SONET/SDH
Yaakov (J) Stein
Chief Scientist
RAD Data Communications
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 2
Background
Y(J)S SONET Slide 3
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 4
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)), …
channels
4 kHz
8 kHz
12 kHz
f
16 kHz
20 kHz
Y(J)S SONET Slide 5
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
timeslots
T1 = 24 conversations per trunk
2 groups per cable!
t
Y(J)S SONET Slide 6
and switching became easier too
Analog Crossbar switch
1
2
3
4
5
6
Digital Cross-connect (DXC)
7
1
2
3
4
5
6
7
8
1
2
3
4
5
t
3
t
processor
2
1
5
4
Complexity increases rapidly with size
Y(J)S SONET Slide 7
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 8
The PSTN circa 1960
trunks
circuits
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)
Y(J)S SONET Slide 9
The Present PSTN
tandem switch
last mile
subscriber line
class 5 switch
PSTN Network
class 5 switch
• 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)
Y(J)S SONET Slide 10
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
digital
signals
analog
signals
Y(J)S SONET Slide 11
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:
3
4
5
6
7
9
1
2
8
1
2
3
4
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
9
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
clock derived from 8000 Hz. quartz crystal
typical crystal accuracy =  50 ppm
So 2 crystals can differ by 100 ppm
5
6
7
8
component
signals
9
i.e. 0.8 samples / second
So difference is 1 sample after 1 ¼ seconds
TDM
Y(J)S SONET Slide 12
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
Y(J)S SONET Slide 13
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
Y(J)S SONET Slide 14
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)
Y(J)S SONET Slide 15
PDH hierarchies
level
0
64 kbps
*
1
E1 2.048 Mbps
*
2
3
E3
4
4
E4 139.264 Mbps
CEPT
24
T1 1.544 Mbps
*
4
T2 6.312 Mbps
4
34.368 Mbps
*
*
4
E2 8.448 Mbps
*
30
*
T3
7
44.736 Mbps
*
6
T4 274.176 Mbps
N.A.
*
24
J1 1.544 Mbps
*
4
J2 6.312 Mbps
*
5
J3 32.064 Mbps
*
3
J4 97.728 Mbps
Japan
Y(J)S SONET Slide 16
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 ?
Y(J)S SONET Slide 17
PDH overhead
digital
signal
data rate
voice
(Mbps)
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 %
Overhead always increases with data rate !
Y(J)S SONET Slide 18
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)
Y(J)S SONET Slide 19
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
Y(J)S SONET Slide 20
SONET/SDH
motivation and history
Y(J)S SONET Slide 21
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)
Y(J)S SONET Slide 22
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
Y(J)S SONET Slide 23
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
Y(J)S SONET Slide 24
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
Y(J)S SONET Slide 25
SONET/SDH
architecture
Y(J)S SONET Slide 26
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
Y(J)S SONET Slide 27
SONET architecture
Path
Termination
ADM
regenerator
ADM
Line
Termination
Section
Termination
Line
Termination
Path
Termination
path
line
section
line
section
line
section
section
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

section – physical link between adjacent elements
multiplex section
regenerator section
Each layer has its own overhead to support needed functionality
SDH terminology
Y(J)S SONET Slide 28
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
*3
STS-12
OC-12
622.080M
*4
STS-48
OC-48
2488.32M
*4
STS-192
OC-192
9953.28M
*4
Y(J)S SONET Slide 29
rates
and
frame structure
Y(J)S SONET Slide 30
SONET / SDH frames
framing
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
Y(J)S SONET Slide 31
SONET STS-1 frame
90 columns
9 rows
framing
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
Y(J)S SONET Slide 32
SDH STM-1 frame
270 columns
9 rows
…
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!
Y(J)S SONET Slide 33
SONET/SDH rates
SONET
SDH
STS-1
columns
rate
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
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
Y(J)S SONET Slide 34
SONET/SDH tributaries
SONET
SDH
STS-1
T1
T3
E1
E3
28
1
21
1
E4
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
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)
Y(J)S SONET Slide 35
STS-1 frame structure
9 rows
6 rows
3 rows
90 columns
Transport
Overhead
TOH
Synchronous Payload Envelope
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 !
