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The 8-bit sequence number SQ which supports values of X up to uses the H4 byte bits [1. In this section only the relevant differences are mentioned. A low order VCAT payload container C—m—Xc provides the trans- port of a continuous payload by using the total transport capacity of X individual payload containers C—m. This extension of the range of signal labels was necessary anyway due to the introduction of virtual concatenation and the requirement to provide some spare values reserved for future applications.

If one of these additional extended signal labels indicates that the application is using virtual concatenation, the low order VCAT OH can be found in the K4 byte bit 2. If the value of the V5 byte bits [5. If the reserved bits are used in the future, care should be taken to ensure that an imitation of the MFAS string is avoided.

The extended signal label multi-frame is repeated every 16 ms, i. In addition, Table 2. It provides a measure for the differential delay with a granularity of 16 ms. The reserved bits shall be ignored by the receiver. The container size for these signals is determined as follows. The payload mapping is shown in Figure 2. The payload mapping is illustrated in Figure 2.

VCAT Eff. As an example, Table 2. Selecting a virtual concatenated payload container of the right size can provide this pipe. However, if the client signal is packet based, the demanded transport bandwidth will vary over time.

This chapter describes the methodology used to dynamically and hitlessly change i. In addition to changing the capacity of the contiguous payload container, this methodology also provides survivability capabilities, i.

The methodology will be applied to the entire VCG, i. The methodology depends on the close coopera- tion of the Source or send side process and the Sink or receive side process.

This is achieved by exchanging control information between Source side and Sink side processes. This chapter will give a detailed description of this control informa- tion and how this control information is used at the Source side and Sink side of the VCG trail. In Chapter 4, the LCAS processes operating at the Source side and the Sink side are described by using state transition diagrams and in Chapter 5 a number of examples explain the operation of LCAS by using time sequence diagrams.

Link capacity adjustment scheme 37 3. Control packets are sent continuously, even if there is no change in the inform- ation that it contains. This ensures that all planned changes of the VCG will be hitless.

The control packet contains both control information sent from Source side to Sink side and control information sent from Sink side to Source side. If that particular member should experience a failure, the Source side can select another member control packet without losing MST and RS-Ack information. Figure 3. Each one of these X successive bytes is transported by one of the X members in the VCG in its smaller member payload container; this is sometimes referred to as byte-slicing or inverse multiplexing.

To be able to reconstruct the original payload container at the receiving end of the link each part has to be reconstructed from the bytes received by the X individual members. To facilitate this, each successive payload con- tainer frame is assigned a unique number. This number is transmitted as control information in each member container.

Due to the limited number of bits available for the counter, the counter starts again at zero when the maximum value is reached. Because container frames are counted, the counter is in general termed frame-counter and the full set of counter values is termed multi-frame. At the Sink side the MFI value has to be used to realign all member container frames of a VCG before the reconstruction of the original payload container frame C—n—Xc can take place.

The MFI is used to determine the difference in propagation delay experienced by the individual members of the VCG caused by the diverse routing through the network. This propagation delay is 5 ms per km of physical path. The difference in delay between the fastest path and the slowest path through the network is referred to as the differential delay. At the Sink side, between the member path termination function and the VCAT adaptation, function buffers are used to compensate the experi- enced delay in order to realign the member container frames C—n before the reconstruction of the original payload container can be achieved.

This makes the multi-framing process of the virtual concatenation methodology with or without support of LCAS identical. The numbering of the members starts at zero and is consecutive, so the last member in a VCG with n members has sequence number n—1.

The use of the sequence numbers is similar to the SQ used in the recommenda- tions for Virtual Concatenation as described in Chapter 2. The value of the sequence number shall be ignored by the Sink side process for all members that are not an active member of the VCG, e. It is important that the Sink side is aware of the state a member is in at the Source side. The state of the member at the Source side is a determining factor in the reconstruction process at the Sink side.

This leaves enough room for possible future expansions of LCAS with more states. Link capacity adjustment scheme 41 Table 3.

This is backwards compatible because in the virtual concatenation standards these bits are set to all zero. It indicates that this is the last member of the VCG transporting valid payload.

