Training PPT,SDH Principle,20040423
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Transcript of Training PPT,SDH Principle,20040423
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Basic Principles of SDH
Pre-sales Comm. And Tech. Support, OTPD
Pang Lipeng
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OutlineOutline
▼ Overview to SDH
▼Rate and frame structure
▼Multiplexing structure and procedures
▼SDH network protection
▼ SDH network synchronization
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OO//EE
Dem
ultip
lexing
Dem
ultip
lexing
EE//
OO
2Mbit/s2Mbit/s (( electrical signalselectrical signals ))
Add/drop 2Mbit/s signals Add/drop 2Mbit/s signals in PDH and SDH systemsin PDH and SDH systems
ADM155Mb/s 155Mb/sOptical
interface
2Mbit/s2Mbit/s (( electrical signalselectrical signals ))
Optical interface
Dem
ultip
lexing
Dem
ultip
lexing
Dem
ultip
lexing
Dem
ultip
lexing
Mu
ltiplexin
g M
ultip
lexing
Mu
ltiplexin
g M
ultip
lexing
Mu
ltiplexin
g M
ultip
lexing
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SDH feature SDH feature -- plentiful overhead bytes-- plentiful overhead bytes
1. SDH system is an intelligent equipment with powerful self-healing function. SDH NMS and dynamic configuration with intelligent check contribute to easy self-healing of SDH network. When a fault occurs to the equipment or system, the services can be recovered rapidly, greatly improving network reliability and lowering maintenance cost.
2. SDH system has good network management function. The overhead bytes (1/10 of the total capacity) in the SDH frame may meet the present requirements in the alarm, performance supervision, network configuration, switching and orderwire, and can be extended further to satisfy the future requirements in the supervision and NM.
SDH advantages: Synchronous multiplexing, standard optical interface and powerful NM.
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SDH shortcomings:1. The frequency band utilization rate of SDH
is lower than that of PDH.2. The pointer adjustment makes the
equipment and interfaces more complex.3. The software control function easily causes
major faults.
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OutlineOutline
▼ Overview to SDH
▼Rate and frame structure
▼Multiplexing structure and procedures
▼SDH network protection
▼ SDH network synchronization
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SSDDHH level SSOONNEETT
115555MM
662222MM
22..55GG
1100GG
STM-1(1920CH)
STM-4
STM-16
STM-64
OC-3/STS3(1440CH)
OC-12/STS-12(8046CH)
OC-48/STS-48(32356CH)
OC-192/STS-192
155.520 Mbit/s
622.080 Mbit/s
2488.320 Mbit/s
9953.280 Mbit/s
Name
40G STM--256 OC-576 39813.120Mbit/s
SDH ratesSDH rates
level Standard rate
STM- Synchronous transfer mode
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9 rows
270 columns (byte)
1
2PAYLOAD
( with POH )
9 columns
SOH
AU-PTR
SOH( section overhead )
capacity = 9 ×270 bytes
8000 frames/s
STM-1 frame structureSTM-1 frame structure
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SOH
SOH
AU pointer SDH payload
( with POH)
1
3
4
5
9
9270N bytes
9N
261 N
Transmission direction
STM-NSTM-N frame structure frame structure
125s
Frame period, frame frequency block and rate
Frame structureFrame structure
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STM-1Signal “A”
STM-1Signal “B”
STM-1Signal “C”
STM-1Signal “D”
Byte in
terleaver mu
ltiplexer
STM-4 signal
(4×STM-1)
t
t
Byte interleave synchronous Byte interleave synchronous multiplexingmultiplexing
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1. Information payloads --They contain various information blocks and some POH bytes used for channel performance supervision, management and control.
2. Section overheads--They are the additional bytes ensuring the normal and flexible transmission of information payload. They provides the frame synchronization and network OAP bytes. SOH consists of RSOH and MSOH. RSOH terminates in the regenerator, and MSOH transparently goes through the regenerator and is assembled/dissembled in AUG.
3. AU-PTR --It indicates the accurate position of the first byte of information payload in STM-N frame, and employs the pointer adjustment technique to resolve the clock deviation of network node so that the information payload can be detached properly at the receiving end of SDH system.
