1. Transport overhead STS-1 Envelope Capacity Section Overhead
90 bytes
3 bytes
Framing A1
Framing A2
Section
Trace J0
BIP-8 B1 OW E1
User F1
Section Overhead
DCC D1
DCC D2
DCC D3
Pointer H1
Pointer H2
Pointer H3
STS-1 Envelope Capacity
9 rows
BIP-8 B2
APS K1
APS K2
DCC D4
DCC D5
DCC D6
Line Overhead
DCC D7
DCC D8
DCC D9
Section overhead is allocated to the first three rows of the transport overhead. Subsequent slides describe each section overhead byte.
Line overhead is allocated to the remaining six rows of the transport overhead. The functions are described in the following slides with the exception of the payload pointer, which has its own section later on.
DCC
D10
DCC
D11
DCC
D12
Sync
S1/Z1
FEBE
M0/M1/Z2 OW E2
Transport
Overhead
125 microsec. 29

2. STS-1 SPE path overhead STS-1 Envelope Capacity STS Path Overhead
90 bytes
3 bytes
STS-1 Envelope Capacity
STS Path Overhead
Transport
Overhead
9 rows
Trace J1
BIP-8 B3
Label C2
The first column of the STS synchronous payload envelope contains the STS path overhead. STS path overhead allows for integrity verification of the end-to-end STS path. In general, it is not modified at line terminations.
Status G1
User F2
Payload Capacity
Synchronous
Payload
Envelope
Multiframe H4
Growth Z3
Growth Z4
TCM Z5 30

3. Framing bytes (A1, A2) A1 Byte A2 Byte 1 1 1 1 0 1 1 0 0 0 1 0 1 0 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Hex: F628
The hex code F628 is transmitted in every frame of every STS-1.
This allows a receiver to locate the alignment of the 125usec frame within the received serial bit stream. Initially, the receiver scans the serial stream for the code F628. Once it is detected, the receiver watches to verify that the pattern repeats after exactly 810 STS-1 bytes, after which it can declare the frame found.
Once the frame alignment is found, the remaining signal can be descrambled and the various overhead bytes extracted and processed. 31

4. Scrambling Scrambler reset to 11111111 Transport Overhead
Envelope Capacity
125 microsec.
Transport
Overhead
Scrambler polynomial = 1 + X6 + X7
Scrambler reset to
Not scrambled
Scrambled
Just prior to transmission, the entire SONET signal, with the exception of the framing bytes and the section trace byte, is scrambled. Scrambling randomizes the bit stream is order to provide sufficient 0-1 and 1-0 transitions for the receiver to derive a clock with which to receive the digital information.
Scrambling is performed by XORing the data signal with a pseudo-random bit sequence generated by the scrambler polynomial indicated above. The scrambler is frame synchronous, which means that it starts every frame in the same state.
Descrambling is performed by the receiver by XORing the received signal with the same pseudo random bit sequence. Note that since the scrambler is frame synchronous, the receiver must have found the frame alignment before the signal can be descrambled. That is why the frame bytes are not scrambled. 32

5. Section trace (J0) J0 Byte 1 2 3 4 5 6 7 8 Trace ID Value ID=2 ID=3
Section trace allows individual fibers to be distinguished from each other. This is particularly useful when systems are initially installed. Without section trace, it can be difficult to detect when the fibers have been incorrectly connected.
In earlier releases of the SONET standard, this byte was used for an STS-1 identifier. All it did was count the timeslots in an STS-N multiplex. The first STS-1 would have hex 01 in this byte, the second 02 and so on.
In order to detect whether this byte is carrying the old STS-1 ID or the new section trace, the section trace does not allow hex 01 as a valid trace ID.
In SONET, the section trace is defined as a one byte code, allowing for up to 255 identifiers (excluding hex 01).
In SDH, the section trace is defined as a 16 byte string that is transmitted repeatedly.
ID=5 33

6. Path trace (J1) Vancouver Recommended Use: Toronto J1 bytes
Path trace code assigned to indicate
physical location of path source
Equipment provisioned to
monitor path trace code =
Vancouver
DS3
DS3
Vancouver
Recommended Use:
Toronto
Path trace provides a way of verifying that the path is being received is coming from the right place. It does not provide any indication of the route that the path took; it only verifies the end points.
The path trace byte carries a 64 byte string that repeats continuously. Each frame of the SPE contains one byte of the string, such that it takes 64 frames to transmit the entire string.
The source of the path inserts a code in the path trace byte that uniquely identifies the source of the path. Although this code is usually user provisionable, ANSI T1.105 recommends the Bellcore Common Language Location Identifier (CLLI) terminated with a carriage return (CR) and a line feed (LF) character. The CLLI code is a string of ASCII characters that identify such things as the state, city, and office that contains the equipment.
The termination of the path monitors the incoming path trace code and compares it to a locally provisioned value. If the codes do not match, an alarm can be raised indicating a potential misconnection.
In SDH, the path trace code is defined to be a 16 byte string, rather than 64.
64 character ASCII string
J1 bytes CR LF n
n+1
n+2
n+63
STS-1 frame
Common Language
Location Identifier (CLLI) 34

