Group
To the AUG we add SOH to make the STM frame

Formally …
C-n n = 11, 12, 2, 3, 4
VC-n = POH + C-n
TU-n = pointer + VC-n (n=11, 12, 2, 3)
AU-n = pointer + VC-n (n=3,4)
TUG = N * TU-n
AUG = N * AU-n
STM-N = SOH + AUG

Multiplexing
An AUG may contain a VC-4 with an E4
or it may contain 3 AU-3s each with a VC-3s with an E3
In the latter case, the AU pointer points to the AUG
and inside the AUG are 3 pointers to the AU-3s
J1
B3
C2
G1
F2
H4
F3
K3
N1
H1 H1H1 H2 H2H2 H3 H3H3

More multiplexing
Similarly, we can hierarchically build complex structures
Lower rate STMs can be combined into higher rate STMs
AUGs can be combined into STMs
AUs can be combined into AUGs
TUGs can be combined into high order VCs
Lower rate TUs can be combined into TUGs
etc.
But only certain combinations are allowed by standards

All SDH mappings
STM-N
AU-3 VC-3 C3
VC-3TU-3TUG-3
C-4VC-4AU-4AUG
…
AUG
AUG
C2
C12
C11
TUG-2 VC-2TU-2
VC-12TU-12
VC-11TU-11
STM-0
ATM 2.144 M
E4 139.264 M
ATM 1.6 M
ATM 149.760M
ATM 48.384 M
ATM 6.874M
E3 34.368 M
T3 44.736 M
T2 6.312 M
E1 2.048 M
T1 1.544 M
* 3
*7
* 3
*7
* 4
* 3

All SONET mappings
STS-N STS-3 SPESTS-3c
STS-1
VT6 SPE
VT2 SPE
VT1.5 SPE
VT6
VT-2
VT1.5
ATM 2.144 M
E4 139.264 M
ATM 1.6 M
ATM 149.760M
ATM 48.384 M
ATM 6.874M
E3 34.368 M
T3 44.736 M
T2 6.312 M
E1 2.048 M
T1 1.544 M
*N
STS-1 SPE
VTG
*7
pointer processing
* 3
* 4

Tributary mapping types
When mapping tributaries into VCs, PDH-like bit-stuffing is used
For E1 and T1 there are several options
 Asynchronous mapping (framing-agnostic)
 Bit synchronous mapping
 Byte synchronous mapping (time-slot aligned)
E4 into VC-4, E3/T3 into VC-3 are always asynchronous
T1 into VC-11 may be any of the 3
(in byte synchronous the framing bit is placed in the VC overhead)
E1 into VC-12 may be asynchronous or byte synchronous

WAN-PHY (10 GbE in STM-64)
There is a special case where the bit-rates work out relatively well
GbE 10GBASE-R (64B/66B coding) can be directly mapped
into a STM-64 (with contiguous concatenation - see later) without need for GFP
MAC creates "stretched InterPacket Gap" to compensate for rate being < 10G
This is the fastest connection commonly used for Internet traffic
Complication: SDH clock accuracy is 4.6 ppm, GbE accuracy is 20 ppm
64*(270-9) = 16704 columns
J1
63 columns of fixed stuff
10GBASE-W 802.3-2005 Clause 50

Protection
and
Rings

What is protection ?
SONET/SDH need to be highly reliable (five nines)
Down-time should be minimal (less than 50 msec)
So systems must repair themselves (no time for manual intervention)
Upon detection of a failure (dLOS, dLOF, high BER)
the network must reroute traffic (protection switching)
from working channel to protection channel
The Network Element that detects the failure (tail-end NE)
initiates the protection switching
The head-end NE must change forwarding or to send duplicate traffic
Protection switching is unidirectional
Protection switching may be revertive (automatically revert to working channel)
head-end NE tail-end NE
working channel
protection channel

How does it work?
Head-end and tail-end NEs have bridges (muxes)
Head-end and tail-end NEs maintain bidirectional signaling channel
Signaling is contained in K1 and K2 bytes of protection channel
 K1 – tail-end status and requests
 K2 – head-end status
head-end bridge tail-end bridge
working channel
protection channel signaling channel