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SDH TM SDH DXC SDH TM
MS MS
PATH
RS RSRS
REG REG
Tributary signals
Tributary signals
VC multiplexing
VC demultiplexing
Path, MS and RS
SDH network segment modelSDH network segment model
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A1 A1 A1 A2 A2 A2 J0 * *
B1 E1 F1
D1 D2 D3
B2 B2 B2 K1 K2
D4 D5 D6
D10 D11 D12
D7 D8 D9
S1 M1 E2
AU PRT
9 columns
9
rows
RSOH
MSOH
STM-1 SOH bytesSTM-1 SOH bytes
SOH bytesSOH bytes
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framing bytes: framing bytes: A1, A2 bytes identifies the initial location of the framA1, A2 bytes identifies the initial location of the frame. The byte code pattern are defined as e. The byte code pattern are defined as A1: 11110110 ( F6H ) A2: 00101000 ( 28H ) (transparent transmission)
RS trace byte: J0RS trace byte: J0 It repeatedly transmits the section AP identity to assure the receiver of the connection with the receiver assigned. The section AP identity adopts the format in section 3 of ITU-T G.831, that is, use a 16-byte frame to transmit the section AP identity.
byte number 8bit value 1 1 C1 C2 C3 C4 C5 C6 C7 2 0 X X X X X X X 3 0 X X X X X X X . …………………………… 16 0 X X X X X X X
Description for SOH byteDescription for SOH byte
note: the first byte is the initial location identity of the frame. C1-C7 is the calculation result of CRC-7 in the previous frame. The other 15 bytes transmit the section AP identity.
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•Data Communication Channel (DCC): D1—D12Data Communication Channel (DCC): D1—D12SOH DCC is the transmission link of SDH management network (SMN). D1~D3 byte transmits OAM information between RS terminals. D4~D12 byte transmits OAM information between MS terminals.One purpose of SDH network management control is to implement the fast distributed control. The best route table calculated by NMS can be delivered quickly to NE via DCC at any time. DCC is the SDH physical channel and has the protocol stack Qecc.
•Orderwire channel: E1 and E2Orderwire channel: E1 and E2E1 and E2 offers the orderwire voice channel. E1 is used for the RS orderwire and E2 for the orderwire between terminals.
•User channel: F1User channel: F1It is for the network provider. It is used for the special maintenance of system operator, e.g., providing temporary 64kb/s data/voice channel.
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•B1: BIP-8B1 supervises the MS bit error in BIP-8 method. After all the bytes scrambled in the previous frame of STM-N make the BIP-8 check, the result is in the B1 byte before unscrambled in the present frame.The bit error supervision is one of SDH characteristics. It can automatically supervise the MS bit error in a simple way. But this mode can not check out the even number of bit errors in one supervise code group. (This case seldom occurs, but a certain error exists.
•B2: BIP-N24It makes the online of supervision of the MS bit error in the BIP-N24 method. The BIP-N24 value of all bytes in the previous STM-N frame is in B2 before scrambled in the present frame. The first three lines of SOH is not for check.
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APS channel: K1 and K2 ( b1-b5 )The two bytes are used for APS instruction. (K1 indicates the switching type and channel No., and K2 indicates the channel No. switched to the protection channel.)•MS-RDI byte: K2 (b6-b8)MS-RDI sends back an instruction signal to the transmitting end, indicating that the receiving end finds an incoming fault or is receiving MS-AIS. After unscrambled, K2 (b6-b8) forms “110”, that is, MS-RDI.•Synchronization state: S1 ( b5-b8 )S1 (b5-b8) transmits the synchronization state information, that is, the synchronization state of the upstream station is transmitted to the downstream station via S1 (b5-b8).
S1 (b5-b8) b5-b8) Clock level 0000 Unknown quality 0010 G.811 reference clock 0100 G.812 exchange slave clock 1000 G.812 local slave clock 1011 SETS 1111 Not for clock synchronization
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MS-REI: M1M1 is used for MS-REI. It sends the quantity of the bit errors checked out by B2.
Bytes related to transmission media: One-fiber unidirection, one-fiber bidirection, etc.Backup bytes -Free byte (for future use in the international standard)(Note: The bytes with “*” will not be scrambled.
STM-N(N>1) SOHIt is formed in the byte interleaving mode. The SOH in the first STM-1 is remained, but the SOH in other N-1 STM-1 is remained only with byte A1, A2 and B2.