7. Section BIP-8 (B1) B1 Byte Even parity calculated over2 3 4 5 6 7 8
Even parity calculated over
all bit 8s from entire STS-N frame
Even parity calculated over
all bit 1s from entire STS-N frame
The section BIP-8 byte contains an 8-bit bit interleaved parity. Bit 1 is even parity calculated over all the bit 1s in every byte of the previous STS-N frame. Bits 2 through 8 are calculated similarly.
Since there is only one section BIP-8 regardless of the value of N, the ratio of parity bits to data bits becomes less at higher line rates. This ratio affects the error detection capability of the mechanism. Therefore, the section BIP-8 is more useful for fault isolation that it is for performance monitoring. 35

8. Line BIP-8 (B2) B2 Byte Even parity calculated over1 2 3 4 5 6 7 8
Even parity calculated over
all bit 8s from entire STS-1 frame
excluding section overhead
Even parity calculated over
all bit 1s from entire STS-1 frame
excluding section overhead
The line bit interleaved parity is similar to the section BIP-8. However, each STS-1 has its own line BIP-8, and it is calculated over the entire STS-1 with the exception of the section overhead.
Since there is a separate line BIP-8 for each STS-1, there is a constant ratio of parity bits to data bits regardless of line rate. This means that the saturation point of the error checking mechanism is constant at about Therefore, the line BIP-8 is used for performance monitoring. 36

9. Line remote error indication (M0/M1)
Example: 2 out of 8 B2 (Line BIP-8)
parity bits detected as wrong in frame n
Line REI sent with binary
value 2 in next outgoing frame
4 bit binary value for OC-1 interfaces (M0)
M0/M1 Byte
8 bit binary value for OC-N interfaces (M1) 1 2 3 4 5 6 7 8
Line REI allows each end of a line system to evaluate the error performance of the signal that it is transmitting.
Each end of the line system is checking the Line BIP-8 on a per frame basis on the received signal. This check allows up to 8 x N errors to be detected in each frame of the STS-N. The result of this check, that is the count of errors, is sent back to the transmitting end in the Line REI byte, so that the transmitting end can maintain the same error count as the receiving end.
Thus, each end can provide performance monitoring statistics of the signal that is being received, using the Line BIP-8, as well as the signal that is being transmitted, using the Line REI. This is referred to as single-ended performance monitoring.
Note: Line REI previously known as Line FEBE (Far End Block Error). 37

10. Path BIP-8 (B3) B3 Byte Even parity calculated over1 2 3 4 5 6 7 8
Even parity calculated over
all bit 8s from entire SPE frame
Even parity calculated over
all bit 1s from entire SPE frame
The path BIP-8 is the same as the line and section BIP-8, except that it is calculated only over the SPE frame. Since the content of the SPE is not modified at intermediate points along the path (line terminations), the path BIP-8 allows end-to-end bit error integrity verification of the SPE. 38

11. Path status (G1) G1 Byte REI = Remote Error Indication2 3 4 5 6 7 8
REI = Remote Error Indication
RDI = Remote Defect Indication
Path REI
RDI
Unused
0 X X No defects
No defects
Payload defects (payload label mismatch,
ATM loss of cell delineation)
Connectivity defects (path trace mismatch, unequipped)
Server defects (path AIS, loss of pointer)
NOTE: Exceptions to handle backwards compatibility
are documented in GR-253.
The path status byte provides a way for each end of the path to monitor the integrity of both directions of transmission. The two functions carried in the path status byte are the path remote error indication (REI) and path remote defect indication (RDI).
The path REI is similar to the line REI. Each end of the path is checking the path BIP-8 on a per frame basis on the received signal. This check allows up to 8 errors to be detected in each frame of the SPE. The result of this check, that is the count of errors, is sent back to the transmitting end in the path REI, so that the transmitting end can maintain the same error count as the receiving end.
The path RDI allows the receiving end of the path to notify the transmitting end when failures have been detected, as opposed to just bit errors. The various error possibilities are classified into 3 groups:
payload defects indicate that although the SPE seems to be OK, the payload is not getting through properly,
connectivity defects indicate a provisioning problem, and
server defects indicates a transmission failure of some sort.
These conditions have different effects on the calculation of performance statistics and unavailability, therefore each end must know the kind of failure in order to come up with the same performance monitoring results. 39

12. Line RDI K2 Byte Channel # 111 Line AIS 110 Line RDI
101 Bidirectional APS
100 Unidirectional APS
Mode
1 1:N Mode 1 2 3 4 5 6 7 8
A line terminal detecting an incoming line failure will send line RDI (110 in bits 6-8 of K2) back to the other end of the line so that both ends are aware that a failure has been detected on the line. 40

13. Section orderwire (E1) Line Terminal Regenerator Line Terminal E1 Byte
Section orderwire is a channel reserved for voice communications between network provider maintenance personnel. Since it is in the section layer overhead, it is accessible at a regenerator, such that a maintenance person working on the regenerator can talk to a colleague at the terminal site to coordinate maintenance activity. 41