Only one member in a VCG shall send this code. Due to wrong provisioning of this connection function, the possibility exists that two or more individual members of two or more different VCGs are interchanged. For example, in Figure 3. The content of the GID bit are the consecutive bits generated by a pseudo-random generator providing a pattern with a length of 1 bits.

Applying the same proce- dure to signals from the LCAS overhead would cause a considerable slowdown of the process. To simplify and speed up the validation of the changes in the virtual concatenation overhead, a CRC is used to protect and validate each control packet. The CRC check is performed on every control packet after it has been received, and the contents are rejected if the check fails.

If the control packet passes the CRC test, then its contents are used immediately. If the control packet containing control information to change the payload size experiences a bit error, the increase or decrease will not be hitless; however, the CRC validation limits this hit to a single container frame. The remainder is a polynomial of, at most, degree n 1. The CRC encoding procedure The control packet is considered to be static, i.

This means that the CRC—n checksum can be calculated a priori over the control packet. The CRC decoding procedure The decoding procedure is as follows: 1 A received control packet is acted upon by the division process as described above. The status of the members at the Sink side is affected by the status of the same members at the Source side and this status is transferred from Source side to Sink side by the CTRL codes in the control packet; the status is also affected by the behavior of the network, e.

Reporting the MST of all possible members in a single control packet would affect the size of the control packets considerably, and conse- quently the refresh rate of the information sent from Source side to Sink side. The quantity of members in the VCG can be any number in the allocated range and can be changed by provisioning from the NMS.

In this manner, the MST value received by the Source side will always correspond directly to the SQ value that it assigned to a member.

To allow the receiver to determine the number of members in the VCG, the following should be noted. In the standard it is recommended to add new members at the end of the VCG. Removing one or more members can happen at any place in the VCG. Changing the size of the VCG at the Sink side will have no effect on the sequence numbers because a change in the sequence numbering is the responsibility of the Source side LCAS process only.

Changes in the sequence numbering at the Source side are detected at the Sink side. Hence, from the time the changed sequence numbers are sent by the Source until the changed MST values are received, the Source side shall not use the received MST values to avoid misinterpretation and losing status synchronisation between Source side and Sink side. To acknowledge from Sink side to Source side that a change in sequence numbers is detected and the changed MST to SQ relation is valid, the re-sequence acknowledge RS-Ack signal is introduced.

The latter occurs when multiple members are added to the VCG at the same time. This addition will increase the payload bandwidth of the VCG. This removal will decrease the payload bandwidth of the VCG. This removal does not affect the payload bandwidth of the VCG. This change does not affect the payload bandwidth of the VCG. Note that following an ADD command from management inter- face i. The RS-Ack bit can only be toggled after the status of all members of the VCG has been evaluated and the sequence change has taken place.

In the event that the RS-Ack toggle is not detected at the Source side, the synchronisation between Sink side and Source side is achieved with the activation of a RS-Ack time-out timer. The expiration of the timer is equivalent to the toggling of the RS-Ack bit and detected at Source side. This means that MST values received in the control packet that contains the toggled RS-Ack bit and MST values received in subsequent control packets correspond to the new sequence.

Note that to avoid losing the status synchronisation between the Source side and the Sink side, no new change in the VCG should be committed until the RS-Ack is received or the RS-Ack time-out has expired for the currently active change request.

When a member is added to the VCG, it is recommended that it shall always be assigned a sequence number one larger than the currently highest sequence number. After the Sink side process has detected and processed the removal of the member, the member can be deleted at the Sink side and the network path can be broken down.

The service provided by each member of the VCG can be operational, degraded due to bit errors or unavailable. When the defect causing the degraded or unavailable condition is cleared, the Sink side will consequently send in the MST of that particular member the status OK.

If the member signal is degraded, i. The bit errors in the payload area of that member have to be handled by the server e. Ethernet layer of the payload transported by the VCG. The following container frames will contain all zeroes in the payload area. Link capacity adjustment scheme 51 3. Changes to the number of members in the VCG will be possible only by provisioning.

M8 is received, i. The value of MFI—2 is used modulo 32 as an index for this table. However, the length of the control packet remains the same: i. The status of all members is transferred in 64 ms, i. Table 3. The lower order control packet shown in Table 3. Link capacity adjustment scheme 55 Table 3.