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STM-4 SOH bytesSTM-4 SOH bytes
15
36 byte
AU PRT9
rows
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A2 A2 A2
E1
D2
K1
D5
D11
D8
144 bytes
A1 A1 A1
B1
D1
B2 B2 B2
D4
D10
D7
S1
9 rows
F1
D3
K2
D6
D12
D9
E2
RSOH
A1
B2
A1
AU PRT
MSOH
M1 。。。
J0/C1 Z0/C1 Z0/C1
注:新协议中有些字节用作 FEC 功能
STM-16 SOH bytesSTM-16 SOH bytes
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A2 A2 A2
E1
D2
K1
D5
D11
D8
576 bytes
A1 A1 A1
B1
D1
B2 B2 B2
D4
D10
D7
S1
9 rows
F1
D3
K2
D6
D12
D9
E2
RSOH
A1
B2
A1
AU PRT
MSOH
M1 。。。
J0/C1 Z0/C1 Z0/C1
注:新协议中有些字节用作 FEC 功能
STM-64 SOH bytesSTM-64 SOH bytes
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FEC FEC (Forward Error Correction)(Forward Error Correction)
Some bytes in SOH of STM-16 ~ STM-256 are used for FEC. FEC means that the signals are coded in a certain format before transmission, then they are decoded with a specific algorithm at the receiver in order to find out the bit errors and correct them.
ITU-I G.975, issued in 1996, employs FEC as a part of the cable communication standards. The new draft, passed in April, 2000, is added with FEC as an option in the 10Gbit/s system.
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OutlineOutline
▼ Overview to SDH
▼Rate and frame structure
▼Multiplexing structure and procedures
▼SDH network protection
▼ SDH network synchronization
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Multiplexing structure in G.707
STM-N× N × 1 140Mb/s
45Mb/s34Mb/s
6.3Mb/s
2Mb/s
1.5Mb/s
×3
× 7
×3
× 1× 3
C-11
C-12
C-2
C-3
C-4
VC-11
VC-2
VC-3
VC-3
VC-4
TU-11
TU-12
TU-2
TU-3
TUG-2
TUG-3
AUG
AU-3
AU-4
VC-12
×4 ×4
×1
Multiplexing
VC
C
TU
AU
TUGAUG
STM
Mapping Positioning
× 7 × 1× 7
Multiplexing structure and proceduresMultiplexing structure and procedures
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注 2: VC-2-mc 主要用于传输图象等业务,具体技术实施方法待定。大约是 4Mb/s34Mb/s 之间。而 SDH 可为其提供 VC-2 、 VC-2 的级联等方式来传输。 图 3.2
Mapping structure in ChinaMapping structure in China
Pointer processing
Positioning
Multiplexing
Mapping
STM-1 capacity: 1. 1×140M signal 2. 3×34M signals
3. 63×2M signals
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New multiplexing structure in G.707
Pointer processing
Positioning
Multiplexing
Mapping
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Byte interleaving multiplexing and Byte interleaving multiplexing and byte block multiplexingbyte block multiplexing
Byte interleaving multiplexing:
(4×AUG-1 are multiplexed into 1×AUG-4)
Byte block multiplexing: When AUG-N (N 4), AUG-16 is≧
Byte block interleaving multiplexing:
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C-n
Multiplexing unitMultiplexing unit
1. Container (C)
C contains service signals at various rates.
G.707 specifies five standard containers for PDH rate series: C-11, C-12, C-2, C-3 and C4.
PDH series indicate the payload by H-n, which is divided into different levels.
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C-n
VC-n POH
VC supports the connection at the SDH path layer. VC, composed of payload (C output) and POH, is the information terminal of SDH path.
VC-n=C-n+VC-n POHVC-n=C-n+VC-n POH
LOVC: VC-1 and VC-2
VC-3 (VC-3 - TU-3 - TUG-3- VC-4)
HOVC: VC-4
VC-3 (in AU-3)
2. VC2. VC
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Adjustment bit
Service/
PDH signal
=
POH
C
=VC
Service signal/PDH signal, C and VC
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VC parameters
VC VC-4 VC-3 VC-2 VC-12 VC-11
125 125 500 500 500
Structure 2619 859 4(129-1)
4(49-1) 4(39-1)
Capacity (byte number) 2349 765 428 140 104
Rate (Mbit/s) 150.336 48.960 6.848 2.240 1.664
Frame frequency and multiframe frequency (Hz) 8000 8000 2000 2000 2000
Frame period and multiframe (s)
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3. TU and TUG3. TU and TUG
TU adapts the low-order path signals into the high-order path layer (e.g., VC-4).