14. Line orderwire (E2) Line Terminal Regenerator E2 Byte 42
Line orderwire is similar to section orderwire, except that it is not accessible at regenerators, which, in general, do not have access to the line overhead. Thus the line orderwire only allows communications between maintenance personnel at line terminal sites. 42

15. Section user channel (F1)
Line
Terminal
Regenerator
F1 Byte
Alarm
Monitor
Monitored
Element
The section user channel is dedicated to network provider use. SONET standards do not define a particular use, protocol or interface for this byte. However, a typical application would be to backhaul alarms from a regenerator site to the central office. 43

16. Path user channel (F2) DS3 F2 Byte User Info Vancouver Toronto 44
The path user channel allows network providers to connect information of their choosing across an STS path layer connection. SONET standards do not define any use, protocol, or interface for this byte; that is left for the individual network providers to specify. 44

17. SONET data communications channels (D1-D3, D4-D12)
D1-3 Bytes = Section Data Comm Channel (192 kbit/s)
D4-12 Bytes = Line Data Comm Channel (576 kbit/s) OS
External
Data
Network
Gateway NE NE NE
The SONET section and line data communications channels (DCC) are used to provide a communications path between a centralized operations system (OS) and the various network elements.
The section DCC is carried in the D1, D2, and D3 bytes which combine to form a single 192Kb/s channel. The line DCC is carried in the D4-D12 bytes which combine to form a single 576Kb/s channel.
The OS connects to a gateway NE, possibly via an external data network. From that point on, the messages are relayed from NE to NE via the DCCs. NEs are able to look at the address specified in received messages and decide whether they are for that NE. If not, the NE knows how to route the message so that it reaches its ultimate destination.
The overall data network created is called the Telecommunications Management Network (TMN).
OSs monitor and control entire networks. They provide operations, administration, maintenance and provisioning (OAM&P) functions such as alarm surveillance and fault management, performance monitoring, configuration management, and connection set-up.
Each network element is modeled using object-oriented techniques and the resulting information model is a standard. Thus the OS views each NE as a set of objects that can be controlled and monitored, regardless of which vendor supplied the NE. In general, all application layer messages in the TMN are from OS to NE and not directly from NE to NE.
The section and line DCCs use the same protocol stack and were originally intended to carry the same messages. The only distinction is that the line DCCs cannot be accessed at a regenerator. The messages that the OS uses to operate on the NE objects are carried over this protocol. NE
OS = Operations System
NE = Network Element 45

18. Synchronization status messaging (S1)
Building Integrated Timing Supply
S1= Stratum 1 traceable
S1= Stratum 3 traceable
S1 = Don’t Use
BITS S1 1 2 3 4 5 6 7 8
Synchronized - traceability unknown
Stratum 1 traceable
Stratum 2 traceable
Stratum 3 traceable
ppm clock traceable
Reserved for network synchronization
Don’t use for synchronization
In a synchronous network, all network elements receive a network clock that is used to transmit all outgoing synchronous signals. Often a network element receives its clock reference via an incoming synchronous signal. A clock will be derived from the incoming signal and used to lock a local system clock. In order to provide robust timing distribution, the network element will usually have a selection of references from which to choose in case the one that is being used fails.
Synchronization messaging provides an indication of the quality of the SONET signal for use as a synchronization reference. It allows a network element to evaluate all of its reference sources and choose the best one.
It also can be used to prevent the possibility of timing loops. 47

19. Path signal label (C2) Hex Value Payload 00 01 02 04 12 13 14 15 E1-FC
Unequipped
Equipped - Nonspecific payload
Virtual Tributaries
DS3
Mb/s
ATM
DQDB
FDDI
Payload Defect Indicator (PDI)
C2 Byte 1 2 3 4 5 6 7 8
The path signal label provides an identifier of the contents of the SPE, since a variety of payloads can be mapped into it.
This can be used to identify problems where the wrong mapper has been plugged into one end of the path. For example, if a DS3 mapper is plugged in at one end and a DS1 mapper at the other end, the connection will obviously not work and the signal label will indicate why.
The signal label can also be used as a way of automatically provisioning mappers that can handle more than one payload type. 48

20. Tandem connection monitoring
Photonic
Convert STS-N signal
to optical pulses
Section
Framing and scrambling
Add section overhead
Line
Multiplex SPEs
Add line overhead
Automatic protection
STS Path
Map payloads into STS SPE
Add path overhead
SONET
Network Element
STS Synchronous Payload Envelope
STS-N with Line Overhead
Light pulses
STS-N with Line and Section Overhead
Payloads (DS3, video)
Payloads (DS1, DS2)
VT Path
Map payloads into VT SPE
Add path OH
VT Synchronous Payload Envelope
Optional
Tandem Connection
Performance monitoring over
path segments
Tandem connection monitoring is a feature that was added to the SONET standard in Its primary purpose is to allow performance monitoring of a segment of a path. It also allows several paths that share the monitored segment to be bundled together, and monitored as a single entity.
SONET, as originally defined, allows for performance monitoring at the section, line, and path layers. However, as the SONET network expands, it will not be unusual for a network provider’s network to consist of many lines, but only segments of the end-to-end paths. This will be especially true for inter-exchange carriers. In order to verify the end-to-end performance of their networks, these network providers will have to either aggregate the line layer performance information, or provide intermediate path layer monitoring at both the entry and exit points to their networks and correlate the results. Tandem connection monitoring provides a built-in mechanism for achieving the same result.
In terms of the SONET layers, tandem connection introduces an optional, intermediate layer between the STS path layer and the line layer. 49