Linear 1+1 protection
Simplest form of protection
Can be at OC-n level (different physical fibers)
or at STM/VC level (called SubNetwork Connection Protection)
or end-to-end path (called trail protection)
Head-end bridge always sends data on both channels
Tail-end chooses channel to use based on BER, dLOS, etc.
No need for signaling
If non-revertive
there is no distinction between working and protection channels
BW utilization is 50%
channel A
channel B

Linear 1:1 protection
Head-end bridge usually sends data on working channel
When tail-end detects failure it signals (using K1) to head-end
Head-end then starts sending data over protection channel
When not in use
protection channel can be used for (discounted) extra traffic
(pre-emptible unprotected traffic)
May be at any layer (only OC-n level protects against fiber cuts)
working channel
protection channel
extra traffic

Linear 1:N protection
In order to save BW
we allocate 1 protection channel for every N working channels
N limited to 14
4 bits in K1 byte from tail-end to head-end
– 0 protection channel
– 1-14 working channels
– 15 extra traffic channel
working channels
protection channel

Two fiber vs. Four-fiber rings
Ring based protection is popular in North America (100K+ rings)
Full protection against physical fiber cuts
Simpler and less expensive than mesh topologies
Protection at line (multiplexed section) or path layer
Four-fiber rings
fully redundant at OC level
can support bidirectional routing at line layer
Two-fiber rings
support unidirectional routing at line layer
2 fibers in opposite directions

Unidirectional vs. bidirectional
Unidirectional routing
working channel B-A same direction (e.g. clockwise) as A-B
management simplicity: A-B and B-A can occupy same timeslots
Inefficient: waste in ring BW and excessive delay in one direction
Bidirectional routing
A-B and B-1 are opposite in direction
both using shortest route
spatial reuse: timeslots can be reused in other sections
A
BA-B
B-A
A
B
B-A
A-B
C
B-C
C-B

UPSR vs. BLSR (MS-SPRing)
Of all the possible combinations, only a few are in use
Unidirectional Path Switched Rings
protects tributaries
extension of 1+1 to ring topology
Bidirectional Line Switched Rings (two-fiber and four-fiber versions)
called Multiplex Section Shared Protection Ring in SDH
simultaneously protects all tributaries in STM
extension of 1:1 to ring topology
Path switching
Line switching
Two-fiber
Four-fiber
Unidirectional
Bidirectional
UPSR
BLSR

UPSR
Working channel is in one direction
protection channel in the opposite direction
All traffic is added in both directions
decision as to which to use at drop point (no signaling)
Normally non-revertive, so effective two diversity paths
Good match for access networks
1 access resilient ring
less expensive than fiber pair per customer
Inefficient for core networks
no spatial reuse
every signal in every span
in both directions
node needs to continuously monitor
every tributary to be dropped

BLSR
Switch at line level – less monitoring
When failure detected tail-end NE signals head-end NE
Works for unidirectional/bidirectional fiber cuts, and NE failures
Two-fiber version
half of OC-N capacity devoted to protection
only half capacity available for traffic
Four-fiber version
full redundant OC-N devoted to protection
twice as many NEs as compared to two-fiber
Example
recovery from unidirectional fiber cut

VCAT
and
LCAS

Concatenation
Payloads that don’t fit into standard VT/VC sizes can be accommodated
by concatenating of several VTs / VCs
For example, 10 Mbps doesn’t fit into any VT or VC
so w/o concatenation we need to put it into an STS-1 (48.384 Mbps)
the remaining 38.384 Mbps can not be used
We would like to be able to divide the 10 Mbps among
7 VT1.5/VC-11 s = 7 * 1.600 = 11.20 Mbps or
5 VT2/VC-12 s = 5 * 2.176 = 10.88 Mbps

Concatenation (cont.)
There are 2 ways to concatenate X VTs or VCs:
 Contiguous Concatenation (G.707 11.1)
– HOP – STS-Nc (SONET) or VC-4-Nc (SDH)
or LOP – 1-7 VC-2-Nc into a VC-3
– since has to fit into SONET/SDH payload
 only STS-Nc : N=3 * 4n or VC-4-Nc : N=4n
– components transported together and in-phase
– requires support at intermediate network elements
 Virtual Concatenation (VCAT G.707 11.2)
– HOP – STS-1-Xv or STS-Nc-Xv (SONET) or VC-3/4-Xv (SDH)
or LOP – VT-1.5/2/3/6-Xv (SONET) or VC-11/12/2-Xv (SDH)
– HOP: X ≤ 256 LOP: X ≤ 64 (limitation due to bits in header)
– payload split over multiple STSs / STMs
– fragments may follow different routes
– requires support only at path terminations
– requires buffering and differential delay alignment