Four TUs are available, i.e., TU-n (n=11, 12, 2 and 3).
TU-n consists of a LOVC-n and a TU-n PTR.
TU-n =VC-n+TU-n PTR
TU-n PTR points the shift between VC-n payload start point and HOVC frame start point.
TUG is composed of one or several TUs at the fixed location of HOVC payload.
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4. AU and AUG4. AU and AUG
AU adapts the HO path signals into MS layer.
AU-3 and AU-4 are available.
AU-n (n=3, 4) comprises a HOVC-n and a AU-n PTR. For example:
AU-n =VC-n+AU-n PTR
AU-n PTR points the shift between VC-n payload start point and MS start point.
AUG is composed of one or several AUs at the fixed location of STM-N payload.
One AU is composed of one AU-4 or three AU-3 in the byte interleaving multiplexing mode.
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TU and AU parameters
TU and AU AU-4 AU-3 TU-3 TU-2 TU-12 TU-11
8000 8000 8000 2000 2000 2000
Structure 2619+9819+3 859+3 4(129)
4(49) 4(39)
Capacity (byte number) 2358 786 768 432 144 108
rate ( Mbit/s ) 150.912 50.304 49.152 6.912 2.304 1.728
125 125 125 500 500 500Frame frequency and multiframe fr
equency (Hz)
Frame period and multiframe (s)
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Multiplexing step 1 Multiplexing step 1 -mapping-mapping
Mapping synchronizes tributary signals with the corresponding VC so that VC can send, multiplex and cross signals independently. (Only STM-1 has the mapping function.)1. Mapping modeThe mapping can be divided into asynchronous mapping and synchronous mapping by the synchronization state between mapped signals and SDH network.Asynchronous mapping --The pointer adjusts the payload to adapt the signals into SDH frame, independent of signal features and network synchronization. The pointer adjusts the frequency or phase difference to synchronize the signals without slide buffer. As a common mapping mode, it is necessary in the long transition from PDH to SDH.
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Synchronous mapping --The mapped signals must be strictly synchronous with SDH network. 125us (the duration of one frame) slide buffer is required to reduce the slide loss in the synchronization. The slide buffer causes 150us delay to the multiplexer, but 10us delay to the demultiplexer.
Synchronous mapping are categorized as bit synchronous mapping and byte synchronous mapping.
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Comparison between asynchronous mapping
and synchronous mapping
Bit synchronous
Synchronous mapping
characteristics
Mapping node
Asynchronous mapping
Byte synchronous mapping
It is a common mode, independent of signal features, network synchronization and slide buffer. The minimum delay caused is 10us. The primary group mapping can not access directly N×64kb/s signals because the de-framing is required. The interface is simple.
It is independent of signal features, but it requires the network synchronization and 125us slide buffer. The delay caused is more than 125us (multiplexer). N×64kb/s signals can not be accessed directly because the de-framing is required. The interface is comparatively simple.
2Mb/s signals should be framed according to G.704. It requires the network synchronization and 125us slide buffer. The delay caused is more than 125us (multiplexer). N×64kb/s signals can be accessed directly because the de-framing is not required. The interface is the most complex.
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VC-4 PAYLOAD
260 columns (byte)
HP-POH9 rows×1 column
J1
B3
C2
G1
F2
H4
F3
K3
N1
C-4 is mapped into VC-4C-4 is mapped into VC-4
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1. Add VC-4 POH
2. HPOH
J1: Path trace byte
It repeatedly sends the HP access point identifier whose content is decided by the transmitter and receiver, thus the transmitter may confirm the connection with the specified receiver. J1 location is pointed by the related pointer.
B3: Path BIP-8 code
It comes from the interleaving parity calculation of all VC-4 bits before scrambling.
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F2 and F3: Path user byte
They offers the orderwire communication between path units.
H4: TU indication byte
It indicates the payload multiframe type and payload location.
K3 (b1-b4): APS path byte
It sends HP APS signals.
K3 (b5-b8): For future use (backup)
N1: Network operator byte
It monitors the HP serial connection.
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Multiplexing step 2 Multiplexing step 2 -Positioning-Positioning
Positioning refers to a process to take the frame deviation information into TU or AU, that is, TU PTR (AU PTR) attached to VC indicates and determined the LOVC frame start point in the TU payload ( the HOVC frame start point in the AU payload.
SDH pointer function:
•When the network works synchronously, the pointer aligns the phases of synchronous signals.