21. Tandem connection monitoring
Path
Terminating
Equipment
(PTE)
Tandem
Connection
Terminating
Equipment
(TCTE)
Line
Terminating
Equipment
(LTE)
Tandem
Connection
Terminating
Equipment
(TCTE)
Path
Terminating
Equipment
(PTE)
DS3
DS3
STS Path
STS Path TC TC
STS Path
DS3
Line
Line
Line
Line
Line
Section
Section
Section
Section
Section
Photonic
Photonic
Photonic
Photonic
Photonic
The introduction of the tandem connection layer results in the definition of tandem connection terminating equipment, which is essentially line terminating equipment enhanced with tandem connection monitoring capability.
The tandem connection may span several line systems, but usually only spans a portion of the paths for which it is providing the monitoring capability.
Section
Section
Section
Section
Line
Line
Line
Line
Tandem Connection
STS Path 50

22. IEC - Incoming Error Count
TCM byte (Z5)
Z5 Byte 1 2 3 4 5 6 7 8
IEC
Data Link
IEC - Incoming Error Count
IEC and Data Link
Tandem Connection
Monitor
Tandem connection monitoring uses the Z5 byte from the STS path overhead.
The path BIP-8 (B3) of the paths that are to become part of the tandem connection is monitored as they arrive from upstream. The incoming error count detected is inserted into bits 1-4 of the tandem connection byte on a per path basis.
At the other end of the tandem connection, the path BIP-8s are monitored again and the results are compared with the values received in the IECs. Any differences are considered to be errors that occurred on the tandem connection.
Bits 5-8 carry LAPD structured messages that are used to send far-end performance reports from each end of the tandem connection back to the other end so that each end can verify the integrity of the both directions of the connection independently.
When paths are bundled together on a tandem connection, the LAPD message is sent only on one path. Paths may be grouped only in bundle sizes that correspond to the supported SONET line rates.
Note that the insertion of the tandem connection byte involves the modification of the SPE at an intermediate point on the path. In order to avoid corrupting the path BIP-8, it must be compensated. When the TCM byte is inserted or removed, the value of the byte that was in location Z5 is subtracted from the BIP-8 and the new value added. This preserves the end-to-end path integrity monitoring capability of the BIP-8.
DS3
OC-12
OC-48
OC-12
DS3
PTE
TCTE
TCTE
PTE 51

23. STS-1 payload pointer offset
90 bytes
Frame N
STS-1 SPE
STS Pointer 1 40 41 87
125 µs
Frame N + 1
The pointer overhead bytes carry a binary number that is an offset, in bytes, into the envelope capacity to the beginning of the next SPE frame.
STS Pointer
250 µs 63

24. STS payload pointer (H1, H2)
I - Increment Bits
D - Decrement Bits
NDF Not Set = 0110
NDF Set = 1001
Concatenation Flag H1 H2 1 X I D 2 3 4 5 6 7 8
Not
Used
New Data Flag
Pointer Offset
The content of the STS payload pointer bytes is shown above.
Bits 7 and 8 of H1 together with bits 1-8 of H2 carry the pointer offset. If the first byte of the next SPE frame was the byte immediately following the H3 byte, the offset would contain the value 00 hex.
The bits of the pointer offset are grouped into I bits and D bits. These bits are used to flag increment operations and decrement operations respectively. A pointer processor monitoring an incoming pointer keeps track of the current offset and continuously compares the next received value with the current value. An increment operation is detected when the I bits of the received value are inverted with respect to the current value. Similarly, a decrement operation is detected when the D bits of the received value are inverted with respect to the current value.
Bits 1-4 of H1 contain the new data flag. This flag is used to notify downstream pointer processors of a change in the pointer value to a new value unrelated to the previous value. This would be used at a cross-connect system for example, when an outgoing STS-1 that was carrying an SPE from one particular incoming STS-1 suddenly starts carrying an SPE from a different incoming STS-1 due to a provisioning command. Since there is no phase relationship between the two SPEs, the pointer value will change. The new data flag is set to 1001 in the first STS-1 frame that contains the new pointer offset value, and then returns to its normal value of 0110.
When an STS-1 is concatenated to another STS-1, its pointer offset contains the value This is the concatenation flag and it effectively disables the pointer processing circuitry for this STS-1.
When a failure is detected at the path layer, path AIS is generated downstream to provide a keep-alive signal. Path AIS is detected by all ones in the H1-2 pointer bytes. 70