Contiguous Concatenation: STS-3c
270 columns9rows
…
9 columns of
section and
line overhead
3 columns of
path overhead
258 columns of SPE
STS-3
270 columns
9rows
…
9 columns of
section and
line overhead
1 column of
path overhead
260 columns of SPE
STS-3c
258 columns * 0.576 = 148.608 Mbps
260 columns * 0.576 = 149.760 Mbps

STS-N vs. STS-Nc
Although both have raw rates of 155.520 Mbps
STS-3c has 2 more columns (1.152Mbps) available
More generally, For STS-Nc gains (N-1) columns
e.g. STS-12c gains 11 columns = 6.336Mbps vis a vis STS-12
STS-48c gains 47 columns = 27.072 Mbps
STS-192c gains 191 columns = 110.016 Mbps !
However, an STS-Nc signal is not as easily separable
when we want to add/drop component signals

Virtual Concatenation
VCAT is an inverse multiplexing mechanism (round-robin)
VCAT members may travel along different routes in SONET/SDH network
Intermediate network elements don’t need to know about VCAT
(unlike contiguous concatenation that is handled by all intermediate nodes)
…
H4

SDH virtually concatenated VCs
So we have many permissible rates
1.600, 2.176, 3.200, 4.352, 4.800, 6.400, 6.528, 6.784, 8.000, …
VC Capacity (Mbps) if all members in one VC
VC-11-Xv 1.600, 3.200, … 1.600X in VC-3 X ≤ 28 C ≤ 44.800
in VC-4 X ≤ 64 C ≤ 102.400
VC-12-Xv 2.176, 4.352, … 2.176X in VC-3 X ≤ 21 C ≤ 45.696
in VC-4 X ≤ 63 C ≤ 137.088
VC-2-Xv 6.784, 13.568, …, 6.784X in VC-3 X ≤ 7 C ≤ 47.448
in VC-4 X ≤ 21 C ≤ 142.464

SONET virtually concatenated VTs
VT Capacity (Mbps) If all members in one STS
VT1.5-Xv 1.600, 3.200, … 1.600X in STS-1 X ≤ 28 C ≤ 44.800
in STS-3c X ≤ 64 C ≤ 102.400
VT2-Xv 2.176, 4.352, … 2.176X in STS-1 X ≤ 21 C ≤ 45.696
in STS-3c X ≤ 63 C ≤ 137.088
VT3-Xv 3.328, 6.656, … 3.328X in STS-1 X ≤ 14 C ≤ 46.592
in STS-3c X ≤ 42 C ≤ 139.776
VT6-Xv 6.784, 13.568, … 6.784X in STS-1 X ≤ 7 C ≤ 47.448
in STS-3c X ≤ 21 C ≤ 142.464
So we have many permissible rates
1.600, 2.176, 3.200, 3.328, 4.352, 4.800, 6.400, 6.528, 6.656, 6.784, …

Efficiency comparison
Using VCAT increases efficiency to close to 100% !
rate w/o VCAT efficiency with VCAT efficiency
10 STS-1 21% VT2-5v
VC-12-5v
92%
100 STS-3c
VC-4
67% STS-1-2v
VC-3-2v
100%
1000 STS-48c
VC-4-16c
42% STS-3c-7v
VC-4-7v
95%

PDH VCAT
Recently ITU-T G.7043 expanded VCAT to E1,T1,E3,T3
Enables bonding of up to 16 PDH signals to support higher rates
Only bonding of like PDH signals allowed (e.g. can’t mix E1s and T1s)
Multiframe is always per G.704/G.832 (e.g. T1 – ESF 24 frames, E1 16 frames)
1 byte per multiframe is VCAT overhead (SQ, MFI, MST, CRC)
Supports LCAS (to be discussed next)
TS0
1st
frame
of
4 E1s
VCAT
overhead
octet
timeeach E1