•When the network dismatches, the pointer aligns the frequencies and phases. When the network is out of synchronization or works asynchronously, the pointer traces and aligns the frequencies.
•The pointer can also accept the frequency jitter and wander in the network.
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AU-4=VC-4+AU-4 PTR
TU-3=VC-3+TU-3 PTR
AU-4 PTR=H1 , Y , Y , H2 , 1* , 1* , H3 , H3 ,H3
Y=1001SS11 : SS --bit without specific value
1*=11111111
TU-3 PTR=H1 , H2 , H3
VC-4/VC-3 positioning in AU-4/TU-3
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Multiplexing step 3 Multiplexing step 3 -synchronous multiplexing-synchronous multiplexing
Multiplexing refers to the adaption of several LP signals (TU-12) into HP signals (VC4) or several HP signals (AU-4) into STM-N frame, that is, adapt TU into VC or AU into STM-N in the byte interleaving mode.
For example: TU12(×3) → TUG2(×7) → TUG3(×3) → VC4
For example: AU-4(×1) → AUG(×N) → STM-N
As VC tributaries are synchronous due to TU and AU pointers, the process is called the synchronous multiplexing.
Multiplexing mode: Byte interleaving mode.
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STM-1signal “A”
STM-1signal “B”
STM-1signal “C”
STM-1signal “D”
STM-4 signal
(4×STM-1)
t
t
Byte interleave synchronous Byte interleave synchronous multiplexingmultiplexing
Byte in
terleave mu
ltiplexer
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#1
1 261
1 9
AUGAUG
123…N 123…N
123…N 123…N
N 9 N 261
#2
1 261
1 9
AUGAUG
STM-N
#N
1 261
1 9
AUGAUG
For example: Multiplex N×AUG into STM-N
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TS0 TS1 TS2 TS15 TS16 TS17 TS18 TS31
2048kbit/s frame
32
R 1
R 32
R 1
R 32
R 1
R 32
R 1
R
C-12 frame
V5 K4N2J2
VC-12 frame
V1 V2 V3 V4
99
TU-12 frameC-12 multiframe
Multiplex 2Mbit/s into STM-1 (1)
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3 个 TU-12 通过字节间插复用成 1 个 TUG-2 的信息结构
TUG-2 information structure
99
1212
2
b
1 2 3 1 3 1 2 3
ca cba
TU-12 a
TU-12 b
TU-12 c
4 4 4
Multiplex 2Mbit/s into STM-1 (2)
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Multiplex 7×TUG-2 into 1 × TUG-3 in the byte interleaving mode
TUG-3 information structure
99
84848686
TUG-2 ( 1 ) TUG-2 ( 2 ) TUG-2 ( 7 )
12
34
5
6
7
1
23
45
6
7
12
34
5
6
7
1
23
45
6
7
Insert bytes
1
2
1
2
3
1
2
3
1
2
33
1
2
1
2
3
1
2
3
1
2
33
1
2
1
2
3
1
2
3
1
2
33
Multiplex 2Mbit/s into STM-1 (3)
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C4 structure
Multiplex 2Mbit/s into STM-1 (4)
Multiplex 3×TUG-3 into C4 in the byte interleaving mode
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In order to supervise the 140Mb/s path signals, it is required in the multiplexing to add a column of VC4 POH before C4 frame. Then the signals is in the VC4 structure.
VC-4 PAYLOAD
260 columns (byte)
HPOH9 rows×1 column
J1
B3
C2
G1
F2
H4
F3
K3
N1
Map C-4 intoVC-4
Multiplex 2Mbit/s into STM-1 (5)
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VC-4 PAYLOAD
261 columns (byte)
J1B3C2G1F2H4F3K3N1
H1 H2 H3 H3 H3Y Y 1 1
AU-4 PTR consists of 9 bytes from column 1~9 at row 4 of AU-4 frame.
Add AU-PTR to VC-4 to make VC-4 become AU-4.
Multiplex 2Mbit/s into STM-1 (6)
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STM-1 PAYLOAD
RSOH
AU-PTR
MSOH
POH VC-4 PAYLOAD
261 column
9 rows×1 column
9 rows
Add RSOH and MSOH to form STM-1.