25. Multiframe indicator (H4)
V4 Bytes H4 00
H4 Byte
V1 Bytes 1 2 3 4 5 6 7 8 H4 01
V2 Bytes
X X X
X X X H4 10
VT SubFrame
Counter
V3 Bytes
Since the VT superframe will span four consecutive STS-1 SPE frames, a method is required to identify which VT subframe is present in the current STS-1 SPE frame. That is the purpose of the H4 STS path overhead byte, which contains a simple modulo 4 counter. All of the VT superframes are aligned such that the H4 byte simultaneously identifies the alignment of all of them. H4 11
V4 Bytes H4 00 88

26. Single-ended performance monitoring
DS3
Path
Terminal
Line
Degraded
Section BIP-8 (B1)
B1 errors
detected B1
Line BIP-8 (B2)
B2 errors B2
Line REI (M0/M1)
Far-end
line errors
counted
M0/M1
Path BIP-8 (B3)
B3 errors
Path REI (G1 bits 1-4)
path errors
Regen
Time
Flow
SONET contains a number of functions that allow the error performance of both directions of the path to be calculated at each end of the path, independently. The above example illustrates how this works.
Errors are occurring in the “degraded” portion of the fiber as illustrated in the slide above. These errors may be caused by a problem with the fiber itself, or with the transmitting or receiving optics.
The regenerator adjacent to the problem section will record the errors in its section layer performance monitoring based on the Section BIP-8 (B1). This will help localize the fault since no other sections are showing errors.
The line terminal at the end of the problem line will also record the errors in its line layer performance monitoring based on the Line BIP-8 (B2). In response, the line terminal sends the error counts back to the source of the line, in the line REI, so that the line performance can be calculated at both ends. Only the line that contains the problem section shows errors.
All path terminals that terminate a path that went across the problem section will record errors in their path layer performance monitoring based on the path BIP-8 (B3). The path terminal will send the error count back to the path source in the path REI so that both ends of the path can observe the error performance of both directions of the path. 99

27. Maintenance signaling
DS1
VT/STS Path
Terminal
Line
STS Path
Regen
LOS
detected
Line AIS
AIS (H1/H2)
STS Path AIS
STS Path RDI (G1)
STS Path RDI
(K2)
Line RDI (K2)
Line RDI
VT AIS (V1/V2)
DS3
DS3 AIS
VT Path AIS
VT Path RDI
VT Path RDI (V5)
(The remaining signaling only occurs if the Line Terminal
cannot restore the traffic with a protection switch)
Time
Flow
DS1 AIS
SONET defines a number of maintenance signals that are used in the event of failures to notify other equipment of the failure, to ensure that alarms are raised only at the point where the failure was first observed, and to allow for the proper conditioning of the performance monitoring.
The above example illustrates how this works. In the example, the fiber into the regenerator has failed resulting in a loss of signal (LOS) detection at the regenerator. This failure may be due to a cut in the fiber, or a failure of the transmitting or receiving optics interfaces. The regenerator raises the appropriate alarm and sends a replacement signal called line alarm indication signal (AIS) downstream. Line AIS is a valid section layer signal with the line overhead and SPE replaced with all ones (before scrambling) or a toggle pattern (after scrambling).
Line AIS is detected at the line terminal by observing 111 in bits 6-8 of the K2 byte. The line terminal will send line RDI (110 in bits 6-8 of K2) back to the other end of the line so that both ends are aware that a failure has been detected on the line. The line terminal will then try to restore the traffic by using protection switching.
If protection switching will not restore the traffic, then STS path AIS is sent downstream on all STS paths that come from the failed line.
At the STS path termination, STS path RDI will be sent upstream to notify the other end of the path. The appropriate AIS will be sent downstream on all payloads or passthrough STS paths that come from the failed link.
In the end, the only alarm raised is at the regenerator. AIS propagates downstream to all affected equipment to notify them that the failure has been detected so they don’t raise additional alarms. RDI propagates to all the appropriate upstream equipment so that they can condition the error counts they are receiving on the REIs.
Note: Line RDI previously known as Line FERF (=Far End Receiver Fail).
Note: Path RDI previously known as path yellow.
100

28. Linear 1+1 Working Channel Protection Channel Line Terminal Regen 102
Automatic protection switching refers to the ability of SONET network elements to detect a failure and transfer the affected traffic to another line.
The most basic protection system is a linear 1+1 system. The term linear differentiates it from ring systems and the “1+1” indicates that there is 1 working fiber and 1 standby fiber and that the traffic in both directions is permanently bridged onto both the working and the standby fiber.
When one end of the system detects that the side from which it is currently selecting the traffic fails, it automatically switches to select the traffic from the other side after first verifying that the switch will restore the traffic and that there are no other reasons why the switch cannot be made.
This will result in a switch in one direction of the traffic; a unidirectional switch. If the system is provisioned for bidirectional switching, the other direction of traffic must be switched as well. However, the failure may not be such that the other end of the system can detect it. In order to ensure that both ends switch, the tail end (the end that initially detected the failure) sends an indication in the APS bytes (K1, K2) that it has switched and why and that the head end should do the same.
A linear 1+1 system can be made survivable by diversely routing the protection channel as is shown in the slide. In this way, a fiber cut on the working channel can still be restored on the protection channel.
102