The status of all 64 members is transferred in ms, i. By provisioning the LCAS protocol can be disabled. The status of all members is transferred in 1. The reduction will only affect the MST multi-frame, as explained below. The primary rate PDH control packet contains: Table 3. The status of all 16 members is transferred in two consecutive control packets. This counter enables the detection of a differential delay of up to ms for an E1 and ms for a DS1.

M8 is reported each control packet. The status of all 8 members is transferred every 2 ms for the E3 signal and every 1. This counter enables the detection of a differential delay of up to ms for an E3 and ms for a DS3. SDL provides two representations: one is graphical and uses symbols to envision the different elements in the language, e.

The textual representation even allows the text to be compiled producing code that can be used to verify the correctness of the protocol. In this chapter, only the graphical representation is used. Due to the diverse routing, it is very possible that some member paths from node A to node Z do experience a network problem, while the member paths from node Z to node A do not experience any network problem.

It was also envisioned that for certain applications the downstream path to the client would require more bandwidth than the upstream link, e. This implies connection asymmetry, i. A virtually concate- nated connection is considered to be symmetrical if each constituent member in the group has an accompanying member in the opposite direction, similar to a bi-directional connection. The Sink side member status is only reported on its partner, similar to the RDI bit in a bi- directional connection.

If it is desired to keep the connection sym- metric, it is recommended that this shall be provisionable per VCG from the network management system. Note that these actions are independent of each other and it is not necessarily required that they are synchronised.

The LCAS protocol provides a hitless increase and decrease of available bandwidth under the control of a network management system. In addition to the changes under management control, LCAS will autonomously remove failed members temporarily from the group.

When the failure condi- tion is remedied, LCAS will add the member back into the group. The autonomous addition of the failed members payload, after a failure is repaired, is again hitless. The value of this parameter will be determined by commands from the network management system. If the failed member is removed by network manager action there will be a renumbering of the remaining sequence numbers.

ITU-T has recommendation Z. It can be used in two ways, either a graphical presentation using symbols representing states, inputs, outputs, tasks, decisions, etc. Figure 4. When a VCG has been provisioned with m members, m instances of this state machine will be active simultaneously.

The messages pertain to the member from which the event is sent. This member is considered to be a provisioned member. It does not have the highest sequence number. This member is considered not to be an active member in the VCG. These events will occur in the Source side Send state machines only.

Now the Source side can, for example, read the returned status information from member No. As long as no return status information is available, the Source end will use the last received valid status information.

This acknowledgement is used to synchronise Source side and Sink side. Due to the renumbering of the sequence at the time of an ADD or a REMOVE request the received member status shall not be used for a time period that is determined by transmission delays and framing delays. Adding a new member is always at the end of the group with a new, highest, sequence number. Procedures at the Sink side receive end Figure 4.

It is used to avoid unwanted effects due to intermittent alarms, as described in ITU-T recommendation G. The HO procedure describes the Hold Off timer activation and deactivation procedure.

It is used to limit the number of temporary removal actions in case of nested protection switches, as described in ITU-T recommendation G. These examples are in the form of time sequence diagrams to explain the time relation between the successive steps taken during the operation of the protocol. As the SDL diagrams are state-machines they do not contain this timing informa- tion.

This chapter also provides some examples of typical LCAS state transitions. Some are based on planned changes of the size of a VCG and some are based on unplanned occurrences in the network affecting one or more members in a VCG.

A scale is not given because some of the time intervals depend on the propagation delay of the physical path between both VCG path termination functions at the Source side and at the Sink side.

There are also time intervals that depend on the internal processing of events and transitions between the different states. The NMS shall also setup the required new member path between the path termination functions at the Source side and the Sink side. The order of provisioning path and termination functions is arbitrary.

The following examples show the preferred order. This code indicates that this member is not yet active in the VCG. In a VCG of size n the highest sequence number is n 1. In the following examples the value max is used to indicate that the SQ number to be assigned is the highest possible value.

For the actual maximum values, refer to Chapter 4. LCAS time sequence diagrams 83 5. The new members will always be added to the VCG after the member that currently is sending the highest SQ number.