Y(J)S SONET Slide 36
STM-1 frame structure
270 columns
RSOH
…
MSOH
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
Y(J)S SONET Slide 37
9*N
columns
Even higher rates
9 rows
270*N columns
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
Y(J)S SONET Slide 38
Byte-interleaving
...
Y(J)S SONET Slide 39
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
Xn
Yn = Xn + Yn-43
Z-43
Y(J)S SONET Slide 40
STS-1 Overhead
section
overhead
line
overhead
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
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)
Y(J)S SONET Slide 41
STM-1 Overhead
RSOH
A1
A1
A1
A2
A2
B1
m
m
E1
D1
m
m
D2
A2
J0
res
res
m
F1
res
res
m
D3
m
– media
dependent
(defined for
SONET radio)
AU pointers
B2
MSOH
B2
B2
K1
K2
D4
D5
D6
D7
D8
D9
D10
D11
D12
S1
M1
res
– reserved for
national use
E2
SOH
Y(J)S SONET Slide 42
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
1 C1 C2 C3 C4 C5 C6 C 7
MSBs are J0 framing (1000…00)
Cs are CRC-7 of previous frame
S are 15 7-bit characters
section access point identifier
0
S
S
S
S
S
S
S
S
S
S
S
…
0
S
S
S
Y(J)S SONET Slide 43
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)
Y(J)S SONET Slide 44
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
Y(J)S SONET Slide 45
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
H1 H2 extra
 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
H1 H2 H3 stuff
 byte after H3 is stuffing
 offset is incremented by 1 (byte)
…
Y(J)S SONET Slide 46
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
Y(J)S SONET Slide 47
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
Y(J)S SONET Slide 48
Payloads
and
Mappings
Y(J)S SONET Slide 49
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)
Y(J)S SONET Slide 50
STS-1 HOP
1
30
59
87
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)
Y(J)S SONET Slide 51
STS-1 Path overhead
J1
B3
C2
G1
F2
H4
F3
K3
N1
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
POH
Y(J)S SONET Slide 52
J1, B3, C2 (path overhead)
C2
(hex)
Payload type
J1 – path trace
enables receiver to be sure
that the path connection is still OK
00
unequipped
01
nonspecific
02
LOP (TUG)
B3 – BIP-8 even bit parity of bytes
04
E3/T3
12
E4
13
ATM
16
PoS – RFC 1662
18
LAPS X.85
1A
10G Ethernet
1B
GFP
CF
PoS - RFC1619
(without scrambling)
of previous payload
C2 – path signal label
identifies the payload type
(examples in table)
Y(J)S SONET Slide 53
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
Y(J)S SONET Slide 54
LOP
1
30
59
7 VTGs
87
1 2 3 4 5 6 7
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
Y(J)S SONET Slide 55
SONET/SDH : VT/VC types
VT/STS
LOP
HOP
VC
column
rate
payload
VT 1.5
VC-11
3
1.728 DS1
(1.544)
4 per group
VT 2
VC-12
4
2.304 E1
(2.048)
3 per group
6
3.456 DS1C (3.152)
2 per group
6.912 DS2
1 per group
VT 3
VT 6
VC-2
12
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
(6.312)
(139.264)
standard PDH rates map efficiently into SONET/SDH !