29. Linear 1:N Working Channel #1 Working Channel #2 Working Channel #N
Terminal
Working Channel #1
Protection Channel
Working Channel #2
Working Channel #N
A linear 1:N (“1 for N”) protection system has one protection fiber providing a restoration path for up to N working fibers. Such a system is not survivable since it is impractical to diversely route all of the fibers. Therefore, a fiber cut, even if it does not affect the protection channel, is liable to affect more than one working channel, and the protection channel can restore only one working channel at a time.
A 1:N system involves more complicated signaling between the two ends of the system to coordinate the switch. This is because the failed channel must first be bridged onto the protection channel before the receiving end can do the switch, and the end that must do the bridge may not even be directly aware that there is a failure.
103

30. APS bytes (K1, K2) - linear protection3 4 5 6 7 8
K1 Byte
Channel #
K2 Byte
111 Line AIS
110 Line RDI
101 Bidirectional APS
100 Unidirectional APS
0 1+1 Mode
1 1:N Mode
Switch
Preemption
Priority
1111 Lockout of Protection
1110 Forced Switch
1101 Signal Fail - High
1100 Signal Fail - Low
1011 Signal Degrade - High
1010 Signal Degrade - Low
1001
1000 Manual Switch
0111
0110 Wait-to-Restore
0101
0100 Exercisor
0011
0010 Reverse Request
0001 Do Not Revert
0000 No Request
The line layer APS bytes (K1, K2) are used to carry the APS protocol between the two ends of the system. The K1 bytes on the protection fiber are used.
The K1 byte acts as the switch request channel. When one end of the system detects a failure on a channel, it indicates the kind of failure and identifies the channel to the other end of the system using the K1 byte. In addition, K1 is used to signal switch requests for reasons other than failures, such as manual and forced switches that are used by maintenance personnel. The different request types are organized in a hierarchy or priority scheme. If the node detects a signal degradation on one channel, but a signal failure on another, it will try to restore the channel affected by the signal failure because the signal failure has a greater impact on traffic.
When the other end of the system receives the switch request, it looks at the priority of the request and compares it to local conditions. If the far end is requesting a switch on one channel due to a manual switch, but it sees a signal failure on another channel on its end, it will ignore the manual switch request and signal back to the far end a switch request for the channel affected by the signal failure and the far end is obliged to comply.
Even after a switch has been established, if a higher priority condition arises, the old switch will be dropped in favor of the new one. This is referred to as switch preemption.
K2 is used to signal which channel has been bridged onto the protection fiber. It also includes some provisioning status related to APS.
When a node receiving a switch request agrees to the request, it will bridge the requested channel onto the protection fiber. It will then signal back to the other end, using the K2 byte, that the channel has been bridged. This will indicate to the other end that it is now safe to complete the switch by taking the traffic off of the protection fiber. A node wanting to switch (that is,. select traffic from the protection fiber) will wait until K2 indicates that the traffic has been bridged so that it knows it is selecting the proper traffic.
When the failure has been corrected, the node requesting the switch will drop its request by signaling no request in byte K1. This will result in the bridges and switches being dropped.
104

31. 1:N APS signaling Working Channel #1 Working Channel #2 Line Terminal
Protection Channel
Working Channel #2
Working Channel #N
Signal Fail
detected on Chn 2
K1 = Signal Fail, Chn 2 K2 = Chn 1, 1:N, Bi
K1 = Reverse Request, Chn 2 K2 = Chn 2, 1:N, Bi
K1 = Signal Fail, Chn 2 K2 = Chn 2, 1:N, Bi
Chn 2 bridged
Chn 2 bridged & switched
Time
Flow
In this example, a failure is detected on channel 2 by the node on the right. Upon detecting the failure, and deducing that it is the highest priority condition, the node sends a switch request in the K1 byte on the protection fiber. Note that the system has been provisioned for bidirectional switching, so that even though the failure is only affecting one direction of traffic, both directions will be switched.
When the other end of the system receives the switch request, it looks at the priority of the request and compares it to local conditions. Since the signal fail request is the highest condition, the node on left bridges channel 2 onto the protection fiber and indicates this in the K2 byte on the protection fiber. Since it is provisioned for bidirectional switching, it also signals a reverse request in the K1 byte, effectively requesting that the node on the right also bridge channel 2.
When the node on the right observes that channel 2 has been bridged, it selects the traffic from the protection fiber, that is, it completes the switch. Upon detecting reverse request it also bridges channel 2 in the other direction. Upon detecting that channel 2 has been bridged, the node on the left completes the switch.
In this protocol, bridges must always be requested from the receiving end, even in the above example where the node on the right knows that it will eventually be requested to bridge. This serves as a safeguard that the receiving node has dropped any other switches that may have been active before the new traffic is bridged, ensuring that channels are not misconnected.
105

