sonet
TRANSCRIPT
SONET/SDHSONET/SDH
Yaakov (J) Stein Chief ScientistRAD Data Communications
Y(J)S SONET Slide 2
Course OutlineCourse Outline
Background (analog telephony, TDM, PDH)SONET/SDH history and motivationArchitecture (path, line, section)Rates and frame structurePayloads and mappingsProtection and rings VCAT and LCASHandling packet data
Y(J)S SONET Slide 3
BackgroundBackground
Y(J)S SONET Slide 4
The PSTN circa 1900The 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 MultiplexingTelephony Multiplexing1900: 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)), …
fchannels
8 kHz
12 kHz
4 kHz
16 kHz
20 kHz
Y(J)S SONET Slide 6
The Digitalization of the PSTNThe Digitalization of the PSTN
Shannon (Bell Labs) proved thatDigital communicationsis always better than
Analog communicationsand 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 tooand switching became easier too
Complexity increases rapidly with size
1 2 4 5 6 7 83
1234567
Analog Crossbar switch Digital Cross-connect (DXC)
processor
t1 2 3 4 5
t2 1 5 4 3
Y(J)S SONET Slide 8
Optimized Telephony RoutingOptimized 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 1960The 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 PSTNThe 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
Y(J)S SONET Slide 11
TDM timingTDM timingTime Domain Multiplexing relies on all channels (timeslots)
having precisely the same timing (frequency and phase)
In order to enforce thisthe TDM device itself frequently performs the digitization
analog
signals
digital
signals
Y(J)S SONET Slide 12
if the inputs are already digitalif the inputs are already digital
If the TDM switch does not digitize the analog signalsthen there can be a problemthe 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
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componentsignals
TDM
Y(J)S SONET Slide 13
The fixThe fixWe 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
0transitions no transitions
Y(J)S SONET Slide 14
Comparing clocksComparing 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 15
PDH principlePDH principleIf 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 16
PDH hierarchiesPDH 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
Y(J)S SONET Slide 17
Framing and overheadFraming and overheadIn addition to locking on to bit-rate
we need to recognize the frame structureWe 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 18
PDH overheadPDH 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 %
Y(J)S SONET Slide 19
OAMOAManalog 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 toOperations, Administration and Maintenance (OAM) functions
including: monitoring for valid signal defect reporting alarm indication/inhibition (AIS)
Y(J)S SONET Slide 20
PDH JustificationPDH JustificationIn addition to FAS, PDH overhead includes
justification control (C-bits) and justification opportunity “stuffing” (R-bits)Assume the tributary bitrate is B TPositive justification
payload is expected at highest bitrate B+Tif the tributary rate is actually at the maximum bitrate
then all payload and R bits are filledif the tributary rate is lower than the maximum
then sometimes there are not enough incoming bitsso the R-bits are not filled and C-bits indicate this
Negative justificationpayload is expected at lowest bitrate B-Tif 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 accommodateso R-bits in the overhead are used and the C-bits indicate this
Positive/Negative justificationpayload is expected at nominal bitrate Bpositive or negative justification is applied as required
Y(J)S SONET Slide 21
SONET/SDH SONET/SDH
motivation and historymotivation and history
Y(J)S SONET Slide 22
First stepFirst stepWith the disvestiture of the US Bell system a new need aroseMCI and NYNEX couldn’t directly interconnect optical trunksInterexchange Carrier Compatibility Forum requested T1 to solve problem
Needed multivendor/ multioperator fiber-optic communications standardThree 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) systemproposal submitted to T1M1T1.119 standard
Rates, formats, definition of network elementsBellcore (Yau-Chau Ching and Rodney Boehm) proposal (Feb 1985)proposed to T1X1term SONET was coinedT1.105 standard (1988)
Y(J)S SONET Slide 23
PDH limitationsPDH 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 24
Idea behind SONETIdea 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 25
Standardization !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 26
SONET/SDH SONET/SDH
architecturearchitecture
Y(J)S SONET Slide 27
LayersLayersSONET 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 28
SONET architectureSONET 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
PathTermination
PathTermination
LineTermination
LineTermination
SectionTermination
path
line line line
ADM ADMregenerator
section section sectionsection
Y(J)S SONET Slide 29
STS, OC, etc.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
Y(J)S SONET Slide 30
rates rates
and and
frame structureframe structure
Y(J)S SONET Slide 31
SONET / SDH framesSONET / 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
Y(J)S SONET Slide 32
SONET STS-1 frameSONET STS-1 frame
Each STS-1 frame is 90 columns * 9 rows = 810 bytes
There are 8000 STS-1 frames per secondso each byte represents 64 kbps (each column is 576 kbps)
Thus the basic STS-1 rate is 51.840 Mbps
90 columns
9 ro
ws
framing
Y(J)S SONET Slide 33
SDH STM-1 frameSDH STM-1 frame
Synchronous Transport Modules are the bit-signals for SDHEach STM-1 frame is 270 columns * 9 rows = 2430 bytesThere are 8000 STM-1 frames per secondThus the basic STM-1 rate is 155.520 Mbps
3 times the STS-1 rate!