PDH VCAT overhead octet
There is one VCAT overhead octet per multiframe, so net rate is
T1: (24*24-1=) 575 data bytes per 3 ms. multiframe = 191.666 kB/s
E1: (16*30-1=) 495 data bytes per 2 ms multiframe = 247.5 kB/s
T3 and E3 can also be used
We will show the overhead octet format later
(when using LCAS, the overhead octet is called VLI)
TS0
frames
of an
E1
VCAT
overhead
octet
…

Delay compensation
802.1ad Ethernet link aggregation cheats
– each identifiable flow is restricted to one link
– doesn’t work if single high-BW flow
VCAT is completely general
– works even with a single flow
VCG members may travel over completely separate paths
so the VCAT mechanism must compensate for differential delay
Requirement for over ½ second compensation
Must compensate to the bit level
but since frames have Frame Alignment Signal
the VCAT mechanism only needs to identify individual frames

VCAT buffering
Since VCAT components may take different paths
At egress the members
are no longer in the proper temporal relationship
VCAT path termination function buffers members
and outputs in proper order (relying on POH sequencing)
(up to 512 ms of differential delay can be tolerated)
VCAT defines a multiframe to enable delay compensation
– length of multiframe determines delay that can be accommodated
H4 byte in member’s POH contains :
 sequence indicator (identifies component) (number of bits limits X)
 MFI multiframe indicator (multiframe sequencing to find differential delay)

Multiframes and superframes
Here is how we compensate for 512 ms of differential delay
512 ms corresponds to a superframe is 4096 TDM frames (4096*0.125m=512m)
For HOP SDH VCAT and PDH VCAT (H4 byte or PDH VCAT overhead)
The basic multiframe is 16 frames
So we need 256 multiframes in a superframe (256*16=4096)
The MultiFrame Indicator is divided into two parts:
 MFI1 (4 bits) appears once per frame
– and counts from 0 to 15 to sequence the multiframe
 MFI2 (8bits) appears once per multiframe
– and counts from 0 to 255
For LOP SDH (bit 2 of K4 byte)
– a 32 bit frame is built and a 5-bit MFI is dedicated
– 32 multiframes of 16 ms give the needed 512 ms

Link Capacity Adjustment Scheme
LCAS is defined in G.7042 (also numbered Y.1305)
LCAS extends VCAT by allowing dynamic BW changes
LCAS is a protocol for dynamic adding/removing of VCAT members
– hitless BW modification
– similar to Link Aggregation Control Protocol for Ethernet links
LCAS is not a “control plane” or “management” protocol
– it doesn’t allocate the members
– still need control protocols to perform actual allocation
LCAS is a “handshake” protocol
– it enables the path ends to negotiate the additional / deletion
– it guarantees that there will be no loss of data during change
– it can determine that a proposed member is ill suited
– it allows automatic removal of faulty member

LCAS – how does it work?
LCAS is unidirectional (for symmetric BW need to perform twice)
LCAS functions can be initiated by source or sink
LCAS assumes that all VCG members are error-free
– LCAS messages are CRC protected
LCAS messages are sent in advance
– sink processes messages after differential compensation
– message describes link state at time of next message
– receiver can switch to new configuration in time
LCAS messages are in the upper nibble of
– H4 byte for HOS SONET/SDH
– K4 byte for LOS SONET/SDH
– VCAT overhead octet for PDH – VCAT and LCAS Information
LCAS messages employ redundancy
– messages from source to sink are member specific
– messages from sink to source are replicated
J1
B3
C2
G1
F2
H4
F3
K3
N1
POH

LCAS control messages
LCAS adds fields to the basic VCAT ones
Fields in messages from source to sink:
– MFI MultiFrame Indicator
– SQ SeQuence indicator (member ID inside VCAT group)
– CTRL ConTRoL (IDLE, being ADDed, NORMal, End of Sequence, Do Not Use)
– GID Group Identification (identifies VCAT group)
Fields in messages from sink to source (identical in all members):
– MST Member Status (1 bit for each VCG member)
– RS-Ack ReSequence Acknowledgement
Fields in both directions
– CRC Cyclic Redundancy Code
The precise format depends on the VCAT type (H4, K4, PDH)
Note: for H4 format SQ is 8 bits, so up to 256 VCG members
for PDH SQ is only 4 bits, so up to 16 VCG members