32. Unidirectional path switched rings
Working
Protection
Ring-based APS provides an increased level of survivability for SONET networks by allowing as much traffic as possible to be restored, even in the event of a cable cut or a node failure.
In a ring, a number of network elements are connected in a closed loop. In the event of a disruption at any point on the ring, the affected traffic can be restored by rerouting it the other way around the ring.
The two main types of ring are the unidirectional path switched ring (UPSR) and the bidrectional line switched ring (BLSR).
In the UPSR, the ring consists of two fibers on each span, transmitting in opposite directions, between adjacent nodes. Traffic entering the ring at any node is transmitted in both directions around the ring until it reaches its exit point. Each node on the ring is line terminating, such that the connection across the ring is a path layer connection. At the exit point, the path layer integrity information is examined for each of the two paths and a selection made.
All the selectors will default to a particular direction, such that under non-failure conditions, all traffic is rotating around the ring in the same direction. Hence the name unidirectional. Since the selection is made on a per path basis, using path layer integrity information, the ring is called path switched.
Ring Node
106

33. Unidirectional path switched rings
Working Fiber
Protection Fiber
Ring Node
Fiber Cut
Path AIS
This example illustrates a cable cut on the ring. The channel traveling from left to right is unaffected by the cut so the selector at the right does not change. However, the channel traveling from right to left is affected. The top node will detect the line failure and send path AIS on downstream to the node on the left. Upon detecting path AIS, the selector at the left will switch to the traffic traveling in the clockwise direction, restoring the bidirectional connection.
Note that no signaling is required to coordinate any of the switching; therefore, it is conceptually simple.
107

34. Bidirectional line switched rings
Clockwise Fiber
Counter Clockwise Fiber
In a bidirectional line switched ring (BLSR), the two directions of a single connection occupy the same segment of the ring, that is,both directions of the ring are used even under non-failure conditions. Hence the name bidirectional.
There are two types of BLSR, the 2-fiber BLSR and the 4-fiber BLSR. In the 2-fiber BLSR, there are two fibers, transmitting in opposite directions, between adjacent nodes. In the 4-fiber BLSR, there are four fibers between adjacent nodes, two in one direction and two in the other. The 4-fiber BLSR supports more traffic on the ring and allows 1+1 span switching between adjacent nodes in addition to ring switching.
Note that the BLSR can support more than one connection on the ring using the same channels, as long as the connections do not overlap on any spans. This is not true of the UPSR that uses an entire channel in both directions all the way around the ring for each and every connection. Therefore, in general, the BLSR can support many more connections than a UPSR, even if they are both operating at the same line rate.
The exception is when all of the connections have one end at the same node, in which case the BLSR and the UPSR have equivalent capacity. Thus, UPSRs are often used in access networks in which all of the traffic is homing in on the local switching office.
Ring Node
108

35. Bidirectional line switched rings
Fiber Cut
Clockwise - Working
CW - Prot.
CCW - Working
CCW - Prot.
In the BLSR, half of the capacity in each direction is assigned to working traffic and half is assigned to protection. If it is a 2-fiber ring, then half of the STS-1s on each fiber are assigned to working and half to protection. If it is a 4-fiber ring, then 2-fibers, one in each direction, are assigned to working and the other two are assigned to protection.
When a failure occurs and a ring switch is required, the nodes adjacent to the failure and on either side of the failure will reroute (loopback) all of the working traffic that was crossing the failed span onto the protection capacity around the ring.
Since the decision to switch is made by the nodes adjacent to the failure using line layer integrity information, the ring is called line switched.
Signaling is required to coordinate the switch between the nodes adjacent to the failure. The line layer APS bytes (K1, K2) are used to carry the protocol.
Ring Node
109

36. APS bytes (K1, K2) - ring protection
K2 Byte
Destination
Node ID 1 2 3 4 5 6 7 8
K1 Byte
Source
0 Short Path Code
1 Long Path Code
Switch
Preemption
Priority
1111 Lockout of Protection
1110 Forced Switch - Span
1101 Forced Switch - Ring
1100 Signal Fail - Span
1011 Signal Fail - Ring
1010 Signal Degrade - Protection
1001 Signal Degrade - Span
1000 Signal Degrade - Ring
0111 Manual Switch - Span
0110 Manual Switch - Ring
0101 Wait-to-Restore
0100 Exercisor - Span
0011 Exercisor - Ring
0010 Reverse Request - Span
0001 Reverse Request - Ring
0000 No Request
111 Line AIS
110 Line RDI
011 Extra Traffic
010 Bridged and Switched Status
001 Bridged Status
000 Idle
The K bytes for ring signaling are similar to those used for linear protection signaling.
The K1 byte is used as a switch request channel as in linear and there is a prioritized selection of switch requests available. However, there are separate requests for ring switches and span switches. The span switch requests are only used in 4-fiber BLSRs. Rather than indicate a channel number, which is not relevant in a ring, the K1 byte identifies a destination node. Each node on a ring has an identifier and all nodes know that ID as well as the location of the node on the ring. A node wishing to initiate a switch needs to be able to identify which node the signaling is destined for.
The K2 byte carries the ID of the source of the signaling. Since ring signaling is generally propagated in both directions around the ring, the K2 byte also indicates whether the received code is arriving on the short path (that is, across the span affected by the failure) or on the long path (that is, the long way around the ring). Finally, the K2 byte also indicates the status of bridges and switches at the source node.
110