270 columns
9 ro
ws
…
Y(J)S SONET Slide 34
SONET/SDH ratesSONET/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
Y(J)S SONET Slide 35
SONET/SDH tributariesSONET/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
Y(J)S SONET Slide 36
Synchronous Payload Envelope
STS-1 frame structureSTS-1 frame structure9
row
s
TransportOverhead
TOH
6 ro
ws
3 ro
ws
Section overhead is 3 rows * 3 columns = 9 bytes = 576 kbpsframing, performance monitoring, management
Line overhead is 6 rows * 3 columns = 18 bytes = 1152 kbpsprotection 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
9 ro
ws
Y(J)S SONET Slide 37
STM-1 frame structureSTM-1 frame structure
SectionOverhead
SOHSTM-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
Y(J)S SONET Slide 38
Even higher ratesEven higher rates
3 STS-1s can form an STS-34 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
Y(J)S SONET Slide 39
Byte-interleavingByte-interleaving
. . .
Y(J)S SONET Slide 40
ScramblingScramblingSONET/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 databut 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
Y(J)S SONET Slide 41
STS-1 OverheadSTS-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
sectionoverhead
lineoverhead
Y(J)S SONET Slide 42
STM-1 OverheadSTM-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
Y(J)S SONET Slide 43
A1, A2, J0 A1, A2, J0 (section overhead)(section overhead)
A1, A2 - framing bytes A1 = 11110110 A2 = 00101000SONET/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 OKenables identifying individual STS/STMs after muxing
J0 goes through a 16 byte sequenceMSBs are J0 framing (1000…00)
Cs are CRC-7 of previous frameS are 15 7-bit characters
section access point identifierSSSSSSS0
SSSSSSS0
C7C6C5C4C3C2C11
…
Y(J)S SONET Slide 44
B1, E1, F1, D1-3 B1, E1, F1, D1-3 (section overhead)(section overhead)
B1 – Byte Interleaved Parity-8 byteeven 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 techniciansfrom regenerator to regenerator
F1 – 64 kbps link for user purposes
D1 + D2 + D3 – 192 kbps messaging channelused by section termination as Embedded Operations Channel (SONET)
or Data Communications Channel (SDH)
Y(J)S SONET Slide 45
Pointers Pointers (line overhead)(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 frameIn all other cases the SPE straddles two frames
When offset is a multiple of 87, the SPE is rectangular
To compensate for clock differenceswe have pointer justification
When negative justification H3 carries the extra data
When positive justificationbyte after H3 is stuffing byte
Y(J)S SONET Slide 46
SONET JustificationSONET JustificationIf 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 neededWhen 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 …
Y(J)S SONET Slide 47
B2, K1, K2, D4-D12 B2, K1, K2, D4-D12 (line overhead)(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 Channelbetween multiplexersusually manufacturer specific OAM functions
Y(J)S SONET Slide 48
S1, M0, E2 S1, M0, E2 (line overhead)(line overhead)
S1 – Synchronization Status Messageindicates stratum level (unknown, stratum 1, …, do not use)
M0 – Far End Block Errorindicates number of BIP violations detected
E2 – line orderwire64 kbps voice link for techniciansfrom line mux to line mux
Y(J)S SONET Slide 49
Payloads Payloads
andand
MappingsMappings
Y(J)S SONET Slide 50
STS-1 HOP SPE structureSTS-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 51