Again a distinction is made between the member of the VCG that has the highest SQ number and any other member experiencing a network problem. The second diagram depicts the addition of a single new member to a VCG where a network problem causes an RS-Ack time-out. The example shows the addition of two members because the addition of a single member can be derived easily from this example and adding more that two members will be similar.

The example illustrates also that the LCAS process can handle situations where the new information is received in a different order as when it was initially transmitted.

Figure 5. The addition is planned and will have to be initiated by the NMS. Step 1 The initial condition. This is reported back to the Source side by toggling the RS-Ack bit. In this example no further re-sequencing is required. The example shows that the LCAS process can handle situations where the expected toggling of the RS-Ack bit does not occur and a timeout is used to recover from this situation. None of these members is the last member of the VCG, i.

Possibly, this member is removed because the network fault cannot be repaired. The removal is planned and initiated by the NMS. The removal of more than two members is also similar.

Step 1 Initial condition. Step 2 The total payload transported by this VCG has to be decreased. This will be achieved in this example by removing member a and member b. At the same time the payload size of the VCG is decreased and distributed over the remaining active provisioned members.

All members with an SQ number larger than the original SQ number of the removed members will be allocated a new sequence number to keep the sequence of the VCG consecutive. The NMS can now also break down the removed members path through the network. The removal of multiple members including the last member of the VCG is a combination of this example and the previous example.

To achieve this member a , the last member in the sequence, is removed. At the same time the payload of the VCG is decreased and distributed over the remaining active provisioned members. Because the last member in the sequence is removed no re-allocation of the SQ numbers is required. LCAS time sequence diagrams 93 5. In both timing diagrams, the temporary decrease is caused by a complete loss of the member signal. If the network problem is a degraded signal due to bit errors, the response of the LCAS process is slightly different as explained in the last section.

This member is not the last member of the VCG, i. The removal of multiple members from within the VCG is similar. The removal is not planned. Step 2 A network fault occurs and affects the path of member a in the VCG. At the same time the payload of member a will not be used anymore to reconstruct the original VCG payload. For a certain time, i. This is indicated in Figure 5. A message reporting the failed member a is sent to the NMS.

Because the sequence numbering is not changed, the temporary change of bandwidth does not have to be acknowledged by the RS-Ack bit toggle. The payload of member a will remain unused. At the same time the payload area of member a is used again and the VCG will be able to transport the full payload. The bandwidth of the VCG is not affected. This member is the last member of the VCG, i. The removal of multiple members including the last one from the VCG is a combination of this example and the previous example.

Step 2 A network fault occurs affecting the path of member a in the VCG. Because the sequence numbering is not changed, the temporary change of bandwidth does not have to be acknowl- edged by the RS-Ack bit toggle.

At the same time the payload area of member a is used again and the VCG can transport the full payload. As a result, the RS-Ack bit will not be toggled due to this planned decrease. The payload bandwidth of the VCG is not affected. Because of the nature of the transported data signals, they are not affected necessarily by the bit errors.

In this case, the affected payload is not discarded. The data layer shall deal with the affected payload. In this case, Steps 2 and 3 in the two examples in sections 5. The payload of member a will continue to be used to reconstruct the original VCG payload. In fact the CBR signal can be transported by any synchronous transport system. Figure 6. SAN Figure 6. More detailed examples of functional models containing the GFP mapping are given in Chapter 7.

This visibility is obtained when the client PDUs are received from a bridge, switch or router function located in a transport network element TNE , e.

In the former case, the client PDUs are received via, for example, an Ethernet interface physical port b in Figure 6. This requires processing of the incoming codeword space for the client signal physical port a in Figure 6. Note that the type of physical port a must be the same to support an interconnection, while the type of physical port b may be different. The transmission order starts at bit 1 of octet 1 up to bit 8 of octet n. Generic Framing Procedure 1 2 3 4 5 6 7 8 1 4 Core Header.

The sink adaptation process performs the same CRC calculation. In the absence of bit errors, the remainder will be zero. A single error in the Core Header can be corrected. A Core Header with multi-bit errors will be recorded for performance monitoring purposes.

The scrambler performs a modulo 2 addition with the hexadecimal number 0xB6AB31E0. This number is the maximum transition, minimum side-lobe, Barker-like sequence of length This variable length area may include from 4 to 65 octets.