37. Survivable ring interconnect
DS1, DS3, STS-N, OC-N
Protection path may
be carried on working
channels or protection
channels
Ring #1
Ring #2
SONET defines a way to provide survivability on connections between rings, regardless of whether they are UPSRs or BLSRs. Essentially, the connection between the rings is provided at two places. As the path signal is dropped from one ring, it continues on to another node on the same ring where the redundant connection will be. The redundant connection is added on the other ring and carried over to the node where the other connection connects to the ring. At this point a selection is made using path layer integrity information. This selector is similar to that used on a UPSR.
The protection path, as shown above, may be carried either on working channels on the rings, or on protection channels. It may also be carried on a working channel on one ring, and a protection channel on the other. There are various tradeoffs involved. However, the key tradeoff is that when working channels are used, working capacity on the ring is sacrificed. On the other hand, when protection channels are used, the redundancy of the ring interconnection will be lost when the ring switches for an unrelated reason.
Drop and Continue
SEL
Path Selection
111

38. Payload defect indicator
Ring #1
STS-1
Ring #2
DS3
An additional complication arises in survivable ring interconnection when there is other equipment inserted between the rings and the interface on one side of the ring is not SONET. In the example above, if a problem occurs with the DS3 line, it will not be visible to the path selector on ring #1 and, therefore, no switch will occur.
The problem also occurs if the STS being dropped from ring #2 is VT structured and the equipment between the rings is STS path terminating, with VT bandwidth management granularity, and has an unprotected equipment failure affecting some number of the passed through VTs. When these VTs are mapped into a new STS-1 by the equipment between the rings, and the selector on ring #1 is an STS path selector, the failure of the constituent VTs will not be visible to the selector so no switch will occur.
SONET uses a payload defect indication (PDI) in the STS path overhead to resolve this. A series of codes in the STS path signal label byte (C2) indicates failures of the STS path payload. In the case of a VT organized STS-1, the codes identify how many VTs failed. In all other cases, the PDI simply indicates whether the payload has failed.
The selector uses the PDI, in addition to the more conventional path layer failure indications, to trigger switches so that the situations described above can be handled.
DCS
STS-1
SEL
STS Payload Defect Indicator (PDI)
triggers the selector in the event of
the DS3 failure
112

39. ITU - SDH recommendations
G.707
SDH bit rates, frame & multiplexing structure, mappings, and functions of overhead
G.781
structure of recommendations on SDH equipment
G.782
types and general characteristics of SDH equipment
G.783
characteristics of SDH equipment functional blocks
G.784
management of SDH equipment and networks
G.803
SDH transport network architectures
G.841
SDH network protection architectures
G.842
interworking of SDH network protection architectures
G.957
optical interface specifications for SDH systems
G.958
interface specifications for SDH systems
The above documents are the key standards for SDH. ITU standards are called recommendations and they are voluntary.
G.707, G.708, and G.709 define the SDH rates and formats. G.957 defines the SDH optical interface specifications.
114

40. ANSI standards ANSI T1.105 basic SONET format, overhead definitions
SONET automatic protection switching
ANSI T
SONET payload mappings
ANSI T
Jitter at (SONET) Network Interfaces
ANSI T
SONET Data Communications Channel Protocols and Architecture
ANSI T
Tandem Connection Monitoring
ANSI T
SONET physical layer specifications
ANSI T
Sub-STS-1 Interface Rates and Formats (VT1.5 and VT Group)
ANSI T
SONET Timing and Synchronization
ANSI T1.106
Original version of ANSI T
ANSI T1.117
Electrical physical layer specifications including STS-1 and STS-3
The original SONET standard was published in two main documents, ANSI T1.105 and ANSI T T1.105 captured the rates and formats information whereas T1.106 captured the optical specifications.
More recently, T1.105 has been divided into a series of documents, each of which captures a different aspect of the interface specification. This allows certain aspects of the specification to be evolved without opening up the entire standard to review and revision.
115

41. Bellcore standards GR-499 GR-253 GR-1377 GR-1230 GR-1400
SONET Transport System Generic Requirements
GR-253
SONET Common Generic Criteria
GR-1377
OC-192 Criteria
GR-1230
Bidirectional Line Switched Rings (BLSR)
GR-1400
Unidirectional Path Switched Rings (UPSR)
These are the basic Bellcore SONET standards.
116

42. ETSI standards ETS 300 147 ETS 300 232/A1 ETS 300 417-1-1
SDH multiplexing structure
ETS /A1
Optical interfaces for SDH
ETS
SDH equipment; Generic processes and performance
ETS
SDH and PDH physical section layer functions
ETS
SDH path layer functions
ETS
Generic requirements for synchronization of SDH
ETS
SDH network protection
ETS
SDH information model
ETR 114
architecture of SDH transport networks
These are the key ETSI SDH standards.
117