Y(J)S SONET Slide 56
LO Path overhead
LOP OH is responsible for timing, PM, REI, …
LO Path APS signaling is 4 MSBs of byte K4
H4=XXXXXX00
H4=XXXXXX01
V1 pointer
125 msec
V5
VC11 – 25B
VC12 – 34B
V2 pointer
J2
500 msec
H4=XXXXXX10
H4=XXXXXX11
V3 pointer
V4 pointer
N2
VC11 – 27B
VC12 – 36B
K4
Y(J)S APS Slide 57
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
Y(J)S SONET Slide 58
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
Y(J)S SONET Slide 59
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
Y(J)S SONET Slide 60
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
Y(J)S SONET Slide 61
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 H1 H1 H1 H2 H2 H2
H3 H3 H3
F2
H4
F3
K3
N1
Y(J)S SONET Slide 62
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
Y(J)S SONET Slide 63
All SDH mappings
AUG
STM-N
AUG
…
AUG
STM-0
AU-4
VC-4
C-4
E4 139.264 M
ATM 149.760M
*3
TUG-3
*3
AU-3
TU-3
VC-3
VC-3
C3
*7
E3 34.368 M
T3 44.736 M
ATM 48.384 M
*7
TUG-2
VC-2
C2
TU-12
VC-12
C12
TU-11
VC-11
C11
TU-2
T2 6.312 M
ATM 6.874M
*3
*4
E1 2.048 M
ATM 2.144 M
T1 1.544 M
ATM 1.6 M
Y(J)S SONET Slide 64
All SONET mappings
STS-N
*N
STS-3 SPE
STS-3c
STS-1
E4 139.264 M
ATM 149.760M
E3 34.368 M
T3 44.736 M
ATM 48.384 M
STS-1 SPE
*7
VTG
VT6
VT6 SPE
VT-2
VT2 SPE
VT1.5
VT1.5 SPE
T2 6.312 M
ATM 6.874M
*3
pointer processing
E1 2.048 M
ATM 2.144 M
*4
T1 1.544 M
ATM 1.6 M
Y(J)S SONET Slide 65
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
Y(J)S SONET Slide 66
WAN-PHY (10 GbE in STM-64)
10GBASE-W 802.3-2005 Clause 50
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
Y(J)S SONET Slide 67
Protection
and
Rings
Y(J)S SONET Slide 68
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)
working channel
protection channel
head-end NE
tail-end NE
Y(J)S SONET Slide 69
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
Y(J)S SONET Slide 70
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
Y(J)S SONET Slide 71
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
extra traffic
protection channel
Y(J)S SONET Slide 72
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
Y(J)S SONET Slide 73
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
Y(J)S SONET Slide 74
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-B
B
A-B
B
B-C
B-A
A
A
C-B
B-A
C
Y(J)S SONET Slide 75
UPSR vs. BLSR (MS-SPRing)
UPSR
Unidirectional
Path switching
Two-fiber
BLSR
Bidirectional
Line switching
Four-fiber
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
Y(J)S SONET Slide 76
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
Y(J)S SONET Slide 77
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
Y(J)S SONET Slide 78
VCAT
and
LCAS
Y(J)S SONET Slide 79
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
Y(J)S SONET Slide 80
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
n or VC-4-Nc : N=4n
 only STS-Nc : N=3 * 4
– 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
Y(J)S SONET Slide 81
Contiguous Concatenation: STS-3c
270 columns
9 rows
…
258 columns of SPE
9 columns of
section and
line overhead
3 columns of
path overhead
258 columns * 0.576 = 148.608 Mbps
STS-3
270 columns
9 rows
…
260 columns of SPE
9 columns of
section and
line overhead
1 column of
path overhead
260 columns * 0.576 = 149.760 Mbps
STS-3c
Y(J)S SONET Slide 82
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
Y(J)S SONET Slide 83
Virtual Concatenation
…
H4
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)
Y(J)S SONET Slide 84
SDH virtually concatenated VCs
VC
VC-11-Xv
Capacity (Mbps)
if all members in one VC
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
So we have many permissible rates
1.600, 2.176, 3.200, 4.352, 4.800, 6.400, 6.528, 6.784, 8.000, …
Y(J)S SONET Slide 85
SONET virtually concatenated VTs
VT
Capacity (Mbps)
VT1.5-Xv 1.600, 3.200, … 1.600X
If all members in one STS
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, …
Y(J)S SONET Slide 86
Efficiency comparison
rate
w/o VCAT
efficiency
with VCAT
efficiency
10
STS-1
21%
VT2-5v
92%
VC-12-5v
100
STS-3c
67%
VC-4
1000
STS-48c
VC-4-16c
STS-1-2v
100%
VC-3-2v
42%
STS-3c-7v
95%
VC-4-7v
Using VCAT increases efficiency to close to 100% !
