To support the massive Internet expansion of recent years, carriers significantly increased the capacity of backbone and core networks. Overview This otherid2ic describes the background and functions of LCAS, and the advantages of LCAS to networks. Basic Concepts Learning the basic . mechanism of LCAS in SDH/SONET network and point out that even though LCAS really brings us much more advantages in SDH transmission network, it also.

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This capability is particularly useful in environments ocas dynamic adjustment of capacity is important. The LCAS mechanism can also automatically decrease the capacity if a member in a VCG experiences a failure in the network, and increase the capacity when the fault is repaired.

Link Capacity Adjustment Scheme – Wikipedia

Although autonomous addition after a failure is repaired is hitless, removal of a member due to path layer failures is not hitless. In the descriptions below, we use the term member to denote a VC. LCAS is resident in the H4 byte of the path overhead, the same byte as virtual concatenation. Virtual concatenation uses 4 of the 16 bytes for its MFI and sequence numbers.

LCAS uses 7 others for its purposes, leaving 5 reserved for future development.

Link Capacity Adjustment Scheme | SONET and SDH: Advanced Topics | InformIT

lcaz Status messages are continuously exchanged and consequent actions taken. The operation of LCAS is unidirectional. This means that in order to bidirectionally add or remove members to or from a VCGthe LCAS procedure has to be repeated twice, once in each direction. These actions are independent of each other, and they are not required to be synchronized.


The protocols behind LCAS are relatively simple. The state machine at the transmitter can be in one of the lvas five states:. This member is not provisioned to participate in the VCG. This member is provisioned to participate in the VCG and has a good path to the receiver.

This member is provisioned to participate in the VCG and has a failed path to the receiver. This member is in the process of being added to the VCG. This member is in the process of being deleted from the VCG. On incoming signal for this member experiences no failure condition.

Or, the receiver has received and acknowledged a request for addition of this member. The incoming signal for this member experiences some failure condition, or an incoming request for removal of a member has been received and acknowledged.

The transmitter and the receiver communicate using control packets to ensure smooth transition from one state to another. The following control words are sent from source to the receiver in order to carry out dynamic provisioning functions.

Each word is associated with a specific member i. Indicate that this VC is currently not a member of the group.

Indicate that this member is normal part of the group and does not have the highest sequence number. The following control words are sent from the receiver to the transmitter. These messages capture the status of all the VCG members at the receiver. The status of all the members is returned to the transmitter in the control packets of each member.

The transmitter can, for example, read the information from member No. As long as no return bandwidth is available, the transmitter uses the last received valid status. This is a bit used to acknowledge the detection of renumbering of the sequence or a change in the number of VCG members.


This acknowledgment is used to synchronize the transmitter and the receiver. The following is a typical sequence for adding a member to the group. Multiple members can be added simultaneously for fast resizing.

The network management system orders the source to add a new member e. The source node starts sending F ADD control commands in the selected member. The source sees the R OKassigns the member a sequence number that is one higher than the number currently in use. At a frame boundary, the source includes the VC in the byte interleaving and sets the control command to F EOSindicating that this VC is in use and it is the last in the sequence. The destination notices the F IDLE command and immediately drops the channel from the reassembly process.

OptiX OSN 7500, 3500, and 1500 Feature Description (SDH Transport Domain)(V200R011C00_04)

In this example, the deleted member has the highest sequence number. If this is not the case, then the other members with sequence numbers between the newly deleted member and the highest sequence number are renumbered.

Although SONET and SDH were originally designed to transport voice traffic, advent of these new mechanisms has made it perfectly suitable for carrying more dynamic and bursty data traffic.

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