STS-1 HOPSTS-1 HOP
1 column of SPE is POH
2 more (“fixed stuffing”) columns are reserved
We are left with84 columns = 756 bytes = 48.384 Mbps for payload
This is enough for a E3 (34.368M) or a T3 (44.736M)
1 875930
Y(J)S SONET Slide 52
STS-1 Path overheadSTS-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
Y(J)S SONET Slide 53
J1, B3, C2 J1, B3, C2 (path overhead)(path overhead)
J1 – path traceenables 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 labelidentifies 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
Y(J)S SONET Slide 54
G1, F2, H4, F3, K3, N1 G1, F2, H4, F3, K3, N1 (path overhead)(path overhead)
G1 – path statusconveys 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 55
LOPLOP
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 MbpsVT 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 typesA VT can have 3, 4, 6 or 12 columns
1 875930 1 2 3 4 5 6 77 VTGs
Y(J)S SONET Slide 56
SONET/SDH : VT/VC typesSONET/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 overheadLO 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 – 25BVC12 – 34B
125 sec
500 sec
H4=XXXXXX00
H4=XXXXXX01
H4=XXXXXX10
H4=XXXXXX11VC11 – 27BVC12 – 36B
Y(J)S SONET Slide 58
Payload capacityPayload 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 59
LOP overheadLOP 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 60
SDH ContainersSDH 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 61
Formally …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 62
MultiplexingMultiplexingAn 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 AUGand inside the AUG are 3 pointers to the AU-3s
J1B3C2G1F2H4F3K3N1
H1 H1H1 H2 H2H2 H3 H3H3
Y(J)S SONET Slide 63
More multiplexingMore 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 64
All SDH mappingsAll SDH mappingsSTM-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 MT3 44.736 M
T2 6.312 M
E1 2.048 M
T1 1.544 M
* 3
*7
* 3
*7
* 4
* 3
Y(J)S SONET Slide 65
All SONET mappingsAll SONET mappingsSTS-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 MT3 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
Y(J)S SONET Slide 66
Tributary mapping typesTributary 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 67
WAN-PHY WAN-PHY (10 GbE in STM-64)(10 GbE in STM-64)
There is a special case where the bit-rates work out relatively wellGbE 10GBASE-R (64B/66B coding) can be directly mapped
into a STM-64 (with contiguous concatenation - see later) without need for GFPMAC creates "stretched InterPacket Gap" to compensate for rate being < 10GThis is the fastest connection commonly used for Internet trafficComplication: 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
Y(J)S SONET Slide 68
Protection Protection
and and
Rings Rings
Y(J)S SONET Slide 69
What is protection ?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 unidirectionalProtection switching may be revertive (automatically revert to working channel)
head-end NE tail-end NE
working channel
protection channel
Y(J)S SONET Slide 70
How does it work?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 bridgeworking channel
protection channel signaling channel
Y(J)S SONET Slide 71
Linear 1+1 protectionLinear 1+1 protectionSimplest form of protectionCan 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 channelsTail-end chooses channel to use based on BER, dLOS, etc.