Multiplex 2Mbit/s into STM-1 (7)
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STM-N
× N × 1C-12VC-12VC-4 TUG-2AUG-4 AU-4 TU-12 2Mb/s
code speed
adjustment
LD POH
TU PTR
AU PTR × 3 multiplexing
×7 multiplexingHD POH
× 3 multiplexing
×N multiplexing
TUG-3
Mapping and multiplexing diagram
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STM-N
× N × 1C-12VC-12VC-4 TUG-2AUG-4 AU-4 TU-12 2Mb/s
2.048Mbit/s
2.176Mbit/s
2.240Mbit/s
2.304Mbit/s49.536Mbit/s
155.520Mbit/s
TUG-3
150.336Mbit/s
6.912Mbit/s
150.912Mbit/s
STM-1 rate adjustment
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Layer standard
Multiplexing Positioning Mapping
STM-N
STM-1
AU-4
VC-4
TUG-3,2
TU-12
VC-12
C-12
Service Telephone and database
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OutlineOutline
▼ Overview to SDH
▼Rate and frame structure
▼Multiplexing structure and procedures
▼SDH network protection
▼ SDH network synchronization
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Link
Star
Tree
Ring
Mesh
SDH network topology
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Self-sealing: When a fault occurs to the network, the service transmission can recover automatically in such a short time that a user can not find it.
Redundant Cross function
intelligence
Network self-healing
SDH network protection and self-healing
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MS 1+ 1 protection
MS 1: 1 protection
MS 1: n protection
( n<=14)
2-fiber unidirectional path protection ring
2-fiber unidirectional MS protection ring
2-fiber bidirectional MS protection ring
4-fiber bidirectional MS protection ring
Link network
Ring network
Dual-Node interconnection (DNI)
SDH network protections
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SDH ring protections
Ring protection categorization:
By fiber quantity: 2-fiber and 4-fiber
By service direction: Unidirectional and bidirectional
By protection object level: Path protection and MS protection
Common ring protection:
2-fiber unidirectional path protection ring
2-fiber unidirectional MS protection ring
2-fiber bidirectional MS protection ring
4-fiber bidirectional MS protection ring
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Several definitions
There are two kinds of SDH protection switching ring: MS protection switching ring and path protection switching ring. What are the most commonly used are 2-fiber unidirectional path protection ring, 2-fiber unidirectional MS protection ring, 2-fiber bidirectional MS protection ring and 4-fiber bidirectional MS protection ring. Before the principles of these rings is described in detail, several definitions should be explained: MS protection switching, path switching, unidirectional ring and bidirectional ring.
What are MS and path? Simply speaking, MS refers to the section between two multiplexers (or the equipment with multiplexing functions), and the multiplexed low-rate signals are called the path.
An extra channel is required to protection signals in the transmission. For the MS switching ring, the protection are based on MS, the switching depends on the MS signal quality between a pair of nodes, and all MS services are switched to another channel in the switching. However, for the path switching ring, the protection is based on path, and the switching depends on the quality of one channel.
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Unidirectional ring Bidirectional ring
Unidirectional ring and bidirectional ring
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a ) in normal case b ) in abnormal case
P
W
A
B
C
D
P
W
A
B
C
D
倒换
2-fiber unidirectional path protection ring
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Working principles of 2-fiber ring
Two fibers respectively form internal ring and external ring. One fiber is used to transmit service signals and the other to protect them. The two rings are unidirectional and transmit services in different directions. The tributary signals from node A to node C are transmitted simultaneously in two fibers and reach C in different direction at one time. C receives the signals of one fiber with better signal quality. If the connection from A to C breaks off, the signals in W fiber will be lost, the switch will be switched from W fiber to P fiber and receives the signals in P fiber. Thus the signals from A to C go on transmission.
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Working principles of 2-fiber ring
A ring has 4 fibers: Two service fibers (one for receiving and the other for transmitting), and two protection fibers (one for receiving and the other for transmitting). Each fiber has a changeover switch. The signals transmit from A to C via W1 fiber clockwise, and the signals transmit from C to A via W2 fiber anticlockwise. This is a bidirectional ring and the two protection fibers are idle. When the connection from A to C breaks off, the changeover switches at B and C connect W1 to P1 and W2 to P2, ensuring the continuity of the ring.
In a 4-fiber ring, the transmission directions of W1 and P2 are the same, and so are those of W2 and P1. W1 and P2 can be integrated into one fiber W1/P2, and W2 and P1 into one fiber W2/P1. A half of W1/P2 timeslots transmit services, and the other half are idle in the normal case and protect W2/P1 services in the faulty case. A half of W2/P1 timeslots transmit services, and the other half are idle in the normal case and protect W1/P2 services in the faulty case. Thus a 4-fiber ring is simplified into a 2-fiber ring.