As shown in Figure 6. The type of extension header is indicated by the content of the EXI bits in the Type Field of the payload header, i. It is intended for scenarios where the transport path is dedicated to one client signal. Optionally, a single error in the Extension Header can be corrected. An Extension Header with multi-bit errors will be recorded for performance mon- itoring purposes.

The complement of this bit sequence is the CRC Payload area scrambling The GFP Payload Area is scrambled to prevent payload information replicating the server layer scrambling word or its inverse. When the de scrambler is disabled its content is retained.

These frames consist of a Core Header and a Payload Area. Payload Extension Header Extension 0 - Header Header. In this way the CMF may be used for multiple purposes. Table 6. An example CMF is provided in Section 6. The Idle frame is used to maintain a constant bit-rate when no client PDUs are available. In this state, the error correction is disabled. Single error correction remains disabled while in this state. In this state, single-bit Core Header error correction is enabled.

Idle GFP frames also participate in the delineation process and are then discarded. If the Payload FCS is present, all its bits are complemented.

These actions ensure that the client sink adaptation process, or the client termination process, will drop the errored PDU. Generic Framing Procedure 6. The Payload FCS is again optional. The pad character is mapped into the GFP frame in the same manner as a control character and is recognised and removed by the GFP demapper. The last two trailing octets are used for a CRC error check over the bits of this superblock.

The minimum value of N depends on the data rate of the client signal, the size of the GFP Header and the size of the payload container. It is the remainder of the CRC calculation over the bits in that superblock performed by the source adaptation process. If the sink adaptation process detects an error, it will output either a 10B error character or an unrecognized 10B char- acter in place of all of the client characters contained in the processed superblock.

This replacement guarantees that the client receiver will be able to detect the presence of the error. This bit sequence is the CRC Note that with this CRC, it is possible to correct single bit errors. However, since the signal is scrambled at the Source and de-scrambled at the Sink, the CRC error correc- tion circuit should account for single bit errors as well as double errors spaced 43 bits apart coming out of the descrambler. The sink adaptation process performs steps 1—3 in the same manner as the source adaptation process.

In the absence of bit errors, the remainder shall be GFP source encapsulation! GFP sink decapsula- tion! This is the minimum size to affect the spare bandwidth the least. This chapter describes the functional models and the de- compositions of the virtual concatenation compound function, the virtual to contiguous concatenation interworking compound functions and the LCAS enabled virtual concatenated compound functions.

Only the Trail Termination functions and adaptation functions are described that have a relation with the concatenation process. Only a brief description of the atomic functions is pro- vided. The functional models described in this chapter are for information only, but complete the description of the concatenation and mapping process.

The operation of the functions is provided in the next sections of this chapter. Figure 7. The SQ numbers will start at 0 and the maxi- mum assigned value will be X—1. X AP Figure 7. X] Figure 7. The received sequence number SQ of each member in the VCG is made available to the network management system.

Upon detection of one or more of the defects the consequent action is to replace the outgoing signal by an all-ones signal, i. The bi-directional model is shown in Figure 7. GFP-T is useful for delay sensitive services. GFP-T Transparent is a layer 1 encapsulation in constant sized frames. Transparent mode accepts native block mode data signals and uses SDH frame merely as a lightweight digital wrapper.

SDH concatenation consists of linking more than one VCs to each other to obtain a rate that does not form part of standard rates. Concatenation is used to transport pay loads that do not fit efficiently into standard set of VCs. The traditional method of concatenation is termed as contiguous. This means that adjacent containers are combined and transported across the SDH network as one container. Contiguous concatenation is a pointer based concatenation. It consists of linking N number of VCs to each other in a logical manner within the higher order entity i.

VC4 and above. The concatenated VCs remain in phase at any point of network. Virtual concatenation maps individual containers in to a virtually concatenated link. Any number of containers can be grouped together, which provides better bandwidth granularity than using a contiguous method.

VCs are routed individually and may follow different paths, within the network, only the path originating and path terminating equipment need to recognize and process the virtually concatenated signal structure as shown in Fig. LCAS is bi-directional signaling protocol exchanged over the overhead bytes, between Network Elements that continually monitors the link.

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