No need for signalingIf non-revertive
there is no distinction between working and protection channels BW utilization is 50%
channel A
channel B
Y(J)S SONET Slide 72
Linear 1:1 protectionLinear 1:1 protectionHead-end bridge usually sends data on working channelWhen tail-end detects failure it signals (using K1) to head-endHead-end then starts sending data over protection channel
When not in useprotection 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
Y(J)S SONET Slide 73
Linear 1:N protectionLinear 1:N protection
In order to save BWwe allocate 1 protection channel for every N working channels
N limited to 144 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 74
Two fiber vs. Four-fiber ringsTwo fiber vs. Four-fiber ringsRing based protection is popular in North America (100K+ rings)Full protection against physical fiber cutsSimpler and less expensive than mesh topologiesProtection at line (multiplexed section) or path layerFour-fiber rings
fully redundant at OC levelcan support bidirectional routing at line layer
Two-fiber ringssupport unidirectional routing at line layer
2 fibers in opposite directions
Y(J)S SONET Slide 75
Unidirectional vs. bidirectionalUnidirectional vs. bidirectionalUnidirectional routing
working channel B-A same direction (e.g. clockwise) as A-Bmanagement simplicity: A-B and B-A can occupy same timeslotsInefficient: waste in ring BW and excessive delay in one direction
Bidirectional routingA-B and B-1 are opposite in directionboth using shortest routespatial reuse: timeslots can be reused in other sections
A
BA-B
B-A
A
BB-A
A-B
C
B-C
C-B
Y(J)S SONET Slide 76
UPSR vs. BLSR UPSR vs. BLSR (MS-SPRing)(MS-SPRing)
Of all the possible combinations, only a few are in use
Unidirectional Path Switched Ringsprotects tributariesextension of 1+1 to ring topology
Bidirectional Line Switched Rings (two-fiber and four-fiber versions)called Multiplex Section Shared Protection Ring in SDHsimultaneously protects all tributaries in STMextension of 1:1 to ring topology
Path switching
Line switching
Two-fiber
Four-fiber
Unidirectional
Bidirectional
UPSR
BLSR
Y(J)S SONET Slide 77
UPSRUPSRWorking 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 networks1 access resilient ring
less expensive than fiber pair per customer
Inefficient for core networksno spatial reuse
every signal in every spanin both directions
node needs to continuously monitorevery tributary to be dropped
Y(J)S SONET Slide 78
BLSRBLSR
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 versionhalf of OC-N capacity devoted to protectiononly half capacity available for traffic
Four-fiber versionfull redundant OC-N devoted to protectiontwice as many NEs as compared to two-fiber
Examplerecovery from unidirectional fiber cut
Y(J)S SONET Slide 79
VCATVCAT
and and
LCASLCAS
Y(J)S SONET Slide 80
ConcatenationConcatenationPayloads 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 VCso 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 or5 VT2/VC-12 s = 5 * 2.176 = 10.88 Mbps
Y(J)S SONET Slide 81
Concatenation Concatenation (cont.)(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
Y(J)S SONET Slide 82
Contiguous Concatenation: STS-3cContiguous Concatenation: STS-3c270 columns
9 ro
ws …
9 columns of section and
line overhead
3 columns of path overhead
258 columns of SPE STS-3
270 columns
9 ro
ws …
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
Y(J)S SONET Slide 83
STS-N vs. STS-NcSTS-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) columnse.g. STS-12c gains 11 columns = 6.336Mbps vis a vis STS-12STS-48c gains 47 columns = 27.072 MbpsSTS-192c gains 191 columns = 110.016 Mbps !