One half of W1/P2 timeslots transmit signals from A to C clockwise and the other half is idle. When the connection between A and C breaks off, the changeover switches at B and C connect the two fiber together. With the timeslot switching technique, the signals are switched from the service timeslot of one fiber to the idle protection timeslot of the other fiber.
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4-fiber bidirectional MS shared protection ring
a) In the normal case b) Switching in the faulty case
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2-fiber bidirectional MS shared protection ring
a) In the normal case b) Switching in the faulty case
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Features and applications of self-healing rings
Advantage: The protection can be implemented easily. No APS protocol is required and the switching is the fastest (<30ms)
Shortcoming: The timeslots between nodes can not be used repeatedly, so the ring transmission capacity is small. And extra services can not be transmitted.
Ring transmission capacity: STM-N
Note: The unidirectional path protection ring found a wide application. It is applied to the centralized services.
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4-fiber bidirectional MS protection ring
Advantage: The timeslots between nodes can be used repeatedly, so the ring transmission capacity is large. And extra services can not be transmitted via the standby fibers P1 and P2.
Shortcoming: The switching is slow because APS protocol and cross connection are required. And the equipment should meet some high requirements.
Ring transmission capacity: k×STM-N (k is the node quantity in the ring)
Note: ADM equipment in a 4-fiber ring should meet some requirements, e.g., system capacity, cross capacity and software function. It is applied to the distributed services.
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2-fiber bidirectional MS protection ring
Advantage: The timeslots between nodes can be used repeatedly, so the ring transmission capacity is large. And extra services can not be transmitted via the standby fibers P1 and P2.
Shortcoming: The switching is slow because APS protocol and cross connection are required. And the equipment should meet some high requirements.
Ring transmission capacity: k/2×STM-N (k is the node quantity in the ring)
Note: The bidirectional MS protection ring found a wide application. It is applied to the distributed services.
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item Unidirectional path ring 2-fiber MS ring 4-fiber MS ring
node
Line rate
ring transmission capacity
APS protocal
swiching time
node cost
system complexity
k k k
STM-N STM-N STM-N
STM-N k/2×STM-N k×STM-N
no yes yes
30ms 50-200ms 50-200ms
low medium high
simple complex complex
Access network and relay net work
(centralized service)
Relay network and toll network (distributed
service)
Relay network and toll network (distributed
service) applications
Characteristics and applications of 3 protection rings
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A
B(primary) D(secondary)
C(primary)
E(secondary)
F
The services from A to F are transmitted in two ways: ABCF or ABDECF.
DNI, based on ITU-T G.842, is very practical to the services across rings.
DNI protection
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OutlineOutline
▼ Overview to SDH
▼Rate and frame structure
▼Multiplexing structure and procedures
▼SDH network protection
▼ SDH network synchronization
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Synchronization
The synchronization is the nervous system of SDH network.
The asynchronization between NEs in one network leads to the unaligned timeslots and no proper connection between transmitter and receiver.
The asynchronization between networks leads to the broken network communication and service disconnection.
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4-level clocks in ITU-T recommendation
• Reference master clock — In compliance with G.811.
• Transit exchange clock — In compliance with G.812 (transit clock at the intermediate exchange)
• End exchange clock — In compliance with G.812 (local exchange clock)
• SDH NE clock — In compliance with G.813 (clock embedded in SDH NE)
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Clock type
1. Caesium atom clock: It has the high long-term frequency stability and precision. The long-term frequency deviation is better than 1*10E-11, but the short-term stability is not good.
2. Quartz crystal oscillator: It has cheap clock source and high reliability, but low long-term frequency stability.
3. Rubidium atom clock: Its stability, precision and cost is between the above clocks. The adjustable frequency range is larger than caesium atom clock, the long-term stability is lower by about one magnitude, but it has excellent short-term stability, low cost and 10-year service life.
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Synchronization mode
1. Full-synchronization mode: The overall network is synchronous with the unique PRC. Its synchronization is highly precise but difficult. In the implementation the hierarchical control scheme is usually adopted, that is, the hierarchical master/slave synchronization mode.
2. Pseudo-synchronization mode: The overall network is divided into several sub-networks, and the master clocks of the sub-networks comply with G.811. The slave clock is synchronous with the master clock in the sub-network. The clocks of the sub-networks are independent of each other, but the differences are so small that they are approximately synchronous.