Y(J)S SONET Slide 87
PDH VCAT
VCAT
overhead
octet
1st
frame
of
4 E1s
TS0
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)
each E1
time
Y(J)S SONET Slide 88
VCAT
overhead
octet
PDH VCAT overhead octet
frames
of an
E1
…
TS0
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)
Y(J)S SONET Slide 89
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
Y(J)S SONET Slide 90
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)
Y(J)S SONET Slide 91
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
Y(J)S SONET Slide 92
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
Y(J)S SONET Slide 93
LCAS – how does it work?
LCAS is unidirectional (for symmetric BW need to perform twice)
LCAS functions can be initiated by source or sink
J1
B3
C2
G1
F2
H4
F3
K3
N1
POH
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
Y(J)S SONET Slide 94
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
Y(J)S SONET Slide 95
reserved fields
0
0
0
0
0
0
0
MFI2 bits 1-4
MFI2 bits 5-8
CTRL
0
0 GID
0
0
0
0
0
0
CRC-8 bits 1-4
CRC-8 bits 5-8
MST bits
more MST bits
0
0 RS-ACK
0
0
0
0
0
0
0
0
0
SQ bits 1-4
SQ bits 5-8
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
MFI1
0
0
0
0
0
1
0
1
1
0
1
0
1
1
1
1
0
0
0
0
0
1
0
1
1
0
1
0
1
1
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
16 frame multiframe
reserved fields
H4 format
Y(J)S SONET Slide 96
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
Y(J)S SONET Slide 97
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”
GID=g SQ=1 CTRL=NORM
Initial state:
GID=g SQ=2 CTRL=NORM
GID=g SQ=3 CTRL=EOS
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=2 CTRL=NORM
GID=g SQ=3 CTRL=EOS
GID=g SQ=FF CTRL=IDLE
Y(J)S SONET Slide 98
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
GID=g SQ=1 CTRL=NORM
GID=g SQ=2 CTRL=NORM
GID=g SQ=3 CTRL=EOS
GID=g SQ=4 CTRL=ADD
Step 3: source sends CTRL=EOS for new member
GID=g SQ=1 CTRL=NORM
GID=g SQ=2 CTRL=NORM
new member starts to carry traffic
sink sends RS-ACK
GID=g SQ=3 CTRL=NORM
GID=g SQ=4 CTRL=EOS
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
Y(J)S SONET Slide 99
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)
GID=g SQ=1 CTRL=NORM
GID=g SQ=2 CTRL=NORM
Example: Initial state
GID=g SQ=3 CTRL=NORM
GID=g SQ=4 CTRL=EOS
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
GID=g SQ=1 CTRL=NORM
GID=g SQ=2 CTRL=DNU
GID=g SQ=3 CTRL=NORM
GID=g SQ=4 CTRL=EOS
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
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
IP
PPP
HDLC
SONET/SDH
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
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
Ethernet
IP
HDLC
other
GFP – client specific part
GFP – common part
SDH
OTN
other
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
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
core
header
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
payload
area
PLI (2B)
cHEC (2B)
payload header
(4-64B)
payload
optional payload
FCS (4B)
Y(J)S SONET Slide
GFP payload header
GFP payload header has
– type (2B)
PTI (3b) PFI EXI (3b)
– type HEC (CRC-16)
UPI (8b)
– extension header (0-60B)
either null or linear extension (payload type muxing)
– extension HEC (CRC-16)
type (2B)
tHEC (2B)
extension header
(0-60B)
eHEC (2B)
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.
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!)
Y(J)S SONET Slide