However, an STS-Nc signal is not as easily separablewhen we want to add/drop component signals
Y(J)S SONET Slide 84
Virtual ConcatenationVirtual Concatenation
VCAT is an inverse multiplexing mechanism (round-robin)VCAT members may travel along different routes in SONET/SDH networkIntermediate network elements don’t need to know about VCAT
(unlike contiguous concatenation that is handled by all intermediate nodes)
…
H4
Y(J)S SONET Slide 85
SDH virtually concatenated VCsSDH 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
Y(J)S SONET Slide 86
SONET virtually concatenated VTsSONET virtually concatenated VTsVT 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, …
Y(J)S SONET Slide 87
Efficiency comparisonEfficiency 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%
Y(J)S SONET Slide 88
PDH VCATPDH VCAT
Recently ITU-T G.7043 expanded VCAT to E1,T1,E3,T3Enables bonding of up to 16 PDH signals to support higher ratesOnly 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 frameof4 E1s
VCAToverhead
octet
timeeach E1
Y(J)S SONET Slide 89
PDH VCAT overhead octetPDH VCAT overhead octet
There is one VCAT overhead octet per multiframe, so net rate isT1: (24*24-1=) 575 data bytes per 3 ms. multiframe = 191.666 kB/sE1: (16*30-1=) 495 data bytes per 2 ms multiframe = 247.5 kB/sT3 and E3 can also be usedWe will show the overhead octet format later
(when using LCAS, the overhead octet is called VLI)
TS0
frames of an E1
VCAToverhead
octet
…
Y(J)S SONET Slide 90
Delay compensationDelay compensation802.1ad 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 pathsso 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 Signalthe VCAT mechanism only needs to identify individual frames
Y(J)S SONET Slide 91
VCAT bufferingVCAT 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 membersand 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 92
Multiframes and superframesMultiframes and superframesHere is how we compensate for 512 ms of differential delay512 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 framesSo 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 93
LLinkink C Capacityapacity A Adjustmentdjustment S Schemecheme
LCAS is defined in G.7042 (also numbered Y.1305)LCAS extends VCAT by allowing dynamic BW changesLCAS 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 94
LCAS – how does it work?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
Y(J)S SONET Slide 95
LCAS control messagesLCAS 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 96
H4 formatH4 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 0CRC-8 bits 5-8 0 1 1 1
MST bits 1 0 0 0more 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 0SQ bits 5-8 1 1 1 1
16 frame m
ultiframeMFI1
rese
rved
fiel
dsre
serv
ed fi
elds
Y(J)S SONET Slide 97
H4 format – some commentsH4 format – some commentsCRC-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 98
LCAS – adding a member (1)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=2 CTRL=NORM
GID=g SQ=3 CTRL=EOS
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 99
LCAS – adding a member (2)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=FAILand 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 onceNote 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=1 CTRL=NORM
GID=g SQ=2 CTRL=NORM
GID=g SQ=3 CTRL=EOS
GID=g SQ=4 CTRL=ADD
GID=g SQ=1 CTRL=NORM
GID=g SQ=2 CTRL=NORM
GID=g SQ=3 CTRL=NORM
GID=g SQ=4 CTRL=EOS
Y(J)S SONET Slide 100
LCAS – service preservationLCAS – service preservationTo preserve service integrity if sink detects a failure of a VCAT memberLCAS 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 2Note: 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=1 CTRL=NORM
GID=g SQ=2 CTRL=NORM
GID=g SQ=3 CTRL=NORM
GID=g SQ=4 CTRL=EOS
GID=g SQ=1 CTRL=NORM
GID=g SQ=2 CTRL=DNU
GID=g SQ=3 CTRL=NORM
GID=g SQ=4 CTRL=EOS
Y(J)S SONET Slide 101
HandlingHandling
PacketPacket
DataData
Y(J)S SONET Slide 102
Packet over SONETPacket 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 interfaceto the SONET/SDH infrastructure
POH signal label (C2) indicates PoS as C2=16 (C2=CF if no scrambler)
Y(J)S SONET Slide 103
PoS architecturePoS 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
IPPPP
HDLCSONET/SDH
Y(J)S SONET Slide 104
PoS DetailsPoS 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 appendedPPP in HDLC framing with byte stuffing43 bit scrambler is run over the SPEbyte 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 105
POS problemsPOS 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 106
LAPSLAPS
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 107
GFP architectureGFP architectureA 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 otherGFP – client specific part
GFP – common partSDH OTN other
HDLC
Y(J)S SONET Slide 108
GFP frame structureGFP 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 payloadFCS (4B)
coreheader
payloadarea
Y(J)S SONET Slide 109
GFP payload headerGFP payload headerGFP 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 110
GFP modes GFP modes GFP-F - frame mapped GFPGood for PDU-based protocols (Ethernet, IP, MPLS)
or HDLC-based ones (PPP)
Client PDU is placed in GFP payload field
GFP-T – transparent GFPGood for protocols that exploit physical layer capabilities
In particular8B/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!)