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3. Quasi-synchronization mode: After the external timing reference is lost, the node clock holds on. The network synchronization quality is not good.
4. Asynchronization mode: The node clocks are different from each other in the synchronization, and the service can not go on normally, so the alarm signals are sent.
SDH network adopts the hierarchical master/slave synchronization mode at present.
Synchronization mode
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Three working modes of slave clock in master/slave synchronization mode
Normal working mode – Upper-level clock tracing and locking mode
The clock reference, traced and locked by the slave site, comes from the upper-level site. It may be the master clock in the network, the clock from the clock source embedded in the upper-level NE, or the GPS clock at the local area. Comparing with the other two working modes of the slave clock, this mode is the most precise.
Hold-on mode
After all timing references are lost, the slave clock is in the hold-on mode. The slave site clock source uses the last frequency information, stored before the timing reference signal is lost, as the timing reference. That is to say, the slave clock has the “memory” function. The function can offer the timing signal complying with the original timing reference, ensuring that the slave clock frequency has a small deviation from reference clock frequency in a long time. This mode is less precise than the normal working mode. The equipment employs the memorized synchronization information, stored before 24 hours, to keep the synchronization state. The precision is required to be 0.37ppm.
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Free-run mode – free-oscillation mode
When the slave clock loses all external timing references and timing reference memory, or works in the hold-on mode for a very long time, the oscillator in the slave clock will work in the free-oscillation mode. This mode is the worst precise. After the memorized synchronization information, stored in the equipment, has been used for 24 hours, the synchronization signals generated by the internal oscillator are used as synchronization signal. The precision is required to 4.6ppm.
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Synchronization network of China Telecom
The digital synchronization network of China Telecom integrates the hierarchical master/slave synchronization and pseudo-synchronization, that is, the distributed timing mode.
1. PRC complying with G.811 in Beijing hierarchically controls the clocks until the lowest-level slave clock, which adopts the hierarchical master/slave synchronization mode.
2. This nation is divided into several synchronization areas. Each area has one LPR – rubidium atom clock. LPR can receive PRC signals or GPS signals. There are small differences in LPR between synchronous areas, but the differences are so small that they are approximately synchronous. So it is called the pseudo-synchronization mode.
As shown in the figure, the slave clock is in Wuhan. When a fault occurs to the master clock (Beijing), the slave clock will replace the master clock.
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Master clock (Beijing)
Slave clock (Wuhan)
Regional reference clock
1
Regional reference clock
2
Provincial exchange
provincial exchange
市 局 市 局
GPS GPS
synchronization area 1
LPR
PRC
synchronization area 2
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G.811
G.812
G.812
G.812
No.1 transit exchange
No.2 transit exchange
No.K transit exchange
} N×G.813 SDH equipment clock
note: K=10;
N=20;
NE clock quantity < 60
SDH synchronization network -synchronization reference link
} N×G.813 SDH equipment clock
} N×G.813 SDH equipment clock
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Synchronization scheme
General principle
Reduce the timing reference transmission length.
The controlled clock obtains the timing from higher-level clock.
The node clock quantity in a synchronization reference link is not more than 60.
Configure more than one external timing references.
Prevent the timing loop – make full use of S1 byte.
Timing information transmission – Obtain the timing from STM-N.
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SSM and S1 byte
SSM
SSM (Synchronization Status Message) directly reflects the synchronous timing signal level in the synchronous timing transfer link. The messages can be used to judge the quality level of the synchronous timing signal received so as to control the operation state of the local node clock, e.g., continue tracing the signal, switch the input reference signal or change to the hold-on state.
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SI b5~b8 Clock level
0000 Unknown quality
0010 G.811 reference clock
0100 G.812 transit exchange slave clock
1000 G.812 local exchange slave clock
1011 SETS
1111 Not for clock synchronization
Note: other utilizations are reserved.
How to use S1 byte
ITU-T G.707 specifies SSM coding mode of STM-N interface, which is shown with MS overhead byte S1 b5~b8.
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Timing protection switching and recovery
The equipment has more than 2 external synchronization signal input interfaces.
A) Timing protection switching function
When the high-level external synchronization source fails, the equipment can automatically switch to the low-level external synchronization source.
B) Recovery function
When the high-level external synchronization source return to normal, the equipment can obtain the timing signals from the high-level external synchronization source.
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