H4 format
MFI2 bits 1-4 0 0 0 0
MFI2 bits 5-8 0 0 0 1
CTRL 0 0 1 0
0 0 0 GID 0 0 1 1
0 0 0 0 0 1 0 0
0 0 0 0 0 1 0 1
CRC-8 bits 1-4 0 1 1 0
CRC-8 bits 5-8 0 1 1 1
MST bits 1 0 0 0
more MST bits 1 0 0 1
0 0 0 RS-ACK 1 0 1 0
0 0 0 0 1 0 1 1
0 0 0 0 1 1 0 0
0 0 0 0 1 1 0 1
SQ bits 1-4 1 1 1 0
SQ bits 5-8 1 1 1 1
16framemultiframe
MFI1
reservedfieldsreservedfields

H4 format – some comments
CRC-8 (when using K4 it is CRC-3)
– covers the previous 14 frames (not sync’ed on multiframe)
– polynomial x8 + x2 + x + 1
MST
– each VCG member carries the status of all members
– so we need 256 bits of member status
– this is done by muxing MST bits
– there are MST bits per multiframe
– and 32 multiframes in an MST multiframe
– no special sequencing, just MFI2 multiframe mod 32
GID
– single bit indentifier
– all members of VCG share the same bit
– cycles through 215-1 LFSR sequence
– different VCGs use different phase offsets of sequence

LCAS – adding a member (1)
When more/less BW is needed, we need to add/remove VCAT members
Adding/removing VCAT members first requires provisioning (management)
LCAS handles member sequence numbers assignment
LCAS ensures service is not disrupted
Example: to add a 4th member to group “1”
Initial state:
Step 1: NMS provisions new member
source sends CTRL=IDLE for new member
sink sends MST=FAIL for new member
GID=g SQ=1 CTRL=NORM
GID=g SQ=3 CTRL=EOS
GID=g SQ=3 CTRL=EOS
GID=g SQ=FF CTRL=IDLE

LCAS – adding a member (2)
Step 2: source sends CTRL=ADD and SQ
sink sends MST=OK for new member
 if it has been provisioned
 if receiving new member OK
 if it is able to compensate for delay
otherwise it will send MST=FAIL
and source reports this to NMS
Step 3: source sends CTRL=EOS for new member
new member starts to carry traffic
sink sends RS-ACK
Note 1: several new members may be added at once
Note 2: removing a member is similar
Source puts CTRL=IDLE for member to be removed and stops using it
All member sequence numbers must be adjusted
GID=g SQ=3 CTRL=EOS
GID=g SQ=4 CTRL=ADD
GID=g SQ=4 CTRL=EOS

Y(J)S SONET Slide
LCAS – service preservation
To preserve service integrity if sink detects a failure of a VCAT member
LCAS can temporarily remove member (if service can tolerate BW reduction)
Example: Initial state
Step 1: sink sends MST=FAIL for member 2
source sends CTRL=DNU (special treatment if EoS)
and ceases to use member 2
Note: if EoS fails, renumber to ensure EoS is active
Step 2: sink sends MST=OK indicating defect is cleared
source returns CTRL to NORM
and starts using the member again
Note: if NMS decides to permanently remove the member, proceed as in previous slide
GID=g SQ=4 CTRL=EOS
GID=g SQ=2 CTRL=DNU
GID=g SQ=4 CTRL=EOS

Y(J)S SONET Slide
Handling
Packet
Data

Y(J)S SONET Slide
Packet over SONET
Currently defined in RFC2615 (PPP over SONET) obsoletes RFC1619
SONET/SDH can provide a point-to-point byte-oriented
full-duplex synchronous link
PPP is ideal for data transport over such a link
PoS uses PPP in HDLC framing to provide a byte-oriented interface
to the SONET/SDH infrastructure
POH signal label (C2)
indicates PoS as C2=16 (C2=CF if no scrambler)

Y(J)S SONET Slide
PoS architecture
PoS is based on PPP in HDLC framing
Since SONET/SDH is byte oriented, byte stuffing is employed
A special scrambler is used to protect SONET/SDH timing
PoS operates on IP packets
If IP is delivered over Ethernet
– the Ethernet is terminated (frame removed)
– Ethernet must be reconstituted at the far end
– require routers at edges of SONET/SDH network
IP
PPP
HDLC
SONET/SDH

Y(J)S SONET Slide
PoS Details
IP packet is encapsulated in PPP
– default MTU is 1500 bytes
– up to 64,000 bytes allowed if negotiated by PPP
FCS is generated and appended
PPP in HDLC framing with byte stuffing
43 bit scrambler is run over the SPE
byte stream is placed octet-aligned in SPE
– (e.g. 149.760 Mbps of STM-1)
– HDLC frames may cross SPE boundaries

Y(J)S SONET Slide
POS problems
PoS is BW efficient
but POS has its disadvantages
 BW must be predetermined
 HDLC BW expansion and nondeterminacy
 BW allocation is tightly constrained by SONET/SDH capacities
– e.g. GBE requires a full OC-48 pipe
 POS requires removing the Ethernet headers
– so lose RPR, VLAN, 802.1p, multicasting, etc
 POS requires IP routers

Y(J)S SONET Slide
LAPS
In 2001 ITU-T introduced protocols for transporting packets over SDH
 X.85 IP over SDH using LAPS
 X.86 Ethernet over LAPS
Built on series of ITU “LAPx” HDLC-based protocols
Use ISO HDLC format
Implement connectionless byte-oriented protocols over SDH
X.85 is very close to (but not quite) IETF PoS

Y(J)S SONET Slide
GFP architecture
A new approach, not based on HDLC
Defined in ITU-T G.7041 (also numbered Y.1303)
originally developed in T1X1 to fix ATM limitations
(like ATM) uses HEC protected frames instead of HDLC
Client may be PDU-oriented (Ethernet MAC, IP)
or block-oriented (GBE, fiber channel)
GFP frames
– are octet aligned
– contain at most 65,535 bytes
– consist of a header + payload area
Any idle time between GFP frames is filled with GFP idle frames
Ethernet IP other
GFP – client specific part
GFP – common part
SDH OTN other
HDLC

Y(J)S SONET Slide
GFP frame structure
Every GFP frame has a 4-byte core header
– 2 byte Payload Length Indicator
PLI = 01,2,3 are for control frames
– 2 byte core Header Error Control
X16 + X12 + X5 + 1
– entire core header is XOR’ed with B6AB31E0
Idle GFP frames
– have PLI=0
– have no payload area
Non-idle GFP frames
– have ≥ 4 bytes in payload area
– the payload has its own header
– 2 payload modes : GFP-F and GFP-T
– optionally protect payload with CRC-32
PLI (2B)
cHEC (2B)
payload header
(4-64B)
payload
optional payload
FCS (4B)
core
header
payload
area

Y(J)S SONET Slide
GFP payload header
GFP payload header has
– type (2B)
– type HEC (CRC-16)
– extension header (0-60B)
either null or linear extension (payload type muxing)
– extension HEC (CRC-16)
type consists of
– Payload Type Identifier (3b)
 PTI=000 for client data
 PTI=100 for client management (OAM dLOS, dLOF)
– Payload FCS Indicator (1b)
 PFI=1 means there is a payload FCS
– Extension Header ID (3b)
– User Payload Identifier (8b)
 values for Ethernet, IP, PPP, FC, RPR, MPLS, etc.
type (2B)
tHEC (2B)
extension header
(0-60B)
eHEC (2B)
UPI (8b)
PTI (3b) EXI (3b)PFI

Y(J)S SONET Slide
GFP modes
GFP-F - frame mapped GFP
Good for PDU-based protocols (Ethernet, IP, MPLS)
or HDLC-based ones (PPP)
Client PDU is placed in GFP payload field
GFP-T – transparent GFP
Good for protocols that exploit physical layer capabilities
In particular
8B/10B line code
used in fiber channel, GbE, FICON, ESCON, DVB, etc
Were we to use GFP-F would lose control info, GFP-T is transparent to these codes
Also, GFP-T needn’t wait for entire PDU to be received (adding delay!)

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